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Arranged by 




from the papers and discussions at the 
Seconcf FAO World Fishing Gear Congress, London, 1963 

Published by 



JUNE 1964 


TUs book embodk* the text and illustrations of the papers presented at the Second 
Worid Fishing Qear Congress, London, 1963, which WM organized by the Food and 
AgrkuUnre Orgmniadoo of the United Nations (FAO). However, tins is not an 
FAO publication. 

The Organization authorized the repnbUcatkHi of the material here presented as 
a service to the fishing industry, hot the responsibility for the reports of the discussions 
fbOowing the presentation of the various papers rests with the edtorial staff of the Fishing 
News Intenutioittlt^ of the pabtohei^ Fishing News (Books) L^ The text of the 
reports of the discussions has been reviewed for accuracy. 

The views expressed, both hi the papers and the discussions are those of the 
htdrridBab concerned and do not necessarily represent the views of FAO. 




All rights reserved 

The views expressed in the papers and discussions are those 
of the contributors and are not necessarily those of the Food 
and Agriculture Organization of The United Nations. 


PRINTED BY MERRTTT & HATCHER LTD., High Wycombe and London 


For con?miMC6 of prawntation tUs book is dhMod into tteoo 
relating to different aspects of the indartry. 

At the end of Parts I and H there are advertisement sections. 
Hew are mdoded because it to appreciated that practical commercial 
odd be readily available about naUng gear and equip- 
procnnubk from Tirious sources for the bettoi'nmt of fluMng 

An indra of ttese idvertisers' ^ BM y^ff fU miffltfi UDMTS in 


Notice to Readers .................. IV 

Copyright ...................... V 

List of Contributors .................. XI 


Section 1 Terminology Page 


Standardisation of Terminology and Numbering Systems for Netting Twines Gerhard Klust 3 

Discussion on Terminology .......... 8 

Section 2 Test Methods 

Test Methods for Fishing Gear Materials (Twines and Netting) A von Brandt and P. J. G. Carrothers 9 

Discussion on Test Methods .......... 49 

Section 3 New Net Materials 

Netting Twines of Polypropylene and Polyamide Compared .... Gerhard Klust SO 

Polypropylene Twines in Japan ..... Katsuji Honda and SMgeru Osada 55 

Use of 'Ulstron* Polypropylene in Fishing . . . C. L. B. Carter and K. West 57 

Synthetic Fibre Fishing Nets and Ropes made in Japan . Japan Chemical Fibres Association 64 

Production and Characteristics of Synthetic Nets and Ropes in Japan . Yoshinori Shimozaki 66 

Japanese Fish Netting of Synthetic Fibres ...... Iwao Tani 71 

New Synthetic Herring Driftnets Used in the North Sea ... Janusz Zaucha 73 

Discussion on New Net Materials ......... 78 

Section 4 Ropes, Knofless Nets and Monofflaments 

Ropes of Polyethylene Monofilaments . . . . C. C. Kloppenburg and J. Renter 81 

Tests on Knotless Raschel Netting . . . . . . .A. von Brandt 88 

Knotless Netting in the Norwegian Fisheries ..... Norvald Mugaas 96 

Knotless Fishing Nets on Raschel Equipment in Italy .... Mario Damiani 97 

Resistance & la Rupture de Filets sans Noeuds .... Francesco Pianarali 101 

Monofilaments in Fishing ...... D.F.C.EdeandW.Henstead 103 

Nylon Monofilament in the Viet-Nam Fisheries . . Tran-Van-Tri and Ha-Khac-Chu 108 

Monofilament Oillnets in Freshwater Experiment and Practice . . . R. Steinberg 111 

Etudes sur to Freinage et l f Usure des FilsdePftche . Maurice Bombeke 115 
Discussion on Ropes, Knotbss Nets and Monofilaments . . . . * .118 

Nets afld HMOS 121*144 


Section 5 Stem Trawling Page 

The Stern Trawler a Decade's Development in Trawl Handling . . Conrad Birkhoff 147 

Some Small Stern Trawlers E. C. B. Corlett 153 

Ross Daring Experiment ....... Dennis Roberts 158 

Discussion on Stern Trawling . * . . . * . . . .160 


Section 6 Bottom Trawling 

Some of the General Engineering Principles of Trawl Gear Design . . . P. R. Crewe 165 

Some Comparative Fishing Experiments in Trawl Design .... W. Dick son 181 

Development of an Improved Otter Trawl Gear .... Chikamasa Hamuro 191 

Towing Power, Towing Speed and Size of Bull Trawl . . . Chikamasa Hamuro 199 

Suggestions for Improved Heavy Trawl Gear Eldon Nichols 204 

Development of Soviet Trawling Techniques . . . . . A. I. Treschev 206 

Double-Rig Shrimp Beam Trawling . . . . . /. Verhoest and A. Maton 209 

Discussion on Bottom Trawling . . . . . . . . . .218 

Section 7 Midwater Trawling 

One-Boat Midwater Trawling from Germany ...... S. Schdrfe 221 

Universal One-Boat Midwater and Bottom Trawl . . . . S. Okonski 229 

Two-Boat Midwater Trawling for Herring with Bigger Boats ... R. Steinberg 235 

Development of the Cobb Pelagic Trawl A Progress Report . . . Richard L. McNeely "240 

Underwater Telemeters for Midwater Trawls and Purse Seines . Chikamasa Hamuro and Kenji Ishii 248 

Reaction of Herring to Fishing Gear Revealed by Echo Sounding ...//. Mohr 253 

Discussion on Midwater Trawling ......... 257 

Section ft Gillnetting, Longlining and Traps 

King Crab Pot Fishing in Alaska R. F. Allen 263 

Les Madragues Atlantique et Sicilienne ....... Vito Fodera 271 

Eel Traps Made of Plastic H.Mohr 277 

Types of Philippine Fish Corrals (Traps) . . Arsenic N. Roldan Jr. and Santos B. Rasalan 279 
A New Fish Trap used in Philippine Waters . . . Santos B. Rasalan 282 and 593 

Dropline Fishing in Deep Water Ronald Powell 287 

Discussion on Gillnetting, Longlining and Traps . . . . . . .291 


Section 9 Purse Seining Page 


Recent Developments in Icelandic Herring Purse Seining . . . Jakob Jakob sson 294 

Sonar Instruction Courses for Fishermen ..... G. Vestnes 306 

Discussion on Purse Seining . . . . . . . . . .310 

Section 10 Deck Machinery 

The Application of Hydraulic Power to Fishing Gear . . . . D. W. Lerch 314 

Advances in Centralised Control and Automation . . . . H. E. H. Pain 338 

Driftnet Hauler for Salmon Fishing ...... Chihlro Miyazaki 347 

Mechanization of Driftnet Fishing Operations . . . . P. A. Kuraptsev 352 

The Complex Mechanization of Beach Seining . . . . . S. S. Torban 355 

Discussion on Deck Machinery . . . . . . . . .361 

Section 11 Fish Detection 

A Comprehensive Echo-Sounder for Distant-Water Trawlers G. H. Ellis, P. R. Hopkin and 363 

R. W. G. Hasten 

Sector-Scanning Sonar for Fisheries Purposes . . . D. G. Tucker and V. G. Welsby 367 

A New Sonar System for Marine Research Purposes . . . T. S. Gerhardsen 371 

Detection et Localisation des Banes de Poissons .... Robert Lenier 376 

Echo-Detection of Tuna ........ Minoru Nishimura 382 

Echo-Sounder Measurement of Tuna Longline Depth Kyotaro Kawaguchi, Masakatsu Hirano and 385 

Minoru Nishimura 

A 200 Kc/28 Dual Frequency Echo-Sounder for Aimed Midwater Shrimp Trawling . . 388 

Masakatsu Hirano and Takashi Noda 

Ddtecteur de Poisson "Explorator" ......./. Fontaine 396 

Bio-Acoustical Detection of Fish Possibilities and Future Aspects . . G. Freytag 400 

Study of Acoustical Characteristics of Fish . . . . E. V. Shishkova 404 

Frequency Analysis of Marine Sounds . . Tomiju Hashimoto and Yoshinobu Maniwa 410 

Identifying Pacific Coast Fishes from Echo-Sounder Recordings . . . E. A. Best 413 

Echo-Sounding through Ice . Tomiju Hashimoto, Yoshinobu Maniwa, Osamu Omoto and 

Hidekuni Noda 415 

Discussion on Fish Detection . . . . . . . . . .417 

Section 12 Fleet Operations 

Japanese Mothership and Fleet Operations for Salmon, Crab, Longlining and Tuna 

Hiroshi Tominaga, (2) Masatake Neo, Nippon Suisan Kaisha Ltd., and Goro Okabe 423 

Las Pesqucrias Espanolas Austro-Atlanticas ..... K. Paz-Andradc 438 
Discussion on Fleet Operations .......... 445 

Advertisement Section on Gear and Equipment etc. 447-468 



Section 13 Gear Reaearck Page 


The Theory of Designing and Testing Fishing Nets in Model . . Tasae Kawakami 471 

The Development of a Midwater Trawl P. Dale and S. Mailer 482 

Fishing Methods and Gear Research Institutes: Their Organization and Scope A. von Brandt 489 

Discussion on Gear Research .......... 493 

Section 14 Instruments for Testing Gear 

Trawl Gear Instrumentation and Full-Scale Testing /. Nicholls 497 

Some Japanese Instruments for Measuring Fishing Gear Performance 

Chikamasa ffamuro and Kenji Ishii 513 

Trawl Studies and Currents . J. N. Carruthers 518 

Performance of the Granton Trawl ....... W. Dickson 521 

Discussion on Instruments for Testing Gear ........ 525 

Section 15-Fish Behaviour 

The Importance of Vision in Fish Reaction to Driftnets and Trawls 

/. H. S. Blaxter, B. B. Parrish, and W. Dickson 529 

Importance of Mechanical Stimuli in Fish Behaviour Especially to Trawls C J. Chapman 537 

The Use of Air-Bubble Curtains as an Aid to Fishing . . . Keith A. Smith 540 

Utilization of Fish Reactions to Electricity in Sea Fishing . . Conradin O. Kreutzer 545 

Problems of Electro-Fishing and Their Solution .... Jurgen Dethloff 551 

Notes on the Importance of Biological Factors in Fishing Operations 

B. B. Parrish and J. H. S. Blaxter 557 

Tuna Behaviour Research Program at Honolulu .... John /. Magnuson 560 

Shrimp Behaviour as Related to Gear Research and Development 

Charles M. Fuss Jr. andF. Wathne 563 

Experimental Dispersion of Chum ..... Kentaro Hamishima 570 

Evolution de la Pdche & la Lum&re dans les Lacs Africains . . . A. Collart 573 

Pump Fishing with Light and Electric Current . . . . . /. K Nikonorov 577 

Discussion on Fish Behaviour .......... 579 

Section 16 Science and the Future 

Prospective Developments in the Harvest of Marine Fishes Dayton L. Alverson and 

Norman /. Willmovsky 583 

Automatic Data Processing and Computer Use in Fisheries . . Benjamin F. Leeper 590 

Discussion on advance in Science and the Future . . . .591 



ALLEN, ROBERT F. 160, 164, 263, 311, 362 

Marine Construction and Design Co., 2300, W, Commodore 
Way, Seattle, Washington, U.S.A. 

ALVERSON, DAYTON, L. 161, 219, 260, 421, 422, 526, 

527, 583, 591, 593 

U.S. Bureau of Commercial Fisheries, Exploratory Fishing and 
Gear Research Base, 2725, Montlake Boulevard, Seattle, 2, 
Washington, U.S.A. 


American Embassy, Copenhagen, Denmark. 


3, South Beach Parade, Great Yarmouth, U.K. 

Federal Fisheries Service, Lagos. 

BEST, E. A. 413 

Marine Resources Operations Laboratory, 411, Burgess Drive, 
Menio Park, California, U.S.A. 

BLAXTER, J. H. S 557, 579 

Marine Laboratory, Tony, Aberdeen, U.K. 

BIRKHOFP, CONRAD . . . 147, 161 

Schiffbau-Dipl, Ing. Fischereitechnische Konstruktioncn 
Schroderstiftstr, 30, Hamburg, 13, Germany. 

BOMBEKE, MAURICE . . . .115 

Ets Cousin Freres, Wervicq-Sud (Nord), France. 

BOSCH, IR. J. V. D. . . . 495, 528 
Mekelweg 2, Delft, Netherlands. 

VON BRANDT, PROF, DR. ANDRES 8, 9, 49, 79, 80, 88, 

119, 489 

Direktor, Institut fur Netz- und Materialforschung, Palmaillc 9, 
Hamburg- AJtona 1, Germany. 

CARLSON, W. W 161, 362 

Blount Marine Corp., Warren, Rhode Island, U.S.A. 


Fisheries Research Board of Canada, St. Andrew's, New Bruns- 
wick, Canada. 

CARRUTHERS, DR. J. N. , . . 495, 518 
National Institute of Oceanography, Wormley, nr. Godalming, 
Surrey, U.K. 

CARTER, C. L. B 57 

Industrial Fibres Division, Imperial Chemical Industries Ltd., 
Harrogate, Yorks, U.K. 

CHAPMAN, C. J 537 

Marine Laboratory, lorry, Aberdeen, U.K. 

CHAPMAN, DR. WILBERT McLEOD 495, 581, 582, 592 
Van Camp Foundation, 739, Golden Park Avenue, San Diego 6, 
California, U.S.A. 


COLE, DR. H. A. . . . 420, 527, 580, 592 
Fisheries Laboratory, Lowestoft, Suffolk, U.K. 

COLLART, A. 573 

FAO/EPTA Economiste des Ptehes, Cotonon, Dahomey. 

CORLETT, E. C. B. . . . 153, 162 

Burness, Corlett & Partners Ltd., Basingstoke, Hants., U.K. 

CRAIG, R. E 417, 421 

Marine Laboratory, Aberdeen, Scotland, U.K 

CREWE, P. R. . . . 165, 219, 494, 495 
Westland Aircraft Ltd., (Saundcrs-Roe Division), East Cowes, 
Isle of Wight, U.K. 

CREWDSON, B. W. .... 79, 80 

The Great Grimsby Coal, Salt and Tanning Co. Ltd., Fish 
Dock Road, Grimsby, U.K. 

Fisheries Laboratory, Lowestoft, U.K. 


DALE, P 482 

Arbeidsutvalget for utvikling av pelagisk enbatstral, C. Sundtsgt, 
3, Postboks 37, Bergen, Norway. 

Society Rhodiatoce, Piazza Erculea, 15, Milano, Italy. 

DE WIT, J. G 219 

Ministry of Agriculture A Fisheries, The Hague, Netherlands. 

DETHLOFF, JURGEN . . . 551, 582 

International Electronics Laboratories, GmbH, 52, Lottestr, 
Hamburg-Lokstedt 1, Germany. 

Pesquerias y Secaderos de Bacalac de Espafta, S.A. (PYSBE), 
Aguirre Miram6n, 2, San Sebastian, Spain. 

DICKSON, W. . 163, 181, 493, 496, 521, 525, 528, 

Marine Laboratory, Torry, Aberdeen, Scotland, U.K. 


MacGregor-Comarain, S. A., 96 bis rue du Ranelagh, Paris, 
16 tone, France. 

DOUST, DAVID J 163, 591 

Ship Hydrodynamics Laboratory, Faggs Road, Feltham, Middx. , 

EDE, D. F. C 103 

British Resin Products Ltd., Devonshire House, Piccadilly, 
London, W.I., U.K. 

EDDIE, GORDON, C. . . . 445, 446 

White Fish Authority, Industrial Development Unit, St. Andrew's 
Dock, Hull, U.K. 




ELUS, G. H. 363 

S. Smith & Sons (England) Ltd., Kelvin Hughes Division, 
St. dare House, Minorics, London, E.C.3., U.K. 

FODERA, Vrro . 
FAO/EPTA, Fishery Adviser, Tunis, Tunisia. 


FONTAINE, JEAN-GABRIEL 362, 396, 418, 420, 421 
Compagnie g6nerale de t16graphie Sans Fil (CSF) 
79, Boulevard Haussman, Paris 8 erne, France. 

FRECHET, JEAN . . . 163, 291, 312 

Federal Department of Fisheries, 
Sir Charles Tupper Building, Ottawa, Ontario, Canada. 

FREYTAG, DR. G. ... 400, 582 

Institut fur Nctz- und Materialforschung, Palmaille 9, 
Hamburg-Altona 1, Germany. 

Fuss, CHARLES M. JNR. . . . .563 
U.S. Bureau of Commercial Fisheries, 
Gear Research Station, Panama City, Florida, U.S.A. 


Simonsen and Mustad A/S, Morten, Norway. 


Flyveien 13, Holmen, Norway. 

HARPER-GOW, L. M. . 80, 161, 219, 260, 581, 582 
Chr. Salvesen & Co., 29, Bernard Street, Leith, Edinburgh 6, U.K. 

GUNDRY, E. F 80 

Joseph Gundry & Co. Ltd., Bridport, Dorset, U.K. 


Fisheries Directorate, Saigon, Viet-Nam. 


S. Smith & Sons (England) Ltd., Kelvin Hughes Division, 
St. Clare House, Minorics, London, E.C.3. 

HAMASHIMA, KENTARO . . . 311, 570 
Nagasaki Prcfcctural Fisheries, Matsugae-Cho, Nagasaki City, 

HAMURO, CHIKAMASA PROF. C. 191, 199, 248, 513 
Fishing Boat Laboratory, Fisheries Agency 1, 
2-Chome Kasumigaseki, Chiyoda-ku, Tokyo, Japan. 

HASHIMOTO, PROF. T. 410, 415 

Fishing Boat Laboratory, Fisheries Agency 1, 
2-Chome Kasumigaseki, Chiyoda-ku, Tokyo, Japan. 

HASLETT, DR. R. W. G. . . . 363, 419 
S. Smith & Sons (England) Ltd., Kelvin Hughes Division, 
Barkingside, Essex, U.K. 

HHNSOHN, DIPL. ING. H. . 160, 261, 362 

Richmers Werft, Postfach 2066, Bremerhaven 2, Germany. 

HENSTEAD, W 80, 103 

British Celanese, 345, Foleshill Road, Coventry, U.K. 

HINDS, V. T 527, 581 

Fisheries Department, Aden. 

HIRANO, MASAKATSU . . . 385, 388 
Sanken Electronics Co., 
1997 Sembon gorin, Numazu City. Japan. 

HONDA, DR. KATSUJI ... 55, 79 

Tokyo University of Fisheries, 
6-Chome Shibakaifcan-dori, Minato-ku, Tokyo, Japan. 


HOWARD, D. S. . 

Bridport Industries Ltd., Bridport, Dorset, U.K. 



HOPKIN, P. R. . . . . .363 

Kelvin Hughes, St. Clare House, Minories, London, E.C.3. 

ISHII, PROF. K 248, 513 

Fishing Boat Laboratory, 2-Chome, Kasumigaseki, Chiyoda-ku 
Tokyo, Japan. 

JAKOBSSON, JAKOB . . . 259, 294, 311, 313 
Fiskideild Skirlag. 21, Reykjavik, Iceland. 

3, 3-Chome, Muromachi, Nihonbashi, Chuo-ku, Tokyo, Japan. 

KAWAKAMI, DR. T. . . . .471 

Department of Fisheries, Kyoto University, Maizuru, 
Kyoto Prefecture, Japan. 

Kanagawa Prefectural, Fisheries Experimental Station, Misaki, 
Miura City, Japan. 

KLOPPENBURG, C. C. . . . 81, 119 

Kunstzijdespinnerij "Nyma" N.V., Nijmegen, Netherlands. 

KLUST, DR. GERHARD . . 3, 8, 50 

Institut fur Netz- und Materialforschung, Palmaille 9, 
Hamburg- Altona 1, Germany. 


W. & J. Knox Ltd., Kilbirnie, Ayrshire, U.K. 


KOBAYASHI, MASAO . . . .119 

Nippon Seimo Co. Ltd., 6, 1-Chome, Ginza, Chuo-ku, Tokyo, 


Smith Research & Development Co., Inc., Lewes, Delaware, 

KRISTJONSSON, HILMAR . 80, 119, 262, 292, 313 
Fisheries Division, FAO, Rome, Italy. 

Chairman Japan Chemical Fibres Association. 


KURAPTSEV, P. A. .... 352 

Institute of Marine Fisheries & Oceanography (VNIRO), 
Moscow, U.S.S.R. 

Fisheries Inspectorate, P.O. Box 630, Accra. 



Consulting Naval Architect, Stadsgarden 10, Stockholm So, 
Sweden. . 

LEAKEY, R. D 292 

R. & B. Leakey, Sutcliffe House, Settle, Yorks, U.K. 


Univac Division, Sperr> Rand Corp., Box 1549, Baton Rouge, 
Louisiana, U.S.A. 

LENIER, CDT. ROBERT . . 218, 311, 376, 419 
Commission Marine Marchande et Peche, S.P.E.R., 
11 bis, rue St.-Augustin, Asnicres (Seine), France. 


Marine Construction and Design Company, 
Seattle, Washington, U.S.A. 



LEVY, HAIM 311, 582 

Place de I'Jndependance, Safi, Morocco. 

LIBERT, Louis ... 261 

Institut des Ptehes Maritimes, Qua! de la Poste, 
Boulogne-sur-Mer (P de Q, France. 

LUSYNE, PIERRE . . 49, 78, 80, 119, 362, 421 
Fishing Gear Section, FAO, Rome, Italy. 

MAGNUSON, J. J. . . . .560 

Bureau of Commercial Fisheries, Honolulu, Hawaii, U.S.A. 

MANIWA, Y. .... 410, 415 

Fishing Boat Laboratory, Fisheries Agency, 1, 2-Chome 
Kasumigaseki, Chiyoda-ku, Tokyo, Japan. 

MARESCHAL, Y. J. A. . . . .311 

Anciens Chantiers Dubigeon, 7, rue Caumartin, Pans, France. 

MAJIGETTS, A. R 218, 219, 220 

Fisheries Laboratory, Lowestoft, Suffolk, U.K. 

MAURIN, C. . . . . .162 

Institut des Ptehes Maritimes, 6, rue Voltaire, Site, France. 

MOCRACKEN, FRANK . . . .260 
Fisheries Research Board, St. Andrew's, New Brunswick, 

McNEELY, R. L 240 

Research Bureau of Commercial Fisheries, Seattle Washington, 

MOHR, DR. H. . . . 253, 277 

Institut fur Netz- und Materialforschung, Palmaille 9, 
Hamburg-Altona 1, Germany. 

MATON, DR. ANDRE . . . .209 
Rijksstation voor Boerdeijbouwkunde, University of Agiiculture, 
Ostend, Belgium. 

MIYAZAKI, DR. CHIHIRO 219, 291, 293, 347, 445, 527 
Tokai Regional Fisheries Research Laboratory, Tsukishima, 
Chuo-ku, Tokyo, Japan. 

MOLLER, S. ..... 482 

Bergens Mekaniske, Verksteder, P.O. Box 858, Bergen, Norway. 

MROSS, SIEGFRIED . . . .418 

Atlas-Werke A. G. Bremen, Paschenburgstr. 63, Bremen, 


Engineer, Statens Fiskeredskapsimport, Bergen, Norway. 

NALL, A. F. B 49 

British Standards Institution, Coronation House, Market Street, 
Manchester, 1., U.K. 

NEO, MASATAKE . . . 423, 428 

Nichiro Gyogyo Kaisha Ltd., Marunouchi Building, Tokyo, 

NICHOLS, ELDON . . 204, 219, 220, 527 
American Telephone & Telegraph Co., Ocean Cables Division, 
32 Ave. of the Americas, New York, 13, N.Y., U.S.A. 

NICHOLLS, J. .... 497, 527 

Westland Aircraft Ltd., (Saunders-Roe Division), East Cowes, 
Isle of Wight, U.K. 


Bergens Notforretning, P.O. Box 868-870, Bergen, Norway. 



NIKONOROV, I. V. . . . . .577 

Caspian Institute of Marine Fisheries & Oceanography, 
Astrakhan, U.S.S.R. 

Tokyo Building, Marunouchi, Chiyoda-ku, Tokyo, Japan. 

NISHIMURA, M. 382, 385 

Fishing Boat Laboratory, 1, 2-Chome, Kasumigaseki, Chiyoda- 
ku, Tokyo, Japan. 

Shibaura Technical Institute, Tokyo, Japan. 


NODA, TAKASHI ..... 388 
Sanken Electronics Co., Ltd., No. 1907 Sembon-Gorin, Numazu 
Shizuoka Pref., Japan. 

NORMAN, J. , . . . . .79 
Ministry of Agriculture, Fisheries and Food, 10, Whitehall Place, 
London, S.W.I., U.K. 

OCRAN, R. K. N 80, 311 

Mankoaeze Fisheries Ltd., P.O. Box 103, Tema, Ghana. 

OKABE, GORO .... 423, 436 
Taiyo Gyogyo Kabushiki Kaisha 4, 1-Chome Marunouchi, 
Chiyoda-ku, Tokyo, Japan. 

OKONSKI, S. ..... 229 

Sea Fisheries Institute, Al Zjednoczenia 1, Gdynia, Poland. 

Fisheries Division, Department of Lands, 3, Cathal Brugha 
Street, Dublin, Ireland. 

OMOTO, OSAMU . . . , .415 
Shibauru Technical Institute, Tokyo, Japan. 

OSADA, S 55 

Nippon Gymo Sengu Kaisha Ltd., Tokyo, Japan. 

PAIN, CMDR. H. E. H. . . . 338, 361 
S. G. Brown Ltd., Shakespeare Street, Watford, Herts., U.K. 

PANZER, HELMUT , . . .119 

Plutte, Koccke & Co., Ottostr 1, Wuppertal-Barmen, Germany. 

Department of Fisheries, Colombo, Ceylon. 

Marine Laboratory, Torry, Aberdeen, U.K. 


PARSEY, M. R 261 

Fibres Division, Imperial Chemical Industries Ltd., Harrogate,' 
Yorks., U.K. 


Revista "Industries Pesqueras", Policarpo Sanz 22, Vigo, Spain. 

PETRICH, BORTI . .311, 312, 581, 582 

U.S. Net & Twine Co., P.O. Box 9206, Long Beach, California, 

Retificio Carlo Badinotti, Viale Beatrice d'Este, 18, Milano, Italy. 


Government of the Cook Islands, Rarotonga, Cook Islands. 

PRAET, MARCEL . . . . .119 
U.C.B., Division Fabelta, Application Industrielle, 18, Chausaee 
de Charleroi, Bruxelles 6, Belgium. 




J. ft. E. Hall Ltd., Dartford, Kent, U.K. 

RASALAN, SANTOS B. . . 279, 282, 593 

Philippine Fisheries Commission, Diiiman, Quezon City, 


Government School of Fisheries, P.O. Box 58, Florce, Norway. 


81, 118, 119 

Nederlandsch Visschery Proefstation, Mailiebaan, 103, Utrecht, 

ROBERTS, CAPT. DENNIS A. 79, 158, 162, 220, 494, 581 
Ross Trawlers Ltd., Fish Docks, Orimsby, Lines., U.K.. 



Wm. Kenyon ft Sons (Rope and Twines) Ltd., Dunkinfield 
Cheshire, U.K. 



Philippine Fisheries Commission, Diiiman, Quezon City, 

ROWE, K. C T. 


Great, Orimsby coal Salt & Tanning Co., Ltd., Battery Oreen, 
Lowestoft, Suffolk, U.K. 

SCHABFBRS, E. A. . . 80, 310, 312, 313, 361 

Branch of Exploratory Fishing, U.S. Bureau of Commercial 
Fisheries, Washington 25, D.C., U.S.A. 

SCHARFE, DR. J. 220, 221, 257, 260, 261, 262, 312, 
361, 418, 495, 526, 527, 528 

Institut fur Netz- und Materialforschung, Palmaille 9, 
Hamburg-Altona 1, Germany. 



Marine Construction ft Design Co., 2300 Commodore Way, 
Seattle, 99, Washington, U.S.A. 



Fishing Gear Division, Tokai Regional Fisheries Research 
Laboratory, Tsukishima, Chuo-ku, Tokyo, Japan. 



Institute of Marine Fisheries & Oceanography (VN1RO), 
Moscow, U.S.S.R. 



Decca Radar Ltd., North Quay Fish Docks, Grimsby, Lines., 



Fish ft Wildlife Service, Gear Research Base, State Fish Pier 
Gloucester, Massachusetts, U.S.A. 

STEINBERG, DR. ROLF . . . ill, 235 

Institut fur Netz- und Materialfonchunf , Palmaille 9, 
Hamburg-Altona 1, Germany. 


&** 1*^4^ *?* Nct 
Chuo-ku, Tokyo, Japan, 

Association, Bchizenbori, 

... 292 


4/0 Ministry of Agriculture, Maiduguri, Northern Nigeria. 

423, 435 

Taiyo Fishery Co., 4, 1-Chome Marunouchi, Chiyoda-ku, 
Tokyo, Japan. 


TORBAN, DR. S ...... 355 

Research Institute of Marine Fisheries (VNIRO) 
17v Krasnoselskaya, Moscow, U.S.S.R. 

TRAN-VAN-TRI . . . . .108 

Fisheries Directorate, Saigon, Viet-Nam. 

TRAUNG, J. ..... 527, 528 

Chief, Fishing Boat Section, Fisheries Division, FAO, Rome, 

TRESCHBV, A. 1 ..... 206, 259 

Chief,' Fishing Technique Laboratory, Institute of Marine 
Fisheries (VNIRO), Moscow, U.S.S.R. 

Fisheries Laboratory, Lowestoft, Suffolk, U.K. 



University of Birmingham, Birmingham]! 5, U.K. % 

TVBDT, JOHN A. . . . 219, 260, 527 
Research and Fishing^Ltd., 1, Chesham Place, London, S.W.I., 

VALDEZ, V. . 79, 80, 313, 119, 420, 421 

Institute de Investigation de los Recursos Marines, Avenida 
Bologncsi 22, La Punta, Callao, Peru. 

VERHOEST, J. . . . 209, 261 

Commissie T.W.O.Z., Zeewezen Rebouw, Natienkaai, Ostende, 

VESTNES, GUDMUND . . . .306 

Institute of Marine Research Nornesparken 2, Bergen, Norway. 


Wakefield Fisheries, Port Wakefield, Kodiac, Alaska, U.S.A. 



U.S. Bureau of Commercial Fisheries, Gear Research Station, 
Panama City, Florida, U.S.A. 


Fisheries Department, Kenya, East Africa. 


WELSBY, V. G. . . . 307, 420, 582 
University of Birmingham, Birmingham, U.K. 


Imperial Chemical Industries Ltd., Fibres Division, 
Harrogate, York*., U.K. 


Institute of Fisheries, University of British Columbia, 
Vancouver, British Columbia, Canada. 


British Nylon 
Pontypool, Monmout] 

Textile Development Dept., 
ire, U.K. 


Sea Fisheries Institute, Zjcdnoczcnia l t Gdynia, Poland. 






THE book Modern Fishing Gear of the World, resulting from the first FAO Fishing Gear 
Congress held in Hamburg in 1957, sold out and had to be reprinted. This confirmed 
the belief that there existed a great need for a comprehensive work on fishing gear and 
fishing technology. The book was widely hailed and has undoubtedly stimulated gear 
development and the application of modem methods in commercial fishing. Innovations that were 
pure experiments when reported at the 1957 Conference have proved themselves and are now widely 
used. Other innovations and developments have occurred and many more studies have yielded 
further data on existing gears. 

These are dealt with in the present book which comes as a result of the Second FAO World 
Fishing Gear Congress, which was held in London at the kind invitation of Her Majesty^ Govern- 
ment in the United Kingdom in May, 1963. Though this Congress dealt with fewer subjects than 
the former one in 1957, the book contains more matter, despite considerable condensation. This 
comes about because subjects are dealt with in 'depth', much more detailed work having been done 
than heretofore. It is hoped that this book, in its turn, will serve to further stimulate interest and 
progress in its field and be useful to the fishing industry in decision making. 

While dealing with Materials, Gear and Fishing, and Gear Research in some 88 papers, 
certain trends stood out. Two of these were the emphasis which the Congress placed upon (1) 
rational gear research and (2) the standardization of testing methods. It was felt that much more 
should be done on these lines to supplement the trial and error approach. Another is the work 
that is reported on the hydrodynamics of trawls. Much has been learned about the movement of 
gear through water and the efficiencies that may be obtained, but it is found that it does not follow 
that the hydrodynamically perfect trawl will necessarily be the best fish catcher. This depends 
upon the reaction of the fish themselves towards moving bodies. This is an aspect of fish behaviour 
a subject which, in spite of its difficulties, is attracting increased attention, though one might say 
that it has scarcely begun. 

When one goes over the papers in this book one is impressed with the growing complexity 
of gear technology, as well as the growing demands upon the skippers who use it. Though there is 
still a great deal of subjective judgment in successful fishing, these judgments are being aided in- 
creasingly, if not being gradually supplanted, by the intricacies of instrumentation. Aviators faced 
the same situation yesterday. Successful flying used to be done 'by the seat of the pants', but this 
is gone today. 

This points to the need for better instruction in the training of skippers in a wide range 
of techniques and of instrumentation as modern fishing vessels become more complex. It is only 
in this way that the increased expenditure of the capital required can be justified. Perhaps this 
book may be useful in this respect. Once again on the matter of training, the facilities offered to 


gear technologists are meagre. Only a few countries have gear research institutes at the present 
time. As has occurred in other kinds of oceanographical research, perhaps the gear scientists will 
find it profitable to organize team work between their institutions, so that they may share their 
skills and observations, especially in the more expensive and more complex operations. 

The papers embodied in this book come as a result of much patient work on the part of 
both the authors and the Secretariat of FAO, who devised and organized the Congress. It is grati- 
fying to think that the two years spent in preparation have yielded this result. Again, it is fortunate 
that the Congress took place in London. The Ministry of Agriculture, Fisheries and Food and 
the Foreign Office generously placed their services at the disposal of FAO, and many thanks are 
due to the Fisheries Secretary, Mr. Hugh Gardiner, and his staff for the excellent arrangements. 
It was fortunate, too, that the Congress coincided with the First World Fishing Exhibition which, 
through the courtesy of World Fishing, enabled the participants to view the latest improvements 
in fishing materials and equipment. 

DR. D. B. FINN, 

Director, Fisheries Division, 
Food and Agriculture Organization 
Rome, January 15, 1964. of The United Nations 





Section 1 Tenninology 

Standardisation of Terminology and Numbering Systems for Netting Twines . . Gerhard Klust 3 

Discussion on Terminology 8 

Section 2 Test Methods 

Test Methods for Fishing Gear Materials (Twines and Netting) A. von Brandt 9 

and P. J. G. Carrothers 

Discussion on Test Methods 49 

Section 3 New Net Materials 

Netting Twines of Polypropylene and Polyamide Compared Gerhard Klust 50 

Polypropylene Twines in Japan Katsuji Honda and Shigeru Osada 55 

Use of 'Ulstron' Polypropylene in Fishing C. L. B. Carter and K. West 57 

Synthetic Fibre Fishing Nets and Ropes Made in Japan. . . .Japan Chemical Fibres Association 64 

Production and Characteristics of Synthetic Nets and Ropes in Japan. . . . Yoshinori Shimozaki 66 

Japanese Fish Netting of Synthetic Fibres Jwao Tani 71 

New Synthetic Herring Driftnets Used in the North Sea Janusz Zaucha 73 

Discussion on New Net Materials 78 

Section 4 Ropes, Knotless Nets and Monofilaments 

Ropes of Polyethylene Monofilaments C. C. Khppenburg and J. Reuter 81 

Tests on Knotless Raschel Netting A. von Brandt 88 

Knotless Netting in the Norwegian Fisheries Norvald Mugaas 96 

Knotless Fishing Nets on Raschel Equipment in Italy Mario Damiani 97 

Resistance & la Rupture de Filets sans Noeuds Francesco Planaroli 101 

Monofilaments in Fishing D. F. C. Ede and W. Henstead 103 

Nylon Monofilament in the Viet-Nam Fisheries Tran-Van-Tri and Ha-Khac-Chu 108 

Monofilament Gillnets in Freshwater Experiment and Practice R. Steinberg 1 1 1 

Etudes sur le Freinage et TUsure des Fils de P6che Maurice Bombeke 115 

Discussion on Ropes, Knotless Nets and Monofilaments 118 

Advertisement Section 

Trade announcements relating mainly to nets, netting and ropes, together with equipment . . 121-144 



Part 1 Materials for Nets and Ropes 

Section 1 Terminology 

Standardisation of Terminology and Numbering 
Systems for Netting Twines 


Some terms and definitions of netting twines recommended by the 
International Organisation for Standardisation are reviewed, as 
well as presently used twine numbering systems, and the need for an 
internationally accepted numbering system is pointed out. The 
tex-system. recommended by ISO, expresses the mass in grams for a 
unit length of 1,000 m of yarn or twine, and has the advantage, 
as an international unit, that it is based on the metric system and 
gives a direct estimation of the twine size, as the heavier the twine 
is the greater the tex value becomes. For the purpose of defining a 
simplified but generally useful numbering system for netting 
twines, the author then discusses the relevant characteristics of 
netting twines in relation to the information required by the fisher- 
men, netmakers and gear technologists. There are two basic ways of 
describing twines by the tex-system; the most detailed one gives 
the tex value of each yarn and the number of yarns and plies, e.g., 
23 texx5x3. To give the weight per unit length of the twine, 
further information must be given as to the degree of twisting and 
chemical treatment. The simpler form gives only the resultant 
linear density of the twine, called R tex, i.e., the actual weight in 
grams of 1,000 m of the twine. This value includes the effect of 
twisting operations, and will thus replace the conventional indication 
of runnage (m/kg and yds/lb). The author feels that R tex alone does 
not provide sufficient information in the case of thin twines but 
would be adequate for thicker twines. The complicated and multi- 
plex construction of braided twines and of twines made of dissimilar 
components only allow designation by R tex values. 

Standardisation de la terminologie et des systfcmes de numeration 


Quelques termes et ddfinitions de fils de filets recommandees par 
TOrganisation Internationale de Standardisation sont recapitulees 
ainsi que les systemes de num6rotage utilised internationalement. 
Le systeme tex, recommand6 et approuv6 par l'"OIS" donne la 
masse en grammes par unite de longueur de mille metres de fil de 
caret ou de fil retprdu. Ce systeme a 1'avantage d'etre bas6 comme 
toutes les unites international sur le systeme metrique et donne 
une estimation directe de la mesure du fil puisque plus lourd sera 
le fil, plus grande sera la valeur en tex. Pour definir un systeme de 
numerotage simplifi6 mais utilisable dans la fabrication de filets, 
1'auteur examine les caracteristiques applicables aux fils de filets en 
relation avec les informations demandees par les pgcheurs, les 
fabricants de filets et les techniciens de peche. II y a deux manieres 
de base pour decrire les fils retordus par le systeme tex; la plus 
detaillee donne la valeur en tex de chaque fil et le nombre de fils et 
de brins, c'est-a-dire, par example: 23 texx5x3. Pour obtenir le 
poids par unitd de longueur d'un fil retordu, on a besoin de donnees 
sur le degrg de retordage et le traitement chimiquc. Le systeme plus 
simple donne seulement la densit6 lineaire resultant du fil retordu, 
appelee R tex, c'est-a-dire le vrai poids en grammes de 1000 m de 
ce fil retordu. Cette valeur comprend Teffet du retordage et rem- 
placera 1'indication conventionnelle m/kg et yds/lb. D'apres 1'auteur 
Vindication en R tex seule, ne donne pas asses d'informations dans 
le cas des fils fins mais decrit d'une fason adequate les fils lourds. 
La construction compliquee et multiplex des fils tresses et des 
fils composes de matieres dissemblables admet seulement la ddsigna- 
tion par les valeurs R tex. 

Normalizadfa de la terminologia y de los sistemas de nmneracion 
dehilos para redes 


Se reseftan apelaciones y definiciones de hilos para redes recomen- 
dadas por la Organizaci6n Internacional para Normalizaci6n, 
sistemas de numeraci6n de hilos empleados actualmente y se pone 
de relieve la necesidad de disponer de un sistema de numeraci6n 
accptado internacionalmcntc. El sistema tex, recomendado por 


Gerhard Klust 

Institut fur Netz und Material- 
forschung, Hamburg 

la OIN, expresa la masa en gramos por unidad de longitud de mil 
metros de hilo; como patr6n internacional tiene la yentaja de que 
se basa en el sistema metrico y de que da una estimacidn directa del 
tamaflo del hilo ya que cuanto mayor es este mas grande es el 
niimero tex. Para dennir un sistema de numeration de hilos para 
redes sencillo y de utilidad general, el autor examina las caracteris- 
ticas principales de dichos hilos con respecto a la informaci6n que 
necesitan Pescadores, fabricantes de redes y tecnicos en material de 
pesca. Existen dos maneras principales de describir hilos por el 
sistema tex ; el mas dctallado da el numero tex y torsibn en cada hilo, 
por ejemplo: 23 tex x 5 x 3. Para poder dar el peso por unidad de 
longitud se necesita mas informaci6n respecto al grado de torsidn y 
tratamiento quimico. La forma mas sencilla solo da la densidad 
lineal resultante del hilo, llamada R tex, es decir: el peso real en 
gramos de mil metros de hilo. Este niimero no s61o comprende 
el efecto de la torsi6n, con lo que sustituira la indicaci6n normal 
de m/kg y y/lb. Considera el autor que el niimcro R tex solo no 
facilita suficiente informaci6n en el caso de hilos finos pero basta 
para los mas gruesos. La complicada y multiple fabncaci6n de 
hilos trenzados y ole materiales distintos s61o permite que sean 
designados por numeros R tex. 


AS this paper is written for the fishing and not the 
textile industry, only those terms are defined which 
should be used for communications in the English 
language for international trade with fishing gear, in the 
exchange of information between fishery scientists and 
in the literature of fishery science. The following terms 
and definitions are based on recommendations of the 
International Organisation for Standardisation (ISO 
Refs. 6 and 7). 

1*1 Material for fishing nets 

(a) Discontinuous fibres (or staple fibres), which may 
be combined by twisting to form spun yarn. 

(b) Continuous filaments, which are usually, but not 
always, combined by twisting to form filament 
yarn, also described as multifilament yarn. 

(c) Monofilaments 1 , continuous filaments which have 
greater diameter and stiffness than those used in 

1 By ASTM Standards, monofilament is a "single filament of 
sufficient size to function as a yarn in normal textile operations**. 
(Ref. 1). 

multifilament yarn. They may be used singly to 
make netting such as for fine gillncts, in which case 
the "y &m " consists of only one monofilament. 
Two or more monofilaments may be twisted 
together to form folded monofilament yarn (or 
plied monofilament yarn). 

1*2 Single yam is "the simplest continuous strand of 
textile material" (Ref. 6). The term "yarn" is a general 
term; when meaning the components of the final 
product the term single yarn should be used. 

M Netting twine. At the first meeting of the sub-commit- 
tee "Textile Products for Fishing Nets" of the ISO 
Committee "Textiles" on 23 May, 1962, in Hamburg, 
it was suggested that the term netting twine should 
be used instead of "net twine", "fishnet twine", 
"fishing twine" or other terms, as the general term 
for any kind of yarn or combination of yarns (inclu- 
ding braided) usable for the manufacture of netting 
(Ref. 7). Fishing nets will seldom be made of single 
yarns. The usual netting twines are: 

Folded yarns (or plied yarns) 
Cabled yarns 
Braided twines. 

The terms "strand" (as a component of twisted 
twine) and "cord" should not be used. 

Netting twine in the form of folded yarn (plied 
yarn) consists of two or more single yarns twisted 
together by a single twisting operation. 

Netting twine in the form of cabled yarn is made 
by two or more twisting operations. Firstly, two or 
more single yarns are twisted together to form a 
folded yarn (plied yarn) and then two or more of 
these folded yarns are twisted together to form the 
cabled yarn (Fig. 1). For very strong netting twines, 
cabled yarns can be combined by a third twisting 

Netting twine in the form of braided twine. A 
variety of single yarns or of folded yams may be 
combined in the braiding process. The possibilities 
of varied constructions are greater than with the 
twisted twines (see 6'3). 

2-1 Conventional systems 

There are many systems in use throughout the fishing 
industries of the world. The most important are probably: 

The international denier system (Td) 

The metric count (Nm) 

The English cotton count (Nee) 

The international denier system gives the weight in 
grams of 9,000 m of single yarn. This system seems to 
obtain in all countries. It is used for netting twines made 

of continuous synthetic filaments and (Ref. 13) also for 
silk twines. Example 1: 210 denx5x3 (abbreviated: 
210 den x 15). 

The metric count is the number of kilometres of the 
single yarn which weigh one kg. It is mainly used in 
European fishing industries for cotton netting twines 
and for netting twines made of discontinuous (staple) 
synthetic fibres and in some countries for hemp netting 
twines. Also netting twines made of continuous filaments 
are so described. Example 2: Nm 20/5/3 (abbreviated: 
Nm 20/15). 

The English cotton count of a single yarn is the number 
of hanks, each of a length of 840 yds, which weigh one 
English pound (Ib). The system is used in Great Britain, 
U.S.A., Japan, Canada and other countries for cotton 
twines and for twines made of discontinuous (staple) 
synthetic fibres. Example 3: Ne c 20/5/3 (abbreviated: 
Ne c 20/1 5).* 

2*2 Conventional systems of less importance or restricted 

The English linen count (lea) of a single yarn is the num- 
ber of hanks, each of a length of 300yds, which weigh one 
English pound (Ib). It is used for netting twines made of 
ramie and flax (Refs. 2, 3 and 13). 

The equivalent English cotton count (C.20). In this 
system yarn count is related to a yarn of Nec20. It is 
used especially in Japan. 

"The numbers of yarn for C.20-equivalent twine are 
obtained by dividing the total denier of the twines with 
the thickness of the C.20-equivalent. It should be remem- 
bered, however, that the thickness C.20-equivalent 
determined for these twines does not necessarily repre- 
sent, in a strictly physical sense, the true thickness of 
cotton 20's." (Ref. 1 2). In Canada and the U.S. A."medium- 
laid, continuous multifilament synthetic twines are 
numbered according to what the manufacturer thinks 
is the equivalent Nec cotton twine, although there is 
disagreement as to what constitutes equivalence". 
(P. J. G. Can-others). 

The rope-yarn number m/kg (N t or N R ) is used in 
some European countries, for instance in Germany 
(Ref. 9) for trawl twines made of manila, sisal and syn- 
thetic filaments. Example 4: N t 3/900 (three single yarns 
are connected in the netting twine; each single yarn with 
a length of 900 m has the weight of one kg). 

The yards per pound number is used for trawl twines 
in Great Britain and Canada and, to some extent, for 
seine twines in Canada. 

2 In the examples 1, 2 and 3, the first figure denotes the s 
yarn count, the second figure the number of single yarns constitu 
the folded (plied) yarn and the third figure the number of fol 
yarns constituting the final product (netting twine). In the inter- 
national denier system, which is a direct numbering system, the 
figures are connected by the multiplication sign x . If an indirect 
system is used (Nm and Ne c ) they are separated from each other 
by the solidus (oblique stroke) /. 

Hemp yarn may be identified by the number of pounds 
per spindle (14,400 yds); (Ref. 3). 

The diameter in millimetre or inch is generally used 
for monofilaments. 

The runnage in m/kg in yds/lb or in g/100 m (the 
fatter e.g., in Italy) of the final product (netting twine) 
is of great importance for netmakers and fishermen. 
It is generally used in connection with other count sys- 

This survey is not complete. In nearly all countries 
further methods of identification of netting twines are 
used, though not all of them can be classed as numbering 
systems. Manufacturers, for instance, sometimes denote 
their products with letters A, B, C ... or with Figs. 1, 
2, 3 and so on. On the other hand, fishermen often prefer 
their own traditional denominations, taking little heed 
of the other systems. (Ref. 13). At the first FAO Fishing 
Gear Congress the need was expressed for replacing 
the various numbering systems by a single system applic- 
able to all kinds of netting twines. The general adoption 
of such a system on an international scale would avoid 
confusion and facilitate international trade and the 
exchange of information between fishery technicians. 
Because of its many advantages and its acceptance as 
universal yarn count system by the textile industry, the 
tex-system established by ISO should be used for netting 


In 1956, Technical Committee Textiles of International 
Organisation for Standardisation agreed to recommend 
a direct system based on metric units for international 
adoption in place of the various traditional methods of 
numbering. The basic unit of the new system is the tex. 
"The linear density (or number) of a yarn in tex expresses 
the mass in grams of yarn having a length of one thou- 
sand m. Thus a thread designated one tex has a mass of 
one gram per thousand m of its length." (Ref. 5). 

TABLE I Conventional numbers and tex values 

1 tex=- 



The higher the tex value, the heavier the yarn. The 
numerical value is followed by the term "tex", e.g., 
23 tex.3 

Formula for converting the various conventional 
yarn numbers into the tex-system (Ref. 5): 

^ 1000 590-5 1000000 496055 

tex-0,llllxTd == - = - = ^-r-zr- 

Nm Ne c m/kg yds/lb 

(Td= international denier system; Nm= metric count; 

Ne c = English cotton count) 

In order to facilitate the conversion into the tex-system 
the following conversion tables are given with values 
rounded off according to ISO (Refs. 4 and 5): 



Nee = tex 

m/kg tex 

70= 7-6 

200- 5 

120= 5 

5000= 200 

75= 8*4 

160= 6-4 

100- 6 

4500- 220 

90= 10 

120= 8-4 

90= 6-4 

4000= 250 

100= 11 

100- 10 

70 8-4 

3600- 280 

110= 12 

90= 11 

60= 10 

3000= 340 

125= 14 

85- 12 

50= 12 

2500= 400 

150- 17 

70= 14 

40= 15 

2250= 440 

180= 20 

60= 17 

30= 20 

2000= 500 

190= 21 

50= 20 

26= 23 

1800= 560 

200= 22 

43= 23 

20= 30 

1600- 640 

210- 23 

36= 28 

12= 50 

1500= 680 

250= 28 

34- 30 

10= 60 

1400= 720 

265- 30 

30= 34 

8= 72 

1200= 840 

300- 34 

20= 50 



420= 46 

18- 56 


630= 69 

14= 72 


840- 92 










3 In order to avoid mistakes, the multiple (kg per 1,000 m) and 
sub-multiple (milligram per 1,000 m) of the tex unit should not be 
used for the fishing industry. It seems to be better for netmakers and 
fishermen to have only one unit for designating all sizes of single 
yarns and netting twines. 


4-1 Technical description 

A complete technical description of a twine in the form 
of folded yarn (plied yarn), based on the linear density 
of the single yarn, comprises (Ref. 6) : 

Linear density (number) of the single yarn in tex 
Direction of twist of the single yarn (Z or S) 
Amount of twist of the single yarn (expressed as the 
number of turns per metre, or of turns per inch, of 
the twisted single yarn) 
Number of single yarns twisted together 
Direction of folding (plying) twist (Z or S) 
Amount of folding (plying) twist (t/m or t/i) 
Example 5 (Ref. 6) : 34 tex S600 x 2Z400; R 69'3 tex. 
For twine in the form of cabled yarns the following 
data must be added: 

Number of folded (plied) yarns cabled together 
Direction of cabling twist (Z or S) 
Amount of cabling twist (t/m or t/i) 
Example 6 (Ref. 6): 20 tex Z700x2S400x3Z200; 
R 123 tex. 

In examples 5 and 6 the values R 69*3 tex and R 123 tex 
are the resultant linear densities of the twines. They are 
given for additional information and separated from the 
proceeding parts of the twine counts by a semi-colon. 
The symbol R is set before the numerical value (Ref. 6). 

4*2 Resultant linear density 

The resultant linear density is the linear density (tex 
number) of the final product (netting twine) resulting 
from twisting, folding or cabling operations. It takes 
into account the increase in mass per unit length by the 
twisting (or braiding) operations. 

Resultant linear density is greatly dependent on the 
amount of twist of single yarns, folded yarns and the 
final product. Some examples for netting twines made of 
polyamide filaments (without chemical treatment) are 
given in Table II. In column 2 of this table the product 
of single yarn tex x number of yarns is given. The values 
in column 3 are the coefficients of twist a of the final 
products which are netting twines in the form of cabled 

yarn. The degrees of twist of single yarns and folded 
yarns have not been considered. The values of a have 
been calculated by means of the formula : 

Nm is the metric number of the twine. The actual 
resultant linear densities in column 4 are rounded arith- 
metic means of different numbers of test results. 

TABLE II Coefficients of twist and resultant tex values 

1 2 
Twine Yarn tex x 
construction number of 

of twine- 
twist, a 

R tex 


in number 






(23 tex x 12) 










(23 tex x 18) 





(23 tex x 21) 





23 texx8x3 
(23 tex x 24) 





23 texx9x3 






In selecting and recommending an international number- 
ing system, the following points of view should be taken 
into consideration: 

5*1 The number of a netting twine is only one of the 
specifications fishermen give when ordering netting 
twine and netting. The fibre material, e.g., cotton 
or nylon, must, of course, also be mentioned. 
Other additional information concerning proper- 
ties of the twine, e.g., its thickness or breaking 
strength, may be given too. But these details do 
not belong to the number of the twine. 

5*2 The new system should be based on the tex values 
and on the recommendations of the International 
Organisation for Standardisation but, for use in 
the fishing industry, simplifications are necessary. 
The complete twine count (see examples 5 and 6) is 
useful for the manufacturers of twine and netting 
and for fishing gear technologists but not very 
practical for the fishermen and most of the net 

5*3 In order to facilitate the introduction of the new 
system into the fishing industry, traditional 
methods of notation should not be entirely dis- 
regarded. They should be respected to a certain 
extent if they are not in contrast to the principles 
of the new system (see point 7). 

5*4 Proposals for simplification (if some details of twine 

construction must be indicated): The meaning of 
the numerical values of the amount of twist is 
scarcely known by fishermen and, therefore, should 
not be indicated. A general term, such as soft-laid, 
medium-laid, hard-laid, would be preferable. 
The twist direction of the final product only is, 
to some extent, of importance to the fisherman 
and netmaker but not the twist direction of the 
components of the twine. With twines in the form 
of cabled yarns it is not necessary to give the 
numbers of the single yarns and of folded yarns 
separately but only the total number of single 
yarns. This notation is already spread in Japan, 
Italy and Germany and in many other countries. 
Examples for this simplified twine indication are 
given in Table II, column 1, in brackets (Ref. 10). 
5-5 Netmakers and fishermen often require the weight 
of the netting twine. Resultant tex values can be 
used for calculating the weight of netting twine and 
netting and thus also the price. In the new system 
they should replace the twine runnage (m/kg or 
yds/lb) which should no longer be used. 


There are recommendations for simplifying the designa- 
tion of netting twine to one value only: the resultant tex. 
On account of the influence of twisting, R tex cannot 
be determined by only multiplying the yarn tex (as in 
column 2 in Table II). Proposals have been to approxi- 
mate the R tex value by applying a correction as twist 
factor of 10 per cent which often holds true for medium- 
laid twines. But the resultant linear density should be 
indicated with sufficient accuracy as the weight of 1000 m 
of the twine to be used as a basis for calculating the 
weight of netting. This R tex value basing on the actual 
weight of the twine could be used alone for thick trawl 
twines (see 6*1), for twines of dissimilar components 
(see 6*2) and for braided twines (see 6'3). 

6-1 Thick netting twines 

The actual R tex values could be used singly, without 
additional description for designating the thick netting 
twines of big bottom trawls. 

Examples for strong twisted trawl twines made of 
continuous poly amide: 

R 8000 tex (125 m/kg) 

R 6250 tex (160 m/kg) 

R 5000 tex (200 m/kg) 

R 4000 tex (250 m/kg) 

R 3333 tex (300 m/kg) 

Hfere the differences between the numerical values are 
so evident that the kinds of twine may be clearly separ- 
ated from each other by the R tex. If necessary, the 
direction of twist of the twine may be put at the end, 
e.g., R 8000 tex S. 

6*2 Netting twines of dissimilar components 

These are produced mainly in Japan and are of some 
importance for the fishing industry. They may be com- 
posed of yarns made of different kinds of fibres (e.g., 

R 3000 tex (333 m/kg) 
R 2500 tex (400 m/kg) 
R 2000 tex (500 m/kg) 
R 1650 tex (600 m/kg) 

'Livlon', made of poly vinylidcnc chloride and polyamidc 
fibres, or 'Marlon', made of polyvinyl alcohol and poly- 
amide fibres) or of spun yarns and filament yarns of the 
same kind of fibre. The clear notation of such twines 
being very complicated (Ref . 6), they should be numbered 
by R tex with additional information concerning the 
fibre material. 

6-3 Braided netting twines 

They are generally round or tubular braided, but braids 
of flat structure are also manufactured for fishing nets. 
There is an infinity of construction possibilities for these 

The number of strands, braided together, depends on 

the number of bobbins of the braiding machine 

(e.g., 6, 8, 12, 16, or others). 

The strand may consist of one or more (Fig. 2) yams, 

twisted together or not. 

Each strand may include the same number of yarns or 


The braid may contain a core (Fig. 2) or not. 

The core may consist of only one yarn, or of several 

yarns, twisted together or not. 

The core yarns may be of the same count as those of 

the strands or of another fineness. 

This complicated structure of braids does not allow 
a simple notation. Only resultant tex can give a practical 
and useful designation. 

5/oQ/f yarns 
tf*oi number: 6 

netting twin* 
(cabltd yarn) 

23 tex x2 x3'j R~.texZ 

Fig. 1. Construction of a netting twine in the form of a cabled yarn. 

braidtd ntttina twir* 

toc/i i frond with 3 aing* 


Fig. 2. Construction of a braided netting twine. 


As can be seen from Table II, the R tex values of netting 
twines of the same single yarn tex and the same number 
of yarns but with different amount of twist can differ 
in a high range. It is even possible that a hard-laid 
twine with a lower number of single yarns has a higher 
R tex than another soft- or medium-laid twine containing 
more single yarns. (See the examples 23 tex x 21, 23 tex x 
24 and 23 tex x 27 in Table II.) 

The resultant tex alone does not supply full indication 
in most cases and netting twine, except those mentioned 
above (see points 6*1-6*3), should therefore be designated 
by the count of the single yarn and the number of single 
yarns constituting the twine and, additionally, by the 
resultant tex. (See Fig. 1.) 

There is another reason for suggesting this notation: 
If the types of fishing gear are classified according to the 
strain on the net material (Ref. 8) most of them belong 
to the medium-strained group, e.g., seines and purse 
seines, bottom trawls of small vessels, most midwater 
trawls, gillnets for herring, cod and salmon, most river 
stownets, dipnets, fykenets, trapnets, castnets and so on. 
Studying fishery literature we can find that the netting 
twines for these types of gear are nearly always designated 
by the single yarn count and number of single yarns. 
The systems used may be different (Td, Nm, Ne or 
others) and so may be the way of writing the notation 
but nearly always we may find the two components 
(often supplied by the runnage in m/kg or yds/lb). 

Fishing industry, netmakers and distributors are 
accustomed to this method of numbering; they are not 
textile experts; for them the number of a netting twine is 
what they ask for when ordering so as to receive a twine 
of a certain construction and breaking strength. Numbers 
such as Td 210 x 15, Nm 20/15 or Ne 20/15 are names of 
these kinds of twine well known to the fishing industry. 
This should be taken into account when introducing the 
tex-system. Example: 23 texx5x3; or simplified: 
23 tex x 15; R 380 tex Z. Here, "23 tex x 15" is the 
unchanging fixed nominator of the twine, similar to the 
conventional systems used in most countries and there- 
fore easily understandable to the fishermen. The resultant 
tex value, rounded to a certain degree, changes according 
to the amount of twist and may be used for calculating 
the weight of twine and netting. 


All numerical values of netting twines should be given in tex. 

The resultant tex value (R tex) could be used alone for 
those types of netting twine which are unmistakably 
identified by this notation or have a complicated con- 
struction (as e.g., braided twines or twines of dissimilar 
components). It should replace the conventional runnage 
in m/kg or yds/lb. 

Types of twine which are not unmistakably identified 
by R tex alone should be designated by single yarn count 
in tex, number of single yarns and, additionally, by the 
resultant tex. (As in Fig. 1.) 

If necessary, the direction of twist (S or Z) of the netting 
twine could be put at the end of R tex, as well as "soft-" 
"medium-" or "hard-laid". 

The author wishes to thank the following experts for their very 
useful advice and comments: Dr. Arzano (Italy), Mr. Can-others 
(Canada), Mr. Hentschcl (Germany), Mr. Lonsdalc (Great Britain), 
Dr. Reuter (Netherlands) and Prof. Takayama (Japan). 
Not all of them concur in all details with his opinion. 

1. (1956) American Society for Testing Materials: ASTM Stand- 
ards on Textile Materials, Philadelphia. 

2. (1958) Canadian Government Specifications Board: Specifica- 
tion for nets; fishing. Ottawa. 

3. (1959) Carrothers, P. J. G. : The physical properties of netting 
and twines suitable for use in commercial fishing gear. 

In: Modern Fishing Gear of the World. 

4. (1958) Deutscher Normenausschuss: DIN 60 905 and DIN 
60 910. Berlin. 

5. (1961) ISO, TC 38: Implementation of the tex-system for 
designating the size of textile fibres, yarns and similar structures. 
Draft ISO recommendation, No. 391. 

6. (1962) ISO, TC 38, Sub-committee 4: Proposal for draft ISO 
recommendation Designation of yams. 

7. ( 1 962) ISO, TC 38, Sub-committee 9 : Basic terms and definitions 
for textile products for fishing nets. 

8. (1959) Klust, G.: The efficiency of synthetic fibres in fishing 
especially in Germany. 

In: Modern Fishing Gear of the World. 

9. (1961) Klust, G.: Fischereiliches Tauwerk aus Hart- und 
Bastfasern. Teil I: SchnUre. Archiv fUr Fischereiwissenschaft, XII, 

10. (1963) Klust, G.: Uber die Verwendung der tex-Werte zur 
Kennzeichnung der Netzgarnfeinheit. Deutsche Seiler-Zeitung, 
82, Nr. 1, 163-164. 

1 1 . (1960) Lonsdale, J. E. : Numbering systems for fishing twines 
Report of Working Party. 

12. (1959) Shimozaki, Y.: Characteristics of synthetic twines 
used for fishing nets and ropes in Japan. 

In: Modern Fishing Gear of the World. 

13. (1959) Takayama, Sh.: Terminology and numbering systems 
used in Japan. 

In: Modern Fishing Gear of the World. 


Dr. G. Klust (Germany) Rapporteur: Following the 
expressed desire of the participants at the First Gear Congress 
(1957) that the different numbering systems should be replaced 
by one single International system usable in all countries and 
applicable to all kinds of net materials, a working party was 
established to review the position and make a recommenda- 
tion. Its Chairman, Mr. Lonsdale, submitted a report in 
December 1960 of which the conclusions were that the uni- 
versal system might be either a conventional system, preferably 
metric, or an entirely new one in which case the tex-system 
should be selected. 

Today the decision is in favour of the tex-system which 
the textile industries of many countries have already accepted. 
The netting industry is only a small special branch of textiles. 
It is therefore important that the tex-system should be accepted 
by the fishing industry without further discussion. The 
unit of the tex-system is the "tex" based on metric values. 
The linear density, or the number of a single yarn, in tex 
expresses the mass in grams of a yarn having a length of 
one thousand metres. For instance, 23 tex means a single 
yarn, which, with a length of 1,000 metres, weighs 23 grams. 
Besides this unit "one tex" the International Organisation 
for Standardisation recommends two other units; one is 


the multiple of tex, kilotex, meaning kilograms per 
thousand metres and the other is the sub-division of tex, 
millitex, meaning milligrams per thousand metres. But 
1 propose that these further terms be not used in fishing in 
order to avoid new confusion. It would seem to be better 
to have high numerical values in tex for thick twines than to 
number by kilotex, and the lighter twine by tex or millitex. 

The advantages of the tex-system are that it is direct and 
can be applied to twine as well as yarn. Because of the 
advantages outlined in detail in the paper the sub-committee 
"Textile products for fishing nets" of ISO, has resolved to 
recommend the following notations for netting twines. 

Notation may give details of the single yarns making 

up the product 

(e.g., 23 tex X 5 X 3 ; R 380 tex) 

or when especially constructed twines are described, may 

be limited to the resultant count and final twist direction 
(e.g., R 380 tex S) 

or when describing braided twines, may give the resultant 

count alone 

(e.g., R 380 tex). 

Netting industries and fishing industries should accept 
these notations and FAO should support them. Manu- 
facturers of twine and netting should use, in the first place, 
the tex value and then the conventional number, maybe in 

Mr. A. Robinson (UK) questioned the wisdom of the 
recommendation that the term "strand" should not be used 
to indicate ply in the twine. He thought, to the trade 
generally, it would be clearer than the alternative of "folded 
yarn". "Strand" was well known because of its use in ropes 
where it was the standard term. In relation to the claim that 
the description should be "unmistakable" he pointed out 
that if the term was given up, it could leave doubt as to whether 
twine was two ply or three ply it could be 6 X 2, 4 x 3, 
or 3 X 4 and manufacturers would often be left to guess. 

Dr. von Brandt: These points have been considered and 
the measures suggested are deemed to be most suitable. 

Mr. B. M. Knox (UK): Since the introduction of 
synthetic fibres there has been a great advance and most 
manufacturers do now designate twine in either metres per 
kilo, or yards per Ib which is an advance on the old method. 
This data does assist the fishing industry in understanding 
the selling numbers of twine. My company will work with 
others to achieve some sort of standardisation. 

Dr. Klust summing up, stressed that descriptions must be 
as simple as possible. The terms "ply" or "fold" are recom- 
mended by ISO for International use. "Strand" is generally 
used for the components of ropes only and not of twines. 
If the first twine numbering proposal of ISO sub-committee 
is accepted (example: 23 tex X 5 X 3), it could leave no doubt 
as to the number of plies. The amount of twist of yarn, ply 
and twine has a great effect on the properties of netting twine 
(as can be seen from tables in the paper). But fishermen do 
not understand numerical values of twist given in turns per 
metre or per inch and only need to know the direction of the 
twist of the final product, the netting twine as well as the 
the terms "soft", "medium", or "hard". The problem of 
trade names was only one for the manufacturer of fibres. 
The main problem was to introduce the tex unit and that 
depended on the manufacturers of twine and netting. The 
question of how to write the number of twine is of secondary 

Part 1 Materials for Nets and Ropes 

Section 2 Test Methods 

Test Methods for Fishing Gear Materials 
(Twines and Netting) 


The present paper is the result of the Working Party on Testing 
Methods set up during the first FAO World Fishing Gear Congress 
and of a recommendation made by the ISO Sub-committee 9 
Textile Products for Fishing Nets. It represents an effort at unifica- 
tion of the various methods used for testing the characteristics of 
fishing twines and netting, resulting from the suggestions and 
exchange of ideas of many gear technologists working in this field. 
For twines the testing methods used in the textile industry can be 
employed; however, properties in the wet condition are of main 
importance in fishing gear. Special methods have to be developed 
for testing fish netting, since no applicable textile methods exist. 
The testing methods described cover both physical, chemical and 
biological tests, and for each item define a characteristic to be 
tested, and the range and accuracy of the instruments to be used. 
The report covers testing of netting materials for the various 
forms of breaking strength, elasticity, elongation, extensibility, 
flexibility, abrasion resistance, weight (under different environ- 
mental conditions), length and thickness, effect of heat, weathering, 
chemicals and deterioration of a biological nature. The paper 
furthermore includes suggestions towards uniform methods for 
testing the properties of netting such as mesh size, strength, knot 
stability and weight under the various forms applicable to fishing 
nets. It had been the intention of the FAO Secretariat to bring this 
matter to the Congress ready for discussion, aimed at definite 
recommendations towards an internationally agreed standardisa- 
tion of testing methods for fishing twines and netting. For the 
testing of some properties, a final selection has not been made from 
among the two or more alternative testing methods described. 
Apart from these few exceptions, the report suggests one set of 
definitions, type of equipment and methodology for testing the 
characteristics of netting twines and netting, and it is hoped that 
these will be accepted as standard in the trade and among fisheries 

Epreuves sur materiaux textiles utilises dans les engins de peche 


La presente communication est le resultat du Groupe de Travail sur 
les Methodes d'Epreuves organist pendant le Premier Congres 
Mondial des Engins de peche et d'une recommandation faite par 
le sous-comite 9 (Produits textiles pour filets de Peche) de 1' Organisa- 
tion Internationale de Standardisation. Elle traduit un effort 
tendant a unifier les diflferentes methodes utilisees pour eprouver 
les caracteristiques des fils et filets de peche et est le resultat d' 
^changes d'idees entre de nombreux tcchnicicns des peches tra- 
vaillant dans ce domaine. En ce qui concerne le fils, les methodes 
utilisees dans 1'industrie textile sont valables cependant il est tres 
important de rechercher des methodes pouvant etre appliquees dans 
le domaine des ptehes a des fils mouillds. Les methodes d'essais 
dtaites couvrent en mftme temps les tests physique, chimique 
ct biologique et pour chacun, definissent les caracteristiques 
a eprouver et aussi la portec et la precision des instruments a 
utihser. Le rapport traite dgalement des epreuves des materiaux 

written by 
Andres von Brandt 

Institut fur Netz und Materialforschung, 
Hamburg, Federal Republic of Germany 

and revised by 
P. J. G. Carrothers 

Fisheries Research Board of Canada, 
St. Andrews, New Brunswick, Canada 

A. von Brandt 

P. J. G. CuTothen 

de filets, des differcnts aspects de force de rupture, elasticity 
allongement, extensibility, flexibility, resistance a rusure, poids 
(sous differentes conditions d'environnement), longueur et 
epaisseur, efTets thermiques, atmosph6riqucs, chimiques et 
deterioration de nature biologique (animaux marins). En outre, 
la publication contient des suggestions pour {'unification des met- 
hodes d'epreuves, dimensions des mailles, resistance a la rupture, 
stability des noeuds et poids sous diflferents aspects, applicables 
aux filets de peche. Le Secretariat de la FAO avail Pintention de 
porter cette question au Congres, prfcte pour discussion et arriver 
ainsi a des recommandations definitives Internationales pour la 
standardisations des methodes d'epreuves des fils et filets mais 
pour les epreuves de certaines proprietcs, une selection finale n'a 
pas encore pu toe etablie. Le rapport cependant suggerc une liste 
de definitions, type d'equipement et methodologie pour eprouver 
les caracteristiques des fils et filets et on espere qu'elle pourra 6tre 
acceptee oomme standard par le commerce et les techniciens des 


Debesc esta ponencia a los rcsultados obtcnidos por el Grupo de 
Trabajo sobre Metodos para Ensayos creado durante el Primer 
Congreso Mundial de Artes de Pesca de la FAO y a una recomen- 
dacion hccha por el Subcomite 9 de la ISO sobre Productos Tex- 
tiles para Redes de Pesca. Apira a unificar los dlversos metodos 
empleados para ensayar las caracteristkas de los hilos y redes de 
pesca, resultado de las sugercncias e intercambio de ideas de muchos 
especiaJistas en material de pesca ocupados en este sector. Para los 
hilos pueden emplearse los metodos de ensayo de la industria tcxtil, 
pero como lo que mas interesa en el material de pesca son sus pro- 
piedades cuando esta hiimcdo, se tiencn que formular metodos 
especiales para dlo, ya que los de dicha industria textil no son 
aplkables. Los metodos descritos, flsicos, qufmicos y bio!6gicos, 
definen para cada caso una de las caracteristicas que se tienen que 
ensayar y dan el alcancc y exactitud de los instrumentos empleados. 
Menciona el autor el ensayo de los materiales para redes en cuanto 
a su resistencia a la rotura, elasticidad, estiramiento, flcxibilidad, 
resistencia a la tracci6n, peso (en diversas condiciones ambientales), 
longitud y diametro, efecto del calor, resistencia a la acci6n 
atmosferica y a las sustancias quimicas y deterioraci6n de naturalcza 
bioldgica. Se hacen tambien sugerencias para uniformar los metodos 
para ensayar propiedades de las redes, como tamafto de la malla, 
resistencia, estabuidad del nudo y peso en las diversas condiciones 
en que trabajan en la pesca. Intentaba la Secretaria de la FAO 
someter esta cuestion al Congreso, lista para discutirla, con objeto 
de que se hicieran recomendaciones definitivas para normalizar 
iriternacionalmente, tras previo acuerdo, los metodos para ensayar 
hilos y redes de pesca. Para la prueba de algunas propiedades 
todavia no se ha hccha una selecci6n definitiva entre los diyersos 
metodos descritos. Salvo estas excepciones, en la comunicaci6n se 
propone un grupo de dcfiniciones, clase de material y metodologia 
para ensayar las caracteristicas de los hilos y las redes y se confia 
que sea aceptado como patron en la industria y entre los tccndlogos 

AS a result of efforts in all parts of the world to increase 
/\thc fishing yield, fisheries science has become more 
interested in fishing methods and fishing gear than ever 
before. A number of research institutes have been founded 
specifically to study problems of fishing techniques and 
others have added such studies to their research pro- 
grammes. The First International Fishing Gear Congress, 
convened by FAO in Hamburg, October 1957, demon- 
strated that this field of study can be approached in 
many different ways and that great possibilities to im- 
prove fishing methods still exist. As a result of these 
studies, fishing gear will be designed and constructed on 
a more rational basis and the fishing effort will become 
more effective. 

It is logical not only to study the fishing gear to improve 
its construction and efficiency but also to study the 
materials from which the gear is made. In particular, 
new man-made fibres will continue to be invented and 
will continually become available for making nets. It is 
important to test these new netting twines and the netting 
made from them to assess their usefulness before trying 
them in full-scale fishing gear. For this purpose it must 
be known what properties of the netting materials are 
important for fishing gear, and appropriate methods for 
testing these properties quantitatively must be available. 
A knowledge of these properties, e.g., of newly developed 
netting twines, permits a decision to be made whether or 
not the material can be used in fishing gear and how 
it can be used to greatest advantage. It is possible to 
avoid unnecessary and expensive tests with commercial 
fishing gear if the properties of the netting materials are 
measured first. 


In some cases netting materials and netting can be 
tested by methods which have already been developed 
by the textile industry and which, in many instances, 
have already been standardised. On the other hand, 
the fishery is often interested in material properties in 
both the dry and wet conditions, whereas these proper- 
ties are of little or no interest for usual textile applications 
and hence have so far not been measured. Thus, the 
fishery must develop new test methods of its own or 
modify existing methods to its own purposes. 

In the Netherlands, a special institute for fisheries 
has tested netting materials since 1911. It is interesting 
that Het Nederlandsche Visscherij-Proefstation en 
Laboratorium voor Materialen-Onderzoek, Utrecht, was 
founded by fishermen organisations. In Germany, tests 
in connection with gear research work began in 1928, and 
a special institute for fishing technique, now located in 
Hamburg, was founded in 1936. In Canada, the Fisheries 
Research Board has been interested in netting properties 
since 1934, and in 1954 the Canadian Government 
Specifications Board founded a Committee on Speci- 
fications for Fishing Gear at the request of the commer- 
cial fishery. In Japan, the Tokai Regional Fisheries 
Research Laboratory in Tokyo became the centre of 
fishing gear research, and, in India, the Central Institute 
of Fisheries Technology, Craft and Gear Wing, in 
Cochin, should be mentioned in this respect. 

Some of these, and other institutions also, have devel- 
oped test methods for netting materials and netting, 
often without knowledge of the other's work, and have 
collected experiences which should be used as the basis 
for future research. Unfortunately, the methods have 
either not been published or exist in literature which is 
difficult to find. Thus, it seems advisable to summarise 
these test methods for general use. 

A Working Party on Testing Methods, founded at 
the First International Fishing Gear Congress, was 
asked by FAO to collect all testing methods used for 
fishing gear materials, with the ultimate objective of 
establishing international standard test methods for 
netting twine 1 and netting. 

A number of interested countries participated in this 
Working Party, and the following submitted contribu- 

Canada (Fisheries Research Board of Canada). 

Denmark (Teknologisk Institut, Kopenhagen). 

France (Chambre Syndicate des Filets de France, 

Germany, Federal Republic (Institut tlir Netzfor- 
schung, Hamburg). 

Germany, Eastern (Institut fiir Hochseefischerei, 

Japan (Tokai Regional Fisheries Research Labora- 
tory, Tokyo). 

1 The International Organisation for Standardisation (ISO) 
Sub-committee 9, Textile Products for Fishing Nets, has prepared a 
draft dealing with basic terms and definitions for textile products 
for fishing nets. In this draft "Netting twine" is denned as a 
"general term for any kind of yarn or combination of yarns (includ- 
ing braided) usable for the manufacture of netting". 

The Netherlands (N.V. Onderzoekingsinstituut Re- 
search, Arnheim). 

(Het Nederlandsche Visscherij Proefstation, 

U.S.S.R. (Laboratory of Fishing Technique, Mos- 

In addition to these papers, recent contributions, 
particularly those published in Modern Fishing Gear 
of the World (1959), have been considered in so far as 
they are concerned with test methods. 

Different methods have reached different stages in 
their development. Test methods for some properties, 
particularly those in which the textile industry is also 
interested, have been highly developed whereas test 
methods for other properties have been only slightly 
developed or do not exist at all. 

Before adoption, a test method should be proven in 
practice; its results should correlate well with experience 
and results in the fishery. It is not possible to develop 
test methods only in the laboratory. The following 
collection of methods includes only those which have 
actually been used in different countries. It is quite 
possible that, as a result of knowledge gained in the 
future, certain methods will have to be modified or 
completely replaced by other methods. As new materials 
become available, new problems will arise and new test 
methods will have to be developed to measure properties 
not yet recognised. 

In spite of this, the following collection of test methods 
for netting twine and netting may be useful to those 
starting investigations in this field which is so important 
to the fishery. 


For testing netting it will usually not be possible to 
base the test method on methods already developed, 
for example, for woven and knitted fabrics. In most 
cases, these other materials differ from netting, even 
from their initial manufacture. The situation is different 
with netting twine, in the form of yarns, twines, and 
monofilaments, which may often be tested by methods 
already standardised as mentioned above (i.e., by national 
standard methods of different countries such as the 
ASTM Standards on Textile Materials in U.S.A, or 
"Deutsche Normen" in Germany, and by the different 
recommendations of BISFA). Yet, very often, these 
methods were developed for finer materials such as 
fibres and yarns and consider only the properties, e.g., 
breaking strength, in the dry condition. Such tests require 
a room having standard atmospheric conditions (see 
below) in order to give comparative results. 

It must be emphasised that the precision which is 
often desirable in the textile industry cannot be achieved, 
and even may not be required, with the rough materials 
often used for making nets. Further, the properties of 
netting materials and netting are usually of interest 
only in the wet condition. The usual dry strength data, 
for example, are normally of no value to the fishery and 
can even be dangerous when they lead to an incorrect 

decision. Only properties in the wet condition are 
important. This results in an advantage in testing 
textile materials for fishing gear because rooms having 
standard atmospheric conditions are not essential. This 
fact is of particular interest to those fishing institutes 
which wish to start testing netting materials and netting. 
On the other hand, it cannot be denied that the price 
of netting twines and netting is usually based on the 
air-dry weight, and this can be determined accurately 
only in a properly conditioned atmosphere. It is not 
good enough to measure moisture content and correct 
the weight to a standard moisture content because the 
amount of moisture in the material affects its physical 
dimensions. For example, a piece of cotton twine is 
longer when it has a low moisture content than when it 
has high moisture content so that measuring its runnage 
(m/kg) when its moisture content is below standard, 
then correcting the weight to standard moisture content, 
results in the measured runnage being too high. This 
error is particularly important if the twine is identified 
by its runnage (e.g., m/kg) or by its weight (e.g., resultant 
tex). Thus, a room having standard atmospheric condi- 
tions is useful even for testing fishing net materials and 
netting. However, absence of such a room is not sufficient 
reason for not starting tests with wet materials. 


The standards already established for textile tests 
specify, as already mentioned, that the specimens be 
conditioned in a standard atmosphere for at least 24 
hours or until the weight becomes constant. The usual 
standard atmospheric conditions are: 

A temperature of 20 2C (70 2F) and 
A relative humidity of 65 2%. 

The test must be performed in the same standard 
atmosphere as that in which the test specimens were 
conditioned. Unless extreme accuracy is required, air- 
dry specimens kept in the normal testing room atmos- 
phere will give dry test results adequate for fishery 

For the much more important wet tests, most investi- 
gators immerse the specimens in clear tap water or sea 
water at ordinary room temperature for at least 12 hours 
before the test. Tests with netting materials made of 
cotton, polyvinyl alcohol, polyamide. or polyester show 
no differences in wet strength whether soaked in distilled 
water, fresh water, brackish water, or sea water. Net 
materials and netting samples which have already been 
used for fishing become soaked in a relatively short time. 
This is also true with new bast fibres and untreated man- 
made netting materials. Other natural fibres require 
more time. Care must be taken that specimens of new 
netting twines made of cotton or of lubricated hard 
fibres are really submerged below the surface of the water 
and do not float without becoming wetted. Soaking 
must be done in containers which are large enough to 
permit clear observation that the samples are wholly 


If, under certain circumstances, the specimens must 
become soaked more quickly, they may be immersed in 
a solution of a wetting agent. 2 Tests may be conducted 
after the specimen has been immersed for 15 minutes. 3 
These agents often have the disadvantage that they 
decrease the slipping resistance of the knots, hence, 
they should be avoided if knots are to be tested. 

Tests with netting twines made of polyamide, polyester, 
or polyvinyl alcohol fibres or polyethylene monofila- 
mcnts have shown that the time of immersion in wetting- 
agent solutions has no effect on the results of wet break- 
ing strength tests. 

A. Testing of netting materials 

The term "netting materials" includes all materials 
used to make netting. They may be made of natural or 
synthetic fibres or even of metal wire. They may be 
monofilaments or they may be twisted or braided 4 into 
one of many various forms. Traditionally, the terms used 
by fishermen do not always agree with the definitions 
used in the textile industry. There have been some pro- 
posals for world-wide standardisation of terms used for 
fishing, especially those for nets. 5 The following remarks 
may be helpful to an understanding of the meaning of 
terms used in the textile industry: 

"Yarn" a generic term for continuous strands of 
textile fibres or filaments suitable for laying, braiding, 
knitting, weaving, etc., into larger structures. It may 
be a number of fibres twisted together, a number of 
continuous filaments laid together with or without 
twist, or a monofilament. The term "netting yarn" as 
used in the fishing industry is very often not in accord 
with this definition. 

"Twine" a balanced, plied yarn made by twisting, 
laying, or braiding several yarns together. Only multi-ply 
yarns are called "twines". The term "netting twine" as 
used in the fishing industry is usually in accord with this 
definition, but it is not always used by the fishermen. 

"Cord" a balanced structure formed by twisting or 
laying two or more plied yams together. 

"String" not generally used in the textile or fishing 
industries because it does not have a precise definition. 
Most tests applied to netting twines or netting may be 
classed as physical, chemical, or biological, but there 
are also other tests related to manufacturing, handling, 
or fishing effectiveness. Many tests have to be conducted 
with the fishing gear in action, but the following descrip- 
tions include only those test methods which may be 
used in the laboratory. 

* The term "vetting ayent" refers to chemical products which 
decrease the surface tension of the water so that the water can 
easily soak into the specimen, displacing the air. Typical wetting 
agents for textiles are 'Aerosol 1 and 'Nekal BX' in a 0-1 per cent 

8 Canada specifies that the specimen be soaked for five min 
(Canadian Government Specifications Board, Specification for 
Nets: Fishing, 55-GP-l, 25 April, 1958). 

4 The terms "braid" and "plait" both mean "to intertwine or 
interweave yarns", but "braid" is usually used with reference to 
twines because "plait" also means "to fold cloth". 

6 See footnote *. 


(a) Physical tests 

Most of the netting twine tests under discussion are 
physical in nature. Surely breaking strength is of greatest 
interest to the fishery and should be reported in every 
case. However, there are numerous other physical 
properties of differing significance. The order in which 
the test methods are presented in the following is arbi- 
trary and does not imply any relative importance. 

(1) Breaking strength 

Breaking strength is defined as that load which causes 
the netting material to break. Wet or dry netting yarns 
may be tested. As already stated, the dry strength is 
usually of no interest to the fishing industry but, for 
its measurement, the test methods standardised in many 
countries may be used. Otherwise, the methods for 
wet netting materials described in the following have 
to be modified to accommodate the dry materials. The 
test report must state whether the materials were dry or 
wet at the time of testing. 


Both the wet strength (e.g., of unknotted twine) and 
the wet knot strength of the netting material are impor- 
tant to the fishing industry, and special testing machines 
are required for their measurement. 
Strength testing machines: Testing machines known 
as dynamometers or tensiometers are used to measure 
breaking strength. The test specimen of netting twine is 
fastened between two clamps and is stretched by an 
increasing load until it breaks. 

Strength testing machines may be classified as follows 
into three basic types, according to the way in which the 
load is applied to the test specimen: 
Constant rate of load. 
Constant rate of extension. 
Constant rate of loading-clamp traverse. 

The widely used pendulum-type tester (Fig. 1 and 2) 
belongs in the last class. The upper clamp is connected 
with the weighted pendulum which, in turn, is lifted by 
the increasing tension in the test material. This tension 
increases irregularly in accord with the elongation 
characteristics of the material, for tension is reduced as a 
result of elongation, particularly at the beginning 
of the test. Similarly, the length of the specimen increases 
irregularly because the upper clamp moves irregularly in 
accord with the irregular increase of tension in the test 
specimen. These pendulum testers have unofficially 
been called "constant rate of nothing" because, so far 
as the test specimen is concerned, nothing changes at a 
constant rate. 

The recently designed, electronic, recording dynamo- 
meters (Fig. 3) are very precise and accurate because 
there is no frictional resistance in die tension-measuring 
unit. In these testers, the first clamp is moved from the 
second clamp at a constant velocity and the tension is 
measured by an electrical element to which the second 
clamp is attached. Only testers with these recording 
electronic dynamometers give a constant rate of exten- 
sion because the clamp on the dynamometer moves a 
negligible amount. 6 

The inclined plane, or constant-rate-of-load, machines 
can be used only for lighter materials up to 2 kg breaking 
strength and hence are not generally used for fishing 
gear materials. 

The clamps usually supplied with the testing machines 
may be used for testing many netting materials. However, 
if smooth or heavy materials, because of their fibre 
properties, construction, treatment, or high strength 
(e.g., twines made of hard fibres or certain man-made 
filaments), slip in the usual clamps before breaking or 
break at the clamps, then special clamps (Fig. 4) have to 

6 These electronic testing machines are relatively expensive. For 
testing only in the field, home-made testers, usually of the spring 
balance type, may be used. Such testers have been described, e.g., 
by Czensny and Meseck: Untersuchungen Uber den Netzfrass 
nicdcrer Wassertiere, Zeitschrift fur Fischerei XXVI, 237-310, 1938; 
by Park, Y.C. and An, U.H.: Tensile Strength of Netting Cords, 
Bull, of Pusan Fisheries College 11, 4-6, 1958; Miyamoto, H. and 
Shariff, A.T.: Experiments on Fishing Net Preservation, Indian 
Journal of Fisheries 6, 145-185, 1959. 

Fig. 2. Pendulum tester: (a) Koch (Schopper) (0-2. 0-5 and 0-10 kg) (b) Frank (0-10, 0-50 kg) (c) Wotpert (0-100, 0-500 kg). 


Fig 3. Electronic recording dynamometer ' Teslatron" Wolpert. 

Fig. 4 

be used. These must be constructed in such a way as to 
avoid sharp edges. 7 

If a pendulum-type testing machine is used, its load 
capacity should be chosen so that the angle of the pendu- 
lum from vertical at the time of breaking is not greater 

7 See Modern Fishing Gear of the World, p. 77, Fig. 3, and sketch 
p. 96. 


than 45 nor smaller than 5. Further, the average break- 
ing load should lie above the first fifth of the measuring 
range. The velocity 8 at which the loading clamp moves 
should be 100 mm/min. 9 

It is convenient to use testing machines with different 
overlapping ranges for testing netting materials, e.g., 
0-2 kg, 0-5 kg, 0-10 kg, 0-50 kg, 0-100 kg and 0-500 kg. To 
a certain extent the testing range of the same machine 
may be altered by changing the pendulum weight (by 
adding or removing weights). To be specific, then, three 
machines are needed, viz. 0-10 kg, 0-50 kg, and 0-500 kg, 
and the other ranges may be obtained by changing the 
pendulum weight. 

There is considerable variation in the length of the 
specimen specified between the clamps at the beginning 
of the test. For normal clamps this should be 200 mm 10 
so that a netting twine sample 30 cm (ca 11 12 in) long 
is required for each test. If, as mentioned before, special 
clamps have to be used for those netting materials which 
slip or break in normal clamps, the samples may have to 
be 100 cm (40 in) or more in length depending on the 
construction of the clamps. The actual test length 
should still be about 200 mm. 

Care must be taken that twisted materials do not lose 
twist while the test specimens are being cut to the 
lengths described above. 

Wet netting materials are tested at a room temperature 
of about 20C (68-70F). It is suggested that higher or 
lower temperatures can affect the result, particularly 
if the material under test has been manufactured from 
man-made fibres (see Section 9: Thermal Reaction). 

The testing machines can become damaged as a result 
of tests on wet specimens. Therefore, the machines 
should be dried and oiled very carefully after use. 

Wet breaking strength: The netting materials which are 
to be tested for wet breaking strength are cut into test 
pieces each 300 mm long and, as described, are immersed 
in water for at least 12 hours. The netting materials are 
tested as soon as they are removed from the water. 
They are straightened and are secured in the clamps of 
the testing machine with 200 mm actual test length 
between the clamps. If breaking strength alone is to be 
tested, it is not necessary to use any particular pre-tension 
contrary to official standards. 

Breaking strength is reported as the average of all 
test results. Thus, the test specimens should be taken from 

8 International standards do not specify the rate at which the 
clamp travels, but, rather, specify that the testing time (time during 
which the specimen is loaded to rupture) shall be 20 2 sec. 

9 Canada (CGSB Schedule 4-GP-2, Method 9.B), Japan (Modern 
Fishing Gear of the World, p. 66), and U.S.A. (ASTM Designation : 
D76-53) specify that the rate of loading clamp traverse shall be 12 
0*5 in/mm = 30 cm/min. 

1 Canada specifies 10 0-25 in in CGSB Schedule 4-GP-2, 
Method 9B; Japan specifies 25 cm in Modern Fishing Gear of the 
World, p. 66, 1959; U.S.A. specifies 10 0-05 in in ASTM Desig- 
nation: D204-57T. Man-made fibres are usually tested according tu 
BISFA rules, viz. 50 cm between grips and 20 2 sec breaking time. 

11 All conversions from the metric system are approximate. 

the netting material in such a way that they are represen- 
tative of the whole lot that is to be tested, e.g., not from 
the first portion of a spool or strand. In many cases, 
particularly if a new material is being tested for the first 
time or if the material is not uniform (e.g., from hard or 
bast fibres), at least 20 test breaks are required to get a 
sufficiently precise result. As a rule, 10 test breaks are 
sufficient for routine tests. Where a series of tests are 
being performed to show loss in strength by rotting of 
preserved, natural fibre netting materials, only five 
test breaks are required for significant results. This is in 
contrast to the usual procedure for tests of yarns and 
twines where the number of test breaks is determined 
by the non-uniformity of the material and by the desired 
precision of the average. Twenty, ten or five test breaks 
are sufficient for fishery purposes under the above 

The test result is disregarded and an additional test 
break is performed if the test specimen slips in the clamps 
of the testing machine, or breaks in or at the edges of 
the clamps, or if the result appears to be different from 
other results in the test as a consequence of unusual or 
irregular faults in the material. 

The breaking strength is reported in kg to two signi- 
ficant figures : averages greater than 1 kg are rounded off 
to the first decimal place and averages less than 1 kg are 
rounded off to the second decimal place. 

Wet knot breaking strength: In testing wet knot breaking 
strength, the type of knot is important. In most labora- 
tories, a simple, overhand knot (Fig. 5) is tied in the 
centre of a single piece of the netting material, because 
only specially trained people can tie typical netting knots 
without losing some of the twist in the netting twine. 
Further, it has been found that the results of tests with 
knots tied by hand in netting twine do not correlate well 
with the results of mesh-strength tests applied to netting 
made by machine using the same knot. The value of 
tests involving the overhand knot is thereby increased. 

In order to test a knot as used for making netting, two 
pieces of the netting twine under consideration, each 

SO cm long, are tied together by the weaver's knot 
(English knot, Fig. 6a) or by the double knot (Fig. 6b) 
developed from it. Also, the reef knot (Fig. 7) and other 
knots may be tested in so far as they are used to make 

However, this test cannot always be conducted satis- 
factorily with all netting materials because the knots some- 
times slip under the increasing load and can slip out 
completely without breaking if only one leg of each side 
of the knot is secured by the clamps (Figs. 6 and 7). 
Where slippage occurs, the two legs on each side of the 
knot must be fixed in each respective clamp (Fig. 37) or, 
unless there are other reasons to the contrary, the over- 
hand knot (Fig. 5) should be used for knot strength 
tests. This latter is particularly true because the usual 
hand-tied or machine-made knots are otherwise tested 
in connection with wet mesh strength and knot stability 
(see Sections 19 and 20). 

The force with which the knots are tightened before 
the test can affect the result but this effect becomes negli- 
gible with the overhand knot. Loosely tied knots are 
usually weaker and their strength more variable than are 
tightly tied knots. The time between tying the knot and 
testing for strength does not affect results significantly 
so long as it exceeds 24 hours. However, all knots should 
be tied shortly before the test and should be tightened 
once more immediately before the test. 

For testing, the wet, knotted twine is fastened in the 
clamps so that the knot is about midway between the 
clamps. The results of breaks which for some reason do 
not occur at the knot are disregarded; only results from 
breaks which occur at the knot are used. 

The wet knot breaking strength is reported as the 
arithmetic mean of all allowable test results, as was the 
case when reporting wet strength. In addition to this 
absolute value, the relative knot strength is also reported 
as a per cent of the wet strength of the unknotted material 
as follows: 

wet knot breaking strength x 100 
wet breaking stength (unknotted). 

Calculations for comparing results: If materials of different 
sizes or weights are to be compared concerning strength 
efficiency, this can be done by converting strength data 
to a common basis such as linear density or cross-sec- 
tional area. Canada proposes to use the term "specific 
strength" to identify all such converted data used for 
comparative purposes. The three following types of 
converted strength data differ in the units used, but all 
may be employed directly to compare strengths of differ- 
ent-sized materials. 

(a) Breaking length: This figure is the length of the 
material whose weight at the surface of the earth 
equals its breaking strength. It is the maximum 
length of the material which can be supported by 
one end of itself without that end being broken by 
the weight of the material. Contrary to common 
practice, it is better to use this parameter for com- 
paring only materials of the same specific weight. 


It is calculated as follows: 

breaking length (km) = 

breaking load (g) x resultant Nm (km/kg). 

However, since netting twines are made of several 
yams, the resultant Nm has to be estimated from 
the yarn Nm, and this is difficult because the various 
hard, medium, and soft twists result in different 
twist contractions. Thus, the breaking length may 
be calculated more conveniently as follows: 

^ ,. , , x breaking load (kg) 
breaking length (km) = 

linear density (g/m). 
To be precise, this calculation can be made only 
with data, particularly for weight, from tests 
performed in the standard atmosphere. In a 
temperate climate, however, tests performed under 
normal room conditions will be sufficiently 
accurate for netting materials. The result is roun- 
ded off to the nearest whole number. 12 

(b) Unit strength: Another way to compare strengths 
of different netting twines is to refer the strength to 
the cross-sectional area of the fibre by calculating 
the unit strength, kg/mm 2 . Measurement of the 
actual fibre cross-sectional area can be affected by 
stretching or swelling the fibre. Thus, the area is 
estimated from the runnage and the density of 
fibre and the unit strength may be estimated as 

unit strength (kg/mm 2 ) = 
breaking length (km) x fibre density (g/cm 3 )- 

(c) Tenacity: The tenacity may be calculated as g/den 
or g/tex. The calculation in g/den is made as 

breaking strength (g) 

tenacity (g/den) ==- 

resultant denier. 

For this formula, the denier figure is normally given 
for the single yarn, as was the case when calculating 
the breaking length above. Netting materials, on 
the other hand, are usually plied twines, as already 
mentioned. Therefore, the resultant denier of the 
whole twine must first be determined. This is the 
weight in grams of 9,000 m of the netting twine. It 
is easier to compute the tenacity from the breaking 
length as follows : 

* / /j ^ breaking length (km), 
tenacity (g/den) = ^ ^ - - 

Tenacity in g/tex is numerically equal to the break- 
ing length in km. 

(2) Extensibility 

Extension is the change in dimension of the netting 
material in the direction of the load, i.e., along the longi- 
tudinal axis. It is measured at the same time and on 
the same machine as strength (see above). For indicating 

12 Where tests are performed in the standard atmosphere, 
results may be reported to the second decimal place. 


extension the testing machine must be equipped with a 
suitable autographic device for recording the elongation 
of the test specimen. The resulting graph shows the 
amount of extension under the various loads. 

Test conditions for extension are the same as those 
described above for strength. Here, too, tests performed 
on wet materials are more important than those perfor- 
med on dry netting materials. For this test, it is essential 
that the netting twine be placed in the tester under a 
standard pre-tension, usually equal to the dry weight of 
a 250-m 13 length of the netting material. As a rule, this 
weight is sufficient to straighten netting materials which 
have become kinked or hardened as by preservatives. 
The pre-tension is applied by hanging appropriate weights 
from the test specimen. There are even some testing 
machines on which the pre-tension can be applied 

One difficulty in measuring the extension of knotted 
netting twines is caused by twine coming out of the knot 
when loose knots are tightened during the test, thereby 
causing a false extension. Thus, extension tests should 
be carried out with unknotted materials unless there are 
particular reasons to the contrary. 

Extension at break: The extension at the moment of 
break is called the breaking extension (total extension, 
stretch at rupture). At least 10 tests should be carried 
out and the average calculated. The result is usually 
reported as a per cent of the initial length : 
Extension at break (%) = 
length at break initial length 

; ~r~. r~i 1 ^ 100. 

initial length 

Usually testing machines plot the increase in length 
directly, and some are even calibrated to give immediately 
the per cent increase in length for certain initial lengths. 
For an initial length of 200 mm half the extension in mm 
is the per cent extension. 

Load-elongation curve: In most cases, e.g., if netting is 
being considered, the extension at break for unknotted 
twine is of little interest to the fishing industry. In 
netting, the knots (or the junctions in knotless netting) 
break at loads lower than do corresponding unknotted 
twines. This is also true of twines used in other ways, 
e.g., for fishing lines. Thus, for the manufacture and use 
of netting materials, it is important to know extensions 
at loads lower than the breaking load. 

The extension under different loads is plotted in the 
so-called load-elongation curve, with the extension on 
the abscissa and the load on the ordinate (Fig. 8a). 
Where this graph is not drawn by an automatic recorder, 
it may be plotted if two persons read off the load and 
extension at the same time during the test. It is not 
advisable for one person to do this by stopping the tester 
periodically to measure the load and extension, because 

ls The amount of pre-tension is variously specified. The 250-m 
weight agrees with most standards. The international BISFA 
specifies the 500-m weight for filament yarns. The measurement of 
the meter-weight is described in Section 5. 

some stress relaxation at constant extension occurs. If 
single values of extension are to be reported, these should 
be at 1/20, 1/10, 1/5, 1/4, 1/2, and 3/4 per cent and at 
the full breaking load. Usually it is better to read off 
extensions under low loads at small, regular intervals, 
and extensions under higher loads at greater intervals, 
e.g., of load in kg. Five tests should be conducted. 


In order to compare the extensibilities of netting mat- 
erials of different breaking strengths, the extension 
values are not plotted against the absolute load in kg but, 
usually, against the per cent of the breaking load, as 
shown in Fig. 8b for netting materials made of different 
man-made fibres. Another procedure is to plot the 
extension against a specific load such as g/den, km- 
weight, or g/tex. Because strength is more variable 
than (and hence cannot be measured so accurately as) 
twine weight, the latter procedure is often the more 

Elasticity: Under stress, netting materials are extended 
by an amount called "total elongation". When the stress 
is removed, this extension may decrease totally or in 
part. This ability to recover extension is called "elas- 
ticity". If, after the stress is removed, the extension 
does not decrease completely, the remaining extension 
is called "permanent" or "irreversible elongation". The 
extension which decreases when the stress is removed is 
called "elastic elongation". With most netting materials, 
part of the elastic or reversible extension decreases as 
soon as the stress is removed and part decreases only 


continuous fiUmint 

continuous fHm*nt 

stapl* Kbr*. 

2 4 6 10 18 14 M M EOttt4MttlOB84aM404B44404t50 

after a period of time. That part of the elastic extension 
which decreases at once is called "immediate elasticity" 
and that which decreases slowly after removal of the stress 
is called "delayed elasticity". Thus, "total elongation" 
consists of three parts, viz. "immediate elasticity", 
"delayed elasticity", and "irreversible elongation", and 
all three parts should be considered when studying 

Elasticity is important in some fishing gear, particu- 
larly that from which fish can escape by slipping through 
the meshes or in which the fish are caught by their heads 
(e.g., gillnets). Further, elasticity is important to mini- 
mise the effect of sudden, jerking stresses (e.g., cyclic 
shock loads). 

The test procedure for measuring elastic extension in 
textiles has not yet been standardised. It is usually done 
on strength testers, as described before, by stressing and 
relaxing the test specimens on a definite cycle. M 

For the fishery, several proposals for testing elasticity 
have been advanced, but not yet compared. The tests 
are conducted with wet, unknotted, netting material. 

Proposal (a): 15 Specimens of netting twine (wet or 
dry) from the same sample are fastened successively in 
the strength tester with 200 mm test length under the 
500-m weight pre-tension. Each specimen is stressed in 
turn by the machine to either 10, 20, 30, 40, 60, or 

14 See DIN proposal 53835, ASTM Designation 1333-54T, or 
the Japanese proposal in Modern Fishing Gear of the World, p. 66, 
items 5-11. 

16 Institut ftir Hochseefischerei, Rostock-Marienehe, according 
to DIN 53835. 


80 per cent of the average breaking load for 10 min. Then 
the load is removed for IS min. For quick tests for 
comparative purposes, one test to SO per cent of the 
breaking load is sufficient. 

On the strength tester, the extension is read as soon as 
the desired stress has been applied. The stress is then 
maintained for the 10 min, by continually adjusting the 
stress on the testing machine to the desired value, while 
the extension of the twine increases further to a constant 

Then, the stress is reduced to the pre-tension value, 
and the immediate change in length for the appropriate 
degree of stress is recorded. To do this, the initial 
200-mm test length is marked on the material and the 
lower clamp is loosened so that only the pre-tension 
affects the specimen. By moving the lower clamp to the 
mark, the remaining extension can easily be read. The 
test specimen is then removed from the tester and im- 
mersed in water without stress for 15 min after which the 
final extension is measured under the above pre-tension. 

Each test to each degree of stress has to be carried out 
with a new specimen. 

According to DIN 53835, the test has to be repeated 
for each load until constant values are obtained. At 
least five tests have to be conducted at each load. 


* foatft* for f *l/r, 

. . jHUMriiatofv ft/far 

e . ess* yair 

Proposal (b): 16 For the test, a 50-cm length is marked 
on a 1-m piece of the netting material under 250-m 
weight pre-tension. 

The material is stressed to 30 per cent of its average 
breaking strength for one hour, then the marked length 
is measured. From this measurement, the total elonga- 
tion at this load is calculated. 

Then the load is reduced to the above pre-tension and 
the marked part is measured once more. All the while, 
the specimen is kept wet by spraying it with distilled 
water. After each pair of measurements, at test load and 


14 16 1* 20 22 24 2t 26 X) & 3435 

f 4A 




i jlU * 


, ,,..i 



at pre-tension, the specimen is soaked in water. These 
measurements are repeated periodically with the same 
specimen until the readings are constant. Some materials 
require several days to reach equilibrium. The final 
length under pre-tension gives the permanent extension. 
Elastic extension may be computed as the difference 
between the final total extension underthetest load and the 
permanent extension. As mentioned, elasticity may be 
computed as the ratio of elastic extension to total extension. 
The test load, equal to 30 per cent of the breaking 
strength, was chosen arbitrarily. There may be advantage 
in carrying out this test to other degrees of stress. 
Fig. 8c demonstrates the method of data presentation 

16 Institut filr Nctzforschung, Hamburg. 


and gives results of extension and elasticity tests on 
'Perlon* continuous-filament trawl twines at stresses 
equal to 10, 20, 30 and 40 per cent of the average breaking 

(3) Flexural stiffness 

The term flexural stiffness refers to the resistance of the 
netting twine to lateral or bending deformation. More 
specifically, it may be defined as the force required to 
cause a unit of bending deflection. By this definition, 
when a fish touches a net, flexural stiffness is a measure 
of the degree to which the netting itself resists the move- 
ment of the fish. Thus, any method for measuring 
flexural stiffness should assess the force which causes the 
deflection. As defined here, flexural stiffness depends 
primarily on the elastic modulus of the twine (governed 
by fibre stiffness and twine construction), on the distance 
between the point of support and the point at which the 
force is applied, and on the diameter of the twine. This 
last is a very important factor. The stiffness of the 
netting material is reported to affect the efficiency of 
many types of fishing gear. Usually, low stiffness in- 
creases catching efficiency whereas high stiffness makes 
the nets easier to handle. Sometimes the antonym 
"flexibility" is used as the characteristic of netting 
materials, as, for example, when the term "softness" 
is used to characterise the desired low resistance to 
deformation of soft-laid materials. 

Different workers, working with different materials, 
have recommended widely differing procedures for 
measuring the stiffness of netting twines. Theoretically 
this property may be measured according to either of 
two basic principles: either the force which causes a 
certain deflection (proposals (b) and (c)) or the energy 
associated with this force (proposal (d)) is measured, 
or deflection of a horizontally mounted specimen under 
its own or an added weight (proposal (a)) is measured. 
This last procedure is widely used for testing textile 
fabrics and paper. 17 

Proposal (a): 18 The flexural stiffness is measured by 
fastening the specimen by one end only in the holding 
device (Fig. 9). 

The wet specimen is slipped horizontally under the 
vertically adjustable upper jaw of the clamp in such a 
way that it does not touch that jaw. The leading end of 
the specimen approaches a horizontal reference line, 
3 cm below the face of the clamp, at a point nearer to or 
farther from the clamp according to the linear density 
and flexibility of the specimen. The specimen is advanced 
until it touches the graduated reference line, and the 
horizontal distance of the point of contact from the jaw 
is read directly as the measure of stiffness. 

Five specimens of each material are each tested 10 
times. The average gives a figure for stiffness which is 
greater for stiffer materials. 

17 Further proposals for measuring the flexibility of netting 
materials, particularly in meshes, are presented in Roessingh, M.: 
Recent experiments on tear and mesh selection in the Netherlands. 
ICES-CM-1959, No. 88. 

18 Institut flir Hochseefischerei, Rostock-Marienehe. 

Care must be taken that the netting material has no 
initial deformation which would cause errors. Such 
errors are particularly possible with wet netting twines 
of medium count. 

Fibres of different specific weights cannot be compared 
by this procedure because the twine is deflected by its 
own weight. The same is true of twines treated with 
preservatives of different weight, even though the twines 
may be of the same number. 

Proposal (b): " The principle of this procedure is to 
measure the force required to deform the netting material 
(cf. the force required for a fish to pass through the mesh). 

Fig. 10 shows the required apparatus. 

19 Institut fUr Netzforschung, Hamburg: Ldtzener 
Mcthode v. Brandt, A.: Arbeitsmethoden der Netz- 
forschung, Stuttgart, 1947, pp. 44-49. 



Twenty cm (8 in) of the specimen are bent in such a 
way that they form a loop (Fig, 10, A-B) and the ends 
are fixed in the metal clamp (Fig. 10, 3). The stiffer 
the material the wider the loop will be. A light 'Cello- 
phane' vessel (Fig. 10, 2) of about 3 gm weight and SO cc 
capacity, is hung from the loop. Care must be taken that 
the vessel is suspended from the bottom of the loop. 
Water is fed into the vessel from a burette, preferably 
automatically filled, adjusted in such a way that the flow 
is by steady drops rather than by a continuous jet (about 
40 cc/min). The flow of water gradually draws the loop 
together. The opening of the loop at its widest point (A-B) 
is constantly observed with the assistance of a gauge and 
the flow of water is stopped as soon as the opening has 
decreased to 5 mm. The weight of the vessel and the 
quantity of water which has dropped into it, as read 
from the burette to the nearest cc, is used as the measure 
in grams for the stiffness of the material being tested. 
Here 1 cc of water is taken to weigh 1 g. The test should 
be repeated 10 times for a good average. 

It is desirable to use the same vessel for all weights of 
material, but it may be necessary to use smaller vessels 
for softer materials and larger vessels for harder materials. 

For comparing different preservatives and the stiffen- 
ing caused by the treatment, the following classification 
is used: 

up to 2 gm = extremely soft 
3-10 gm = very soft 
11-1 5 gm = soft 
16-25 gm = medium hard 
26-50 gm = hard 
more than 50 gm = wire-like. 

Special variations of the procedure have been proposed 
for particularly soft net materials and for small samples. 

Because a vessel must be used to contain the water, a 
certain initial weight cannot be avoided. Thus, if the 
specimens are very soft, measurement may be impossible 
because the loop opening is drawn together to less than 
5 mm by the weight of the vessel alone. The weight of 
the vessel may be counterbalanced by the arrangement 
shown in Fig. 11. 20 In this, one pan of the balance is the 
vessel into which the water is allowed to flow, and its 
weight is off-set by the weight of the other pan. A little 
glass hook is hung into the loop of twine and is 
attached to the balance by a fine strand ('Perlon' or 
nylon). The initial stress on the loop can be decreased to 
0*1 gm in this way. Water flow is regulated to about 
5 cc in 60-100 sec by a burette with a very small opening, 
thus permitting easier regulation for softer materials. 
With this modification, results can be reported to the 
nearest 0*1 gm. Otherwise, the procedure is the same as 
described above. 

But even with this modification, it is possible that 
extremely soft materials cannot be tested because the 
loops formed with 20 cm of the material close under 
their own weight to less than 5 mm loop-opening width. 

Proposal (c): 21 Twenty turns of the material are 
wound close to each other around a rod 4 cm in diameter 
and are held together by a narrow strip of adhesive 
tape. This coil is removed from the rod and its diameter 
is reduced from its initial value of about 4 cm to 2*5 cm 
(1 in) by an increasing diametrical load. 22 

Two flat plates about 10 cm in diameter are fitted to 
the clamps of an electronic dynamometer. The moving 
clamp is lowered a few centimetres and the cord cylinder 
is placed on the lower flat plate (Fig. 12). The lower 
clamp is then moved upwards until the cylinder becomes 
an ellipse with a short axis of 2-5 cm. The force required 
for this is taken as the measure of flexural stiffness. 

Proposal (d): 23 For measuring the flexural stiffness 
of coarser netting materials (and cordage) the specimen 
is suspended with a weight at one end like a pendulum 
and is allowed to swing. Stiffer materials resist the swing 
more strongly and stop the swinging sooner. 

20 According to an unpublished proposal by E. Schoeninger. 

21 van Wijngaarden, J.K. : Methods of testing net cords and nets 
especially made of synthetic material. Rayon Revue 1 1, pp. 146-160, 

22 This testing technique is similar to those used in the textile 
industry for cloth. 

28 Taylor, H. F. and Wells, A. W.: Properties and values of 
certain fish-net preservatives. Doc. No. 947, U.S. Bureau of Fish- 
eries, Washington, D.C., 1923. 


A wet specimen 30-cm (1 ft) long is attached to a clamp 
and is stressed by a 50-g 24 weight (Fig. 13a). The wooden 
base is sawn to form a sector of a circle of 30-cm radius 
and is placed 30 cm from the clamp. The pendulum is 
raised along the sector for a distance of 15 cm (6 in) and 
is released without acceleration. The number of swings 
for the amplitude to decrease to half its original value 
are counted as the measure of stiffness. 

The test is repeated 10 times with each of 10 samples. 
If the sample becomes unduly softened at the clamps 
by the test, the number of swings may be reduced by a 
stopping device as shown in Fig. 13b. 

If fine, twisted netting materials are being tested, 
care must be taken that the twine does not lose twist. 
If it does, the pendulum will not swing always in the 
same plane and the number of swings to half amplitude 
cannot be counted. 

Fig. I 3 

(4) Abrasion resistance 

The term "abrasion resistance" refers to the ability of 
the material to withstand abrasion under defined, 
conventional conditions. It is said to correspond to the 
mechanical wearing out with use. But it is difficult to 
simulate in the laboratory the conditions which are 
experienced by fishing gear materials during fishing. It 
is not possible to obtain absolute values from test pro- 
cedures because actual mechanical wear is a complex 
process. The original nature of the material and subse- 
quent damage by light or micro-organisms can affect 
abrasion resistance. Also, the results of abrasion tests 
are affected by the type of testing machines. 

There are two ways in which abrasion can occur in 
fishing gear: 

(a) The netting materials may be abraded against 
other harder objects such as the sea floor or the hull 
of the boat. 

(b) The netting materials may be abraded against one 
another as, for example, in the knots or in the 
seams of the net. 

In textile-abrasion tests, the specimens may be rubbed 
until they break or otherwise may be rubbed for a certain 

period, then the change in other properties such as 
strength studied. 

Abrasion against hard objects: Abrasion of netting mat- 
erials against comparatively hard objects is usually 
regarded as the more significant procedure. For this, a 
special testing machine is used in which the test specimen 
is stressed and is rubbed on or by a hard object. The 
number of rubs required for a given amount of wear 
decreases as the load is increased. Fig. 14a represents the 
Sander test machine, 25 while Fig. 14b represents a 
design reported by Shimozaki. 26 Both testing machines 
work according to the same principle and the specimens 
are kept constantly wet during the test, but the angle 
described by the netting material at the point of contact 
with the abradant is different on the two machines. In 
the Sander testing machine, the specimen assumes an 
angle of about 90 at the abradant as shown in Fig. 14a, 
but, in the design reported by Shimozaki, the correspond- 
ing angle is about 150. In the Sander testing machine 
the specimen is pulled back and forth, whereas in the 
design reported by Shimozaki, the specimen is held fast 
while the abradant is moved back and forth. 27 

24 It would be more suitable to use the 250-m weight. 

26 Klust, G.: Untersuchungen liber die Scheuerfestigkeit von 
Fischnetzschnurcn. Protokolle ziir Fischereitechnik 3, pp. 64-88, 
1954. This machine uas originally developed for testing textile 
fabrics, not netting materials. 

26 Shimozaki, Y.: Characteristics of synthetic twines used for 
fishing nets and ropes in Japan. Modern Fishing Gear of the World, 
pp. 19-29, 1959. 

27 The Institut fur Hochseefischerei in Rostock-Marienche uses 
an abrasion testing machine designed by Bobeth-Hahn (DRP No. 
8961). On it, the load on the specimen may be held constant or it 
may be changed between maximum and minimum values at every 
complete rubbing cycle. 


The water pouring steadily over the specimen serves 
not only to keep the specimen wet but also to keep it 
cod and to wash away loose particles as they are rubbed 
off the material. 

The netting twines are mounted as shown in the draw- 
ings. 20 Several specimens may be mounted beside one 
another. Loads are attached to the free ends of the 
netting material, equal to or greater than the 250-m 
weight according to the purpose of the test. 

On the Sander machine, the test specimen is pulled 
back and forth over a corundum rod in which particles 
of corundum are bound in a ceramic material. The size 
of the corundum particles is standardised, is chosen 
according to the material under test, and must always 
be reported. The machine described by Shimozaki used 
oil stones for (he abradant. The Sander machine imposes 
63 rubbing cycles per minute, whereas the machine 
described by Shimozaki imposes 80, and these cycles 
are counted automatically on both machines. On the 
Sander machine the stroke amplitude, E-F, is 6 cm 
(2f in) if the machine is set up as shown in Fig. 14a. 
The specimen is rubbed until it is completely worn and 
breaks. The test is repeated at least 25 times with each 
sample, and the arithmetic mean of the results is taken as 
the abrasion resistance. 

The type of abradant, the magnitude of the load on 
the specimen, the speed of rubbing, the angle described 
by the specimen at the abradant, and the length of the 
rubbing stroke all affect the result and should be included 
in the report. 

Abrasion against itself: For testing abrasion resistance 
between similar or different netting twines an apparatus 
was proposed by Taylor and Wells. 29 As can be seen 
in the diagram (Fig. 15a), the two twines A-B-F-C and 
D-F-E are rubbed against one another. The first twine 
is fastened eccentrically to disc A, is led over the small 
roll B, through the loop in the second twine at F, and is 
attached to the table at point C. The second twine is 
fastened to the table at point D and is led through the 
loop of the first twine at point F. The free end of the 
second twine is stressed by the weight E. 80 

The weight should be between a fifth and a twentieth 
of the average breaking strength of the specimen and 
should be chosen so that between 100 and 200 complete 
rubbing cycles are required before the specimen breaks. 
Taylor and Wells recommend that SO tests be averaged 
for each result. The load, the speed of rubbing, and the 
angles of contact should be reported with each result. 

AC ]A 

18 A-B= netting material under test, C = abradant, D= weight, 
E-F = direction of reciprocating motion, O = clamp for test 
specimen, H = cycle counter, 1 = crank, J = motor, K = water 
tank supplying water to specimen, L = water tank catching water 
from specimen. 

" Taylor, A. F. and Wells, A. H.: Properties and values of 
certain fish-net preservatives. Document 947, U.S. Bureau of 
Fisheries, 1923. 

86 It is important to keep the angles of the abrading materials 
equal for all comparative tests. 


T S 


Old Mthod Ntw Mtthod 

(5) Weight 

Accurate tests for material weight, as required for the 
calculation of breaking length, can be conducted only 
in a standard atmosphere which complies with official 
international standards for temperature and relative 
humidity (see above). In addition to tests for twine 
count which are not within the terms of reference for 
these discussions, the weight of the netting materials in 
these standard atmospheric conditions is of interest for 
comparison with the wet weight. Also, the weight of the 
netting material in water is important. 81 

Dry weight: The old Canadian procedure, 32 as shown in 
Fig. 15b, is to fasten the specimen of netting twine, T, 
in a clamp, A-A, by its upper end, allowing it to hang 
vertically (not horizontally as shown in Fig. 20 and Fig. 
21). The required pre-tensioning weight, W, is hung 
from the lower end of the specimen by means of the clamp 
B-B. This vertical arrangement avoids the effect of 
friction in pulleys, etc. The specimen is cut first, while 
under appropriate pre-tension, against the scale S, at a 
point one m below the upper clamp as shown at K, and 
is then cut a second time at the lower face of the upper 
clamp, A-A. Ten such 1-m specimens are weighed to- 
gether and the linear density or runnage of the netting 
material is computed. 

A refinement of this apparatus is shown as the new 
method in Fig. 1 Sb. It consists essentially of two clamps, 
A-A and B-B, mounted on a vertical support. A steel 
meter-scale, S, divided to the nearest 0*5 mm, is mounted 
behind the clamps so that the distance between the 
clamps may be set accurately. A trip balance for pre- 
tensioning the specimen is mounted above the upper 

81 See Chapter 21 : Weight of netting. 

** Can-others, P. J. O.: Fisheries Research Board of Canada, 
St. Andrews, N.B. 

clamp, A-A. The upper end of the specimen is fastened 
to the hook, H, under the pan of the balance and the 
specimen is allowed to hang between the open jaws of 
the clamps. The weights, W, on the balance beam are 
set to exert a prc-tcnsioning force on the netting material 
equal to the 500-m weight and the clamps are set so that 
their facing edges are exactly 1 m apart. The lower end 
of the specimen is pulled by hand as shown at P until 
the balance pointed reads "O" on the balance scale, and 
the jaws of the clamp B-B are tightened. The jaws of 
clamp A-A are then tightened. The specimen is cut at 
the facing edges of both jaws as shown at K-K and the 
1-m piece is removed for weighing. As before, 10 such 
1-m pieces of the netting material are weighed together 
and the runnage (m/g) or the linear density (g/km) can 
easily be calculated. 

Wet weight: For measuring the wet weight of netting 
twines out of water, the test specimens are cut as des- 
cribed and soaked. The resulting shrinkage is not 
considered because the greatest interest lies in how much 
more the specimen weighs wet than dry. 

When the netting material has become completely 
soaked, it is removed from the water, allowed to drain, 
and is weighed. 33 The wet weight is reported as a per 
cent of the dry weight according to the formula : 

Wet weight (%) = - 

wet weight (g) x 100 

dry weight (g) 

or as the per cent increase in weight according to the 
formula : 

,^ wet weight dry weight </wv 

Water absorption ( %) =- f r~ - X 1 00. 

dry weight 

Weight in water: The weight of netting materials as well 
as of netting in water depends not only on the specific 
gravity of the fibre but also on the speed with which 
water soaks into the material and on the tendency for 
air bubbles to remain in or on the material. Because 
of this air entrainment, quite dense materials can float 
at or near the surface of the water unless they are loaded 
to cause complete immersion. 

The apparatus shown in Fig. 16 may be used to mea- 
sure the weight in water. 34 A specimen of netting twine 
of known weight (e.g., 15 g) is completely soaked as 
described above and is fastened to the bottom of the 
balance pan as shown in Fig. 16. It is suspended in 
water (distilled, fresh, or sea) at about 20C (68F). As 
distilled water contains no dissolved air, the formation 
of bubbles on the specimen may be minimised by its use. 
The weight of the specimen under these conditions is 
measured and is reported as a per cent of the weight of 
the air-dry specimen. 

88 In Modern Fishing Gear of the World, p. 21, Shimozaki gives 
factors for the estimation of the dry weight (linear density mg/m) 
and the wet weight of several man-made and cotton netting mater- 

84 v. Brandt, A.: Arbeitsmethoden der Netzforschung, Stuttgart, 
1947, pp. 107-108. 

Fig. I 6 

u. * /o/x weight in water (g) x 100 

Weight in water (%) = ; r-rr- ; : 

e air-dry weight (g) 

It is also possible to compute the weight in sea water from 
the formula: 35 36 

Weight ( %) in sea water = 

specific gravity of sea water\ 

x 100. 

\ specific gravity of material 

Similarly, the specific gravity of the fibres may be com- 
puted from the following formula: 


Specific gravity = r-r- - .. ... 

weight m distilled water 

air-dry weight in air. 

Floating ability and sinking speed: Of particular interest 
with netting twines and still more important with certain 
complete nets, is the time for which they remain buoyant, 
i.e., whether they float for a longer or shorter time or 
whether they sink quickly. Whether the ability to float 
for a long time (e.g., netting materials made of fibres 
having specific gravity less than one) or the ability to 
sink quickly is important depends on the type of fishing 
gear. Different procedures for measuring this property 
have been developed for different reasons. 

Proposal (a), Floating ability: 37 The ability of a speci- 
men to retain its lightness in water can be measured on 
the same apparatus as was used for measuring the weight 
in water. 

The specimen of netting material is suspended in a 
vessel which is filled with fresh water or sea water. 

85 Can-others, P. J. G.: The physical properties of netting and 
twines suitable for use in commercial fishing gear. Modern Fishing 
Gear of the World, pp. 69-74, 1959. 

In Modern Fishing Gear of the World, p. 22, Shimozaki gives 
factors for estimating the weight in water from the dry weight for 
materials made of several man-made fibres and of cotton. 

87 v. Brandt, A.: Arbeitsmethoden der Netzforschung, Stuttgart, 
pp. 107-108, 1947. 


Weights are placed on the specimen to hold it below the 
surface of the water. At intervals of time, first of hours 
and later of days, the added weights are removed and 
the weight of the specimen alone in the water is measured. 
This procedure is repeated over a period of time until 
the weight in water remains constant. 

The weight in water may be reported as a per cent of 
the weight of the air-dry specimen as described above. 
For better understanding, the changes in floating ability 
may be plotted graphically. The time in hours or days is 
plotted against the abscissa and the increase of weight in 
water, expressed as a per cent of the dry weight, is 
plotted against the ordinate. Fig. 17 shows three cotton 
twines prepared by different methods. According to the 
preparation the floating ability will be lost immediately 
(I and II) or after some hours (111). 



Proposal (b), Sinking speed: 38 For determining sinking 
speed, a piece of netting twine 2-cm ( it in) long is knotted 
in the middle and is soaked in clear water for 12 hours. 
A glass vessel is filled to a height of 50 cm with water at 
20 5C (Fig. 18) and the test specimen is allowed to 
fall from the surface of the water. The time required for 
the sample to fall the distance of SO cm is measured in 
seconds and the average sinking speed is calculated in 
cm per sec. Three tests should be performed with each 
specimen. Results from tests in which the sample sinks 
at an angle or close to the wall of the vessel should be 

(6) Diameter 

For measuring diameter, generally speaking, special 
thickness gauges of different types are used. With these 
a certain diametrical pressure on the netting material 
cannot be avoided so that the results of measurements 
on soft-laid or loosely braided materials or on those 
made of staple fibres are too small. 39 Therefore, it has 
been proposed to test these materials according to an 
ASTM procedure (proposal (b)). The facts that the 
diameter of netting twine is decreased as a result of 
extension under tensile load and that the cross-section 

w Japan Chemical Fibres Association. The manufacture and 
testing of synthetic yarns and fibres used in Japanese fishing gear. 
Modern Fishing Gear of the World, p. 68. 

89 Carrothers, P. J. G. : The physical properties of netting and 
twine suitable for use in commercial fishing gear. Modern Fishing 
Gear of the World, pp. 69-74, 1959. 


of the twine may be elliptical or even rectangular rather 
than circular must be taken into consideration. 

Proposal (a), for measurement of monofilaments and 
firm netting twines: A manual thickness gauge 40 (Fig. 
19a) may be used for this measurement. The test specimen 
is placed between the two circular pressure feet (A^A* in 
Fig. 19a) which are completely flat and are set parallel 
to one another. The feet separate to 10 mm ( ft in) apart 
without pressure so that all kinds of netting materials and 
even thinner cordage used in the fishing industry can be 
measured. By pressing lightly on the key (B in Fig. 19a) 
the thickness of the material may be read directly from 
the scale (C in Fig. 19a) to the nearest 1/100 mm. The 
results of at least 20 measurements should be averaged, 
and with very irregular materials even more measure- 
ments may have to be made. As mentioned, it must be 
noted whether or not the specimens really have a circular 

Alternatively, a dead-weight dial gauge 41 (Fig. 19b) 
may be used for this measurement. In this, four lengths 
of the specimen are laid parallel on the plane anvil, A, 
and the movable foot, F, which is plane and parallel to 
the anvil, is lowered gently onto the specimen by means 
of the handle, H. The weight, W, is chosen so that the foot 
exerts a standard total force of 6 oz (170 g) on the 
specimen. The thickness of the specimen is read to 
the nearest 1/1,000 in directly from the dial. 

Proposal (b): Particularly for soft, compressible netting 
materials. Canada has suggested that ASTM Desig- 
nation D578-61 for glass yarns be used for measuring the 
diameter of netting materials. This specification reads: 

"(a) This method is based upon the use of a microscope 

equipped with either a micrometer eye-piece with 

a movable scale or a filar micrometer eye-piece. 

(b) Apparatus. A microscope having a movable stage 

which can be rotated to bring the yarn parallel 

40 Klust, G.: Verflnderung des Durchmessers in Netzganen 
durch Wasserung. Protokolle zur Fischereitechnik IV, pp. 84-106, 

41 ASTM Designations D204-57T and D885-62T, CGSB 
Schedule 4-GP-2, Method 37, and B.S. 2544: 1954. 

to the cross hair shall be used. The magnification 
shall be of such power that the yarn shall cover 
approximately one-quarter of the field of view. 

(c) Procedure. Mount the yarn on the movable stage 
of the microscope by means of a yarn carrier which 
is provided with suitable guides to maintain a 
constant tension. Take care that no change in 
twist occurs in mounting the yarn. Rotate the 
stage until the yarn is parallel to the cross hair. 
Determine the diameter of the yarn as the differ- 
ence in the micrometer settings when the cross hair 
is moved from one edge of the yarn to the other. 

(d) Number of measurements. Make 20 measurements 
at least 1 ft apart, and take the average as the 
diameter of the yarn." 

The Fisheries Research Board of Canada has found a 
projection microscope to be just as accurate as, and more 
convenient to use than, the movable stage micrometer 
eye-piece instrument described above. First, the image 
of a stage micrometer is projected by the microscope 
on the screen on the bench and an appropriate scale and 
vernier for the screen, reading to the nearest 0*01 mm at 
the stage, is prepared from the image. The netting 
material is then mounted on the stage of the projection 
microscope, as described above by ASTM, and its 
diameter is read directly by adjusting the specially pre- 
pared scale and vernier to the image of the material on 
the screen. As in the ASTM designation, 20 measure- 
ments should be made. An F = 32 mm Macro-Tessar 
lens has been found suitable for most materials. 

Besides avoiding compression of the material, these 
optical procedures permit study of the profile of the 
material for irregularities and fuzziness, and may be 
adapted to studying diameters of the material in water 
and with different tensile loads on the material. 

Proposal (c): Coil winding method. 42 Twenty turns of 
the netting material are wound closely and parallel to 
one another around a cylinder of about 5 cm diameter. 
The overall width of the 20 turns is measured with callipers 
and is divided by 20. The result is reported as the thick- 
ness of the material in mm. Five tests should be per- 
formed. The difficulty is that uneven tension in the speci- 
men and variations in the angle at which the material 
is fed on to the rod can affect the result. 43 

(7) Surface roughness 

Even if there is little danger of the netting material 
becoming compressed during the measurement of dia- 
meter, there is still the possibility that netting materials 
made of staple fibres will be much thicker in water as a 
result of protruding fibres than is indicated by the meas- 
urements described in Section 6. 44 This surface roughness 

42 Japan Chemical Fibres Association. The manufacture and 
testing of synthetic yarns and fibres used in Japanese fishing gear. 
Modern Fishing Gear of the World, p. 65. 

43 Hamilton, J. B.: Direct method for measuring yarn diameters 
and bulk densities under conditions of thread flattening. The Journ- 
al of the Textile Institute, Transactions, 50, pp. T655-672, 1959. 

44 Can-others, P. J. G.: The physical properties of netting and 
twines suitable for use in commercial fishing gear. Modern Fishing 
Gear of the World, pp. 69-74, 1959. 

also affects several other properties, such as knot 
stability (see Section 20), pollution of nets (see Section 
22), hydrodynamic drag, and selectivity of catch. So 
far, however, a procedure for measuring surface rough- 
ness has not been developed and, until now, tests have 
been restricted to visual observations, such as by photo- 
graphs. 45 

(8) Shrinking and lengthening 

Generally speaking, the length of netting materials 
changes when they are soaked in water. Usually a 
shrinkage occurs, but sometimes a lengthening takes 
place. Of greatest importance is the fact that this change 
in length affects the size of the mesh (see Section 18). 
These length changes may be determined either on the 
unknotted material or by measuring the size of mesh. 
However, the two results are generally not in agreement 
because of the effect of the knots, quite apart from the 
effect of any difference between the measuring procedures. 
The main source of the difference in results lies in the 
fact that shrinkage in the netting material is usually 
measured under a pre-tension which is graduated accord- 
ing to the linear density of the material, whereas the 
mesh size is measured either without pre-tension or with 
a pre-tension which has been agreed upon by international 
conventions but which is not related to the runnage of the 

Tests for length change may be conducted to deter- 
mine the effect of cold tap water or they may be conducted 
to determine the effect of higher temperature, e.g., by 
boiling during dyeing or hot treatment with a preserva- 
tive. The test procedures which have been proposed for 
the fishing industry to date do not differ significantly 
from one another. 

Proposal (a): 46 For measuring length, the specimens 
are mounted in the apparatus as shown in Fig. 21. To 
avoid errors resulting from pulley friction, scale parallax, 
and twine sag, it has been recommended that the appa- 
ratus of Fig. 21 be mounted vertically and that the test 
specimen be hung very near to the scale. The specimen, 
more than 2 m long, is attached to point A (Fig. 21) 
and is led over the roll B, 200 cm away. A load equal to 
the 250-m weight 47 is applied to the free end. 

45 v. Brandt, A.: Zur Knotenkonstanz von Fischnetzen. Archiv. 
fUr Fischerciwissenschaft 9, pp. 244-265, 1958. 

46 Klust, G.: Lftngenverftnderung von Netzgarnen, Protokolle 
zur Fischercitcchnik VI, pp. 70-83, 1956. 

47 BISFA rules for synthetic fibres specify a pre-tension equal to 
the 500-m weight. 


For determining changes in length due to wetting, 
two or more points are marked on the dry, pre-tensioned 
specimen before it is soaked, and the distance between 
these points is measured. The specimen is soaked for 
at least 12 hours in fresh or sea water at normal tempera- 
tures. Otherwise, the specimen may be boiled or treated 
in any other way as by preservatives or by dyeing. Then 
the distance between the marked points is measured 
once more in the manner described above. 

The final length is reported as a per cent of the initial 
by length: 

Final length^ Final length (cm) x 100 

Initial length (cm). 

Proposal (b): 48 Five knots are tied in a piece of netting 
twine at intervals of about 1 m and each part of the 
material, as defined by the knots, is mounted in turn 
on the measuring board as shown in Fig. 20. As men- 
tioned for Proposal (a) above, the apparatus would 
preferably be mounted vertically. The specimen is 
stressed by a standard pre-tension and the distance 
between each pair of successive knots is measured to the 
nearest 1 mm. The sample is then placed, unstressed, in 
cold or warm water as required, and the distances 
between the knots are subsequently measured again. The 
changes in length may then be reported as: 

_. . , /oyv Initial length final length 
Shrinkage (%) ,.*... - x 100 

Initial length 


Length change to wetting ( %) = 

Final length initial length 

x 100. 

Initial length 

In this latter case, shrinkage is reported as a negative 
quantity and elongation is reported as a positive quan- 
tity/ 9 

Proposal (c): 50 Tests in cold water. A distance of 
1,000 mm is marked on the test specimen under standard 
pre-tension and the specimen is tied outside the marks 
to form a loop. The specimen is immersed in water at 
room temperature for 12 hours or until it has become 
thoroughly soaked, and then it is dried. The new length 
is then measured in mm, as above, and the shrinkage is 
calculated by the first formula in Proposal (b) above. 

M van Wijngaarden, J. K. : Methods of testing net cord and nets, 
especially those made of synthetic material. Rayon Revue, 11, pp. 
146-160, 1957. 

49 Can-others, P. J. O. : The physical properties of netting and 
twine suitable for use in commercial fishing gear. Modern Fishing 
Gear of the World, pp. 69-74, 1959. 

60 Japan Chemical Fibres Association: The manufacture 
*nd testing of synthetic yarns and fibres used in Japanese fishing 
gear. Modern Fishing Gear of the World, pp. 62-68, 1959. 


Tests in boiling water: A test specimen more than 1 m 
long is folded in half and the two free ends are secured 
together. A standard, prc-tensioning load is applied to 
the loop and both sides of the loop are marked at a 
distance 50 cm from the fold. The test specimen is 
then boiled, unstressed for 30 min and is dried. The 
marked distance is measured again, as above, and the 
shrinkage is calculated as follows: 

et. i ,o/x 
Shrinkage (%) = 

500 final length (mm) 
~ - v - ' 

x 100. 

At least five tests should be carried out for each average. 

(9) Thermal reaction 

The actual properties of netting materials, particularly 
those made of man-made fibres, can be different from 
the measured properties at temperatures which differ 
from the specified standard temperature at which the 
properties were originally measured. This is true both 
for the higher temperatures experienced in the tropical 
fishing industry and for the lower temperatures experi- 
enced in the winter fisheries of most countries in the 
temperate and arctic climates. In particular, changes 
can be expected in strength, extensibility, and stiffness. 
Special equipment, required for measuring physical 
properties of netting materials at extreme temperatures, 
has not yet been developed by fishery researchers. To 
determine whether or not netting materials can with- 
stand temperatures required for preserving or dyeing, 
boiling tests are usually carried out, particularly to 
establish resulting changes in length. Such tests demon- 
strate that the materials can be affected by temperature. 
The methods developed by the textile industry for 
measuring resistance to cold and heat should be remem- 
bered. 51 

(10) Weather resistance 

Some properties of netting twines, especially strength, 
are adversely affected by light, particularly by ultra- 
violet radiation. Usually, light damage is studied by 
exposing the unprotected samples to natural sunlight in 
the open air. Since light is not the only factor in such 
exposures, the tests are often described as being for 
weather resistance or atmospheric resistance. In this 
test, quite a number of other factors affect the material : 52 

Primary exposure factors: 

chemicals in the air oxygen, carbonic acid, 
water (steam, fog, dew, rain, snow, ice) 


air movements (wind) 

solar radiation. 
Secondary exposure factors : 


shifting sand 

micro-organisms and insects. 

81 E.g., Sommer, N. and Winkler, F.: Prilfung der Tcxtilien. 
In: Handbuch der Werkstoffprilfung V, 1312-1335, 1960. 

69 Hofmeier, H.: Klimapriifung zur Ermittlung der Gebrauch- 
seigenschaften von Kunstatoffen und anderen Werkstoffen. Kunst- 
stoffe41,pp. 179-180, 1951. 

In every case, changes in strength or abrasion resis- 
tance or fading of dyed materials have to be measured. 
When different materials are being compared, it should 
be recognised that light rays do not penetrate deeply so 
that materials of different diameter are differently 

For determining the effect of the sun's rays and weather 
in the open air, the samples are stretched in a frame 
(Fig. 22). These frames are set in an unshaded place 
(e.g., on a roof) free from dust and smoke if possible. 
They are faced in a southerly direction and are secured so 
they are not damaged by wind. The frames are built of 
wooden laths, 3 cm wide by 2 cm thick, and are varnished 
for protection against the weather. The size of the frame 
is governed by the size and number of the samples. 
Where strength is used as the measure of weather damage, 
a test length of 200 mm is required (see Section 1) so 
that the frame should be 30 cm wide. Several frames 
may be exposed together. 

The materials are suspended, without tension if possible, 
on aluminium nails or screws set 1 cm apart on the frame. 
The nails or screws on opposite sides of the frame are 
off-set \ cm so that the test material assumes a zig-zag 
shape (see Fig. 22). 

Every two or three months test specimens are removed 
for measuring changes in strength or other properties. 
The measured result is referred to the total time of 
exposure or to the number of hours of sunlight measured 
during the exposure as reported by the local weather 
station. " 

Resistance to sunlight: For determining the effect of 
sunlight completely exclusive of rain and dust, the 
materials may be exposed in a box covered with ultra- 
violet transparent glass. As for the weathering test, the 
box is set at an angle of 45 facing south, and the bottom 
of the box is ventilated to avoid overheating. 54 

51 In connection with weathering tests, the amount of sunlight 
and precipitation and the relative humidity should be measured 
each day. 

M Standard procedures specify that the ventilation be such that 
temperatures inside the box are no more than 2C above the outside 

The sample is wound around an opaque board, 30 cm 
wide, which is mounted in the box so that its upper face 
is 5 cm from the glass cover. The specimens on the 
shaded side of the board are used as controls for com* 
parison with the specimens on the exposed side of the 
board. As mentioned above, the resulting change in 
properties is related to the number of hours of sunlight 
during exposure. 

Resistance in weathering machines: It has also been 
proposed that accelerated tests be conducted with 
artificial radiation instead of with sunlight which requires 
a long exposure time. These weathering machines are 
constructed so that the samples may be exposed to 
artificial rain as well as to artificial sunlight, e.g., The 
American Weather-Ometer. 55 

(b) Chemical tests 

(11) Resistance to preservatives, oik, etc. 

Netting twines made of man-made fibres may be damaged 
by substances such as creosote oils and tannins which 
are used to preserve natural fibres as well as by fish 
slime and by other chemicals (e.g., light and heavy oils) 
used in the fishing industry which also damage natural 
fibres. Their effect, particularly on strength, should be 
studied. 56 

(c) Biological tests 

Fishing gear made of natural or synthetic fibres can 
become damaged by various organisms both in storage 
and in use. However, storage tests which study damage 
in net warehouses will not be included here. Only those 
tests which assess damage to fishing gear by water-borne 
organisms will be discussed. 

For netting materials made of plant fibres, cellulose- 
digesting microbes and fungi living in the water and in 
most storehouses are the major cause of rot. Similar 
damage by micro-organisms can also be experienced by 
fibres of animal origin. 

Netting materials or natural and synthetic fibres can 
also be damaged by several higher-order organisms in 
the water. Here, our thoughts are restricted to those 
organisms which eat dead vegetable matter or gnaw at it 
for some other reason and, hence, which attack netting 
made of plant fibres, too. Those water animals which 
have become trapped in fishing gear and damage it in 
an effort to become free will not be considered. Among 
these could be members of all types of animals living in 
the water, including the fish themselves. 

Finally, within the scope of biological tests is a study of 
that damage by communities of living animals or plants 
known as fouling, whereby the organisms move actively 
onto fishing gear which has been in the water for a 

65 Japan Chemical Fibres Association: The manufacture and 
testing of synthetic yams and fibres used in Japanese fishing gear. 
Modern Fishing Gear of the World, pp. 62-68, 1959. 

M See Chapter 16: Dyeability and treatability, and Chapter 
17: Storability. . , 

57 Concerning organisms which are moved passively onto the 
nets, see Chapter 22: Pollution. 


(12) Rotting rabtance 

If treated or untreated netting materials made of natural 
fibres are to be tested for rotting resistance, the condi- 
tions experienced by the fishing gear during fishing 
must be considered. Of great importance is the constant 
rinsing action by the water. 58 Tests, as conducted in 
textile research with pure cultures of certain micro- 
organisms such as fungi or with mixed cultures enriched 
to a greater or lesser degree with suitable micro- 
organisms as in the soil burial test, 59 can yield incorrect 
results with fishing gear materials, unless storability tests 
are being conducted (see Section 17), because there is no 
constant rinsing and consequent leaching of preservative 

The two tests described below refer, in the one case, to 
a field test in natural water and, in the other case, to a 
laboratory test. The objection 60 to the field tests in 
natural water is that non-biological factors such as 
currents, rinsing time, and mechanical effects can affect 
the result. The objection to laboratory tests, e.g., in 
aquaria, is that the test vessel can become poisoned by 
preservative agents leached from the test specimens and 
that leaching is slower than in the field so that rotting 
may be lower in the experiment than during fishing. So 
far, these difficulties have not been overcome. Neverthe- 
less, by eithei procedure, comparable results may be 

Proposal (a): test under natural conditions. 61 For 
measuring the resistance to rot under natural field con- 
ditions, the specimens are exposed in a suitable location 
under the surface of the water. Damage by the micro- 
organisms is measured by testing the strength of the 
specimen both before and periodically during the exposure. 
The lower the rot resistance of the material, the quicker 
it will decrease in strength, and vice versa. However, 
the rate of strength decrease depends also on the rotting 
activity of the water. Therefore, this must be measured 
at the same time. Thus, the duration of the test is deter- 
mined on the one hand by the rotting activity (exogene 
factor) of the water and on the other hand by the rotting 
resistance (endogene factor) of the netting material. 
The exposure should be continued, if possible, until the 
specimen has lost at least 50 per cent of its initial strength. 
The rotting activity which causes the netting material 
to lose 30 per cent of its initial strength is taken as the 
measure of rot resistance. A greater required rotting 
activity to cause this loss indicates greater rotting 
resistance of the material. 

M For netting materials made of synthetic fibres, which as a rule 
are not destroyed by cellulose- or protein-digesting micro-organisms, 
the methods described here may be used to measure their resistance 
to damage by the water or other agents in it. 

M See proposal DIN 53933 and ASTM Designation D684-54. 

60 Zaucha, J.: Evaluation of rot-retarding net preservatives. 
Modern Fishing Gear of the World, pp. 128-132, 1959. 

61 v. Brandt, A.: Arbeitsmethoden dcr Nctzforschung, Stuttgart, 
pp. 64-83, 1947, and Method of testing resistance of net materials to 
micro-organisms. Modern Fishing Gear of the World, pp. 133-136, 
1959. This method is based on the so-called "physical-activity- 
detennination" by Meseck (1929). 


Thus, the test consists of two parts: first, the rotting 
activity of the place where the exposure is to be made is 
determined and, second, the strength loss experienced 
by the specimen is measured. The rot resistance of the 
specimen is computed from both these values. 

If the rotting activity of several test places is known, 
comparative tests can be conducted in these different 
places even during different seasons. Thus, the test 
results are independent of differences in exposure site 
and exposure time. 

The monthly rotting activity of a new exposure site 
may be measured as follows : 

Special cotton test twine of metric No. 50/1 5 (20 tex x 
5 x 3) 62 is suspended in the water at the site in question 
on the first day of the month. Prior to exposure, the 
twine is first extracted by boiling in distilled water and 
its initial strength in the wet condition is measured; 
then pieces of it, each 30 cm long, are tied together by 
means of a rot-resistant material into four bundles of 
10 pieces of twine each. After the first, second, and third 
week and on the last day of the month, one of the bundles 
of test twine is removed and its loss in strength is deter- 
mined. The per cent loss in strength after one month is 
the rotting activity of that site during that month. For 
the above procedure, the loss in strength should not 
exceed 70 to 80 per cent of the initial strength. 

In areas where rotting activity is high, a loss in strength 
equal to 70 to 80 per cent of the initial strength may be 
reached before the end of the month, even before the 
end of the first week. In this case, it is not possible to 
determine the monthly rotting activity by the above 

The procedure must be modified so that the exposed 
specimens are replaced by new ones after they have lost 
| of their initial strength. The time at which this loss 
occurs may be found easily by testing exposed specimens 
for strength loss every week as described above. For sites 
which have been used experimentally for several years, 
the time required for the strength loss to equal of the 
initial strength during any particular month is already 
known approximately. In such cases, the specimens may 
be replaced every week or 10 days as indicated. The 
per cent strength losses of all successive specimens, 
replaced one or more times during the month, are added 
together. 63 The total result is the rotting activity of that 
site during the month in question. Similar determination 
of the monthly rotting activity of a given site, by exposing 
the untreated specimens until they have lost only 

62 In tropical areas, where the rotting activity is higher, heavier 
twines such as metric No. 20/45 (50 tex x 15 x 3) have been used. 

63 The rate of strength loss with time is not constant, being much 
slower at the beginning of the microbiological destruction than at 
the end. For this reason, strength losses greater than 70 to 80 per 
cent of the initial strength are not included in rotting activity 
estimates. Even so, some error cannot be avoided, but the simple 
summation of per cent strength losses can still be useful. 

SO per cent of their initial strength, has already been des- 
cribed elsewhere. 64 Because the moment of the desired 
standard strength loss does not always fall at the end of 
the month, or because the exposure is not always started 
at the beginning of the month, daily values for per cent 
strength loss may be calculated as the basis for estimating 
the rotting activity which affects a specimen for periods 
less than a month. 

In only very few cases are untreated netting materials 
tested for rotting resistance. Usually the materials have 
been treated with a preservative. 

For measuring the efficiency of a preservative in 
reducing rot, cotton twines, e.g., metric No. 50/15 
(20 tex x 5 x 3), or materials made of other fibres sub- 
ject to rot, are treated according to directions for the 
preservative. For comparing different preservatives with 
one another, the same size twine should be used through- 

The twine is cut into 60 cm lengths, the ends are knotted 
to conserve twist, and the specimens are treated. Then 
they are tested for increase in weight, content of certain 
active preservative agents, changes in flexural stiffness 
(Section 3), changes in length (Section 8), changes in 
strength (Section 1), etc. The pieces of treated netting 
material are folded in two and tied into bundles 30 cm 
long. Care must be taken that the material used for 
tying the bundles does not affect nor is affected by the 
treated test specimens, and that it does not rot itself. 
Otherwise, the bundles will become loosened before the 
end of the test period. 

In test places where the current is strong, e.g., offshore 
or in rivers, etc., the specimens may have to be tied to- 
gether at their lower ends as well as at their upper ends to 
prevent mutual damage or entangling. It must be noted 
that the specimens are rinsed more in sites having strong 
currents than in still locations. This stronger rinsing 
can decrease the resistance to rot. Results from places 
which differ widely in these non-biological factors should 
not be used for comparative tests. 65 

Each bundle of treated netting twine should consist 
of 60 test pieces (30 double twines). This number is 
sufficient for six tests of 10 twines each or 12 tests of 
five twines each. Larger bundles than this are not satis- 
factory because the outer twines protect the inner ones 
from the rinsing action of the water, particularly if the 
bundles are tied at both ends. 

It is not necessary to rinse the bundles of treated twine 
before the test because, as with fishing nets, rinsing 
occurs automatically during the first few days of exposure. 

64 Zaucha, J. f Evaluation of rot-retarding net preservatives, 
Modern Fishing Gear of the World, pp. 1 28-1 52, 1 959, has proposed 
to use the 50 per cent strength loss rather than the 70 to 80 per cent 
strength loss as the limiting value. In Proposal (b), the per cent loss 
in strength is not specified at all. Rather, the time for total rotting 
of the untreated twine is called a period, and the number of periods 
required for the treated twine to lose a specified per cent of its 
initial strength is used as the measure of rotting resistance. 

66 Results from test sites having low rotting activity will usually 
be lower than results from test sites having high rotting activity 
because rinsing in the former sites has greater effect through the 
longer test period. 

Specimens which have been treated with different pre- 
servatives should not be exposed close together, other- 
wise they may affect one another. 

The specimens are exposed in such a way that they do 
not touch the bottom, or piles, or any other object. In 
offshore regions in the presence of tide, the specimens 
should be mounted deep enough so that they do not 
become exposed to the air at any time. 

For assessing a new preservative, twines treated with 
it are exposed together with twines treated with a known 
standard preservative for comparison. All samples are 
exposed in the water at the same time. Under European 
conditions, the first test for strength loss is applied to the 
required number of specimens, e.g., 10, each 30 cm long, 
removed from the bundle after two or even three months 
of exposure. 66 As a rule, tests are repeated every two 
months in summer and every three months in winter 
unless an abnormal strength loss requires more frequent 
examination. The rotting activity is measured at the 
same time by the procedure described above. 

The specimens taken from the exposed bundle are 
rinsed, any fouling, etc., is removed, and the breaking 
strength is determined immediately. If it is not possible 
to conduct the strength test immediately, the specimens 
are dried quickly and thoroughly and are stored under 
suitable conditions for later test for wet strength. 67 

The loss in strength of the treated specimens is com- 
pared with the rotting activity of the water, measured 
at the same time (see above). As already described, the 
rotting resistance of the treated specimens is reported 
as that rotting activity of the water which causes the 
treated specimens to lose 50 per cent of their initial 
strength. Usually, the exact rotting activity for this 50 
per cent strength loss cannot be determined directly by 
experiment but must be computed by linear interpolation 
from the test strengths and rotting activities before and 
after the 50 per cent strength loss as follows: 

. . 

Rotting resistance =* 


(ti tO X 


+ ^ 

tj = rotting activity which causes less than 50% 

strength loss 
t 2 = rotting activity which causes more than 50% 

strength loss 
R! = breaking load after exposure to rotting acti- 

vity ti 
R 2 = breaking load after exposure to rotting activity 

t a 
RBO =50% of initial breaking load 

66 In tropical areas, specimens should be removed for test after 
two or three weeks, or, at most, one month. 

67 It has also been proposed to preserve specimens in formalin 
when they cannot be tested immediately. The German DIN 53932 
(draft) has proposed 0-1 per cent Trcventol CMK' for this purpose. 
This procedure should be satisfactory particularly for untreated 
twines which are being used to measure the rotting activity of the 


the desired value for rotting activity is between t t and 

The results may be grouped as follows for classifying 
the efficiency of various preservatives: 

rotting resistance up to 200 
200 to 500 
500 to 1,000 
1,000 to 2,000 

no practical effect 
minor effect 
medium effect 
: good effect 

more than 2,000 : very good effect. 

Proposal (b): tank test in laboratories. 69 The tank 
consists of a 12-litre bottle with a tightly fitting rubber 
stopper carrying a fermenting tube (Fig. 23). The tank is 
placed in a constant temperature water bath at 28C (82F). 
Ten litres of sewage sludge is collected in a bottle from 
the local sewage treatment plant as the source of supply 
for the rotting medium. This bottle of sludge is placed 
alongside the rotting tank in the water bath as soon as 
possible. Meanwhile, the following nutrient solution 
is prepared and placed in the rotting tank : 
8 litres of tap water 
10 g secondary potassium phosphate 
10 g ammonium sulphate 
0*1 g sodium chloride. 

To this solution two litres of sludge from the supply 
bottle and a few sheets of filter paper torn into small 
pieces and shaken in water are added. 

The rotting tank is allowed to remain at 28C until the 
bacteria have begun to multiply (usually between one and 
two days). Then the pH is measured with an electric 
pH meter and is corrected to 7-5 with potassium car- 
bonate. The rotting tank is now ready for use. 

Samples of cotton twine treated with the preservative 
being tested are placed in the rotting tank along with 
samples of untreated twine as a control. The tensile 
strength of the untreated sample is checked daily. The pH 
is measured and adjusted every three days. When the 
bacterial growth ceases to cause a surface scum, more 
filter paper is added. The tank is stirred daily. 

As soon as the tensile strength of the untreated twine 
has dropped to 0, all the samples are removed from the 
tank and are rinsed. This concludes the first exposure 
period. Thus, in this procedure, one period is defined 
as the time required for the complete deterioration of a 
sample of untreated twine of the same metric number as 
the treated samples. 

Before a new exposure period is started, the samples of 
twine are rinsed in running water for three days. Mean- 
while half the contents of the rotting tank are removed, 
one litre of sewage sludge is added from the supply bottle, 
the rotting tank is topped up with nutrient solution at 
28C, and the pH is adjusted to 7*5. Then the test samples 
are returned to the rotting tank along with a fresh sample 
of untreated twine as a control. 

After each exposure period the tensile strength of the 
wet test samples is measured by performing five test 

48 For an example of this method see: v. Brandt, A. : Method of 
testing resistance of net materials to micro-organisms. Modern 
Fishing Gear of the World, pp. 133-136, 1959. 

* Teknologisk Imtitut, Copenhagen. 


breaks on the strength-testing machine using a 10 cm test 
length. The results of these tests are tabulated, reporting 
first the tensile strength after treatment with the preserva- 
tive, then the percentages of this strength which remain 
after the first, second, third, etc., exposure periods. 

Experimental procedure: Cotton twine, Nm 20/6 
(50 tex x 2 x 3) (tensile stength = 6'0 to 6-5 kg), is 
used for this experiment. For each test, a total of about 
40 pieces of cotton twine, each about 50 cm long, are cut. 
These pieces are folded in half, gathered into small 
skeins about 25 cm long, and treated with the preserva- 
tive as desired. 

Nylon lines, 30 cm long, are fastened between two 
circular plates made of a dense material. The test skeins 
of twine are fastened to these lines, and are identified by 
numbers marked on the circular plates. 

In addition to the hole drilled for the fermenting tube, 
another hole is drilled in the centre of the large rubber 
stopper used to seal the rotting tank, and a glass tube 
20 mm in diameter is inserted. This glass tube can be 
sealed with a smaller rubber stopper. 

When the two circular plates with the sample skeins 
attached are lowered into the rotting tank, the lower 
plate settles to the bottom of the tank while the upper 
plate is suspended just below the surface of the fluid by a 
nylon line which is secured between the central glass 
tube and its smaller rubber stopper. Thus, the sample 
skeins are held in a vertical position in the rotting tank 
without touching each other. The contents of the tank 
are stirred daily by vertically moving the nylon line from 
which the upper circular plate is suspended. 

The upper end of the sample of untreated cotton twine 
is similarly led through the central hole in the large 
rubber stopper, while a 250-g weight (less than five per 
cent of its initial tensile strength), tied to the bottom end 
of the sample, rests on the bottom of the rotting tank. 
After this, the tensile strength of the control twine is 
checked daily simply by lifting it by hand. When the un- 
treated twine breaks, its strength must be between and 
5 per cent of its initial tensile strength (see Fig. 23) 

This procedure gives results which agree quite well 
with those from field experiments, although there are 
certain exceptions. 

1. When the preservative treatment gives the netting 
twine a coating of tar, plastic, etc., the results in 
the rotting tank are often better than in practice 
because the coating frequently forms an effective 
physical barrier against the bacteria in the tank 
whereas in practical use flexing and natural rinsing 
cause local breaks in the coating, thereby laying the 
twine open to bacterial attack at these places. 

2. Certain preservatives are simply deposited on the 
twine. Since these agents may be washed very slowly 
from the twine, it is possible that the three-day 
rinse is insufficient and that the loading of preserva- 
tive on the twine does not reach equilibrium 
until after the sample has been placed in the rotting 
tank. In this case, the preservative agent will 
migrate from this test sample into the water and 

onto the other test samples, thus causing erroneous 


(13) Resistance to macro-organisms 

Because damage by other water-borne organisms (some 
insect larvae particularly of caddis and various sand- 
hoppers which gnaw the material, or wood borers such 
as ship-worms and gribbles which also attack cordage) 
cannot be tested during practical use, this must be done 
in laboratory aquaria 70 or in lath boxes which are im- 
mersed in natural waters. 71 The boxes are constructed in 
such a way that the organisms approach the test material, 
which is stretched and fastened in the boxes, from the 
side. The bottoms of the boxes are solid so that organisms 
which fall off the test material during exposure are not 

In laboratory aquaria the test may be conducted in 
such a way that the organisms are offered their ordinary 
food as well as the specimen of netting material. Other- 
wise, according to procedures developed for wood and 
textile vermin, 72 only the specimens of netting material 
are offered, nothing else. In the first circumstance the 
animals can select their food, whereas in the second 
circumstance they are forced to eat the netting material 
or die. The latter arrangement is the more severe test, 
and it will indicate whether or not the netting material 
will be taken as food in an emergency. Caddis use 
netting materials particularly to build their tubes. 

(14) Fouling resistance 

If netting materials and cordage are allowed to stand in 

70 v. Brandt, A.: Arbcitsmethodcn dcr Netzforschung, Stuttgart, 
pp. 83-85, 1947. 

71 Meseck, G.: Untersuchungen liber den Netzfrass niederer 
Wassertierc, etc., Zeitschrift ftir Fischerei XXVI, pp. 237-310, 1938. 

72 Becker, G.: Biologische Untersuchungen an Tcxtilien. 
Handbuch der Werkstoffjprttfung, 5 : Prttfung der Textilien, pp. 971- 
1007, 1960. 

water for a long time, plant and animal organisms can 
actively settle on them. The settling of these communi- 
ties of living organisms on dead objects is known as 
fouling. Special test procedures have been developed for 
studying fouling on ships. 73 Such a test will be successful 
only if it is carried out in the field under natural condi- 
tions. The fouling conditions at the test site must be 
known. In this matter, the following has to be reported : 

(a) The general development of the fouling, its com- 
position and intensity during the course of one year 

(b) Whether there is only one or several periods of 

(c) When the fouling periods begin and end 

(d) The seasonal appearance of the different types of 
organism, singly or in groups, and the order of their 

(e) All organisms which settle first prerequisite to 
subsequent organisms. 

As fouling can consist of many species of different animal 
and plant organisms, 74 it must be realised that treatments 
which can prevent fouling of netting material in one 
place can be useless in another where there are completely 
different organisms. 

It is possible to assess the fouling activity either by the 
density or by the weight of the fouling organisms. The 
test may be conducted with gear used for fishing or with 
netting twines or cordage exposed specially for this 
purpose. The procedure for the test depends on the type 
of fouling. 

Proposal (a): 75 Netting materials, netting, or cordage 
is exposed under water in a suitable place, if possible before 
the beginning of the fouling period. Fouling is observed 
continuously, particularly more frequently during the 
initial period of immersion. 

For measuring fouling, the test sample is removed from 
the water for a short time and the fouling intensity is 
recorded photographically. For comparative values, the 
fouling density is reported as the per cent of the surface 
area which has become covered during the exposure 
period. Fouling density should be reported more preci- 
sely when it is light than when it is heavy. For example, 
the following classes of fouling intensity are proposed : 

free of fouling =- 0% 

partial fouling = 1, 3, 5, 10, (15), 20, 30, 50, 

60, 80, 90% 
maximum fouling =100%. 

Finally, the fouling is removed at the conclusion of the 
exposure to determine to what extent the surface fibres 
of the netting materials have been affected by the fouling. 

Proposal (b): 76 The specimens of netting material are 
immersed in water for a sufficiently long time to become 

78 KUhl, H.: Arbeitsmethoden fUr Untersuchung von Bewuchs- 
schutzmitteln. VcrdfTentlichungen des Institute fur KUsten-und 
Binnenfischerei, Nr. 16, 1957. 

74 Ray, D. L.r Marine boring and fouling organisms. Seattle, 

76 In accord with fouling tests for ships. KUhl, H. : see n above, 

76 Takayama, S. and Shimozaki, Y.: Development of fishing 
net and rope preservation in Japan. Modern Fishing Gear of the 
World, pp. 113-122, 1959. 


thoroughly soaked, then they are weighed on a spring 
balance and exposed at the test site. Periodically, the 
specimens are removed from the water, allowed to drain, 
and weighed, complete with the organisms which have 
settled on them, on the spring balance. Results with 
variously treated materials are shown in Fig. 24. 

of touting (%) 

^W A 


(d) Tests for suitability 

The way in which different netting materials are best 
used in fishing gear and the way in which they are best 
handled during fishing can be determined only during 
practical use. Usually the assessment will be a subjective 
one. Sometimes quite effective netting material is rejected 
by the fishermen because it has other disadvantages, e.g., 
if it is too hard or cuts the hands when handled. Only a 
few of these properties, which are important during 
practical fishing, can be measured in the laboratory. 

Also in this category should be mentioned the ease with 
which the netting twine can be made into netting, its 
amenability to or need for dyeing or treating with a 
preservative or stiffening agent and its storability when 
not in use. 


(15) ProcessabilHy 

Netting materials, e.g., twines, can be manufactured into 
netting by manual, semi-mechanical, or mechanical 
means. This is usually accomplished by knotting the 
netting materials. Recently it has also become possible 
to manufacture netting mechanically without knots. 
Whether or not the netting material can be knitted easily 
by manual or mechanical means must be determined by 
practical tests. This processability depends upon strength, 
particularly knot strength (Section 1), elasticity (Section 
2), flexibility (Section 3), and on surface roughness 
(Section 7). The last two properties also affect knot 
stability (Section 20) and mesh strength (Section 19). 

Manufacturing time can be measured, for either man- 
ual or mechanical netmaking, to determine whether the 
processing speed for a material under test is greater or 
less than that for a known material of equal value. 78 

77 v. Brandt, A. : Arbeitsmcthoden der Netzforschung, Stuttgart, 
pp. 54-63, 1947. 

78 Different netting twines are of equal value if they may be 
interchanged for practical fishing. Between twines made of different 
fibres, there are seldom two of identical count. Very often substitu- 
tion is made on the basis of equal wet knot strength, but this cannot 
always be done, e.g., if one of the twines is so fine that it is difficult 
to handle. 


The major causes of delay during manufacture of netting 
are breakage of the netting material, excessive surface 
roughness, adhesion as a result of static electricity, and 
the formation of unwanted loops and kinks. For asses- 
sing netting materials in this way, certain types of fishing 
gear or pieces of netting are manufactured by experienced 
netmakers. Manufacturing time is measured and manu- 
facturing difficulties are recorded as the basis for com- 
parison between the new and conventional materials. 
This test procedure can be similarly applied to mechanical 
netting manufacture. Where the netting manufacture is 
fully mechanical, the average number of rows of knots per 
unit time and the amount of waste are measured. The 
average number of rows of knots tied per unit time will be 
decreased every time the machine is stopped because of 
difficulties. Also the amount of waste cut from the netting, 
expressed as per cent of the amount of good netting 
manufactured, will be increased by a greater difficulty 
in processing. 

(16) Dyeability and treatability 

Before new fibres can be used for practical fishing, it 
should be determined whether or not they can be dyed or 
otherwise treated for stiffening, etc., without damage. 
Some synthetic materials are thermoplastic, and soften 
and shrink (see Sections 8 and 9) even at temperatures 
which are commonly used without damage for dyeing 
or treating netting materials made of natural fibres. 

It is also possible that other chemicals, which are used 
for treating fishing gear or which are used for other 
purposes in the fishing industry and come in contact 
with the netting twines, can damage these netting mater- 
ials (see Section 11, Resistance to preservatives, oils, etc.). 

Often the textile industry offers dyes for man-made 
fibres which are too expensive for the fishing industry 
or which cannot be used by the fishing industry because 
they require special dyeing equipment. In such cases, 
other suitable processes must be sought. 

In this matter, the fastness of the dyes, e.g., against 
exposure to light (Section 10), as well as dyeability 
must be considered. The fastness of dyes or of other 
treatments may be measured during the tests for weather 
resistance (Section 10), for rot resistance (Section 12) or 
by one of the following proposals. 79 

Proposal (a), salt-water test: Two samples of the 
netting material are placed in a solution containing three 
per cent sodium chloride (NaCl) and 0-5 per cent mag- 
nesium chloride (MgCla). After 24 hours immersion the 
samples are rinsed in fresh water and dried. The changes 
in colour and in stiffness (through loss of treating agent) 
are noted. 

Proposal (b), agitating test: Samples of netting 
larger than 10 cm 2 are subjected to the action of a suitable 
agitator (washing machine) for 10 hours in water at 
20 5C. Then they are rinsed in fresh water and dried. 
The change in colour is noted and the loss of treating 
agent is measured by weighing. 

79 Japanese Industrial Standard (draft) J.I.S.L., 1958, Method 

(17) Storability 

Untreated or treated netting materials can also suffer 
damage even by nothing more than protracted storage 
under normal climatic conditions. This can be observed 
particularly with treated materials which have not yet 
been used but which have experienced a loss in strength. 
It is possible, too, that netting materials stored under 
pressure can be damaged by spontaneous combustion, 
particularly if they are damp. 80 

For testing storability, netting materials should, as a 
rule, be held for a long time in a dry, shaded, and well 
ventilated place to simulate storage conditions. Strength 
of the netting material is measured periodically as des- 
cribed in Section 1. Of course, the intervals of measuring 
have to be very long. 

For testing spontaneous combustion, the netting 
materials are stored in large piles which may be moist. 
Thermometers may be inserted into the pile of netting, 
even by the fishing industry itself, to measure its tempera- 
ture. Smaller netting samples, or even the agents which 
cause spontaneous combustion, may be tested by the 
Mackey test for which the procedure has not been 
standardised. 81 

The effect of warm and moist storage can be deduced 
from soil-burial tests. The test described above (Section 
12) for rot resistance in water cannot be used for testing 
storability. Similarly, the soil-burial test cannot be used 
for testing rot resistance under fishing conditions. 

Proposal (a), soil-burial test for assessing storability 
under damp conditions. 82 For this test, soil-burial boxes 
(e.g., 45 X 30 X 25 cm), of an air-permeable material 
(e.g., wood or asbestos-cement, but not glass), are filled 
with soil to at least 1 50 mm deep. The soil should be a 
well-seasoned compost, and should have a moisture 
content of about 20-35 per cent, based on the dry weight. 
Soils which contain loam or clay are not suitable. The 
boxes are kept at 29 1 U C (84' F) in a room whose 
atmosphere is saturated with moisture (100% RH). 

For determining soil activity six cotton test twines, of 
metric number 50/15 (20 tex x 5 x 3) and each 30 cm 
long, are buried vertically in the soil so that only a short 
part (ca 5 cm) is not covered, and the soil is pressed 
lightly toward the test twines. The samples should be 
at least 50 mm apart. The boxes are weighed at the 
beginning of the test and the weight is checked daily. 

Water lost by evaporation is replaced by spraying warm 
(29C = 84F), chlorine-free water over the test soil. 
After four days in the soil the twine samples are removed 
and tested for strength. When the twine strength decrea- 
ses by 50% 10 during the four-day burial period 
(80 10% during a seven-day period), the mixture in 
the box is active enough for use. 

80 v. Brandt, A.: Arbcitsmethoden der Netzforschung, Stuttgart, 
pp. 87-91, 1947. 

81 Schniffher, R.: Untersuchungen von textilien Hilfsstoffen. 
Handbuch der WerkstoffprUfung Vol. 5: Prufung der Textilien, 
pp. 858-912, 1960. 

88 In accordance with DIN 53933. See also ASTM Designation 
D684-54, ASA L 14.55-1960, Norme Fran$aise NF X 41-503-1955. 
Indian Standard 1623 and 1633, 1960. 

Meanwhile, the samples which have already been 
treated with the preservative are rinsed in tap water 
for 24 hours at 20C (68F) a flow rate at such that the 
water is renewed five times per hour. They are then ready 
for testing. 

If the boxes of soil are sufficiently active, up to 20 
specimens of treated twine each 30 cm long are buried as 
described above along with six similar, untreated twines 
for control. The untreated control twines are replaced 
every four days. Five of the treated twines under study 
are removed every two weeks, rinsed, and tested for 
strength. The test is seldom continued for longer than 
eight weeks. Usually four weeks is sufficient. 

By testing the untreated control twines it is possible to 
determine the rotting activity or the number of rotting 
periods affecting the treated samples during the test 
(see Section 12). 

It must be pointed out that the results of soil-burial tests 
are inherently affected by experimental conditions, and 
since conditions of soil burial differ somewhat from condi- 
tions in damp storage, the results from the soil-burial 
test should be interpreted cautiously. 

A better test for storability may be to inoculate sam- 
ples of the netting material with a standard culture of 
fungi, to incubate the infected sample in a warm, moist 
atmosphere, and to check the strength samples at regular 
time intervals. Such tests require special bacteriological 
laboratories which are usually not associated with fishery 
research institutes. 

B. Testing of netting 

The term "netting" refers to textiles which consist of 
one yarn or one or more systems of yarns which are 
crossed or joined so as to form meshes in the final pro- 
duct, or to meshed structures which are formed by other 
means such as by stamping or cutting sheet materials 
or by extrusion. 83 Usually the mesh assumes a diamond 
shape, seldom square. In knotless netting the meshes are 
sometimes more or less honeycomb shaped (hexagonal). 
Netting may be classified as follows, according to the 
structure of the fabric, 84 see Fig. 25: 

1. Knotted netting usually made with the weaver's 
knot or reef knot or double-knot modifications of 
both types. 

2. Knotless netting either by the twisting technique or 
by the Raschel procedure. 

3. Marquisette leno or mock leno weave (cf. gauze). 

4. Woven netting open plain weave (cf. cheese cloth). 
The chemical, biological and suitability tests described 

above for netting twines may be similarly used for testing 
netting. However, it is usually more convenient to 
apply such tests to netting twines than to netting. 

There are, however, quite a few physical tests which 
refer particularly to the mesh or to the knot. These can 
be of great importance in the evaluation of netting 
fabrics and, hence, also of netting twine. 

88 According to draft proposal by ISO/TC38/SC9. 
84 According to Japanese Industrial Standard (draft) J.I.S.L. 
1948: Testing methods for synthetic fishing nets, Sections 2-8. 


knotted netting 

straight type 
(ordinary type) 

minnow net 
(twined net) 

jtigzag type 

N 222 

woven net 

Fig. 25 

tortoise type 

(18) Netting Dimensions 

Two types of dimension pertinent to netting have to be 
considered: the overall proportions of the netting fabric 
and the size of the meshes. Particularly for measurement 
of mesh size, quite a variety of instructions have been 
issued, because most regulations for the protection of 
fish populations are based on the specification of mesh 

Length and breadth: The length of a piece of netting 
may be indicated either by the number of meshes or by 
the number of meters. The breadth (depth) is always 


indicated by the number of meshes which lie in one row 
oriented in the direction at right angles to the direction 
of the length indication. 85 

Mesh size: The size of the mesh may be indicated as 
the length or as the opening of the mesh. The mesh 
length in knotted (or knotless) netting is the distance 
between the centres of two opposite knots (or opposite 
joints) in the same mesh when the mesh is fully extended 
at right angles to (or parallel to) the continuing direction 
of the twines (Fig. 26, (a) (b)), whereas the opening of 
the mesh in knotted (or knotless) netting is the inside 
distance between two opposite knots (or opposite joints) 
in the same mesh when the mesh is fully extended at right 
angles to (or parallel to) the continuing direction of the 
twines (Fig. 26, (c) (d)). 8 


The mesh size may be measured in many different 
ways. In addition to the fact that with hand-made 
netting the circumference of the spool is taken as a 
measure of mesh size, there are three basic principles 
underlying all these different ways for measuring mesh 
length. These are: 87 

" According to DIN 61250 (draft). 

86 ISO/TC38/SC9: Basic terms and definitions for textile pro- 
ducts for fishing nets. Draft, May 1962. 

87 v. Brandt, A.: Zur Technik der Bcstimmung der Maschen- 
grttsse. Archiv fUr Fischereiwissenschaft 6, pp. 54-64, 1956. 

(a) By direct measurement of knot spacing. The dis- 
tance between knots on opposite sides of the same 
mesh or beside one another in the same mesh is 
measured with a graduated rule. The place on 
the mesh where the rule is read depends on whether 
the mesh length or the mesh opening is being 
measured. It makes a difference whether or not 
the knot is included. 

(b) By inserting a mesh gauge into the mesh to measure 
the space surrounded by the mesh. Recently, the 
type of mesh gauge and the required force have 
been standardised. 88 

(c) By counting the number of knots per unit length. 
In this procedure, the netting is fully extended and 
the number of knots within a known length are 

Since the recommendations for mesh size are designed 
to allow smaller fish to escape, only the mesh opening is 
significant. Therefore, only those procedures which 
have been developed for measuring mesh openings will 
be treated here. Knot size and twine diameter of the 
netting material are not considered. Particularly in 
heavy netting it makes a difference to the result whether 
or not they are included in the measurement. In finer 
(ratio of twine diameter to mesh length is small) netting 
it makes little difference whether or not the knot is 

In the netting trade it is customary to measure the 
mesh size by gathering the netting lengthwise and by 
measuring the distance between opposite or successive 
knots, including one knot. 

In the following proposals for mesh-length measure- 
ment, only wet netting will be considered because gear 
selectivity is determined by the size of the wet meshes. 8g 
For the netting manufacturer it is important to know the 
relationship between the sizes of wet and dry meshes. 
Thus, the length of dry meshes may also be measured by 
the following procedures to determine changes in mesh 
size (shrinking or lengthening): 

As indicated above, a mesh gauge is inserted into the 
mesh to measure its opening. For this, there must be a 
certain force to straighten the mesh. There is no general 
agreement as to how great this force should be for diff- 
erent materials. As discussed above for unknotted 
netting materials, any standard should relate the magni- 
tude of the straightening force to the weight of the 
material, e.g., equal to the weight of a certain length of 
the material, preferably the 250 m weight. (See proposal 
(d) in this Section). Because it is difficult to measure the 
weight per unit length of knotted netting materials 
which have already been used, it becomes necessary 
simply to specify a certain force during measurement of 
mesh size and to relate the measurements with different 
materials to the fishing selectivity of those materials. 

88 Prospectus of the ICES Mesh Gauge. Le Bureau du Conscil 
Permanent International pour 1'Exploration de la Mer, Charlotten- 
lund, Denmark, 1962. 

89 However, gear selectivity depends not only on mesh size but 
also on the shape of the meshes and on other internal and external 

So far, no method has been developed for measuring 
mesh size whereby the relation between the measured 
mesh size and the selectivity of the fishing gear is the same 
for all types of material. 

Proposal (a): mesh measurement in trawls for the 
marine fishery. 90 The mesh is measured by the so-called 
"ICES Mesh Gauge" with locking device, 91 which acts 
longitudinally to exert a standard force on the mesh. 
(Fig. 27) This gauge yields more consistent results with 

90 According to a proposal of ICES, Report of the Mesh Selec- 
tion Working Groups Meeting in Copenhagen, December 1959 and 
1960, Part IV, General Considerations on Trawl and Seine Mesh 
Selection and its Measurement. 

91 Fig. 27 shows different types of gauges which exert a force on 
the mesh. (1) ICNAF type, (2) Scotch longitudinal-force mesh 
gauge on which the ICE Smesh gauge was based, (3) Polish longi- 
tudinal-force mesh gauge, (4) Lowestoft "scissors" gauge. 

AM 4 

a * * a r * 

n M m H an**9j f t i 4 t i i t 





different people 92 than does any other gauge by virtue of 
its design and construction. Because measurements of 
mesh size must be exactly c nparable for mesh selec- 
tivity studies, it is recommended that all other gauges 
used for such work be calibrated against this type of 
gauge as standard. 

In particular, a force of 4 kg is exerted on the mesh 
when measuring mesh size in codends of trawls and seine 
nets. It is recommended that the gauge be recalibrated 
regularly. The codend must be completely wet. It is 
recommended that the meshes chosen for measuring 
should be in straight lines running fore and aft along the 
top side (not near the selvedges) of the after half of the 
codend, starting from the third row behind the codline. 
Meshes adjacent to strengthening ropes, meshes with a 
join in any bar, and meshes in any repaired pait of the 
codend should not be measured. The measurements 

ra v. Brandt, A. and Bohl, H.r Ma&chenmessung 
mit Druckrnessgerftten bei Schlcppnetzen. Protokoile 
ztir Fischereitechnik 5, pp. 277-295, 1959. 



1,00 KO 

mit Mttstob 

Fig. 2 8 


should be made immediately after every haul so that 
any change in average mesh size with time can be detected 
and reported. 

The number of meshes to be measured should not be 
fixed arbitrarily because the minimum number of mea- 
surements is governed by the desired accuracy of the 
average and on the standard deviation of the mesh size 
which, in turn, varies with the original construction and 
use history of the codend. Usually, the average mesh 
size must be determined with an error less than two per 
cent. For this, it will usually be sufficient to measure 
25 meshes if the observed 95 per cent range of mesh sizes 
is 20 mm and the average of measurements must be 
within 2 mm of the actual mesh size. In scientific reports, 
the number of measurements and the standard error of 
the average mesh size should be reported along with the 
average mesh size and range of mesh sizes. 

The mesh size is reported in mm and the average is 
reported as the mesh size for that codend. 

Proposal (b): mesh measurement in gillnets made of 
fine material. 93 This measurement is made on a special 
gauge as represented in Fig. 28. A panel of the netting, 
five meshes long by five meshes broad (deep) is cut from 
the gillnet. The rows of meshes along the upper and lower 
edges of the panel are respectively attached to hooks A 
and B and the gauge is suspended vertically from point C 
as shown in Fig. 28. The load D (1 kg) applies a tension 
to all five rows of meshes and the total length of five 
meshes is read at point F on scale E. This length divided 
by 10 gives the average length in mm of each mesh bar 04 
taken from one knot to the next, including one knot. 

Proposal (c): 95 The average measurements of 12 
individual meshes in gillnets is taken as mesh size for the 
net. Three of these meshes are selected from each 
lengthwise quarter of the net, one randomly from the 
top third of the net (i.e. from the third nearest to but not 
including the corkline selvedge), one from the middle 
third, and one from the bottom third exclusive of the 
leadline selvedge. 

The following measuring devices are recognised for 
measuring the mesh size (Fig. 29): the Allen net rule, the 
Hovey gauge, the Selkirk gauge, the flexible rule, and the 
straight rule. These devices apply a tension between 
three ounces (85 g) and eight ounces (225 g) to the mesh 
at the time of measurement. 

The Allen gauge consists of a threaded rod along which 
the internally threaded sleeve C (Fig. 29) can travel. The 
block D is moved along the rod by turning sleeve C 
to which it is attached, but the block itself is restrained 
from turning on the rod by flats which are milled on 
either side of the rod. The mesh to be measured is placed 

98 Florin, J.: Ein Messger&t ztir einheitlichen Bestimmung der 
Netzmaschenweiten. Schweizerische Fischerci Zeitung 65, pp. 243- 
244, 1957. 

94 According to the ISO draft mentioned above, the term "bar" 
is defined as the distance between two sequential knots or joints, 
measured from centre to centre. 

95 According to Canadian Government Specification Board, 
Specification for Nets: Fishing, 25 April, 1958, and Fisheries 
Research Board of Canada, St. Andrews, N.B. 


in the notches at A and B, and the sleeve C is turned by 
hand, moving the block D and the hook B away from 
hook A until the mesh becomes tight. The tension in 
the mesh pulls on the lever at B so that it pivots about 
point P and compresses the spring until the dog on the 
lever drops into one of the notches in sleeve C, stopping it 
from turning. The mesh opening is then read directly from 
the scale which is engraved on one of the flats on the rod. 
By proper selection of the spring, this gauge exerts a 
consistent tension of about five ounces (140 g) on the 
mesh at the time of measurement and gives more consis- 
tent results with different persons than do the other 

Alltn Gouge 

Hovey Gouge 

Fig. 2 9 


- n - 





ilkirk Goug 



The Hovey gauge (Fig. 29) operates on the same prin- 
ciple as the 1CN AF gauge. The weight W is inserted into 
the mesh until the edges of the wedge rest against oppo- 
site knots in the mesh. The mesh is held horizontal so 
that the gauge hangs freely and the weight W (8 oz) 
exerts a standard force on the mesh. Care must be taken 
not to overstress the mesh while holding it. The mesh 
opening is then read directly from Scale S. 

The Selkirk gauge (Fig. 29) consists of a cylindrical 
tube with a longitudinal slot cut in its wall, with a scale 
S engraved against the slot, and with a hook A fastened 
near its upper end. A rod W of standard ( 8 oz) weight is 
inserted into the lower end of the tube and the hook B 
is fastened near its upper end. The mesh to be measured 
is placed in the notches at A and B and the gauge is 
suspended from its ring R so that the weight W exerts the 
standard tension on the mesh. The mesh opening is then 
read directly from Scale S. 

The flexible rule is a go-no-go gauge used for enforcing 
fishery regulations. It is a piece of flexible steel, \ in 
(1 cm) wide by 1/100 in ( mm) thick and the same 

length as the minimum allowable mesh opening. The rule 
is flexed to a slight curvature and is inserted into the 
mesh so that each end rests against one of the diagonally 
opposite knots in that mesh. The rule is then released. 
If the rule straightens out, the mesh opening is satisfactory 
if the rule is held curved by the mesh, the mesh opening 
is too small. 

The straight rule is a thin, graduated rule, usually made 
of steel. The index end of the rule is placed against one 
of the knots in the mesh to be measured and the opposite 
knot is pulled along the rule until the mesh is straight 
but not stretched. The mesh opening or the mesh 
length is then read directly from the scale on the rule. 

The measurement is usually made by these gauges 
inside the mesh, i.e., between the knots (mesh opening), 
and with the mesh fully extended in the direction normal 
to the selvedge. 96 The result is reported in inches to three 
significant figures. 

Proposal (d): 97 Although in this method, as in Pro- 
posal (b), the knot is included in the mesh size measure- 
ment, this is of little importance when the mesh opening 
in fine netting is being measured. 

The mesh size is measured by a metric scale as the 
distance between the centres of opposite knots (mesh 
length) while the mesh is loaded by a pre-tension which 
causes no noticeable extension of the sample. For man- 
made fibres, this pre-tension in grams equals 1/30 of the 
total denier of the netting twine. Neglecting twist 
contraction, this is equivalent to the 300 m weight. 

At least three measurements are made and the average 
is reported in cm to the first decimal place. 

(19) Strength and extensibility 

The term "strength of netting" is usually used to mean 
mesh strength, although, less frequently, it refers to the 
bursting strength of a piece of netting or to the tearing 
strength. If the usual strength-testing machines are used, 
the extension of the piece of netting or of the single mesh 
under load can be measured at the same time as the 

Mesh strength: To determine the strength of finer 
netting, the fisherman places two fingers in the mesh 
and tries to break it. He judges the mesh strength not 
only from the resistance to breaking but also from the 
sound of the break. The quantitative test is carried out 
in a similar way. The strength-testing machine is fitted 
with two pins which go into the mesh and pull it by an 
increasing force until it breaks. 

Proposal (a): 98 The test for the strength of dry or wet 
netting is applied to single meshes. These test meshes 
are cut from the netting in such a way that the cut ends 
of twine are at least | in (12 mm) long if the mesh is 
large enough, and, in any case, no closer to the test 
mesh than the mid-point of the cut bar. 

96 The selvedge is considered to run parallel to the general course 
of the twine through the netting. 

97 Japanese Industrial Standard (draft) J.I.S.L., 1958: Testing 
methods for synthetic fishing net (2-9). 

98 According to Canadian Government Specifications Board. 
Specification for "Nets: Fishing", 55-GP-l, 25 April, 1958. 


The teat is conducted on the usual strength-testing 
(gee Section 1) on which the damps have been 

replaced by pins which stand at right angles to the direc- 
tion of pull. These should be sufficiently rigid that they 
do not bend noticeably when stressed by the test mesh. 
On the other hand, they should be fine enough that the 
mesh can be placed on them without any of the knots 
touching them, because, for the test, the mesh should 
be placed over the two pins in such a way that none of the 
four knots touches either of the two pins (Fig. 30). 
Thus, the test is conducted with the mesh stressed diag- 
onally to the usual mesh shape. 

The average of ten test breaks is reported as the mesh 
strength, and this may also be reported as a per cent of 
twice the strength of the netting twine to indicate mesh- 
strength efficiency. 

Proposal (b):<"> For this test, too, the usual strength- 
testing machine on which the usual clamps have been 
replaced by hooks five to eight mm in diameter is used. 
Either a piece of netting or a single mesh may be tested 
(Fig. 31). The mesh is stretched over the hooks in its 
usual shape or at right angles to it, i.e., so that one of the 
knots touches each hook. Otherwise, the mesh may be 
stretched diagonally without any of the knots touching 
either of the hooks as described in Proposal (a), Fig. 30. 

Japanese Industrial Standard (draft) J.I.S.L., 1958 (5-10). 

The initial distance between the two hooks is 10 cm 
and the rate of loading hook traverse is 15-30 cm/min. 
The mesh strength is taken as the average of 10 tests and 
is reported in kg to three decimal places. 

Proposal (c): 100 Knotless netting is tested in the same 
way as described in Proposal (a), except that loops of 
twine of sufficient diameter and strength are used instead 
of the pins. 

Sometimes a single mesh which has been cut from a 
piece of netting cannot be tested because of loosening 
and slipping in the junctions. In this case, instead' of 
cutting off a single mesh, small pieces of netting are cut 
and the single mesh in the centre of each piece is tested 
according to Proposal (a). 

In contrast to knotted netting, knotless netting which 
has been made by the Raschel process has noticeably 
a different strength when stressed in its usual shape 
(i.e., in the direction of the breadth or depth of the netting 
normal to the general course of the yarns) than when 
stressed at right angles to this (i.e., in the direction of the 

100 Institut fur Netzforschung, Hamburg. 



length of the netting). The strength when stressed vertically 
(former case) is usually the lower one. For this test, rows 
of single meshes are cut from the netting both vertically 
and horizontally and each row is stressed in the usual 
fabric clamps on the strength-testing machine (Fig. 32). 
The initial distance between the clamps is 10 cm. Care 
must be taken that no knots or joints are caught between 
the faces of the clamps, otherwise the twine may become 
cut at that knot or joint before the mesh breaks. 

The strength in both directions is measured and the 
difference is reported as a per cent of the higher value. 

Netting strength: The term "netting strength" is used 
in more than one way. Sometimes it means the bursting 
strength of uniformly stressed netting and sometimes 
it means the diagonal tearing strength of pieces of netting 
such as those used for mesh-strength tests in Proposal (b). 

The bursting-strength testers used in the textile industry 
cannot be used for measuring the bursting strength of 
netting under uniform load, particularly if the netting 
has large meshes. The knots are squeezed by the frame 
and the netting breaks there. m 102 

101 Results of bursting-strength tests with fine (Nm = 70 and 
higher) netting on makeshift apparatus have been reported by 
v. Brandt, A.: Untersuchungen iiber die Maschengrosse. Proto- 
kolle ziir Fischereitechnik, 5, 1951. 

102 For woven or other very small mesh netting the usual bursting- 
strength tester of the textile industry may be used. The average 
of the results of 10 tests is reported in kg/cm 2 , Japanese Industrial 

Standard (draft) J.I.S.L., 1958 (5-13). 


Fig. 33 a 
Fig. 33a. Preparation of net sample for the determination of net strength. 

Fig. 33b. Way of hooking a net sample onto the dynamometer for 
determining the net strength. 

Two methods for netting strength have been proposed, 
the first applying the load to a single mesh, and the second 
applying the load to several meshes at once. 

It is not always true that the measurement of mesh 
strength by Proposal (a) in Section 19 is preferred to the 
following proposal by van Wijngaarden even though the 
former can be conducted more easily and requires less 
material, because the latter permits a better study of knot 
stability (see Section 20) at the same time. 


Proposal (a): 108 For the test, machine-made netting is 
cut in the manner shown in Fig. 33a. The twines from 
the creel on the netting machine must be distinguished 
from the twines from the shuttle. The twine from the 
shuttle makes the knot whereas the twine from the creel 
only forms the loops. The mesh bars are cut at a, b, c, 
and d in Fig 33a, but not at e, yet again at f. The netting 
is stressed in a strength-testing machine, the same as is 
used for measuring mesh strength, by the upper hook A 
and the lower hook A 1 , shown in Fig. 33b. 

The netting strength is that force in kg which breaks the 
netting at the uncut mesh bar shown as e in Fig 33a and 
b. The speed of the loading jaw traverse equals 
one per cent of the initial specimen length per second. 

For the subsequent test break, the netting is mounted 
on the testing machine with the hooks in meshes B and 
B 1 (Fig. 33a) and the mesh bar at h is cut. 

This test cannot be conducted if the knots slip. 


a. 5 knots 
9 ttgs 


Proposal (b): 104 For measuring netting strength, 
strips of netting four meshes (five knots) wide are cut in 
both the lengthwise and breadthwise (depthwise) 
directions from the net, and nine bars of the strip are 
fastened between the clamps of a suitable tensile tester 
for each test break (see Fig. 34a). The initial distance 
between the clamps should be no more than 20 cm and 
the speed of the loading-clamp traverse should be 15-30 
cm per minute. If necessary, fewer meshes in breadth or 
in length may be taken to accommodate the limitations 
of the tester, but in such cases the size of the specimen 
must be reported (see Fig. 34b). 

At least 10 test breaks should be conducted, and the 
average is reported as the test strength. 

The way in which the specimen is mounted in the 
clamps on the tester is of utmost importance. The knots, 

109 van Wyngaarden, J. K.: Method for determining the tear 
resistance of nets, the so-called net strength. N.V. Research, 
Arnhem, Preevenverslag, 1959. 

104 Japanese Industrial Standard (draft) J.I.S.L., 1958 (5-9). 

being thicker than the twine, can be broken by the pres- 
sure cutting the twine in the knot if the knots are caught 
between the faces of the clamps. Also, it is difficult to 
mount the specimen in such a way that the load is equally 
distributed among all the mesh bars across the sample. 
Therefore, Carrothers 106 threads a rod through the 
meshes on the upper edge of the specimen and another 
rod through the meshes on the lower edge, and hooks 
these rods onto the strength-testing machine so that 
the machine pulls these rods one from the other until the 
specimen breaks. For such mounting, the test specimen 
must be a whole number of meshes wide, not, for example, 
\\ meshes as shown in Fig. 34b. 

Rending strength: 106 The specimen may be cut from 
the netting either lengthwise or breadthwise (depthwise) 
as shown in Fig. 35. The specimen should be larger than 
ten knots = nine meshes square. A cut about 15 cm 
long is made into the specimen so that five knots = four 
meshes remain uncut. The parts marked A and B in 
Fig 35 are pulled apart by the strength-testing machine. 
The initial distance between the clamps is 10 cm and the 
speed of loading jaw traverse is 15-30 cm per minute. 
The rending strength is that force which is required to 
tear the sample for five knots or more. At least 10 test 
breaks should be made. 

Fig. 35 

Frictional strength in minnow netting: 107 The proce- 
dure for testing the strength of woven minnow netting is 
different from that for other types of netting. The test 
specimen is cut 30 cm long in the direction of the warp 
by 10 cm wide in the direction of the woof or filling. 
The specimen is prepared as shown in Fig. 36. At the 
upper end of the specimen, all but the central warp are 
cut short and only the central warp is attached to the 
upper clamp of the strength-testing machine. At the 
lower end of the specimen, the central warp is cut short 
and all the warps but it are attached to the lower clamp. 
The number of woof or filling threads is limited to 15. The 
specimen is pulled apart at 15-30 cm per min and the 
force required to pull the central warp out of the specimen 
is measured. At least 10 tests are made. This procedure 
can be applied only to minnow netting which has been 
treated with a resin or some such similar bonding material. 

105 Modern Fishing Gear of the World, p. 72, 1959. 

106 Japanese Industrial Standard (draft) J.I.S.L., 1958 (5-11). 

107 Japanese Industrial Standard (draft) J.I.S.L., 1958 (5-12). 


I I 

cut off 
*c*pf- only c*nt*r warp 

~-Jr c*nt9r warp cutoff 

cutoff woof Fig. 36 

(20) Knot stability 

When netting materials of synthetic fibres came into use, 
the question of knot stability or knot constancy arose. 
The term "knot strength" is sometimes used wrongly 
in this context. Knot stability is the ability of the knot to 
retain its original form, both resisting inversion into an- 
other form without twine slip and resisting loosening with 
resulting twine slip but without inversion. Knot stability 
is a prerequisite for constant mesh size in a fishing net. 
The following test methods refer to the resistance of the 
knot to inversion, of the twine to slipping in the knot, 
and of the knot to loosening. 

Inversion resistance: When netting is made with the 
weaver's knot, it is possible that the loop C-D (Fig. 37) 
does not fall correctly when the knot is tightened so 
that it forms an insecure knot with the mesh of the pre- 
ceding row. This insecure knot can also be formed from 
a proper weaver's knot if the mesh legs A and B are 
pulled apart strongly while legs C and D remain slack. 
A similar insecure knot can also be formed in the same 
way, and usually more easily, from a reef or square knot. 
It can be seen, as shown for the weaver's knot in Fig. 37, 
that, in these insecure knots, the netting material C-D 
can readily slip on netting material A-B. 

Proposal (a): 108 This procedure is based on the fact 
that the knot can be inverted into a slip knot as described 
above. The legs A and B (Fig. 37) are stretched between 
the clamps of the strength-testing machine while the 
legs C and D remain free. The force at which the inver- 
sion of the knot occurs is measured and may be reported 
in kg or as a per cent of the knot strength. 

108 van Wungaarden, J. K.: Testing methods for net twines and 
nets. Modern Fishing Gear of the World, pp. 75-81, 1957. 

D C 

When the knots for this test have not been cut from 
netting but have been formed by hand, the force by which 
the knots are tightened is important. To minimise the 
effect of this variable, the knots should be pulled together 
by a force equal to about 20 per cent of the knot strength. 

Proposal (b): 109 For this test, several weaver's knots 
C and D are tied in the netting material A and fi as 
shown in the first drawing of Fig. 37 and Fig. 38. The 
material A and B is loaded by a series of frequent jerks 
in an effort to invert the knots. The arrangement is 
shown in Fig. 38. 



Fig. 38 

The knotted test specimen is stretched horizontally 
so that the resulting deformation of the knots may be 
better observed. The loading mass E equals that of the 
500-m weight. For the test, the mass is lifted 50 cm and is 
allowed to fall every two seconds, thereby jerking the 

109 Treschev, A. J.: Laboratory of Fishing Technique. Research 
Institute of Marine Fisheries and Oceanography, Moscow. 

5 10 


40 50% 

Fig. 39. Tightening force of knots In per cent of the cord strength: 
(/) treated cotton knots: (2) untreated cotton knots: (3) Capron knots 
fixed at temperature 100C: (4) Untreated cotton flap knots. 


The number of jerks required to invert the knot is 
taken as the measure of its stability or resistance to 
inversion. This, in turn, is largely dependent on the 
force by which the knots are tightened, usually taken as a 
per cent of the twine strength as shown in Fig. 39. 

Proposal (c): no By this proposal, the inversion resis- 
tance discussed above and the knot-slip resistance men- 
tioned below are both measured while testing for mesh 
strength by Proposal (a) for mesh strength in Section 19 
above. The strength tester is the usual pendulum type 
with an automatic load-elongation recorder and reverse 
stop-pawls on the pendulum. The test mesh is mounted 
on the tester as shown in Fig. 30 with the pins a known 
distance apart. Thus, the distance for which the recorded 
line follows the elongation axis (load = 0) when added 
to the initial spacing of the pins, gives the mesh opening 
under zero load. If one of the knots inverts, this shows as 
a short distance on the recorded line which is parallel to 
the elongation axis (recorded load = constant = inver- 
tion resistance). Twine slip in a knot also shows as a 
straight portion in the recorded line parallel to the elonga- 
sion axis, but this is usually longer than the knot inversion 
indication (recorded load=constant ^knot-slip resistance). 
In any one mesh, only one knot can invert and only two 
can slip. Therefore, it is necessary to watch only three 
knots during test to distinguish inversion from slip for the 
record. For this test, none of these knots should touch 
either of the pulling pins. If the mesh eventually breaks, 
the inversion resistance and the knot-slip resistance may 
be reported as a per cent of the mesh strength. Thus, by 
this one procedure, as many as four different properties 
may be measured. This procedure has the further advan- 
tage that variations resulting from variously tightened 
hand-tied knots are avoided by using netting meshes. 

Knot-slip resistance: 111 Slipping can also occur 
without any deformation of the knot. If the knot is 
stressed from one side only, that leg can slip through the 
knot without the knot becoming broken. 

For measuring slippage in a knot, the legs C and D vs. 
A (Fig. 40) are stressed in the strength-testing machine. 
The force which causes 5-cm of the free leg B to slip out 
of the knot is reported. (The 5-cm length is used only 
with knots tied specifically for the test.) In this test, the 
angle between legs C and D should be 0, and the rate 
of loading-clamp traverse should be one per cent of the 
initial separation per second. Manually tied knots should 
be tightened for 10 sec, 5 min before the test, by a force 
equal to 20 per cent of the knot strength. 

The knot-slip resistance is reported as that force, 
expressed in kg or as a per cent of the knot strength, 
at which slipping first occurs for more than one second. 
Shorter jerks which result from tightening of the stressed 
knot are ignored. Knot-slip can be seen particularly 

well if the extension of the specimen is recorded auto- 
graphically at the time of testing. Then the following may 
be observed (Fig. 41): 

110 Can-others, P. J. O.: Fisheries Research Board of Canada, 
St. Andrews, N.B., Canada. 

111 van Wyngaarden, J. K. : Testing methods for net twines and 
nets. Modern Fishing Gear of the World, pp. 75-81, 1957. Also, 
Methods of testing net cord and nets. Rayon Revue 11, pp. 146-160, 



Fig. 40 

Dthnung ilongttion 
Fig. 41 

(a) The knot does not slip but the specimen is extended 
and breaks in the knot (Fig. 41a). 

(b) After a certain load has been applied the knot slips 
in a jerking manner. However, the material breaks 
before the free leg B has slipped completely out 
of the knot (Fig. 41 b). 

(c) A more or less steady slipping occurs until the free 
leg B has slipped completely out of the knot 
(Fig. 41c). 

At least 10 tests are carried out. Knot-slip resistance 
may also be evaluated by reporting the frequencies at 
which the above alternatives occur as per cents of the 
total number of tests. 112 

Loosening resistance: For this, the knots are placed in 
a rotating box and their opening-up is observed. 

Proposal (a) : 118 For testing loosening resistance, knots 
are tied between pieces of the netting material about 
20-cm (8 in) long. The knots are tightened by fastening 
a weight equal to 1 g per five resultant twine tex (200 m 
weight) to both ends of the same twine and allowing 
the weight to fall 20 cm before the knot takes the load. 

Five of the knots thus formed are placed in a rotating 
wooden box (20x30x20 cm) which turns at 60 revolu- 
tions per min (Fig. 42). The box is stopped at regular 
time intervals and the number of knots which have 
become loosened is counted. 

118 v. Brandt, A.: Zttr Knotenkonstanz von Fischnetzen. 
Archiv flir Fischereiwissenschaft, 9, pp. 244-265, 1958. 

118 von Wyngaarden, J. K.: Testing methods for net twines and 
nets, etc. Modern Fishing Gear of the World, pp. 85-91, 1959. 
Also, Methods of testing net cords and nets. Rayon Revue 11, 
pp. 146-160, 1957. 

JO em 

>i 20cm 

Fig 42 

Proposal (b): 114 Knots from netting or knots formed 
under a known tension from netting twine are cut in 
such a way that their legs are 1'5-cm ( tf in) long. For 
synthetic netting twines the twine ends are fused and for 
natural fibre netting twines the twine ends are cemented 
to prevent them from unravelling. 

At least 10 of these knots are put into a rubber-lined 
box which rotates at 60 turns per min. A small rubber 
cylinder is also placed in the box. The box is made of 
metal, to permit wet tests, and is 15 x 10 x 10 cm. After 
10, 20, and 30 min, the number of knots which have 
become loosened during each period is counted, and is 
reported as a per cent of the total number of specimens. 

(21) Weight of netting 

The dry weight of uncut netting is of interest for com- 
mercial purposes. However, the wet weight is also of 
interest because the netting is usually wet when lifted 
through the air during fishing operations. Finally, the 
buoyed weight of netting while still in the water is of 
interest. The manner used for stating the weight of netting 
varies because of the different ways in which the measure- 
ments are made, e.g., length may be stated as the number 
of conventional length units or as the number of meshes 
(see Section 18) and weight may be stated in metric or in 
other units. 

Dry weight of netting: stated exactly, the weight of 
netting for commercial purposes is not its actual, air-dry 
weight, but is its bone-dry weight plus a standard mois- 
ture content. The corrected weight of the netting can be 
computed from the following formula: 116 

100 + Re 
Standard weight == measured weight x R 

where R = measured moisture content ( %) 
Re = standard moisture content ( %). 
Standard moisture contents for various textile materials 
may be different in different countries, and this should 
be recognised when "official" moisture ratios are quoted. 
Standard moisture contents for natural fibres are all 
higher than the contents usually found actually to exist. 

114 Stutz, H.: Lateral strength and knot-firmness of synthetic 
twines for fishing purposes. Modern Fishing Gear of the World, 
pp. 87-92, 1959. Also, v. Brandt, A.: Zttr KnotenkonsUuu : von 
Fwchnetzen. Aichiv fur Fischereiwisscnschaft 9, pp. 244-265, 1958. 

115 Japanese Industrial Standard (draft) J.I.S.L., 1958 (2-12). 

The actual moisture content is found by drying the 
specimen at 105-1 10C until its weight is constant. This 
determination cannot be made if the thermoplasticity of 
synthetic fibres is such that they must be dried below 

The following is an official moisture content for various 
textile fibres: 

Synthetic fibres 

polyamide 4*5% 

polyester 0-4 

vinyl chloride 

polyvinyl alcohol 5*0 

Natural fibres 

cotton 8-5% 

flax 12 

hemp 12 

ramie 12 

sisal 12 

manila 12 

Proposal (a): 116 The proposal is based on the state- 
ment of the length of the netting in metres and of its 
breadth (depth) in the number of meshes. If the weight 
(c) in kg is known for a piece of netting of a given twine 
number and mesh size, 100 m long by 100 meshes broad 
(deep), the weight of any other piece of the same style of 
netting may be estimated from the formula: 

, . , x Ien 8t h x breadth (depth) x c. 
We,ght (kg)= 1M ^ 

Values for the constant c are given in Tables 1 , 2, 3, and 4. 
They are the weights of pieces of netting, 100 m long 
when fully extended by 100 meshes broad (deep), 117 
which have been measured by the netting manufacturers 
under prevailing conditions of temperature and humidity, 
not in conditioned rooms with the standard atmosphere. 
Thus, these values may vary between different factories. 
Also, differences in these weights may be caused by the 
knots being more or less strongly tightened. 

These tables give the values for air-dry netting made 
with the single weaver's knot using medium-laid twines 
of cotton (Tables 1 and 2), flax (Table 3), or hemp (Table 
4). Of course, these tables are valid only in a climate 
similar to that of Central Europe, hence they are presented 
only as an example. These lists of data may be examined 
further and presented graphically. The resulting curves 
should not have a discontinuity. If, for a given twine 
size, the weights for pieces of netting of only a few mesh 
sizes are known, the weights for intermediate mesh sizes 
may then be determined by graphical interpolation. 

In these tables, the mesh sizes have been reported as 
the distance between the centres of neighbouring knots 
(mesh size = mesh-side length = \ mesh length). 

Proposal (b): 118 This proposal is based on the state- 
ment of the length of the netting in fathoms (1 fm = 
6 ft = 1*83 m) and of its breadth (depth) in the number 
of meshes. The weight is estimated in Ib. 

116 v. Brandt, A.: Arbeitsmethoden der Nctzforechung, Stutt- 
gart, p. 101, 1947. 

117 Schmidt, K., and Amwand, K., have also published con- 
stants for estimating the weights of netting which is not fully 
extended but is hung to various ratios: Tabcllcn zur Gcwichts- 
bcrcchnung von Baumwollnctzen untcr BerUcksichtigung cincs 
Einstellungsverhaltnisses. Deutsche Fischcrci Zeitung 5, pp. 100- 
104, 1958. 

118 Fisheries Research Board of Canada, Technological Station, 
Vancouver, B.C., Canada. 


Table 1 

" ^ 

5 6 8 10 12 14 




1,00 0,90 0,82 0,76 0,73 0,71 



1,20 1,10 1,00 0,90 0,88 0,86 



1,45 1,35 1,20 1,10 1,07 1,04 



1,95 1,80 1,60 1,35 1,33 1,31 



2,15 2,00 1,87 1,80 1,76 1,73 



3,80 3,50 3,10 2,80 2,70 2,60 



2,95 2,80 2,45 2,20 2,14 2,08 



3,90 3,80 3,74 3,50 3,40 3,33 



3,60 3,20 2,90 2,84 2,75 



5,20 4,70 4,10 4,00 3,90 


-H 18 20 25 30 40 50 


0,54 0,52 0,50 


0,69 0,69 0,6? 0,65 0,62 


0,80 0,78 0,74 0,73 0,72 


0,98 0,96 0,94 0,92 


1,27 1,25 1,20 1,15 


1,67 1,65 1,60 1,55 1,50 


2,52 2,50 2,45 2,40 2,30 


1,98 1,95 1,90 1,88 1,85 


3,17 3,10 2,95 2,90 2,82 2 



2,57 2,50 2,45 2,40 2,30 


3,70 3,65 3,60 3,55 3,50 

Table 2 








18 20 22 25 




4| B^ 


















8,9 8,8 8,6 8,5 
11,2 11,0 10,6 10,6 
13,5 13,2 13,0 12,6 












8,9 8,7 8,5 8,3 
14,7 14,0 13,8 13,4 












21,7 20,4 20,1 19,3 
27,8 27,0 26,0 25,3 
34,8 34,4 33,6 32,3 








2 5 




50 60 70 


100 i 









30 29 28,5 
5 36 34 33,5 
42 40 39 











57 54 
67 65 
76,5 74 


Tbl 3 
















7,7 7,6 7,5 










11,6 11,0 10,8 










16,4 16,1 16,0 










5,9 5,8 5,7 










8,9 8,7 









11,8 11,6 11,4 










19,1 18,7 18,4 










24,7 24,0 23,7 












30, * 30, 4 29,9 












14,1 13,8 13,2 










33,3 31,0 29,1 



TftbU 4 

lhi l 
















3 a 






























4 AA / 









































































































































































































(23) Visibility 

The assessment of the absolute visibility of netting, partic- 
ularly of gillnets under different conditions, or the 
study of whether or not the netting blends with the 
surroundings in the water, can be achieved by direct 
observation as by a SCUBA diver or with the help of a 
bathyscaphe or television. As a record is usually required 
for comparisons, photography is used. Probably these 
visibility studies should be made with natural light 
rather than with artificial light, and the turbidity of the 
water as measured by oceanographers should be reported 
with the test results. 

The visibility can be varied by lowering the netting 
and the camera together to the desired depth and photo- 
graphing by remote shutter release. In this instance it is 
proposed to study only pieces of netting. These are 
stretched on a frame and are mounted with the camera 
on a base as shown in Fig. 44. 126 

Fig. 44 

The apparatus consists of a base bar A, about 1-5 m 
long, on which the waterproof case with the camera, B, 
and the frame carrying the netting under study, C, have 
been fixed. The frame takes a piece of netting 50 x 50 
cm which may be interchanged with similar pieces of 
different types of netting for comparison. 

The camera should preferably be a model with auto- 
matic winding. In this way, several photographs can 
be made in sequence with the help of cable D without 
removing the camera from the water. It should be noted 
that, for example, at an actual distance of 1*3 m between 
the netting and the camera, the camera should be focused 
at 1*0 m because of the higher index of refraction in the 
water. At an actual distance of 1*2 m an optical distance 
of 0-9 m should be used. Preliminary test photographs 
should be taken to determine the exposure, particularly 
if colour pictures are being taken. 127 

126 Ulrich, B.: Bin ncues Gebiet ftir den Robot-Fernausldser. 
Neue Fotolinic, No. 6, pp. 15-19, 1951. 

117 Park, Y. J. : Visibility of webbing dyed with several kinds of 
colours under the ocean. Bull, of Fisheries College, Pusan National 
University, Vol. VI, pp. 7-10, 1 962. Describes measuring the visibility 
of netting by fastening pieces of netting to a frame and by carefully 
and slowly lowering the whole into the water until the observer 
cannot obviously recognise the netting through a water glass. The 
depth in cm is taken as a measure of the visibility of the netting 
under test. 


(24) Hydrodynamic resistance of netting 

Since the hydrodynamic resistance of netting cannot 
readily be estimated from the towing resistance of towed 
gear, the netting can be stretched on a frame for measur- 
ing this property. Such measurements can conveniently 
be made in a tank in which other objects, such as ship 
models, can also be tested. The netting is towed at 
different speeds, preferably those at which full-scale 
gear is dragged, expressed in nautical miles per hour 
(knots) or metres per second (Fig. 45). One square 
metre of netting has been used for such tests. 128 The 
resistance of the frame alone is measured separately, 
and this is subtracted from the resistance of the mounted 





Manila 3/900 No. 1 

! /Ptrlon 400 m/kg braid* No. 2 



As the use of ship-model tanks is very expensive, 
simpler makeshift apparatus has been used. 129 Fig. 46 

128 Institut fur Netzforschung, Hamburg. Also, Deutsche 
Seiler Zeitung 79, pp. 146-197, 1960. 

129 Miyamoto, H.: Hydrodynamic resistance of a plane net: 
International Shipbuilding Progress 2, pp. 291-295, 1955. Nomura, 
M., and Mori, K.: Resistance of a plane net against flow of water 
HI Effect of kind of fibres on the resistance of net. Bull. Japanese 
Soc. Sc. Fish., Vol. 21, pp. 1110-1113, 1956. 

represents the required arrangement. A piece of netting 
is stretched on a metal or wooden frame, loaded by a 
weight and balanced by a counter-weight. In this the 
netting may be set at right angles to the direction of 
motion or at any other desired angle to it. The frame is 
allowed to sink through the water under a known force 
both with and without the netting to be tested. The 
resistance of the netting to motion through the water 
may then be computed either from the times required 
for the frame to travel a certain distance or from the 
distances travelled by the frame in a certain time. 


Dr. A. von Brandt (Germany) Rapporteur: There is no 
doubt that clear definitions are the basis of all our discussions. 
Just as we need a numbering system, we also need an agree- 
ment on testing methods for the properties of materials. It 
was therefore a very important achievement that, at the first 
International Fishing Gear Congress, a working group was 
set up to advise on and recommend standard methods of 
testing net materials. To help that working group scientists 
of Canada, Denmark, France, West and East Germany, 
Japan, the Netherlands and the USSR gave co-operation in 
providing information about the methods actually used in 
their country for testing netting twine and netting. The 
group had to limit its work to these two items, but there 
was no doubt that agreement was also needed on testing 
methods for ropes and lines as used in fisheries as well as an 
agreement on methods of testing the different types of fishing 
gear either in model or full scale. 

The version of the work on testing netting twine and netting 
now presented by P. J. G. Carrothers (Canada) and myself 
should be considered as a draft and not as a final form. It 
does, however, review the situation as it is now and we hope 
it will help all who want to begin testing materials for fishing 

In the meantime, national working groups for definitions 
and testing methods have been set up in different countries. 

Furthermore, some international groups have discussed 
special items, e.g., the International Council for the Explora- 
tion of the Sea and the International Council for Northwest 
Atlantic Fisheries have discussed methods for testing mesh 
size; and a new international working group on biological 
deterioration of material, including the rotting of fish nets in 
water, was established two weeks ago by the Organisation for 
Economic Co-operation and Development in Paris. 

Standardisation, however, could not be effected by these 
bodies but only by the International Organisation for Stand- 
ardisation, ISO, in London. This organisation had set up 
last year a special sub-committee for textile products as used 
for fishing nets and the second meeting of this sub-committee 
was held in London recently. Actually many fishery experts 
in this field were members of this ISO sub-committee and 
they were in a position to endeavour to make certain that 
ISO would decide on standards which were in accordance 
with the wishes of fisheries as well as of the textile industry. 

In order to rationalise and to avoid duplication of effort, 
I suggest the FAO working committee for standardisation 
of testing methods set up during the First Gear Congress be 
dismissed and its functions transferred to that sub-committee 
of the International Organisation for Standardisation. 

Mr. A. F. B. Nail (UK) a Member of the British Standards 
Institution, outlined the position of ISO. It was the recog- 
nised body for international work on standards of quality, 
performance, size or terminology. It was set up by the 
United Nations during the period 1941-1948 and it worked 
through the national standards organisation of member 
countries. There were over 100 technical committees 
belonging to JSO and among them was Committee No. 38 
dealing with textiles. This comprised representatives of 41 
countries from all parts of the world. There were specialised 
sub-committees dealing with textile products for fishing needs. 
Germany was responsible for the Secretariat of that committee 
and at the last meeting there were representatives of nine 
countries present and some of those representatives have been 
members of the FAO working group. There were strong 
links between the two bodies in both membership and 



Part 1 Materials for Nets and Ropes 

Section 3 New Net Materials 

Netting Twines of Polypropylene and Poly amide 


This paper concerns experiments to date conducted with polypro- 
pylene, a synthetic fibre developed in 1954. Properties of polypro- 
pylene netting twines which were tested include breaking strength 
dry and wet, knot breaking strength, extensibility and influence 
of twine construction. A rather complete comparison is made 
between polypropylene twines and pplyamide twines. It is pointed 
out that at the first International Fishing Gear Congress the authors 
generally judged the efficiency of the synthetic net material by 
comparing their properties with those of natural fibre twines. 
At the present time, however, new net materials have to compete 
with high-class synthetic twines, especially polyamide. Comparisons 
of new synthetic twines with twines of natural fibres rather than with 
established synthetic twines cause little interest among members of 
the fishing industry. Comparative tests between polypropylene 
twines and polyamide twines reported on in this paper include 
breaking strength and R tex, breaking strength and diameter, 
abrasion resistance, extensibility and elasticity. A resume is given 
of actual field work undertaken by other researchers on polypro- 
pylene fishing nets in gillnetting for salmon and cod and in bottom 
trawl fishing. All sections of this paper are thoroughly supplemented 
with graphs and tables. 

Fib et filets de potypropyKne et polyamide comparaison de lews 


La communication traite des experiences conduites jusqu'a ce 
jour sur le polypropylene, une fibre synthetique developpec en 1954. 
Les proprietes des fils de filets polypropylene eprouvees 6taient les 
suiyantes: resistance a la rupture & sec, mouilles et noues, extensi- 
bilite et influence de la fabrication. II y est fait une comparaison 
assez complete des fils de filets polypropylene et polyamide. Alors 
qu'au Premier Congres des Engins de Pechc on jugeait refficacitd des 
fibres synthetiques en comparant leurs proprietes avec celles des fils 
en fibres naturelles, aujourd'hui par centre, les nouveaux materaux 
de filets doivent fctrc compares avec des fibres synthdtiques 
superieures et speciakment, le polyamide. La comparaison des 
nouveaux fils synthetiques avec des fils en fibres naturelles n'a 
plus aucun interdt pour 1'industrie des pcches. Les epreuves com- 
paratives cntre fils en polypropylene et fils en polyamide mention- 
nees dans cette communication ont trait a la force de rupture et 
la grosseur en R tcx, la force a la rupture et le diametre, la resistance 
a 1'abrasion, 1'extensibilite et 1'elasticite. Un resume des essais 
actuels entrepris par les chercheurs sur des filets en polypropylene 
dans la pcche au saumon, au cabillaud et dans le chalutage de 
fond est compris dans cette etude. Toutes les parties de cette 
documentation sont largement completees par des graphiqucs et 
des tableaux. 

HOos para redes hechas de propileno y poliamido Comparackta de 


Alude esta ponencia a los expcrimentos realizados hasta ahora con 
polipropileno, una fibra sintetica descubierta en 1954. Entre las 
propiedades de los hilos para redes de polipropileno se ensayaron 
la resistencia a la rotura estando humedos y secos, resistencia a la 
rotura del nudo, estiramiento y efecto de la manera de fabricar el 
hilo. Se hace una comparacibn bastante completa entre los hilos de 
polipropileno y los de poliamidos. Se menciona que en el Primer 
Congreso Internacional de Artes de Pesca los autores, en general, 
juzgaron la eficacia de un material para redes sintetico comparando 
sus propiedades con las de hilo de fibras naturales. En la actualidad 
los nuevos materiales para redes tienen que competir con hilos 
sinteticos de gran calidad, particuiarmente poliamidos. Las com- 
paracipnes de los nuevos hilos sinteticos con los de fibras naturales, 
mas bien que con los hilos sinteticos cstablecidos, des picrtan poco 
interes entre el personal de la industria pesquera. Los ensayos 
comparatives entre hilos de polipropileno y de poliamidos de que 



Gerhard Klust 

Institut fur Netz und Materialforschung, 

se informa en esta comunicacidn comprenden resistencia a la rotura 
y R tex, resistencia a la rotura y diametro, resistencia al desgaste, 
estiramiento y elasticidad. Se hace un resumen de experiencias 
practicas realizadas por otros investigadorcs con redes de poli- 
propileno empleadas para pescar al enmallc salmon y bacalao en los 
artes de arrastre de fondo. Todas las sccciones de esta ponencia estan 
muy bien documentadas con ilustraciones y tablas. 

THE introduction of synthetic net materials is certainly 
one of the three main technological revolutions in 
modern fishing. Nowadays fishing nets made of poly- 
amide (nylon) are found all over the world. Polyvinyl 
alcohol nets have also gained great importance in some 
countries, while the use of polyvinyl chloride, poly- 
vinylidene chloride, polyethylene and polyester for 
fishing nets has remained more limited. In 1954, a new 
synthetic fibre, polypropylene, was developed in Italy 
and is now manufactured in several other countries. 
The author has tested more than 40 samples of netting 
twines made of this fibre, ranging in size from fine gillnet 
twines to thick trawl twines, and originating from Great 
Britain, Denmark, Italy, Japan and Germany. In this 
paper only continuous multifilament twines of high 
fibre quality are considered. 

Testing methods 

R tex values i.e., the actual weight in grams of 1,000 m 
of the netting twine and runnage in m/kg were calcula- 
ted from the weight of 10 m of the samples. The values 
of diameter are average data of 20 single measurements 
taken on a twine length of 10 m with a dial gauge 

Twines which were tested in wet condition were im- 
mersed in water for one day or, if not enough time was 
available, for one hour with the wetting agent 'Nekal BX' 

Breaking strength was tested by a modern electronic 
recording dynamometer (Testatron'). The average 
data are based on IS to 20 single tests. Two kinds of 
knots were used for testing the knot breaking strength, 
the common overhand knot and the English knot 
(weaver's knot). In the latter case the knot was tied with 
two pieces of netting twine and the two ends (bars) of 
each piece, were fixed in one of the two clamps of the 
testing machine respectively. 

For comparing the results of the weaver's knot with 
those of the overhand knot the former values must be 
halved. The knot strength efficiency is related to the 
breaking strength of the unknotted twine in wet condi- 
tion. The load-elongation curves were recorded by the 
strength-testing machine. They extend only to a load 
corresponding to the knot breaking strength. 

The elasticity was tested by loading the freely hanging 
netting twine samples with a weight of 30 per cent of 
their breaking strength. 

Amount of twist (the number of turns per metre of 
twine), was determined with a twist tester (Frank). With 
these values (ts/m) the coefficient of twist was calculated 
by the formula: 


a = where Nm is the metric number of the netting 

twine (not of the single yarn). The abrasion resistance was 
tested with the Sander test machine by rubbing the 
samples (constantly kept wet) against a rough corundum 
rod under a load of 200 g. The measured value is the 
number of double friction movements until the twine 
breaks. At least 30 tests on each sample. 

Properties of polypropylene netting twines 

The average data for results of most tests are in Tables 1 
and II. The comments are supplementary. 

Breaking strength dry and wet Unlike twines made 
of polyamide and polyvinyl alcohol but like twines made 
of polyethylene and polyester, polypropylene twines have 
practically the same breaking strength wet as dry. 
In breaking length, polypropylene twines are of the same 
high quality as polyamide twines with respect to strength. 
Their tenacity is higher than that of netting twines made 
of all other groups of synthetic fibres except polyamides. 

Knot breaking strength With twisted twines, the 
English knot gave better results than the overhand knot. 
With braided twines, there was no such difference, thus 
being similar to polyamide twines. Losses in strength by 
knotting somewhat depend on twine fineness. 

Losses in strength by : 

Finest netting twines (twisted) 

Twisted netting twines of medium strength, 
approximately from R 130 tex to R 500 

Thicker netting twines, twisted 

Braided trawl twines 

English Overhand 
knot knot 
24% 30% 



Extensibility Continuous polypropylene twines have 
a relatively low extension. The form of the load-elonga- 
tion curves is very remarkable and typical for polypro- 
pylene material Figs. 4, 5 and 6 show they are almost 

Influence of twine construction Physical properties 
of twines depend on the quality of the fibre but also 
to a high degree on the twine construction. The single 
yarns composing the twines described differ in fineness. 
Most have the relatively low yarn number of 170 or 190 
denier and are combined to twines in the form of cabled 
yarns. Trawl twines numbered 35,21 and 22 (Table II) 
are made of a few single yarns with the high yarn number 
3,000 denier; 36 is made of a single yarn of 2,000 denier. 

Samples 23 to 28, single yarns of equal fineness about 
800 denier. Although differences in fibre quality, fineness 
of single yarns and the number of yarns in the twines 
may explain some of the variations shown, the influence 
of the amount of twist is definitely more evident. It is 
already known from other continuous multifilament 
materials, that the breaking strength decreases with 

i. NettfnantviiiM 

Ho of testing 1 U 12 13 14 2 15 
Promotion A B B B B 1 C 














No in Denier 190x2 170x3 170x4 170x6 170x9 190x9 190x9 
Rtex (1000 *) 43,5 63,9 63,4 130 189 211 208 
Bunnage, /* 22900 15657 11990 7686 5291 4740 4805 












507 0,23 0,33 0,35 0,44 0,52 0,60 0,58 












Brkg. strength 
etraifiht,dry,kg. 2,36 3,67 ,4,50 7,38 11,21 13,1 11,0 
trsight,wet,kg. 2,35 3,63 4,13 7,43 11,31 12,9 10,8 













Brkg. strength, wet 
o/nand knot, kg. 1J6 2,29 3,02 4,14 6,34 6,5 6,1 
fegliah knot, kg. 3,63 5,07 6,73 9,2 13,1 14,9 14,2 
Engl. knot, half kg. 1,62 2,54 3,37 4,6 6,6 7,5 7,1 














*Knot atrength efficiency, 
o/nand knot % 74,9 63,1 73,1 55,7 56,1 50,4 56,5 
Ingl,knot,half <f> 77,7 70,0 81,6 61,9 58,4 58,1 65,7 












Breaking length 
atraight,dry,Jrm 54,3 57,4 54,0 56,7 59,3 62,1 52,9 
straight,wet km 54,0 56,6 49,5 57,1 59,8 6l,l 51,9 












Bkg.lsngth wet 
o/nand knot, to. 40,5 35,8 36,2 31,8 33,5 30,8 29,3,half kn41,6 39,7 40,4 35,4 34,9 35,5 34,1 













Bxt. at knot 
bkg.strength % - - - 12 12 10 11 












* Knot strength efficiency in the Tables I and II relates to straight breaking strength of wet twines. 


increasing twist and that extensibility as well as diameter 
and weight per-unit-length increase. In Table I the 
samples 2, 3, 4, 5, 6 and 7 are soft laid with an average 
coefficient of twist of a = 118. The samples 8, 9 and 
10 are medium laid (a = 164 in the average) and there- 
fore have lower breaking lengths and higher extension 

Yabl* II Tr.wJ tin* Md of continuous Polypropyln 

to. of toting 

Kim (MOO we) 
! of laglt jrtn 
., 7kg. 

1191 1456 3116 
869 696 321 




















2131 4795 
22 46 
469 209 


1,46 1,64 2,31 2J1 2,61 2,66 - 

Bkg . toongth 
trtigfct, dzjr kg. 
tMlgnt, *t kg. 

52.8 59,7 156,7 206,0 164,6 226,5 94,6 114,0 210,0 260.0 

52.9 60,4 160,3 210,4 172,1 237,6 94,9 Ho, 4 W,? 282,0 


o/hand knot, kff. 
fe. knot, kg. 
feg. knot, ike. 


o/hnd knot, kg. 

tog. knot **. 

Dkg. Imgth 
talht, dry ka. 
t, wt lou. 

27,8 33,9 77,5 101,2 

63.8 76,4 161,0 234,9 

31.9 Jb,2 60,5 117,5 

73,5 96,7 46,7 
160,9 200,2 96,7 
6U,5 100,1 46,4 

57,5 114,5 
113,6 216,3 
5b,6 109,* 


kg. iMgtb t 
o/hand knot. kn. 
tog. knot, ftto. 

52,6 56,1 4B.3 48,1 42,7 41,5 51,3 49,7 57,6 47,3 
60,3 63,2 50,2 55,6 4b,6 4^,1 51,0 4b,b 54,9 4b,U 

45,9 41,0 50.2 48,2 47,0 4,1 55,5 53,5 4:5,8 47,4 
45,9 41,4 51,4 40,7 4J/.2 50,5 5b,& 54,6 41,5 47, 

24,1 23,3 24,9 23,4 21,0 21,0 2tt,5 27,2 23,9 22, b 
27,7 26,2 2>,8 27,2 23,0 21,3 2b,J 26,7 2c',b 23,3 

breaking itrongth % 
* Th tiM 25-26 

10 11-12 13-15 12-13 9-10 9-11 13-14 H-12 

art bridd 

Comparing polypropylene and polyamide twines 

Here the properties of polypropylene twines for fishing 
nets are compared with those of polyamide twines. 

The breaking strength of the two types of twines 
(see Figs. 1 and 2) is compared wet and knotted with 
the weaver's knot, because the tests then come nearest 
to the normal strains of fishing. The knot breaking 
strengths are compared on the basis of R tex values, Fig. 1 
containing those of fine and medium strong twines, Fig. 2 
those of heavy twisted and braided trawl twines. The 
regression-line for polypropylene in Fig. 1 relates to the 
round black points only and not to the crosses which 
characterise the breaking strength of twines of lower 
quality. Both figures show that with the same R tex 
value both types of fibre have nearly the same breaking 
strength wet, knotted. 

Netting twints mod* of 

Polyamid* o 


300 800 


900 flOO 

1300 1500 

Figs. 1 and 2. R tex values and knot strength of netting twines made of 
continuous polypropylene compared with those of polyamide. 



$ no 



Trowt twttm fnodtof 

1200 1900 2000 2400 2600 ' 3200 ' 3600' 400o' 4400 4600 3200 5&0 6000 


Fig. 2. See caption Fig. 1. 

Breaking strength and diameter Apart from its high 
tenacity the most remarkable property of polypropylene 
is its low specific gravity of 0*91 as compared with 
1-14 for polyamide. Specific gravity and specific 
volume being reciprocally proportional, the diameter 
increases with decreasing density. Polypropylene twines, 
therefore, must be thicker than polyamide twines of 
equal weight per-unit-length. Since twines of both fibre 
types with equal twine number have nearly the same 
breaking strength, the diameters can also be compared 
on the basis of the latter property. Fig. 3 shows the result 
of this comparison. Very thin twines of both fibres do not 
differ in breaking strength and diameter. With stronger 
twines those made of polypropylene are on average about 
15 to 20 per cent thicker than polyamide twines. 



If W 

Fig. 3. Knot strength and diameter of fine and medium strong netting 
twines made of continuous polypropylene and polyamide. 

Abrasion resistance Results of these tests depend very 
much on the method and mainly on the test machine 
used and it seems almost impossible to imitate the actual 
conditions of friction, which affect nets during fishing. 
Table IllAbrasion resistance of netting twines, vet (average data) 

PnlvnrnnvUnA filament t~i. J_ m .\ . e iay 

Polypropylene filament 
R tex Diameter Double 



547 0-99 










Polyamide filament 









Twines of exactly the same R tex or the same diameter 
were not available. Comparisons based on R tex values 
showed no significant differences between polypropylene 
and polyamide. This indicates that polypropylene has a 
very high abrasion resistance, for polyamide is known to 
be one of the best fibrous materials in this respect. 

If twines of equal diameter are compared, the abrasion 
resistance of polyamide twines is higher than that of 
polypropylene twines. 

Fig. 4. Load-elongation curves of polypropylene and polyamide netting 
twines (wet, %). Pp= polypropylene. Pa ^polyamide. 

Extensibility The inherent extensibility of synthetic 
fibres varies, being relatively large for the polyamide 
group and comparatively small for the polypropylene 
and polyester groups. The amount of stretching during 
manufacture is of great influence. The more the fibres 
are stretched, the more does extensibility decrease and 
their breaking strength (also per centage losses by knot- 
ting), increase. Nearly all polypropylene twines, men- 
tioned in this paper, are of highly stretched material, 
similar to nylon twines. The form of fibre also influences 
twine extensibility; staple fibre twines are more extensible 

Fig. 5. Load-elongation curves of trawl twines (wet t %). (As in 

Fig. 4 the curves only reach up to a load corresponding to the 

knot breaking strength.) 

than continuous filament twines. The influence of the 
amount of twist is shown in Fig. 6 with the nylon twines 
2 and 3. It is possible to double extensibility by increasing 
the coefficient of twist from, e.g., 150 to about 200. 

The influence of increasing loads on twine extension is 
demonstrated by the load-elongation curves of Figs. 4 
and 5. Here polypropylene and polyamide twines 
approximately corresponding in twist and knot breaking 
strength, are compared. They show remarkably great 
differences of extensibility polyamide stretch much more 
than polypropylene twines particularly under low 
loads. (Figs. 4, 5, and 6.) 

Fig. 6. Load-elongation curves of netting twines (wet, %) made of 

polypropylene (Pp) and polyamide (Pa). Twines I and 2 medium laid 

(a- 750-760), twine 3 hard laid. 






4 - 
3 - 


Fig. 7. Elongation and elasticity of wet netting twines loaded for 
one hour with 30 per cent of their (straight) breaking strength. 
Elongation: (a)^ immediately after loading. (b)= loaded for one hour, 
(^immediately after removal of load. (d)=one hour after removal 
of load, (c) **permanent elongation. 

Elasticity Because of the factor of duration of time 
of loading, elasticity was tested by special trials, the results 
of which are compared in Figs. 7 and 8 and they confirm 
once more that polyamide twines are more extensible 
than polypropylene twines but the permanent elongation 
of polyamide is lower. A further remarkable difference 


between the two kinds of twine becomes evident 
after longer loading, for instance 24 hours, as in Fig. 8. 
Polypropylene twines show a "creep" when subjected to 
loads for prolonged times. Polyamide twines do not 
change their length during a long period of loading. 
They show no creep and have a better elasticity. 


Fig. 8. Elongation and elasticity of dry netting twines loaded for 24 
hours with 30 per cent, of their (straight) breaking strength. Elongation: 
(a) immediately after loading. (b)^loadedfor one hour. (c)=loaded 
for three hours. (d)= loaded for 24 hours. (e)= immediately after 
removal of load. (/) * one hour after removal of load, (g) = permanent 
elongation (measured four days after removal of load). 

The first trials with polypropylene fishing nets seem to 
have started in 1961. Experiences, therefore, are still 

A report by Carrothers on trials of polypropylene and 
nylon salmon gillnets in Canada says polypropylene nets 
were easy to handle and their knot stability was superior 
to that of nylon nets. In capacity to catch fish, there was 
no conformity in the experiences of the fishermen. 
In trials with polypropylene cod gillnets during the 
Lofoten season carried out by Norwegian fishermen, the 
polypropylene nets were about 10 per cent lighter and 
lost less strength during fishing than nylon nets, with 
about equal catching efficiency. 

In a cruise by the USA research vessel Delaware an 
otter trawl, "in which the top wings, top belly and square 
were constructed of polypropylene fibre twine", was 
compared with a trawl, entirely made of manila twine. 
After equal numbers of tows, "The polypropylene- 
equipped net took 57 per cent fish compared to 43 per 
cent with the all-manila net". It also gave easier handling, 
lower water resistance and easier towing. 

Trials of British trawlers with trawls entirely made of 
polypropylene, brought similar results. Especially 
braided twines were very suitable. These trawls can be 
towed more easily through the water than comparable 
manila trawls and give greater headline heights than nylon 
using the same number of floats. 


Mid water trawls 

No experiences with polypropylene for this modern gear 
type are known to the author. Some general remarks 
can, however, be given. 

Fishing gears subject to heavy strain, especially 
trawls, should on principle be constructed of net material 
which combines high breaking strength with a twine 
diameter as small as possible. The small diameter gives 
less towing resistance. The twines should also have a 
relatively high extensibility with a high share of elasticity 
in order to absorb kinetic energy so that the trawl nets 
can withstand shock loads during fishing and heavy 
stress when hauling big catches. 

For midwater trawls these demands are of particularly 
great importance. The nets have to be larger and be 
towed faster than bottom trawls, because pelagic fish 
are usually fast swimmers. Light and strong nets are, 
therefore, indispensable. Since twine efficiency depends 
not only on strength but also on extensibility, midwater 
trawl twines are relatively high twisted. As shown in 
Fig. 6, the degree of extension can remarkably be increased 
in this way. As a high extension obtained by twisting is 
connected with a considerable decrease in strength, nylon 
twines should not be manufactured with a coefficient of 
twist higher than 200. If twine thickness is really of such 
great importance for the towing resistance, polyamide 
twines are superior to polypropylene twines (see Fig. 3). 
Considering extensibility the same fact has to be 
stated. Because of the low original extension of polypro- 
pylene fibres, twines made of these fibres must be twisted 
to a much higher degree than polyamide twines in order 
to reach an extensibility suitable for midwater trawls. 
But with this hard twisting the breaking strength would 
considerably decrease and the diameter would increase 
even more. 


v. Brandt, A. : Das Schwimmschleppnetz. Protokolle z. Fisch- 
ereitechnik, V, Nr. 22/23, pp. 201-224, 1958. 

v. Brandt, A. and Carrothers, P. J. G. : Test methods for fishing 
gear material (Twines and Netting): 2nd World Fishing Gear 
Congress, 1963. 

Carrothers, P. J. G. : Results of laboratory and field evaluation of 
continuous filament isotactic polypropylene fibre for salmon gill- 
nets. Fish. Research Board of Canada, Circular No. 66, 1962. 

Imperial Chemical Ind. Ltd. : 'Ulstron' polypropylene for fishnets, 

Imperial Chemical Ind. Ltd.. 'Ulstron' trawl trials, 1962. 

Imperial Chemical Ind. Ltd.: Trials with 'Ulstron' polypro- 
pylene cod gillnets, Lofoten season, 1961, 1962. 

Klust, G.: The efficiency of synthetic fibres in fishing, Modern 
Fishing Gear of the World, 1959. 

Klust, G. : Net materials for trawlnets. Comp. Fishing Committee 
No. 87, 1961. 

Klust, G. : Polypropylen-Schntire als Fischnetzmaterial. Dtsch. 
Seiler-Zeitung, Nr. 4, pp. 203-208, 1963. 

Klust, G.: Netzmaterial fur Schwimmschleppnetze der Hoch- 
sccfischcrei. Allgcm. Fischwirtschaftszcitung H20/21, 1963. 

Mohr, H. : Reaction of herring to fishing gear studied by echo 
sounding. 2. World Fishing Gear Congress, 1963. 

N.N.: Delaware report on polypropylene trawl. Bureau of 
Commercial Fisheries, Report No. 95, 1961. 

Polymer Ind. Chimiche S.p.A.: General description of 'Mera- 
klon' Booklet No. 1, Milan, 1960. 

Schftrfe, J. and Steinberg, R.: Neue Erfahrungen mit Schwimm- 
schteppnetzen (Juni bis Dezember, 1962). Protokolle z. Fischcrei- 
technik, VIII, No. 37, pp. 160-230, 1963. 

Schftrfe, J.: One-boat midwater trawling in Germany. 2. World 
Fishing Gear Congress, 1963. 

Polypropylene Twines in Japan 


Several Japanese firms started production of polypropylene net 
twines in 1961. The first products varied in quality, but recently 
more stable material has been produced and polypropylene twines 
are now used in the construction of nets in Japan. Although 
efforts are still in progress to improve the present twines and mono- 
filaments, tests were made to ascertain their properties in relation 
to other synthetics. Polypropylene has a lower specific gravity 
(0-91) and a higher breaking strength (g/den) both dry and wet 
straight and knotted, than any other fibre material. Water absorp- 
tion is negligible and consequently there is no loss of strength through 
wetting. On the basis of equal size (same diameter but lower weight 
per unit length) it is equal in strength to 'Amilan' but slightly 
inferior to Tetoron* multifilaments. Elasticity and elongation is 
small at low loads, but breaking energy is 50 per cent greater than 
that of 'Amilan*. Resistance to weathering and abrasion is still 
poorer than that of 'Amilan*. No appreciable difference in catch- 
ability or ease of handling could be noted during fishing tests 
when compared with 'Amilan', except that polypropylene was more 

Les fils de polypropylene au Japon 


Plusieurs entreprises japonaises ont commence en 1961 a fabriquer 
des fils de polypropylene pour les filets de pechc. La qualite des 
premiers produits a beaucoup varie et tout r&emment on a mis au 
point une matiere plus stable que Ton utilise dej& pour la structure 
des filets de peche. Bien que de gros efforts soient en cours pour 
ameliorer encore les fils et monofilaments actuels, des essais ont 
deja et6 effectues en vue de determiner les qualites specifiques de ces 
fils par rapport aux autres fils synthetiques. Le poids specifique du 
polypropylene est inferieur (0-91) et sa resistance & la rupture 
superieure (g/den) (que la fibre soit seche ou mouillee, nouee ou 
non) a ceux de toute autre fibre. Son absorption d'eau est minime et 
de ce fait le mouillage n'entraine aucune diminution de sa force. 
Sur la base de mfimes dimensions (c'est-a-dire du meme diametre, 
mais d'un poids inferieur par unite de longueur), la force du poly- 
propylene est egalc a celle de T Am ilan' mais un peu inferieure a 
celle des multifilaments de Tetoron 1 . Son elasticite et son elongation 
aux charges faibles sont restreintes, mais sa resistance & la rupture 
est de 50 pour cent plus grande que celle de rAmilan'. Sa resistance 
a I'usure du temps ou par frottemen reste inferieure a celle de 
PAniilan'. Pendant les essais de peche, on n'as pas constate de 
difference appreciable entre P Amilan* et le polypropylene quant a 
la capacite de capture et la facilite de I'amenagement du poisson, 
sauf que le polypropylene se faisait plus volumineux. 

Hilos de polipropileno en el Japon 


Varias empresas Japonesas iniciaron en 196 1 la producci6n dc 
hilos de polipropileno para redes. La calidad de los primeros 
productos variaba mucho, pero ultimamente se ha obtenido un 
material mas estable y estos hilos se emplean actualmente en la 
construcci6n de redes. Continuan las investigaciones para mejorar 
los hilos empleados actualmente, realizandose, ademas, ensayos para 
determinar sus propiedades con respectoaotrosmaterialessintdticos. 
El polipropileno tiene una gravedad especifica ms baja (0-91) y una 
mayor resistencia a la rotura (g/den) tanto humedo como seco, 
anudado como sin anudar, que cualquier otro material para fibres. 
La absorci6n de agua es insignificante y por lo tanto no hay perdida 
de resistencia a causa de la mojadura. A igualdad de dimensipnes 
(el mismo diametro, pero menos peso por unidad de longitud) 
es tan fuerte como * Amilan', pero un poco menos que los multi- 
filamentos de Tetoron'. Le elasticidad y el estiramiento son peque 
nos a cargas reducidas pero la resistencia a la rotura es un 50 por 
ciento mayor que la de 'Amilan'. La resistencia al desgaste y a la 
abrasidn es todavia menor que la de 'Amilan'. No se observarpn 
diferencias sensibles en cuanto a captures o facilidad de manip- 
ulacidn en ensayos de pesca en los que se compar6 con 'Amilan', 
aunque el polipropileno result6 ser mas voluminoso. 

"EXPERIMENTAL production of polypropylene was 
JLJ started by several manufacturers in 1961. Of the 
first products quality was unstable and the resultant 

Katsqji Honda 

Tokyo University 
of Fisheries 


Nippon Gyomo 
Sengu Kaisha Ltd 

fibres unreliable. Products are now more stable and such 
fibres are beginning to be used in fishing gears. 

Investigations towards improvement are still being 
carried out and the data provided here is not final. 

Properties of polypropylene yarns are given in Table I, 
compared with others productions. From this it can be 
clearly seen that: 

(a) Specific gravity of polypropylene is the lowest 
among all synthetic fibres. This is a definite advan- 
tage for light fishing gear. But it can be a drawback 
in some gears where sinking speed is important. 

(b) Breaking strength, dry and wet, both straight and 
looped, is greater than that of other synthetic 
fibres. However, when compared with multifila- 
ment yarns of the same size (diameter), polypro- 
pylene has the same strength as 'Amilan*, but is 
inferior to Tetoron' (polyester). In spun yarns it 
equals both Tetoron' and 'Kuralon' of the same 

(c) As polypropylene absorbs practically no water, 
its characteristics undergo no changes in wet 
conditions so that wet strength is as high as dry 
strength which is an advantage. 

(d) The stress-strain curve under loop test is given in 
Fig. 1. (Mark P.P. as used throughout Figs. 1 to 7 
and Table I means polypropylene.) Compared 
with 'Amilan', polypropylene is difficult to stretch 
even under low load and its elongation is com- 
paratively small. Its breaking energy is SO per cent 
greater than that of 'Amilan' ; but if the curves are 
made for equal diameter (Fig. 2) then the breaking 
energy is 30 per cent lower. This means that it is 
weaker than 'Amilan' of the same diameter. 

Abrasion tests (Fig. 3) show that polypropylene has a 
relatively low abrasion resistance and is therefore not 
well suited for trawls, etc. 

Tests for weathering on its strength and elongations 
(Figs. 4 and 5) made it clear that polypropylene has a 
much lower resistance to weathering than either 'Amilan' 
or Tetoron'. 


As a monofilamcnt polypropylene was found to be 
superior both in straight and loop tensile strength. For 
elastic recovery, the results in Fig. 6 show that the elas- 
ticity decreases much faster with increasing load than 
for 'Amilan* of the same diameter. 

Impact tests and abrasion tests showed that polypro- 

pylene was inferior to * Amilan* (Table II and Fig. 7). 

Polypropylene is more transparent than 'Amilan' but 
rather stiffen 

Two hundred and fifty polypropylene gillnets made of 
No. 10-5 (2315d) monofilament twine in 121-mm mesh 

continued on page 57 



\ f 

' I 








AMMH 110 









tO If 14 Elong*ti*n 

Fig. 1. Stress-strain 


PP 100 D 







lion CIO 




Fig. 2. Stress-strain diagram 
for equal diameter. 

Fig. 3. Abrasive 

10 20 50 40 r 

Fig. 4. Loss in strength 
from exposure. 



Totoron 210 D/, 5 90 

24 6 8 10 12 Kg 

Fig. 5. Elongation decrease from exposure. Fig. 6. Monofilament elastic behaviour. 




500 1000 *" "" 

Fig. 7. Monofilament abrasive resistance. 





Zton tablo Ib 



P.P. Milan TotoroB SATOA Toriroa 
160 P 210 D 210 D 360 9 300 D 
20.24 P15P 24P i P 60 P 

p.p. luntloa Spun Bpuft i 
nylon Totoroi 
20 8 20 8 22 8 20 8 

Jbawrod. doniov (a) 



pooiflo fmritj 

0.91 1.14 1.38 1.70 1.40 

0.91 1.30 1.14 1.38 

Crooo oootion 



MOkiMf itTOnfftk 

Brookini otronfth (kf) 



(/o) to. 

6.5-7.7 6.20 6.50 2.0 2.50 

4.3-4*5 4.50 2.70 2.60 
" 4.20 2.40 2.65 

Broftfcinf olongntlon (%} 



IrMtiBg rttmgtttlM 

Loop otronfftk (kf ) 



"* wit 

20.5 18.0 6.0 26.0 20.0 
* 19*0 6.5 * 21.0 

16.0-16.5 10.0 20.0 23*5 
" 13*0 22.0 24.5 

Loop olonAtion (%) 



Loop *ty+Mffftk 

AOOMAM % of otronf th, by loop 



(/4-) gy 

4.8-5.3 5.10 4.60 1.3 1.90 
* 4.53 " " 2.00 

3.4-3.6 3.40 2.30 2.25 
11 2.80 2.10 2.35 

BrooJcinc otroM (kf/M2) itrftikt 



(F) Dy 

11*5 11.0 7.0 13.5 12.0 

12.5-13.5 75 16 5 21 5 

(in tho oto of MM finonoso) loop 




13.5 7.5 " 13.0 

9.5 14.5 23.5 

tar loop wot 

28.0 13.0 29.0 35*0 22.0 

20.0 33.0 13.0 11.0 

Item T*bl II 



^ nj| ttrta . 

2515D, 121 m 

26$00 t 121 HP 

(0t) (kf/BII*) 


r^ 1 ** 

53-46 53 81 31 32 
4<MJ 47 53 20 25 

35-37 49 14 33 
28-30 32 22 29 

Brakia nry (k.n) (Trtiol) 



^ w 

0.1 4*5 0.4 0.1 0.2 


Br*kin nry (kff.n) (horizontal) 




Use of < Ulstron ' Polypropylene in Fishing 


Production of 'Ulstron' for fishing nets will, by the end of 1963 
be greater than the total equivalent consumption of all other U K' 
synthetic fibres. An oil derivative, polypropylene was invented in 
1954 in Miten. Total world production of polypropylene now 
exceeds 300,000 tons per annum but only a small percentage in the 
form of fibres, the majority being used as moulding. High-tenacity 
Ulstron' at 8-5 g/den competes with nylon on strength and price 
basis. Plaited cords may be produced on conventional machinery 
*Ulstron' nets can be constructed on ordinary netting looms such 
as Zang, Dandy, Amita and Porlester, and are said to offer advan- 
tages in both runnage and mesh strength over equivalent nylon. 
Knot firmness is relatively good but loom tensions in netmaking 
should aim at 1 -0 g/den followed by knot stretching at 1 -0 g/den at 
100/1 10C for 15 minutes while held to fixed dimensions on a 
stretching frame. 'Ulstron' should not be heated over 110C. 
'Ulstron' yarn may be obtained in and dyed into various colours. 
Lighter than water, 'Ulstron' is now being used in a variety of 
fishing gears such as bottom trawls and midwater trawls, gillnets, 
Danish seines and purse seines, etc., as well as in ropes, where it is 
in the same strength bracket as nylon and polyester ropes, but has a 
considerably greater runnage and is cheaper per unit length of 
equal strength. 


C. L. B. Carter 

K. West 

Fibres Division, 

Imperial Chemical Industries Ltd 

L"Ulstron' ne doit pas etre chauffe audessus de 110C U fil de 
filet 'Ulstron' peut 6tre obtenu dans diverses tcintes. II flotte sur 
1'eau et est maintcnant utilis& pour des engins de pechc varies tels 
que: chaluts de fond, chaluts pelagiques, filets maillants, sennes 
danoises, sennes coulissantes, etc., aussi bien que pour des cordes ou 
1" Ulstron' est l'egal du nylon et du polyester en ce qui concerne la 
resistance a la rupture, ma is est beaucoup plus long au kilo et est 
beaucoup moins cher par unite de longueur pour la mdme resistance. 

L'Utilisation du polypropylene 'Ulstron' dans 1'industrie de la peche El empleo de pollpropileno 'Ulstron' en la industria pesquera 


La production d' 4 Ulstron' pour les filets de peche sera, a la fin de 
1963, superieure a la consommation totalc de toutes les autres 
fibres synthetiques pour le Royaume-Uni. Le polypropylene, 
deriyS d'une huile minerale, a etc invente a Milan en 1954. La pro- 
duction mondialc totale de polypropyleneexcede maintenant 300,000 
tonnes par an, mais un faible pourcenlage seulement sert a la 
fabrication des filets, la majorite etant utilised pour les plastiques. 
L"Ulstron' dc haute tenacite a 8,5 g/den peut rivaliser avec le nylon 
au point de vue resistance et prix. Des cordes tressees peuvent 
etre produites sur les machines traditionnelles et les filets d" Ulstron' 
peuvent etre fabriques sur des m6tiers ordinaires comme Zang, 
Dandy, Amita ct Porlester. Us sont sensds offrir des avantages sur 
les filets en nylon de denier equivalent, en ce qui concerne le rapport 
m/kg et la resistance des mailles. La stabilite des noeuds est relative- 
ment bonne mais la tension sur le metier pendant la fabrication devra 
6tre dirigee vers 1 g/den, suivie d'un etiragc des noeuds a un g/dcn, 
100/1 10' C pendant 15 minutes, en tenant le filet fixe sur un cadre. 


La producci6n de 'Ulstron' para redes de pesca sera, para fines de 
1963, mayor que la de todas las demas fibras sinteticas en el Reino 
Unido. El propileno es un derivado del petrbleo descubierto en 
Milan en 1954. La producci6n mundial total de la sustancia excede 
de 300.000 tons anuales, pero s61o una pequena proporcibn en la 
forma de fibra y la mayor parte para moldes. El 'Ulstron' de gran 
tenacidad a 8,5 g/den compite con el nylon en robustez y precio. 
Las cuerdas trenzadas pueden fabricarse en las maquinas normales. 
Las redes de 'Ulstron' se manufacturan en los telares corrientes 
como el Zang, Dandy, Amita y Porlester y sc dice que resultan 
ventajosas en cuanto at peso por longitud y resistencia de ia malla 
con respecto al nylon equivalente. Elnudo es bastante fir me, 
pero las tensiones en el telar, cuando se hacen redes, deberian ser 
del orden de 1,0 g/den, seguidas de estiramiento del nudo a 1,0 
g/den a 100-1 10 C durante 15 rnin mientras se sujeta a dimensiones 
fijas en un armaz6n de estiramiento. El 'Ulstron' no debera calen- 
tarse a mas de HOC. Sus hi los pueden prepararsc de cualquier 

continued from page 56 

size nets were tested in the North Pacific Ocean from May 
to July, 1962, for salmon and trout. The catch ratio per 
net appeared to average approximately that of 'Amilan' 
nets of the same diameter and were as follows: 
Polypropylene 1 '30- 1 -45 

4 Amilan' nets made in 1 96 1 1 -00 

4 Amilan' nets made in 1 962 1 '50- 1 -65. 

The polypropylene nets were found to be two to three 
times more voluminous than 4 Amilan' ones and tended to 
blow about easily, making them liable to fouling during 
hauling and shooting. 

As regards fishing damage it would appear that the 
number of tears is twice that of the 4 Amilan' nets but the 
knot slippage was very much lower. 

After the trials, the nets were again tested and it 
appeared that the strength had increased during use so 
that it was now higher than that of 'Amilan'. No change 
in mesh size as a result of the trials could be ascertained. 
Polypropylene nets made of multifilament 180d/15 
were made in 121 -mm mesh size and operated together 

with 'Amilan' and 'Tetoron' nets made of equal diameter 

Catches per net were as good as those of contemporary 
'Amilan' nets and normally better than older nets. 

In handling they were more bulky and tended to blow 

About 600 nets made of polypropylene staple yarn 
20s/ 15 (resin treated, singed) were operated from April 
to August 1962, for a period of 103 days, together with 
nets made of 'Kuralon'. 

The experiment showed that catches were higher than 
those of the 'Kuralon' nets and, while the handling of the 
nets was fairly good, there was more difficulty in retriev- 
ing the crabs than with the 'Kuralon* nets. 

The nets were tested after use and it was noticeable 
that, whereas the monofilamcnt and multifilament nets 
slightly increased in breaking strength and decreased in 
extensibility, the opposite occurred with these staple 
yarn nets. 


color o tefiirse de Igual manera. Es menos denso que el agua y se 
noes de enmalle, artes daneses y redes de oerco de jareta, asi como 
en caboi de todas dases en los que tiene la misma robustez que los 
de nylon y poiiesteres, pero entran muchos mas metres en kg y es 
mis barato por unidad de longitud de la misma resistencia. 

4 T TLSTRON' polypropylene is rapidly becoming the 
vJ leading synthetic fibre for the production of 
fish netting in the U.K. It is already used for a wider 
range of fish nets than any of the other synthetics, and 
production of 'Ulstron' for nets and other fishing appli- 
cations will, by the end of 1963, be greater than the total 
equivalent consumption of all the other U.K. synthetic 

Polypropylene is the latest synthetic fibre to be devel- 
oped for the fishing industry and this paper outlines the 
properties of 'Ulstron', I.C.I.'s brand of polypropylene, 
and describes the experience to date in net production and 

Discovery and development 

Polypropylene has been known for many years as an 
oil, although, for various reasons, it had no commercial 
importance. In 1954, at the Polytechnic Institute of Milan, 
Professor Natta, whose work had been concerned with 
the stereospecific polymerisation of defines using organo- 
mctallic catalysts of the Zeigler type, discovered how to 
make isotactic material with a regular ordered molecular 
structure. This raised the melting point for 20 to 40C to 
165C and altered the whole future of polypropylene as 
both a plastic and fibre material. Propylene is derived from 
oil, being a gas which is obtained at the same time as 
ethylene by cracking petroleum. As a point of interest. 
'Terylene' and polythene are also derived from oil while 
nylon is derived from coal. 

I.C.I, recognised the great potential for this new 
material and negotiated a patent licence from the Italian 
chemical firm of Montecatini in August, 1960. This 
agreement covered the exclusive production and sale in 
the U.K. by I.C.I, of polypropylene filament yarn, staple 
fibre and textile monofilaments. To the high tenacity 
multifilament and monofilament polypropylene yarns, 

I.C.I, has given the trade name 'Ulstron'. A large plant, 
with an eventual capacity of 5,000,000 Ib per annum, is 
now in production and work is proceeding to extend 
this capacity still further. 

Total world production of polypropylene is now 
estimated to exceed 300,000 tons per annum but at 
present only a small percentage is in the form of fibres, 
the majority being supplied for plastic uses such as 
moulding. With most brands of polypropylene fibre 
the tenacity (6 g/den) has been considerably below that 
of nylon (7^-9 g/den) and this has undoubtedly held 
back a more rapid expansion in the fishing industry than 
would otherwise have been expected from its outstanding 
properties of lightness and low price. 'Ulstron' at 8*5 
g/den is the first (and, at present, only) brand of poly- 
propylene to be fully competitive with nylon on strength 
as well as price. This accounts for its rapid progress at 
the expense of nylon and other synthetics which it is 
replacing, a pattern which is likely to occur in other 
parts of the world during the next few years as other 
brands of polypropylene become more fully developed. 

Some details of this new material will therefore be of 
particular interest. 

Properties of 'Ulstron' 

'Ulstron' has a low density, high wet strength and extreme- 
ly high capacity for withstanding shock loads. Its pro- 
perties are compared with those of other yarns used in 
fish net manufacture in Table I. Other advantages include 
a good resistance to chemical and micro-biological 
attack (Table II) and negligible water absorption. It 
is currently produced in the following deniers: 

190 den (Decitex 21 1), 380 (Decitex 422), 570 (Deci- 
tex 633), 760 (Decitex 844), 1,140 (Decitex 1,270), in 
both natural and melt coloured green. 

Twine making 

* Ulstron' twines and nets are constructed in much the 
same way as other synthetic materials but, because of 
their lower liveliness compared with nylon or Terylene' 

Property 'Ulstron' 

Tenacity (g/den) . . 8-0-8-5 
Extension at Break ( %) 1 8-22 

Elastic Recovery (% from 5% 88 


Working Load Extension 1-11 

Initial Modulus (g/den) 90 

Toughness x 10*(joiiles/g) 80-100 

Wet Tenacity (g/den) . . 8-0-8-5 
Wet Extension (%) .. 18-22 

Moisture Regain (%).. 0-1 

Softening Point .. 160-170 
Specific Gravity of Fibre 0-91 

TABLE I Comparative filament yarn properties 
Polyethylene Terylene' 























at 3% Ext. 

































* The tensile properties quoted for cotton, manila, sisal of necessity refer to spun yarns, which have an overall 
tenacity lower than that of the individual single fibres. 




















Caustic Soda 

Caustic Soda 

Caustic Potash 

Trichlorethylene . . 

Trichlorethylene . . 

Carbon Tetrachloride 




Oxidising Agents: 

Sodium Hypochlorite 

Hydrogen Peroxide 

Cone. W/W 




5% active C1 2 
12 volume 

TABLE II Chemical resistance data 
Temp.(C) Time 




at the boil 


at the boil 












Residual Strength (%) 

Nylon Polyester 'Ulstron' 

89 100 

71 100 



83 90 

100 100 

86 92 100 

90 90 

100 100 

90 90 

N.A. N.A. 75 

N.A. N.A. 70 

100 100 100 

100 100 100 

85 92 95 



N.A. 95 85 

N.A. 100 90 

N.A. No data available 

twines, twist setting is less important. In twisting * Uls- 
tron' twines, it may be found necessary to employ ring 
travellers one or two sizes lower than are used for nylon 
twines of equivalent numbers of plies, owing to the lower 
density of the material. 

'Ulstron' twines, matching in twist-hardness nylon 
twines of 210-den yarns, will be obtained by using one 
190-den yarn for each 210-den yarn and employing twist 
levels at all stages which are some five per cent lower than 
for the nylon twine. Where high extensibility and high 
bending stiffness are required hard twines will be con- 
structed; where the net is to be subsequently bonded, 
soft twines will be adequate. Twist levels which have been 
employed in constructing hard and soft 'Ulstron' twines 
based on 190-den yarn are given in Fig. 1. 


Fig. 1. Ply twist for three-ply twines based on 190-denier ' Ulstron 9 yarn 

Equivalent twist hardnesses for 'Ulstron' twines, 
based on the same number of plies of any other denier, 
may be calculated from the formula 
T 8 = TWWO 


where d is the yarn denier to be used, T! the twist (tpi) 
for twines based on 190-den 'Ulstron' and T t the twist 

to be used in the twine to be constructed. Balanced 
twines are produced exactly as for other materials by 

cable twist (tpi) = ply twist (tpi) 


no. of plies 

although, because any unbalance in 'Ulstron' twines 
tends to decay, there is more latitude for error than in 
producing balanced Terylene' or nylon twines. 

Plaited or braided cords may be produced on conven- 
tional machinery into either hard extensible braids or 
soft inextensible braids. Ends/spindle, spindles/braid 
and picks per inch can be almost identical for 'Ulstron' 
braids based on 190-den yarn to those employed for 
210-den nylon braids with equivalent numbers of ends. 

Comparative properties 

All the advantages which 'Ulstron' yarn offers over com- 
petitive yarns are retained when twine properties are 
compared. 'Ulstron' yarns are produced in multiples of 
190 den and so, when compared with 210-den nylon 
twines of equivalent number of plies, they have approxi- 
mately 10 per cent runnage advantage. Table III shows 
that 'Ulstron' twines have, in addition, a small wet 
strength advantage over their nylon equivalents. 

TABLE III Comparative wet knot strength of 'Ulstron' and 
nylon twines 

Wet Knot 
Bid (Ib) 



No. of Plies 

190-den ier 'Ulstron' 

210-denier Nylon 6,6 




*Ulstron f 

Wet Knot 


Bid (Ib) 











N.B. (1) Runnage advantage to 'Ulstron 1 - 210190+9$% 

(2) Twines constructed at comparable twist hardnesses. 


Similar comparisons and conclusions can be made 
about the strength and weight advantages of 4 Ulstron' 
in netting form. It can be seen from Table IV below that 
'Ulstron' nets offer an advantage in both runnage and 
mesh strength over equivalent nylon nets over a wide 
range of mesh sizes and constructions. Nylon is used for 
a basis of comparison because of its former pre-eminence 
in the industry. 

TABLE IV Physical properties of gillnets 

Net Construction (1) Runnage (2) 

Ply Mesh size 'Ulstron' Nylon 


2 2-5 165-5 154 

2 2-6 172 156 

2 3-1 184 160 

2 4-5 433 411 

3 4-5 292 268 
9 6 84-4 75-5 

15 5-5 44 39 

18 4-1 26 23 

Wet mesh strength (Ib) 
* Ulstron' Nylon 

















1 I ) Number of plies of 1 90-denier 'Ulstron' or 2 1 0-denier nylon. 

(2) The runnage is expressed as the number of square yards of 
net per Ib, the meshes being fully open and squarely aligned. 

Net making and finishing 

'Ulstron' nets may be constructed on any of the netting 
looms in common use such as the Zang, Dandy, Amita 
or Porlester. Table V and Fig. 2 show that such nets 
show very little knot inversion in fishing and less ten- 
dency for knot opening in transportation from loom to 
stretching frame than equivalent Terylene' and nylon 
nets subjected to identical stretching and stabilising 
treatments. Knot slippage may nevertheless occur unless 
adequate precautions are taken. 

TABLE V Load in kg to invert double knots tied under a tension 
of 625 gms 

Nylon 6 x 210 1-73 

Early 'Ulstron' 6 x 190 1-85 

Current 'Ulstron' 6 x 190 2-30 

Current 'Ulstron' 6 x 190, relaxed in boiling \\ater for 15 

minutes 2-7 

Current 'Ulstron' 6 x 190, stretched under a load of 1 

gpd at 1 1 0C for 5 minutes 3 1 



Fig. 2. Relation between slippage and knotting tension. 

Loom tensions in netmakmg should be as high as is 
consistent with smooth running, and maximum levels 
of 1*0 g/den should be aimed for. Subsequent stretching 
should be at a tension of at least 1*0 g/den if carried out 
cold, but preferably the net should be subjected to a 
temperature of 100/1 10C for 15 min whilst held to 
fixed dimensions on a stretching frame. * Ulstron' nets 
should never be subjected to temperatures exceeding 
1 10C If these precautions are followed, the knot stabi- 
lity will be found satisfactory without using twine or 
net bonding agents. Good knot stability can also be 
obtained by heat relaxing the net at 100/1 10C for 15 min 
after stretching, but only at the expense of some 8 to 10 
per cent shrinkage compared with six to eight per cent for 
nylon. Satisfactorily stable knots are obtained in 'Uls- 
tron' nets made from braided cords even when no heat 
treatment is used and at deniers in excess of 20,000 there 
is no need to stretch the nets. 

Where facilities for heat treatment of 'Ulstron' nets 
made from fine twines are not available, it is necessary 
either to employ considerably higher net stretching 
tensions than usual or to apply a suitable knot bonding 
agent. Such agents may also be applied where the fisher- 
men require a high degree of stiffness, as for wing trawls. 
A number of bonding agents have been examined for 
their effect on knot inversion. Not all the agents exam- 
ined produced an improvement. In general, bonding 
of knots produced an improved stability whilst bonding of 
the twine gave a lowered stability. Among those agents 
which produce a noticeable increase in stability are 
'Bitumen PE4', 'VinamuF N6515, N6530, 'Cuprinol' and 
'Marstein'. In most cases, the solution should be diluted 
to ensure the correct pick-up which will vary with the 
dilution, the size of twine in the netting, and whether 
the net is mechanically dried or not but should be as low 
as possible compatible with the required improvement 
in knot stability and stiffening of the twine. The 'Bitumen 
PE4' treatment which is often used for bottom trawls, 
besides stiffening the twine, protects it from the abrasive 
wear of the sea bed and reduces the uptake of sand and 

'Ulstron' yarn may be obtained in natural colour, 
melt dyed green, or in special cases melt dyed black. 
If other colours are required, the nets can be dyed with 
one of the following range of disperse dyestuffs : 
'Supracet' Yellow 2G 'Cibacet' Blue OF 

'Resolin' Yellow 5R 'Dispersol' Yellow PP 

'Serisol' Fast Pink RGL 'Dispersol' Red PP 
'Foron' Brilliant Violet BL 'Duranol' Blue PP. 
All these give adequate depth of shade and rub fastness. 
These dyestuffs are applied at 85 to 100C and full 
depth of shade may be obtained in some 30 min. Unless 
the net can be held to length in dyeing, a net shrinkage of 
up to nine per cent must be allowed for. 

Mounting requirements 

'Ulstron' nets are themselves lighter than water and 
therefore need adequate weighting. The mounting 
requirements vary according to the type of net used: 


in mounting gillnets which have already a weighted 
sinkerline, it is merely necessary to increase the weighting 
to compensate for the lower weight of 'Ulstron'. When 
netting which weighs 1 Ib in air is immersed in water, the 
actual weight in water is D- 1 where D is the specific 


gravity of the fibre. Hence the weight of 1 Ib (air weight) 
of various nets immersed in water for long enough to 
displace all entrapped air is: 
1 Ib (air weight) of cotton netting 

1 Ib (air weight) of nylon netting 

0'33 Ib negative 

0*12 Ib negative 

1 Ib (air weight) of 'Ulstron' netting (HO Ib positive 


Thus in changing from cotton to 'Ulstron' netting an 
extra 0*43 Ib of weighting should be added (if necessary) 
to the sinkerline for every 1 Ib (air weight) of cotton 
netting previously used. 

In mounting trawlnets the aim is often to secure 
maximum headline/footrope separation, which depends 
principally on the design of the net. In changing to 
4 Ulstron' from the denser fibres, therefore, no great 
increase in headline/footrope separation should be 
expected but instead it should be possible to reduce the 
number of floats without impairing the shape of the net 
in fishing. The use of fewer floats will reduce the drag 
of the net, which may lead to valuable savings in power. 

Fishing performmnce 4>ottoro trawls 

The most important property of a bottom trawl is its 
fishing efficiency, which depends upon many factors, 
some of which favour the use of 'Ulstron'. Among the 
favourable factors is the lower drag which can be obtained 
by reducing the number of floats on the headline whilst 
keeping the same trawl mouth opening. One unfavour- 
able factor is the smooth surface and small knot size of 
'Ulstron' multifilament trawl meshes, which may permit 
more of the smaller fish to escape, but this applies equally 
to other synthetic fibres. 

Since catches are highly dependent upon the efficiency 
of the skipper, the season, the weather, and the state of 
the sea bed, it is difficult to make a meaningful compari- 
son of fishing power. All that could be said after a trial, 
in which four trawling companies fished 'Ulstron 1 
trawls for a total of 12 trips, was that there was no signi- 
ficant difference between their catching power and of the 
manila trawls previously used. Thus although the 
'Ulstron' trawls caught an average of 82- 1 kits/net/day 
against 69-7 kits/net/day for equivalent-sized boats 
fishing the same grounds over the same periods, most of 
the difference could be explained by the superior fishing 
ability of the boats used in the trial; before the 'Ulstron' 
nets were supplied, the boats concerned in the trial 
caught on average 12 per cent more fish/net/day than 
the mean for all the boats fishing at the same periods. 

Although no clear-cut answer emerges on fishing 








'8 so ,80 


Fig. 3. Price / life relationship from trials on fishing efficiency. 



efficiency from the trials, the price/life relationship is 
very much clearer (Fig. 3). In coarse ground fishing the 
average life of braided 'Ulstron' trawls (if not lost) is 
some five weeks 9 fishing. Making allowances for acci- 
dental losses by snagging on submerged objects and 
similar mishaps, the average life is reduced to some two to 
three weeks. The corresponding average life for manila 
trawls, again taking accidental loss into account, is some 
1 to 1 J weeks, or rather less than one full trip. Since 
'Ulstron' Granton trawls in braided multifilament cost 
in the region of 160 to 210 compared with some 
130 for manila trawls based on equivalent diameter 
cords, the 'Ulstron* trawls have an advantage of about 
SO per cent over manila trawls on a price/life basis. 
Similar considerations apply in fine ground fishing 
where the rates of accidental loss and severe damage are 
much lower. Fig. 3 shows that trawls in more costly 
materials than * Ulstron' would be uncompetitive in 
the market, even if they had a higher resistance to normal 
sea-bed abrasion, simply because of the chance of com- 
plete loss. 

Measurements have shown that 'Ulstron' trawls 
constructed from braided cords show no mesh-size 
changes greater than three per cent in practical fishing 
up to three trips. 'Ulstron' trawls braided from twisted 
twines show a small initial mesh-size increase associated 
with knot tightening. After fishing for three trips, the 
meshes decrease in size owing to penetration of sand 
and other particles into the cords; but this shrinkage is 
less with 'Ulstron' than it is with other trawl materials 
which absorb moisture and so suffer from direct yarn 
shrinkage as well as sand penetration. Accordingly, it 
is possible to braid 'Ulstron' trawls to their desired 
final mesh size more accurately than for other materials. 
Closely associated with the relative constancy of mesh 
size in an 'Ulstron' trawl, and stemming from its knot 
stability and its low moisture uptake, is the fact that the 
International Meshsize Regulations governing minimum 
mesh size are less likely to be infringed by 'Ulstron' 
trawls which are very light. A full bottom trawl weighs 
about 250 Ib compared with 450 Ib for the equivalent 
net in manila. They are easy to handle and because of 
their low moisture uptake behave well in freezing condi- 

Eighty per cent of Norway's large trawlers now use 
'Ulstron' nets and they are being increasingly adopted 
by nearly all the U.K. trawler companies. The Silver 
Cod Trophy, awarded annually by the British Trawlers 
Federation to the skipper landing the greatest volume of 
fish, was won in 1962 by Somerset Maugham which was 
largely using 'Ulstron' netting. 

MUwater trawls 

Midwater trawls are generally treated with a stiffening 
agent to ensure that the nets retain their hydrodynamic 
shape. The benefit of using a netting material sufficiently 
light to permit a reduction in floats is greater with mid- 
water trawls than for bottom trawls and the American 
report on the cruise of the U.S.S. Delaware suggests 


that fuel bills for towing 'Ulstron' midwater trawls may 
be somewhat less than for heavier materials. 

Results so far available show a wide variation in 
reported catching power between boats, with some skip- 
pers unenthusiastic whilst their neighbours are reporting 
much higher catches. Undoubtedly much depends upon 
intelligent mounting of the new material and upon the 
use of an adequately rigid bonding agent. Nevertheless, 
all the reports welcome the lightness and ease of handling 
'Ulstron' midwater trawls. 


As in trawlnets, fish-catching power is of critical impor- 
tance. In this case the relative fish-catching power of 
nets in two materials is somewhat easier to measure 
since the nets to be compared can be fished at the same 
time and under the same conditions. The comparisons 
have been chiefly with nylon, since the superiority of this 
fibre in fish-catching power over other netting materials 
such as cotton was already well established before 
'Ulstron' was introduced. 

Fig. 4 shows measurements of the fish-catching power 
of a range of different types of 'Ulstron' and nylon nets. 
In some cases the differences can be simply explained. 
Thus, the salmon leader nets did not incorporate weights 
on the "non-return valves" leading to the net bags so 
that on the ebb tide the 'Ulstron' valves, formed of netting 
panels, closed too rapidly and prevented the entry of 

Fig. 4. Fish-catching power of " Ulstron* nets. 

The other differences are less easy to explain. It was 
emphasised in the section on mounting that where an 
'Ulstron' net is set on to a shorter support rope it will bag 
upwards, whereas a nylon net would bag downwards. 
In these circumstances the 'Ulstron' net would catch more 
of the fish swimming near the surface and this may explain 
the superiority of the 'Ulstron' salmon gillncts. A further 
reason for genuine differences in catching power stems 
from the difference in extensibility of the two materials. 
'Ulstron' meshes are less extensible at low loads than 
are nylon meshes so that they will release fewer fish 
whose girth is only slightly greater than the mesh size. 
Conversely, fewer fish of girth appreciably greater than the 
mesh size will be able to penetrate the 'Ulstron' meshes 

than could become entrapped in a nylon mesh. To some 
extent, therefore, the relative fish-catching power of 
'Ulstron'and nylon nets will depend upon the size of the 
fish being caught in relation to the mesh size. The differ- 
ence in extensibility may be the explanation of two 
features reported by a number of fishermen: namely, the 
ease of removing the fish by hand from the net and the 
reduced damage to fish compared with nylon. 

The most extensively documented trial so far carried 
out on 'Ulstron' gillnets was a trial with cod gillnets 
in the Lofoten Islands. (Fig.4). The result on fish-catching 
power showed that in this particular trial, in which six 
boats fished a total of 30 nets for 42 days, there was no 
statistically significant difference in fish-catching power 
between the 'Ulstron' and nylon nets. The trial was 
interesting in that it showed that the average loss in 
strength of the nets after one year's fishing was 16 per 
cent for the 'Ulstron' nets and 24 per cent for the nylon 
controls. These results are again so close as to be statis- 
tically inseparable but they serve to show that the 
strength losses in fishing from all causes are no greatei 
than those found with nylon netting. 

Some outstanding results were reported from Nyasa- 
land where a single 'Ulstron' net caught nearly as much 
as 14 nylon nets of similar construction being fished 
alongside. Catches during three fishing periods were: 
Single 'Ulstron' net 14 Nylon nets 
20 dozen 52\ dozen 

25 J dozen 50 dozen 

104| dozen 82 dozen 

150 dozen 184^ dozen 13 dozen per 

single nylon 


In another trial, supervised by one of the East African 
Fisheries Officers, similar 'Ulstron' and nylon nets were 
fished for 36 days on small shark. During this period 
4 Ulstron' caught 416 Ib and nylon 319 Ib. At the end of 
this period, however, the nylon nets were no longer 
serviceable whereas the 'Ulstron' was still considered to 
have half its life remaining. 

It is well known that wide variations can occur in fishing 
results but it has been reasonable to deduce from these 
and other trials that the performance of 'Ulstron' is at 
least as good or better than that of nylon and contrary 
to some expectations the slightly greater diameter of an 
'Ulstron' gillnet twine, compared with equivalent ply 
nylon, does not have any detrimental effect on catching 

Other nets 

'Ulstron' has been found very satisfactory for the produc- 
tion of Danish Seine nets and a large number of seiners, 
particularly from Grimsby, have now changed to it. 
Twisted twine has generally been used in runnages from 
460 m/kg to 180 m/kg although there has been some 
usage of braided twine, particularly for the codend. 

'Ulstron' has made some progress in the wing trawl 
market against considerable competition from polythene 

monofil, which has been established for some time. The 
greater strength, lower price and ease of mending of 
'Ulstron' are, however, finding favour with many fisher- 

An interesting recent development has been the use of 
'Ulstron' in ringnets, a small type of purse seine used 
largely in Scotland for herring. In some instances only 
part of the net has been made from 'Ulstron 9 , the balance 
being in cotton or nylon. A number of 100 per cent 
'Ulstron' ringnets have, however, been produced and 
fished successfully. In spite of the buoyancy of 'Ulstron' 
no difficulty has been reported in getting the nets to sink 
properly and it is therefore likely that, with proper 
rigging and weighting, it will be possible to produce 
purse seines in 'Ulstron' where the reduction in weight 
will give lower prices and easier handling. 

Other uses for 'Ulstron' include trap netting, lobster 
pots, anglers' keep netting, lines, snoods and dyed 
mending twine. Trial quantities of knitted, knotless 
netting have also been produced. Further trials are in 
progress evaluating 'Ulstron' monofilament yarn and 
twines in various types of net. 


The second largest use for 'Ulstron' is in the production 
of ropes, many of which are used for fishing purposes 
particularly in Norway. 

'Ulstron' rope is in the same strength bracket as nylon 
and polyester rope and considerably stronger than poly- 
thene rope but has considerably greater runnage than 
nylon and polyester as shown below (for 1 in circ) : 

Ib per 

4 Ulstron' .. 16-0 
Nylon . . 21-5 
Terylene' .. 24-75 

B/load(lb) Cost per pence/ton/ 

2,100 Ib 
2,240 Ib 
2,100 Ib 

37s. 6d. 
48s. 9d. 
58s. 8d. 

7 50 

* Typical U.K. price 

The 'Ulstron' rope has been largely used as mounting 
ropes for cod, herring and other types of gillnets; in 
some cases the netting itself has been 'Ulstron' but in 
many cases 'Ulstron' rope has been used with nylon or 
other types of netting. The reports on the ropes have 
been very satisfactory. 

One particularly interesting development in the rope 
field has been the introduction of ropes made partly or 
wholly from 40-in length 5,000 diameter polypropylene 
staple fibre. This long staple 'Ulstron' has been specially 
developed by I.C.I, and a leading U.K. ropemaker. It is 
processed on standard hard fibre machinery into either 
100 per cent spun 'Ulstron' rope, which is as strong as 
'Ulstron' multifil rope, or into 35/65 or 50/50 polypro- 
pylene/sisal blended ropes which are cheaper but not 
quite so strong and to which the trade name 'Systron' 
has been given. These ropes are cheaper than other 
synthetic ropes and have performed well in trials as 
trawl quarter ropes and for other uses. 

Twines can only be produced from this new staple fibre 
at runnages less than 900 yds/lb, so its use in the fishing 
industry is restricted to ropes and heavy trawl twines. 

continued on page 64 

Synthetic Fibre Fishing Nets and Ropes Made 
in Japan 

The estimated production of synthetic fibre for fishing nets and 
ropes in Japan is estimated at 45,000,000 Ib in 1 962, of which about 
32,000,000 Ib was used for making fishing nets, the latter having 
increased from 21 ,000,000 Ib in 1 959. On the other hand, the produc- 
tion of natural fibre fishing nets (cotton, hemp, etc.) declined from 
some 28,000,000 Ib in 1949 (when synthetic fibre net manufacture 
started) to only about 3,000,000 Ib in 1962. About 85 per cent of 
Japanese fishing nets are now made of synthetic fibres. Initially 
nylon, vinylon, polyvinylidene chloride and polyvinyl chloride 
fibres were mainly used but after 1959 polyethylene and poly- 
ester were added, and polypropylene in 1962. Exports of syn- 
thetic fibre fishing nets have increased from 400,000 Ib in 1955 to 
about 70,000,000 Ib in 1961, shipped to more than 100 nations. 
In 1961 nylon and vinylon each accounted for 41 per cent of syn- 
thetic fibre nets, polyethylene six per cent, polyvinylidene five per 
cent, polyvinyl chloride two per cent, polyester one per cent and 
blended fibres four per cent. In lines and ropes vinylon accounts 
for 53 per cent and polyethylene for 23 per cent. Almost the entire 
output of ropes of blended fibres is exported. 

Filets et cordes fabriques en fibres synthetiqties 

Au Japon, la production de filets et cordes en fibres synthdtiques 
est estimee pour 1962, a 20.000 tonnes, dont 15.000 pour la 
fabrication de filets, celle-ci ayant augmente de 9.500 tonnes 
riepuis 1959. D'autre part, la production de filets en fibres 
naturelles (coton, chanvre, etc.) a diminue de 13.000 tonnes en 1949 
(dbut de las fabrication des filets en fibres synthdtiques) a 1.330 
tonnes en 1962. Aujourd'hui environ 85 pour cent des filets japonais 
sont fails en fibres synthdtiques. Initialement, les fibres de nylon, 
vinylon, polyvinylidene chloride et polyvinyl chloride etaient 
surtout utilises mais, depuis 1959 on a introduit le polyethylene et 
le polyester puis, en 1962, le polypropylene. Les exportations de 
filets en fibres synthdtiques vers plus de 100 pays, sont passees de 
180 tonnes en 1955 a 32.000 tonnes en 1961. En 1961, 41 pour cent 
des filets en fibres synthdtiques etaient fabriquds en nylon et vinylon, 
six pour cent en polyethylene, cinq pour cent en polyvinylidene 
deux pour cent en polyvinyl chloride, un pour cent en polyester et 
quatre pour cent en fibres mixtes. Quant aux filins et cordes, 53 
pour cent dtaicnt faits de vinylon et 23 pour cent de polyethylene. 
Presque toutes les cordes en fibres mixtes sont exportdes. 


Japan Chemical 

Fibres Association 

Redes y cabos de pesca de fibras sinteticas fabricados en Japon 


La producci6n japonesa de fibras sintdticas para cabos y redes de 
pesca se estima en 45 millones de libras en 1962, de las que 32 
millones de emplearon para fabricar redes. En 1959 se emplearon 
con el mismo objeto 21 millones de libras. For otro lado, la pro- 
duccion de redes de pesca de fibras naturales (algoddn, caflamo, 
etc.) disminuy6 de unos 28 millones de libras en 1949 (afto en que 
comenzo la fabricaci6n de fibras sinteticas), a unos tres millones de 
libras en 1962. Actualmente, el 85 por ciento de las redes de pesca 
japonesas se hacen de fibras sintdticas. Inicialmente, las que mas 
se usaben eran de nylon, vinylon, cloruro de polivinilideno y 
cloruro de polinivinilo, pero a partir de 1959 empczaron a usarse 
polietilenos y poliesteres y desde 1962 polipropileno. La exportacibn 
de redes de pesca de fibras sintdticas ha aumentado desde 400.000 
libras en 1955 a unos 70 millones de libras en 1961, que se han 
enviado a mas de 100 naciones. En 1961 se fabricaron de nylon y 
vinilon el 41 por ciento de las redes de fibra& sintdticas, de polietileno 
el seis por ciento, de polivinilideno el cinco por ciento, de cloruro de 
polivinilo dos por ciento, de poliesteres el una por ciento y de fibras 
mixtas el cuatro por ciento. En cabos y cuerdas el vinilon representa 
el 53 por ciento y el polietileno el 23 por ciento. Se exporta casi toda 
la produccidn de cabos de fibras mixtas. 

THE production of synthetic fibre fishing nets and ropes 
increased annually since its commencement in* 1949, 
and 10 years later, in 1959, the output totalled some 
25,000,000 Ib of which 21,000,000 Ib were used for 
manufacturing fishing nets. The production is estimated 
to increase further in 1962 to some 45,000,000 Ib of 
which 32,000,0001 bare expected to be used for fishing nets. 
The 1 949 output of natural fibre fishing nets, lines and 
ropes cotton, hemp, and silk products totalled some 
216,000,000 Ib of which 28,000,000 Ib were used in 

continued from page 63 

For trawls the material has certain obvious advantages 
such as hairiness and relatively large knot size so that 
more fish may be retained at a given mesh size than in 
equivalent continuous multifilament 'Ulstron' trawls. 


Nylon, with its outstanding advantage of high strength, 
was introduced to the fishing industry just after World 
War II. Polyester with its better resistance to stretching, 
was added to the fishermen's resources some five years 
later and polythene followed at a similar interval having 
the advantage of being lighter than both. 

With the introduction of 'Ulstron' polypropylene, 
the fishing industry now has a synthetic fibre which to a 
large measure combines the outstanding properties of all 
these three groups which have formerly been used to meet 
the exacting requirements of the fisherman. 


Appendix: Trademarks 

Ulstron', Terylene', 'Disperse!' and 'Duranol' are registered 
trademarks, the property of Imperial Chemical Industries Limited. 

'Supracet' is a registered trademark, the property of L. B. 
Holliday and Co. Ltd., Huddersfield, England. 

'Foron' is a registered trademark, the property of Sandoz AG. 
Lichtstrasse 35, Basle, Switzerland. 

'Rcsolin* is a registered trademark, the property of Farben- 
fabriken Bayer Aktiengesellschaft, Leverkusen, Federal Republic 
of Germany. 

'SerisoF is a registered trademark, the property of Yorkshire 
Dyewares and Chemical Co., Kirkstall Road, Leeds, 3, England. 

'Cibacet' is a registered trademark, the property of Ciba AG. 
Klybeckstrasse 141, Basle, Switzerland. 

'Systran' is a registered trademark, the property of Wrights 
Ropes Ltd., Universe Works, Birmingham 9, England. 

'Vinamul' is a registered trademark, the property of Vinyl Pro- 
ducts Ltd., Carshalton, Sumy, England. 

'CuprinoF is a registered trademark, the property of Cuprinol 
Ltd., 7 Upper Belgrave Street, London, S.W.I., England. 

fishing nets. The output dropped to 94,000,000 Ib in 
1962, only 3,000,000 Ib were used for fishing net manu- 

At present nearly 85 per cent of the total fisheries need 
is met by synthetic fibre nets. 

In early days, synthetic fibre nets and ropes were 
made principally of nylon, vinylon, polyvinylidene 
chloride and polyvinyl chloride fibres. In 1959, however, 
polyethylene and polyester fibres were added and, in 
1962, polypropylene fibre joined the ranks of materials 
for manufacturing fishing nets. The use of combinations 
of fibres began in 1955. 

While the demand for fishing nets is increasing, prin- 
cipally for export, demands for ropes and the replace- 
ment of manila ropes with those of synthetic fibres have 
also risen steadily. 

In 1955, exports of synthetic fibre fishing nets totalled 
400,000 Ib but by 1961 the amount jumped to some 
70,000,000 Ib which were shipped to more than 100 


Fluctuations in production of synthetic fibre fishing 
nets and ropes are shown in Table I. 

TABLE I Production of synthetic fibre nets and ropes 

In t,000lh 



Polyvinyl Chloride. . 
Twisted blended . . 

(Reference) . . 
Natural Fibre 

















































_ . 










































A review for 1961 reveals that, for fishing nets, nylon 
and vinylon each accounted for 41 per cent; polyethylene 
fibre for six per cent; polyvinylidene fibre for five per 
cent; polyvinyl chloride fibre for two per cent; polyester 
fibre for one per cent; and blended fibres accounted for 
four per cent. It is evident that the majority of fishing 
nets is made of nylon and vinylon. In ropes and lines 
(longlines and those for land use included), vinylon 
accounts for 53 per cent; polyethylene fibre for 23 per 
cent; nylon for 12 per cent; polyvinyl chloride fibre tor 
six per cent; polyester fibre for four per cent; polyvinyli- 

dene fibre for two per cent, showing the importance of 
vinylon, nylon and polyethylene in this field. 

Combinations of fibres are comprised principally 
of nylon and vinylon, nylon and polyvinylidene, and 
nylon and polyvinyl chloride fibres. Almost the entire 
output of ropes of blended fibres is exported. 

Production of synthetic fibre fishing nets is expected to 
expand steadily in future, as in the past, but even greater 
increase of rope production is expected. The output of 
natural fibre ropes in 1962 has shown a decrease of some 
7,000,000 Ib from the preceding year, but the synthetic 
fibre rope output has increased some 1,000,000 Ib, thus 
showing a progress in replacing manila ropes with syn- 
thetic fibre ropes. 

The principal uses of fishing nets made of different 
synthetic fibres are shown in Table II. 

TABLE II Principal uses of synthetic fibre 


Nylon: Gillnet, round haul net, longlines, various ropes 
Vinylon: Round haul net, setnet, gillnet, longlines, trawlnet and 

other small dragnets, various ropes 
Polyvinylidene Chloride: Setnet, trawlnet and other small dragnets, 

various ropes 
Polyvinyl Chloride: Setnet, trawlnet and other small dragnets, 

various ropes 

Polyester: Longlines, various ropes 
Polyethylene: Various ropes, gillnet, trawlnet and other small 

Polypropylene: Gillnet, various ropes. 


Expansion of export of synthetic fibre products is 
notable see Table 111. Nylon accounted for 60 per 
cent of the total, while vinylon accounted for 21 per 
cent and blended fibres for 12 per cent. These products 
are shipped to more than 100 nations, with South-east 
Asia, Africa and North America as the principal markets. 
Others importing Japanese fishing nets and ropes in 
large quantities include Thailand, Pakistan, Malaya, 
the United States, Canada and Iceland. 

TABLE 111 Exports of synthetic fibre nets and ropes 
In 1,000 Ib 



Polyvinylidene Chloride 
Polyvinyl Chloride . . 
Polyester Fibres 
Polyethylene Fibres . . 
Twisted blended yarn . . 

New synthetic fibres 

New synthetic fibres used for fishing nets include 
polyethylene and polyester fibres, which were adopted in 
1959, and polypropylene fibres first used in 1962. Among 
the three types of fibres, polyethylene is used in greatest 
quantity, while polyester and polypropylene fibres are 
still in an experimental stage. Polyethylene and polypro- 
pylene fibres are characterised by high tenacity and low 
specific gravity, while polyester fibre is outstanding for its 
high resistance against wear and tear. 

continued on page 66 






Production and Characteristics of Synthetic Nets 
and Ropes in Japan 


The paper reviews the production and consumption of synthetic 
materials in Japan and tries to clarify some of the technological 
problems regarding their rational use. The total amount of webbing 
and ropes presently used in Japan is about 87,000 tons. Webbings: 
27,000 tons of which 90 per cent synthetic. The conversion has been 
more complete for netting than for ropes as only about 19 per cent 
of the ropes hi use are of synthetic fibres. Summing up, the paper 
gives the characteristics of the various synthetic materials, supported 
by numerous tables. The main requirements for various fishing gears 
are then discussed. For fixed trap nets twines are required which have 
a high initial weight in water and smooth surface texture to give 
a minimum resistance to the current ; tarred polyvinyl alcohol is 
sometimes preferred. For purse seines which also need a high 
sinking speed, twines of at least two g/den loop strength are required, 
and tarred nylon is mainly used. For the straight gilling types of 
gillnets, such as sardine nets, the Japanese fishermen prefer poly- 
vinyl alcohol yarn. For other gillnets the polyamide and polyester 
groups are generally used. For bottom trawls nylon and vinylon 
normally have been used but polyethylene and polypropylene are 
gaining ground. However, due to the cost of synthetics, the extensive 
wear and occasional loss of such nets, the Japanese fishermen often 
use natural fibres when towing on a rough bottom especially for 
the underparts of trawls. One of the reasons why man! la rope is 
largely preferred by the fishermen is its low elongation (12 to 
20 per cent) as compared with 25 to 50 per cent of synthetic fibre 
ropes. Recent production of ropes made of mixed materials 
and subsequent treatment with resin has improved the handling 
of polyethylene ropes but further improvements to rope-making 
techniques are required before synthetic material ropes may 
be fully exploited in fisheries. Monofilament gillnets are said 
to have a 1*2 to 2-3 times higher catch ratio than other types of 
twine; nevertheless, these nets present difficulties in bulkiness and 
knot-fastness, and further research and development is required. 

Production et caracteristkjues des cordes et filets synthftiques au 

La communication traite de la production et de la consommation de 
materiaiix synthetiques au Japon et essaie d'6claircir certains 
problemes technologiques concernant leur utilisation rationclle. 
Sur 87,000 tonnes de filets et cordes utilises au Japon, environ 90 
pour cent sont de matdriaux synthetiques. La conversion a et 
plus complete pour les filets que pour les cordes dont 19 pour cent 
seulement sont en fibres synthetiques. La communication contient 
eaalemcnt de nombreux tableaux dormant les caracteristiques des 
diffcrcnts materiaux synthetiques utilises ainsi que les principals 
conditions requises par divers engins de peche. Pour les trappes 
fixes, les fits doivent fitre lourds et avoir une surface lisse afin 
d'offrir le minimum de resistance au courant; le polyvinyl alcohol 
goudronne* etait pr^ferd jusqu'ici. Pour les sennes coulissantes, une 
grande vitesse de coulee et des fils ayant au moins deux gr/d de 
resistance en mailles sont n&essaires et le nylon goudronne est le 
plus utilised Pour les filets maillants tels que les sardinicres, les 
pecheurs Japonais preferent les fils de polyvinyl alcohol; pour les 
autres filets, ils emploient gdneralement le polyamide et le polyester. 
Pour les chaluts de fond, le nylon a et6 normalement utilise mais 
le polyethylene et le polypropylene gagnent du terrain. Cependant, 


Yoshinori Shimozaki 

Tokai Regional Fisheries Research 
Laboratory, Tokyo 

etant donn6 le prix 6leve des fibres synthetiques, Fusure excessive 
et les pertes occasionnelles de ces filets, les pgcheurs Japonais 
utilisent souvent des fibres naturelles, particulierement pour le 
dessus des chaluts. 11s preferent aussi les cordes de manille dont 
reiongation (12 a 20 pour cent) est tres faible compared a celle 
des cordes synthetiques (25 a 50 pour cent). Une recente production 
de cordes faites de materiaux mixtes puis traitees avec de la restne, a 
ameiiore la manipulation des cordes de polyethylene, mais d'autres 
ameliorations dans les techniques de fabrication des cordes seront 
necessaires avant que la peche puisse exploiter compietement les 
cordes faites de fibres synthetiques. Les filets maillants en mono- 
filaments sont senses avoir un coefficient de capture superieur de 
1-2 a 2-3 a celui des autres fils, mais ils sont trop volumineux, 
leurs noeuds manquent de stability et leur developpement requiert 
de plus amples recherches. 

Produccion y caracteristicas de redes y cables sinttticos fabricados 
en el Japdn 


Resefia el autor la producci6n y consumo de materiales sinteticos 
en el Japon y aclara algunos de los problemas tecno!6gicos que 
plantea su uso racional. Actualmente en el pais se emplean unas 
87,000 tons de paftos y cables, el 90 por ciento de los cuales es de 
materiales sinteticos. La transformaci6n ha sido mas completa en 
el caso de las redes que en el de los cables pues solamente el 19 
por ciento de los que se emplean actualmente es de fibras artificiales. 
Menciona el autor las caracteristicas de varios materiales sinteticos 
que las da en numerosas tablas. Discute las necesidades principles 
de los di versos equipos de pesca. Para las trampas fijas se necesitan 
hilos de mucho peso inicial en el agua y de superficies suaves para 
reducir la resistencia a la corriente; hasta ahora se ha mostrado 
predilecci6n por el alcohol de polivinilo alquitranado. En el caso 
de las redes de cerco, que se tienen que hundir rapidarnente, se 
necesitan hilos de, por lo menos, dos g/den y se usa principalmente 
nylon alquitranado. Para las redes que son principalmente agall- 
eras, como los sardinales, los Pescadores Japoneses prefieren hilos de 
alcohol de polivinilo; para otras redes de enmalle se usan general- 
mente poliamidos y poliesteres. Para los artes de arrastre de fondo 
se hap usado normalmente redes de nylon, pero las de polietilcno 
y polipropileno ganan terreno. Sin embargo, dcbido al costo de las 
redes sinteticas, su gran desgaste y perdida ocasional, los Pescadores 
Japoneses emplean con frecuencia fibras naturales, especialmente 
en las secciones inferiores de los artes de arrastre. Una de las razones 

continued from page 65 

Nearly 60 per cent of polyethylene output is used for 
manufacturing ropes, including floatline, buoyropes, 
seine net ropes, anchor ropes and guyropes. For ship 
use, towlines, hawsers, guyropes, lifelines and flaglines 
are made of these fibres. Fishing nets made of these 
fibres include trawlncts and other small drag-nets, fixed 
nets and round haul nets. Polyester fibre is used predomi- 


nantly for manufacturing trawlnet ropes and longlines. 
Polypropylene fibre manufacturing was limited last 
year to trial products, and is now being used in crab 
gillnets, round haul nets and ropes. Improvements in its 
weathering resistance, dyeability and abrasion resistance 
are likely to make it popular for manufacturing various 
types of fishing nets. 

de que muestrcn tanta preddecci6n por el abaca de Manila es su 
poco estiramicnto (de 12 a 20 por cicnto) con respccto al de las 
fibras sintetteas (de 25 a 50 por ciento). La fabricaci6n reciente de 
cables de matenales mixtos y tratamiento posterior con resina 
ha mejorado la manipulaci6n de los de polietileno, pero se tcndran 
que introducir otras mejoras en las tecnicas de fabricaci6n antes 
de que los cables sintdticos puedan aprovecharse completamente 
en la pesca. Se afirma que los artes de enmalle de monofilamentos 
pescan de 1-2 a 2-3 veces mis que las de otras hilos, a pesar de lo 
cual su adopcion no es universal porque son voluminosas y estan 
expuestas a que se corran los nudos, defectos que sc tendiin que 
corregir mcdiante ulteriores investigaciones y perfeccionamientos. 

THE total amount of netting and rope presently 
used in Japan is about 87,000 tons, an increase of 
15,000 tons since 1940. 

Of this tonnage an increasing volume is made from 
synthetic fibres as is shown in the range of tables on 
various aspects given in this article. 

The rapid increase in local consumption is due to the 
expansion of high-sea fisheries. The local utilisation of 
synthetic fibre for fishing ropes is much less than for 
fishing nets. Out of an estimated 57,000 tons of rope in 
service in 1962, 10,688 tons (only 19 per cent) consisted 
of synthetic rope including the polyvinyl alcohol fibre 
used for longline and seine warps. The much slower 
conversion is due mainly to the low cost of manila which 
is about one-quarter the price of synthetic fibre. 

Much care has been taken in assessing the best 
material for each particular fishery. 

Fixed nets. Knotless nets and netting made of con- 
tinuous heavy filament yarn are recommended. Resistance 
to sunlight should be high. Netting twines of polyvinyli- 
dene chloride and vinyl chloride groups have been 
mainly used. Polyvinyl alcohol has poor weather resis- 
tance and a low specific gravity but, to compensate, 
coal tar is often applied to those parts of the net receiving 
the main load or to the whole net when set in an area 

where the sea is generally rough. Nylon, though strong 
enough, is not very satisfactory as regards specific 
gravity and sunlight resistance and is furthermore too 

The polyethylene group has low specific gravity (0*96) 
but is strong and not costly, and in black-coloured 
twines is adopted to make the upper parts of set nets. 
The black-coloured fibre stands up well to sunlight. 

Purse seines. Optimal physical properties of webbing 
twines for purse seines : over2*0g/denin wet loopstrength, 
20 per cent in elongation, as well as high specific gravity. 
Knotless or weaver's knot webbings of nylon, vinylon and 
Tetoron' yarns are used in greater amounts for purse 
seines than for set nets. For the landing part of purse 
seines, however, fishermen prefer nylon or 'Tetoron' to 
vinylon even if they use vinylon for most of the other 

Sinking speed is important in purse seines and the 
amount of lead used for a nylon net is 30 to 40 per cent 
more than for a cotton net. The use of polyester group 
for purse seines is developing though these materials 
are rather costly. Tarring of the nylon and vinylon 
groups has increased the weight and sinking velocity of 
large-sized purse seines for skipjack, tunas, and mackerel, 
but this old technique cannot increase the specific gravity 
of a net. 

Gillnets require invisibility in water, fineness and 
pliability, elasticity and abrasive resistance. Polyamide 
yarns are preferred for gillnets but some fishermen have 
turned to polyester yarn for large types of gillnets. 
Polyvinyl alcohol yarn, less fine, is mainly used for 

Trawlnets. Even when nylon, polyethylene or polypro- 
pylene is used for other parts of a trawlnet, cottoiTor 

Table I Cost ratio of fibre material to other materials used in fishing gear (1957) 

tind of foar 

flbro iiatorial 

Othor Mtorialo 

Ooot ratio 

(ra * i) 


Xton Aaount Ooot 
(Motrio ton) (Ton) 

Itoa Aaount Ooot 
(luabor or (Ton) 
Motrio ton) 

rixd not for 

lottinf, 19*6 5t 660, 000 
Ropooi/ 27.8 3t 520, 000 
Othor fibro 
wtoriolo 10.0 1,580,000 

Bamboo 2,163 
01*10 float* 441 300,000 
Straw bag 17,000 
Xgrootuff 480,000 


ftro-boat trawl 
(T5.35 ton, 25 hp) 

Vottinf 0.191 185,000 
Ropoo 2.428 268,000 

Olaoo f lotto 100 10,000 
Trawl board 2 50,000 
Othoro 40,000 


Two-boat puroo 
oino for tuna 
and okipjaok 

Wotting 8.894 12,800,000 
Bop** 3.04 
Mtorlalo 1.0 1,400,000 

Loado 1.6 1,490,000 
float* 0.5 420,000 

Puroo Bin** 0.08 120,000 


I/ Wire rep* ie inoluAeA in the fibre wrterll ae thie Mjr be replaoed by fibre Material in future. 


vinylon webbings arc used for under parts of the net be- 
cause of their low cost. Fishermen have found it practical 
through experience not to use costly net materials, which 
are often lost when working rough grounds and this 
explains why cotton nets are still used side by side with 
polyamide, polyvinyl alcohol and polyethylene material. 

Table II gives the following percentages of synthetic fibres used in 
various fishing gears by 1962: 

fixed nets large 95, medium 85, small 70. 

seine nets purse seines 100, nuikiri ami (lampara type) 80, 

beach seine 70, others 90. 
gillnets for salmon 100, crab 95, sardine 90, herring 90, 

others 95. 

dipnet for suary 75, others 80. 
trawl other 100, two-boat 100, medium-sized 50, small-sized 

45, others 45. 
longline oceanic tuna 100, coastal tuna 95, others 70. 

Longlines. Comparative experiments conducted in 
regard to the tensile strength, elongation, and potential 
energy left in disused longlines of cotton and synthetic 
lines, indicate that vinylon line tested unknotted 
showed better characteristics than cotton line, whereas 

similar sizes of both lines when tested for loop strength 
did not always give comparable results in regard to the 
strength and potential energy. This implies that the loop 
strength is probably the best criterion for safe operation 
of longlines. 

In the near future ropes mixed with polyethylene or 
polypropylene are likely to be preferred for the mainline 
of the longline gear, with improvement of the operation 
system, though the vinylon group has until now sur- 
passed the other groups of synthetic fibres for the con- 
struction of tuna mainlines. 

For the branchlines, polyester lines are preferred 
because of their high catch efficiency and their favourable 
characteristics of specific gravity, tenacity, and fineness. 

Ropes. Cost of synthetic fibre ropes, over three times 
higher than manila rope, is one of the drawbacks. 
Another defect of synthetic fibre ropes in general is 
excessive elongation. In contrast to 12 to 20 per cent of 
manila ropes, synthetic fibre ropes are susceptible to as 
much as 25 to 50 per cent. This can make the ropes 
inefficient and sometimes even dangerous. 



Sub Total 

Table III Fishing Gear synthetic fibres produced and exported 

Output in metric tons 
1955 1957 1961 1962 1955 

Export in metric tons 
1957 1961 

Polyvinyl alcohol 
Polyvinylidene chloric] 
Polyvinyl chloride 
Polyethylene . . 











495 ! 
2,466 4,1 
315 t 














Polyvinyl alcohol 
Polyvinylidene chloric! 
Polyvinyl chloride 
Polyethylene .. 
Polypropylene. . 






Sub Total 



























"~~ ~ 








Polyvinyl alcohol 
Polyvinylidene chlorid 
Polyvinyl chloride 
Polyethylene .. 














Sub Total .. .. 293 








TOTAL .. .. 4,515 








* Ropes made of mixed yarns of polyamide and polyvinylidcne chloride or of polyvinyl chloride and polyamide. 
t Includes materials for longline fisheries throughout. 


(Metric tons) 
Fixed net 


Vinylidcne chloride 
Vinyl chloride . . 



Table IV Local consumption of synthetic fibres for fishing gear 
I9 *l 1962 (Metric tons) 

Trawls and other* 

732 651 

490 343 

89 2 




Purse seine 

Vinylidene chloride 
Vinyl chloride 










Nylon . . . 
Vinylon . . . 



Vinyl chloride 







Nylon . . 
Vinylon .. 
Vinylidene chlon 
Vinyl chloride 






















Nylon ... 525 


Vinylon . . 



Vinylidene chlorid 




Vinyl chloride . 









Polypropylene . 




Sub-total 4,271 


Total of netting and ropes . . 12,785 


All nets include twinejjsed for constructing or mending. 
Table V Tensile strength and elongation of various kinds of twine for comparable diameters 

Kind of twin* fibre 





Straight (wst) Trawlar Knot (wst) 

Psnsils strsngth 


Irsaking Strsngth 



(Kg) (g/d) 

(Kg) (g/d) 

Polys thy Isns A Continuous 

400d/F 9x3 










Polys thy Isns B Continuous 

400d/F 9x3 










Polys thy Isns C Continuous 

350d/F 10x3 










By Ion 6 Continuous 

210d/15F 20x3 










Polys star Continuous 

2?0d 20x3 










Polyvinyl ohlorids 


300d/60F 20x3 










Polyvinylidsns ohlorids 


360d/F 18x3 










Polyvinyl alcohol 


20s 15x3 











20's 16x3 

















Polysthylsns A, B, and C diffsr in basic natarial as wsll as in aanufaoturing proosssss, aooording to 

ThS s amp Is spsoimsns wsrs givsn a medium twist and tsstsd undsr ths following conditions t 
room tanpsraturas 20 to 23C, Isngth of apsoinsm 60 om, pulling vslooityt 0.22 oa psr ssoond 

Table VI Number of major fishing gears by kind and size in Japan, 

Kind of gear Small- Medium- Large- 

Fixed net. . 
Purse seine 
Minor drag nets 
Gillnet . . 

Small-type nets are 













in fixed nets, those operated by under 10 

fishermen, and in other gears, those operated with non-powered 
boats or less than 10 GT. 

Medium-type nets are, in fixed nets, those manned by 10 to 50 
men, in other gears, those operated with boats from 10 to 50 GT. 

Vinylon ropes are used greatly for warplines of Danish 
seine type of trawls seems because of their durability 
which is three to four times greater than that of manila 
ropes. Recently, a vinylon rope combined with wire 
came into existence for the warpline. Fishermen who 
employed this combination rope reported that the new 
warpline enabled them to operate more efficiently. 

Polyethylene ropes are particularly suitable for 
the mainline of bottom longlines for cod and cod- 
like fish and for floatlines of salmon gillnets. Such 
ropes are very often twisted together with spun 


Table 711 Some properties of tuna longline 

VMM ConatruotiQH of linti ftiMMttr 

9nil *trnfta 
frf) (w.t) 


( <**: 

Main * 

OMMM 80*8 50 x 3 x 3 *00 



feinUm BnrnofcliM 

0mm** 20*8 J5 x 3 * 3 6.10 




tfttwft Io.1l(lOOd/> x 10 x 11 + 20*8 11 z t)*J 




Mftn**n JTo.12(lOOd/r x 10 x 12 * 20*8 12 l)x3 




foU*tqrlvM 300d/F x 9 x 33 6.09 




Vjlen 210 d/1?F * 36 x 33 4.55 




fetoron 250 d/241 1 * 35 * 33 4*50 




M*n**n Voe 9 
(100 d/7 x 10 x 9 4 20 f 8 9 x 1) x 3 5*3 




Ihipxp Ho. 10 
(100 d/f x 10 x 10 + 20*8 10 x 1) 5.7 




im oonstruotvd with iOOd of Tiny Ion onofilMMnt* whoro vary 10 yarn* ! ooToxod with Tingrlon 
pan yarn. Tttoron ia a ooMtroial naM of polyaatar produoad in Japan. 

Table VIII Result of creep test and abrasion teat of twinee 


Relation between time (hour) and elongation (urn) when 2 fi/d of load is Frictions against Frictions 
given until creep occurs in twine steel ed^reJJ against oil 
stone 2J 

Beginning 0.5 1 6 25 50 46 102 152 318 600 (hours. 

load 10 (kg) 

load 5 (kg) 

load 0.5 (kg) 

Polyethylene A 

10.8 16.6 17.8 22.6 23*6 Creeping 








11 B 

9.2 11.4 13.2 16.6 18.2 20.6 Creeping 








11 C 

12.0 15.4 16.8 18.2 18.6 19.2 24,0 Creeping 









20.0 21.7 21.7 21.7 21.7 21.7 22.3 22.3 Cree- 
3.0 3-3 4-0 4.0 4.3 4-7 Creeping 

7.0 7-3 7-3 7-3 7-3 Creeping 




} broken 
within 1,000 
tines of 
friction Jj 

JJ Angle of the steel edge is 90 . _2J fear strength against oil stone is represented by the ratio between the tensile 
strength after abrasing twine 5tOOO tines and the original strength. 3 1 No comparable data available for these materials 
as they were broken before abrasing 1,000 tines. 

yarns of either 'Teviron' or vinylon in order to prevent 
slippage that may occur in handling them. The mixing 
rate of Teviron' or vinylon with polyethylene twines is 
from 10 to 30 per cent, depending upon the producers' 
or fishermen's requirements. Laboratory experiments 
indicated that the abrasion coefficient of polyethylene 
twines against oil stone was 0*18 to 0*20 as compared 
with 0*3 to 0*34 for spun yarn twines of Teviron' and 
vinylon, the former being more slippery than any other 
kind of synthetic twine. 

Twisting together with other materials and subsequent 

treatments with resin has much improved the handling 
of polyethylene ropes. 

Rope-processing techniques will require further im- 
provement before synthetic fibre material ropes can be 
adopted for fishing rope to a greater extent. 

Table IX gives abrasion coefficients for continuous filament netting 
twines by oil stone friction test. 

PolyethyteneO-20, 'Amilan* (all nylon), 025 'Tevi-Ny' (partly nylon) 
0-27, 'Kyokmin' (partly nylon) 0-28, 'Tcviron' 0*28, 'Saran' 0-30, 
'Kurehalon' 0-31. 
Spun yarn: 'Kremona' 0-24, cotton 0*37 manila 0-41. 

Japanese Fish Netting of Synthetic Fibres 


The total production of fishing nets in Japan for 1961 was approxi- 
mately 9,000 tons; half of this was nylon. About 5,500 tons were for 
domestic use while about 3,500 tons were exported. Research on 
polypropylene fishing twines was recently undertaken and in 1962 
several firms started industrial production. Tests on the polypro- 
pylene fibres produced in Japan showed that it is stronger than 
polyamide 6 for the same denier and has a higher degree of tough- 
ness. It is, however, stiffer and more bulky which, at present, seems 
a disadvantage in its use as gillnet material. However, development 
continues and indications are that this fibre should produce equal 
if not superior quality fishing twines. The heat treatment used on 
synthetic yarns is aimed at reducing elasticity and increasing 
strength; such treatment applied to twines is aimed at stabilising 
the twist, while the heat treatment of netting is directed toward 
heat-setting the knots. Most heat-treatment methods can be classi- 
fied under either the dry or the wet systems, although some methods 
use both. Several methods have been evolved using electricity, 
chemical baths and even high-frequency. The latest innovation is 
based on heat radiation and makes use of ultra-red lamps. The 
boiling water treatment of the early days has long since been super- 
seded by the use of steam or other heated vapours. Liquid treatment, 
however, has the advantage that it avoids all risk of overheating 
as the temperature of the bath can be fully controlled. A major 
point in all heat treatment is that the shrinkage which accompanies 
all such treatments must be within the range of the specific elonga- 
tion of the material. 

Filets de pfcche Japonais, en fibres synthftiques 


La production totalc de filets au Japon pour 1961 etait approxima- 
tivement de 9.000 tonnes dont la moitte gtaient de nylon. Environ 
5.500 tonnes gtaient destinies a ('utilisation domestique et 3.500 
tonnes, a ('exportation. Des 6tudes surles fils de polypropylene ont 
6te entreprises recemrnent, et en 1962, plusieurs firmes en commen- 
Caient la production industrielle. Les epreuves des propri6tes du 
polypropylene produit au Japon ont montr que sa resistance a la 
rupture etait plus grande que celle du polyamide 6 pour un mdme 
denier et que le polypropylene 6tait aussi plus tenace. Le polypro- 
pylene par contre est plus raide et plus encombrant ce qui, a 
present, constitue un desavantagc pour son emploi comme matdriau 
de filets mai Hants. Cependant le developpement continue et il 
semble que cette fibre pourra produire des filsdequalitesuperieure. 
Le but du traitement par la chaleur utilise pour les fils synthetiques 
est de reduire l'lasticit et accroitre la resistance a la rupture. Un 
meme traitement applique aux fils retordus permettra de stabiliser 
le retordage tandis que ce traitement applique aux filets fixera 
les noeuds. Les diflferentes m&hodes de traitement peuvent 
6tre classees dans deux groupes: le systeme "sec" ou le systeme 
"mouilte" bien que certaines methodes utilisent les deux 
systdmes. Plusieurs m&hodes utilisent I'electricitS, les bains 
chimiques et meme du courant a haute frequence. La derniere 
innovation est basee sur 1'irradiation de la chaleur a partir de lampes 
a rayons ultra-rouges. L'eau bouillante des premiers temps a 6t6 
remplacee par la vapeur chauffee. 

Redes de pesca Japonesas de fibras sinteticas 


En Jap6n en 1961 se fabricaron cerca de 9.000 toneladas de redes 
de pesca; la mitad eran de nylon. Unas 5.500 tons las absorbi6 el 
mercado interne y unas 3.500 tons se exportaron. Recientemente se 
inici6 la investigaci6n de los nibs de polipropileno para redes de 
pesca y en 1962 varias empresas iniciaron la producci6n industnal. 
Los ensayos de las fibras de polipropileno fabricadas en Japon 
demuestran que a igualdad de denier son mas robustas y duraderas 
que las de poliamido 6, pero tambi&i son ma> rigidas y voluminosas, 
lo que actualmente parecc ser un inconveniente para su empleo en 
los artes de enmalle. Continuan los estudios y las indications son 
que con esta fibra se obtendran hilos iguales o superiores. El trata- 
miento tdrmico reduce la elasticidad e incrementa la robustez de las 
hilazas sinteticas : cstabiliza la torsidn de los hilos y fija los nudos de 
las redes. Casi todos los tratamientos termicos pueden clasificane 
como secos o hiimedos, aunque en algunos cases se emptean ambos. 
Se recurre tambitn a la electricadad, baftos quimicos e incluso la 


Iwao Tani 

Japan Synthetic Fibre Net and 
Rope Association 

aha frecuencia. La mas reciente innovacidn se basa en la termo- 
radiaci6n de las lamparas de rayos ultra-rojos. Los baftos en agua 
hirviendo de los primeros dias han sido sustitufdos por el empleo de 
vapores de agua u otros supercalentados. El tratamiento liquido tiene 
la ventaja de que evita todo peligro de sobrccalentamiento porqu la 
temperature del bafto puede regularse exactamente. Un factor de la 
mayor importancia en todos los tratamientos termicos es que el 
lincogimiento que los acompafta tiene que quedar dentro do los 
emites del estiramiento especifico del material. 

Q YNTHETIC fibres used in the manufacture of fishing 
>3 nets in Japan are: 

Group Type Some brand names 

Polyamide Nylon 6 * Amilan* and 'Grilon' 

Polyvinyl alcohol Vinylon 'Manryo', 'KuralonVKrcmona' 


Polyvinylidene Vinylidene 'Saran', 'Kurehalon' 
Polyvinyl chloride Teviron*, 'Envilon' 

Polyester Tetoron' 

Polyethylene *Hi-Zex', 4 Pylen-E', 'Ethylon' 

Polypropylene 'Pylene' 

These fishing twines are each made up of one-fibre 
material. Twines made up of a combination of yarns or 
fibres of different fibre materials are also produced, 
mostly with nylon as the basic material, although twines 
are available with vinylon as the basic material with 
other fibres twisted in. 

Total production of fishing nets in Japan for 1961 was 
approximately 9,000 tons; half being nylon. About 
3,500 tons were exported, the balance being for domestic 

The numbering systems presently used in Japan are 
not officially established and are for business dealings 

The quality standards of Japanese twines and nets for 
use within the country are covered by the Japanese 
Industrial Standards (J1S) which has been set up by the 
Ministry of International Trade and Industry, These 
standards comprise the following: 

JIS L 1033-58 Testing method for vinylon spun plied 

yarn for fishing net 
JIS L 1034-58 Testing method for filament nylon 

plied yarn for fishing net 
JIS L 1035-58 Testing method for filament vinyli- 

dene chloride yarn and filament 

vinylchloride plied yarn for fishing 



JIS L 1043-58 Testing method for synthetic fibre for 
fishing net 

The JIS further comprises inspection standards for the 
use of net manufacturers as quality norms for their 
product intended for export. 


Polypropylene fibres industrial production only started in 
1962. There is therefore very little experience of it as a 
netting twine. 

It is presently produced by two different techniques- 
that introduced by Montecatini of Italy, and by the 
Abisun Corporation of U.S.A. Eight firms have started 
manufacture with a total production capacity of 38 tons 
per day. 

The technique used in spinning, dyeing and general 
manufacture of polypropylene filament differs from one 
manufacturer to another. 

TABLE I Average characteristics of Japanese polypropylene 

Polypropylene fila- Polypropy- 
ment I80d (20-24 f) lene spun 
No. 20S 

Specific gravity 
Tenacity (g/den) .. 

Extensibility, straight ( / ). . 
Loop strength (g/den) 

Decrease of strength caused 

by knotting (%).. 
Wet breaking strength 

(kg/mm 2 ) 

Moisture absorption ( %) . . 


6 -57 -7 
6 -57 -7 
4 -85 -3 
4 -8-- 5 -3 

4 -34 -5 
4 -3-^4 -5 
16-016 -5 
16 -016 -5 
3 -4-3 -6 
3 -4-3 -6 






The study and research on polypropylene filament and 
twines for netmaking is continuing and there are indica- 
tions that, with more experience, twines will be produced 
of equal if not superior qualities to other synthetic 
fibres. The fibre has unique features and characteristics 
which have already proved to a certain extent that it can 
be used to advantage for the manufacture of twines and 
ropes. There are, however, a few problems to be over- 

Heat treatment 

Heat treatment of synthetic netting twines varies accord- 
ing to the kind of fibre, twine count, type of knot and the 
end-use intended. Some synthetic fibres do not require any 

Heat treatment is applied for three main reasons: 
(a) Twist-setting of twine; (b) Setting of knots in netting; 
(c) Stabilising the shape of meshes. 

It is primarily aimed at reducing the extensibility and 
at increasing the strength of twines although, at the 
same time, it sets the twist in the twine. There are many 
methods for heat-setting and a variety of devices are in 
use. For certain fibres heat treatment is more difficult 


than for others and only heat treatment of nylon 
netting is considered here. 

There seems no theoretical or scientific principle for 
establishing an optimum heat treatment method. The 
techniques in use have been developed and improved by 
practical experience, while new methods and new devices 
are continually introduced. 

Early heat treatment processes were mainly intended to 
obtain an "ironing" effect, and usually consisted of 
heating and pressing at the same time. 

The first 'Amilan' netting twines (type 100) were heat 
treated by the "heated ironplate" system. The apparatus 
consists of a chamber with a heated ironplate on top. 
This heated chamber contains 'Anilene' or ethylene glycol, 
and the webbing is run through it. 'Amilan' softens at 
180"C, so the heat should be kept below this point. 

This treatment is expensive and can only be used for 
twines of 210d/6 and finer. 

To heat treat coarser twines than 210d/6, a new type of 
'Amilan', type 300, was developed. This type did not prove 
successful for gillnet twines, being too extensible. The 
next ' Amilan' yarn was of type 700 with a very low exten- 
sibility and it proved excellent for all types of gillnets. 

Heat-treatment methods are of two types dry and 
wet. Sometimes both methods are incorporated in one 
type of heat treatment, so that the so-called dry system 
also uses water steam. The dry system is mostly used by 
net manufacturers while the wet system is mainly applied 
to yarns and twines. 

The method and the construction of the necessary 
equipment vary according to production capacity, type 
of nets and nature of heat used. Various methods, based 
on heat conduction, use either electric regulators, 
'Anilene' baths, melamin-resin, metal baths or high- 

A different principle is heat radiation by, for example, 
the use of ultra-red lamps. Here the net is run over the 
radiating elements or alternatively the element is passed 
over the netting. In general this method has not proved 
very effective. 

One heat-treatment method used largely in early days 
was to run the nets through boiling water, but today's 
wet-treatment systems all make use of water steam or 
other vapours. 

Treatment with liquid, such as boiling water, has a 
big advantage in that it avoids the risk of over-heating. 
Nylon, which shrinks in boiling water, lends itself well to 
this treatment for setting knots. 

Twines usually require heat-setting to reduce their 
extensibility and increase the strength, whereas heat 
treatment is normally aimed at setting the knots in the 
nets; in both cases, however, the heat should be applied 
while the material is under tension. For nets, this is 
normally obtained by running them between friction 
rollers, which furthermore help to reduce irregularities 
in the braiding and at the same time pre-set the knots 
firmly. The tension to be applied should be chosen 

continued on page 73 

New Synthetic Herring Driftnets Used in the 
North Sea 


In driftnctting the technique and nets differ from area to area and 
even between the nationals operating them, due to the peculiar 
traditional habits of the fishermen themselves. Experiments carried 
out using a great variety of net constructions made of polyamidc 
('Perlon'. 'Steelon', 'Dederon'), polyyinyl alcohol CKuralorT) and 
cotton, showed that while polyamide is by far the strongest material, 
polyvinyl alcohol lends itself better for herring driftnetting due to 
less damage to the fish. Notwithstanding, it was observed that all 
synthetic materials must be stiffened to avoid damage to the herring 
while shaking out. Two years of subsequent commercial fishing 
with *Kuralon' nets have shown that while their catch efficiency is 
equivalent to that of cotton nets, they need only one-fifth as much 
replacement for loss and damage. By the end of 1962, more than 
2,000 'Kuralon' driftnets were in commercial use compared to 
8,000 cotton driftnets showing that Polish fishermen are now turning 
over to synthetic materials for the driftnet fishery. 

Nouveaux materiaux synthetique* utilises dans la peche au hareng en 
mer du nord 


La technique de la peche au filet mi i I lam d iff ere solon Ic lieu et 
merne la nationality des operateurs, difference due aux habitudes 
traditionnelles particulieres des pcheurs eux-memes. Des essais 
effectues avec des filets de constructions tres variees, faits de poly- 
amide rPerlon", "Steelon\ *Dederon'), de polyvinyl alcohol 
('Kuralon') et de coton, ont montrc que bien quc le polyamide soit 
le plus fort des materiaux le polyvinyl alcohol est meilleur pour 
fabriqucr des filets maillants pour le hareng parcc qu'il endommage 
moins les poissons. Cenendant on a pu observer que tous les 
materiaux synthetiqucs doivent etre durcis pour qu'ils n'abiment 
pas les harengs lorsqu'on secoue les filets. Deux annces de peche 


Janusz Zaucha 

Sea Fisheries Institute, Poland 

commercial avec des filets maillants faits en 'Kuralon* ont montre 
que leur efficaeite de capture est equivalente a celle des filets en 
coton et ne doivent fitre remplaces pour perte ou dt6rioration que 
dans la proportion cTun cinquieme. A la fin de 1962, plus de 2000 
filets maillants en 'Kuralon' etaient employes commercialement 
com re 8000 en coton, ce qui demontre que les pecheurs polynesions 
commsncent nuintenant a utiliscr les mat6riaux synth6tiques pour 
la peche du hareng au filet maillant. 

Nuevos materiales sintetlcos para las redes de deriva arenqueras 
usadas en el mar del norte 


En la pesca a la deriva, la tcnica ya las rejes varian de un pais a 
otro incluso dentro del mismo pais dd*!j a las peculiar! dades 
tradicionales de los Pescadores. Los expjrim^ntos realizados con 
muchas redes de poliamidos ('Perlon, 'Steelon', *Dederon*), 
alcohol de polivinilo (' Kuralon') y algoddn, demos traron que 

continued from page 72 

according to type of fibre and size of material, mesh size 
and volume of netting. 

Very often, a resin product is applied to set the knots 

Treatment of * Amilan 9 nets 

For heat-setting of salmon gillnetting, this procedure is 
general : 

(a) As the net comes from the braiding machine, it is 
passed through a resin solution or polyvinyl 
alcohol and is wound on a roller. During winding 
the netting is pressed to prevent loosening of the 

(b) The netting is then passed in a stretching machine 
to tighten the knots. While under stress, the netting 
passes through a vacuum pipe which enhances the 
heat conductivity of the material. 

(c) After stretching, the webbing is usually inspected 
and, if necessary, mended. 

(d) A steel bar is inserted in the meshes at both ends 
of the section immersed in the boiling water, and 
force is applied to the bars in accordance with the 
amount of stretch required. 

The maximum extensibility of 'Amilan' netting is 1 7 to 
20 per cent and netting of 21 Od/ 12 should be stretched 

while under treatment 13 to 14 per cent of its original 

Steam can be applied as heating agent but the material 
should not come into contact with it. The webbing is run 
through a steel-plate box while the steam passes through 
the double wall of this box and heats the air in the 
chamber, where the webbing is run. 

Another method is to run the webbing, in firm contact, 
over the heated surface of a hotplate. Tensionless heat 
treatment in resin emulsion at a temperature of 90 to 
100"C is still another method. 


Nets are normally dyed after drying, and dyeing should 
take place only after the net has been thoroughly washed 
and all chemicals used removed. One recent method 
applies the dye automatically, by spraying to the net while 
it passes through a dye-beck. Such dyes are complex in 
composition and are normally applied by a special pro- 
cess so that the dye permeates and reaches the filaments 
of individual twines. 

After dyeing, a resin treatment is sometimes applied ; 
either urea-resin or thermoplastic resin may be used; 
the latter enhances the effect of dyeing. After such treat- 
ment, the net is usually stretched and dried for about 
five minutes at a temperature of ISO to 160 C. 


aunque los poliamidos son, con mucho, los mis robustos, el 
alcohol de polivinilo cs m6s a proposito para la pesca a la deriva del 
arenquc porque peijudica menos al pescado. Tambien se observ6 
que a todos los matcriales sintcticos hay que darles rigidez para 
evitar que estropccn al arenquc cuando se sacuden. Dos aflos de 
pcsca industrial con redes de 'Kuralon' ban demostrado que su 
rendimknto de pcsca es equivatente al de las redes de algod6n, pero 
que las averias y perdidas se reducen a la quinta partc. Para fines 
de 1962, los Pescadores polacos empleaban 2.000 redes de deriva 
de 'Kuralon' y 8.000 de algodon, lo que demuestra que adoptan 
los materialcs sinteticos para dichas redes. 

T71SHING techniques and gear differ from area to 
r area even though they have been developed for 
catching the same species offish. This is partly due to the 
different types of vessels used by different nations but is 
also due to the peculiar traditional habits of the fishermen 
themselves. The same applies to the mechanisation and 
modernisation of boats and gear for different communi- 
ties, in spite of the fact that such communities operate 
the same areas for the same species and are in close 
contact with one another. 

Herring driftnetting, operated for ages by Dutch, Bri- 
tish, German and Polish fleets, are all carried out simul- 
taneously for the same species and one could expect 
that the gear would have developed one standard type. 
But this is not so and investigation shows the basic 
purpose is different. For most driftnets used it would 
seem the main purpose has been to construct strong and 
durable nets; with cotton nets numerous and costly 
preservation treatments are necessary to prolong working 
life and even with conversion to synthetic materials, this 
purpose seems to have been maintained. In other words, 
Dutch experiments with nylon herring driftnets seem 
to be entirely directed towards producing more durable 

The purpose behind the driftnets used by the Soviet 
herring fleets is vastly different as the nets are made of 
cheap and remarkably thin cotton yarn of only average 
quality, with all lines made of hemp. Although the nets 
are treated with a preservative they are designed to be 
discarded after one fishing season. This, of course, saves 
repeated preservative treatment and storage between 
seasons. The reason is probably plenty of cheap cotton. 

Since countries such as Poland do not have their 
own source of cheap natural materials, efforts to moder- 
nise the herring gillnets by replacing cotton materials 
with synthetics are necessary, so Polish experiments have 
aimed at determining the best type of synthetic material 
to use. 

The need for synthetic gillnets in Polish fisheries is 
also based on the practice of trawler-drifters to switch 
from gillnets to trawls and vice versa. In these circum- 
stances, cotton gillnets, even though well preserved, 
were frequently found to deteriorate after only a few 
days storage. The first small-scale experiments were 
initiated in 1956, and now over 2,000 synthetic gillnets 
are used in Polish fisheries. 

Experiments to date have resulted in the use of syn- 
thetic nets with sisal lines, Their superiority over cotton 
gillnets has been proved but it is still felt that gillnets 
made entirely of synthetics would be more desirable. 

Materials and methods 

Even though synthetics such as the polyamide or poly- 
vinyl alcohol fibres are well known, they are still not 
used in certain types of gear. The Material Research 
Section of the Laboratory of the Sea Fisheries Institute, 
Gydnia, Poland, selected two basic types of synthetic 
products: the polyamide group ('Steelon', 'Perlon', 'Ded- 
eron') and the polyvinyl alcohol group ('Kuralon') for 
experiments, aimed at developing synthetic driftnets for 
the drifter-trawler fleet. The technical data of these 
materials, as well as for the cotton (used as a control), 
is given in Table I which is linked with Table II. 

The coefficient of change in mesh size was calculated 
by the formula: 

w 100a 
Ww - _~ 

Where Ww is the coefficient of change in mesh size. 

a is the elongation of the mesh at break in 

b the barlength plus one knot (mesh size). 

The coefficient of the mesh stability in connection with 
the percentage of untied meshes during the mechanical 
analysis, gives an indication of the probable behaviour 
of each material during actual fishing operations. The 
coefficient of stability Wt of the mesh is evaluated by the 
formula : 

Wt = 100 I 
where Ws is the mean mesh breaking strength 

Wo is the mean strength of the meshes untied 

during the analysis 
R is the percentage of meshes untied during 

the analysis 
R is obtained from the dependence 

R - i+T. 

where I is the number of breakings in the analysis 
which served to evaluate the mean 
breaking strength 

I s is the number of untied meshes during the 

The yarn diameter was measured with a thickness 
gauge with accuracy of 0-01 mm with an initial charge of 
50 g. 

The investigations were carried out during two voyages, 
each lasting for about two months. 

Eighty experimental driftnets were made of these 
materials for the first voyage and 1 10 for the second. 

Experimental nets were constructed to the same design 
and specifications as those successfully used by all 
western nations fishing herring in the North Sea. The 
net sections consisted of a central panel of 780 meshes 
long and 340 meshes wide, with 10 mesh wide selvedges 
made of thicker yarn. The webbing of the central panel 
and the side selvedges were hung so that the horizontal 
strain was "with the knots" while the upper and lower 
selvedges were hung so that the vertical strain was "with 
the knots" (see Fig. la). 

In order to investigate if the arrangement of the netting 
and the selvedges have an influence on the practicable 
use, nets were made with the netting arranged in the 
different ways illustrated in Fig. 1. 



1 - 





Fig. 1 Mesh direction arrangement of gillnets. 

& normal arrangement 

b normal simplified 

c unstrengthened 

d simplified inverted 

e nonnal inverted 

The experiments were carried out on a typical com- 
mercial herring fishing vessel with a crew partly composed 
of scientists for recording the necessary data. Data 
recording during and after use of experimental nets 

(a) Breaking strength of netting after the fishing period. 

(b) Fishing yield for each type of gillnct. 

(c) Estimation of the amount and the degree of 
damage to the fish. 

(d) Estimation of the number of fish gilled. 

(e) Degree of deterioration of the gillnets after the 
fishing period. 

(f ) Estimation of each particular type of net material 
for commercial use. 

(g) Evaluation of work required of the crews for opera- 
ting the different kinds of materials. 

Experimental operations 

The two experimental voyages lasted approximately 
two months each. The first one took place during 
October-November 1959, and the second in August- 
September 1960. Due to bad weather in 1959, the number 
of observations, experiments and measurements carried 
out were fewer than in 1960. About 40 operations 
during 1960 gave catches totalling 77 tons of herring 
compared to about 20 tons in 1959. 

After each fishing operation samples of netting from 
all driftnets were analysed for mechanical strength and 
to define the eventual decrease in strength sec Table 11 
(linked with Table I). 

Polyamides have a relatively high strength, and the 
nettings of thinner yarns, e.g. 265 den/9 were nearly twice 
as strong as cotton netting; the thicker yarns of 265 den/ 
12 were 2-5 times stronger than standard cotton ones. 

The average strength of 'Kuralon' gillnets (omitting 
the badly chosen Nm 40/9) corresponds in general 
to the strength of high-quality cotton gillnets. 

As indicated in Tables 1 and II, polyamide material 
has higher breaking strength than 'Kuralon', whose 
strength is near to that of the very good cotton. 
The mesh size of the experimental nets in 1959 was not 
suited to the mean size of the fish caught and the nets 
made of different materials had various mesh sizes. 
The nets in 1960 were all of more or less equal mesh size. 

Ttfchnicol .jLt_ 

in k,; 

= of !i.rriii 

Table 1 
, drift net i u 

ieu uurin 

. the ttptruiOTltti 

?<K3f<I.JC(ll UDfOlfiC tiO 

Table II 
ntt of oat*} 

il.. uned uariru 1959-60 



hnixe I'enii change 

Leah of broken 
tenacity mebhes 

Breaking Low. in trtngth 
Mtrtngth through braiding 
in ke 

T e hnite in an 
(Ik; prt-ttnuion) 


MtshMist KnotfuttncN 

Mater itl 








Dry Wet 

l>ry -tt Dry 








210 d'9 








99. H - 11 

1S9 12.6 -12.6 








** i/ 

r -.-1 







99.9 99. 6 

15.3 15.9 -0.6 








trt d/'> 







41. t? 

9'>9 9^.7 9- 

15.1 12.5 








.'. d/9 


14. . 






'/i. 9 yy.9 

15.9 14.5 xl.y 








26S d/y 








99.. 9*^.6 11 

16.5 14.1 O.6 








265 d/9 







99.5 99-5 25 

15.4 14.0 -0.6 








265 d/9 


If,. 1 






99.6 99.0 1 

17.1 15. n -4.5 













265 d/12 








100 100 

18.2 16.4 -5.2 








265 d/12 








100 100 

22.2 20.7 -0.*9 








265 d/12 








100 100 

20.2 -1.0 








265 d/12 

2?. 2 







ion 100 

22.1 21.2 -0.5 








H 40/9 






/ ( n.O 


100 100 15 

5.1 4.6 -16.4 








Vm 54/15 








9% 5 99.8 5 

6.4 5.5 -ll.o 








54/1 5 








KX) 100 

8.0 6.6 








Km 40/15 








99.6 99.7 5 

7.8 7.1 -10.0 








H 40/15 








100 100 

9.6 6.5 -1.0 
















100 100 

7.9 6.2 -4.8 








fa 20/9 








100 100 

9.5 7.4 -6.0 








Mm 54/15 









100 100 
100 100 
100 100 

6.6 6.5 -2.9 
6.4 7.0 -19.0 
8.0 8.1 










The mesh size of polyamidc nets increased appreciably 
when wet, while the increase in the mesh size of 'Kura- 
lon' nets was much less. The coefficient of change in 
mesh size is characteristic for the particular type of 
material and increases very much as a result of the tar 
treatment. The meshes of the untreated polyamide 
nets became easily distorted, causing considerable 
change in their dimension. It should be stressed that all 
the synthetic nets tested are commercially adaptable 
except those appearing as items 1 and 12 in Tables I and II. 
Among examined materials, dyed 'Per Ion' must be 
rejected for commercial use owing to its lack of mesh 
stability, and 'Kuralon' Mm 40/9 also because of in- 
sufficient initial strength. 

Fishing efficiency 

The most important factor is, of course, fishing efficiency. 
It also plays an important part in experimental catches 
in the interpretation of the obtained indices. However, 
due to the small number of operations carried out, no 
final conclusions concerning commercial possibilities 
can be drawn at this time. 

The fishing results obtained with the different kinds 
of nets in both 1959 and 1960 are presented in Tables 
111 and IV, and, as can be seen, the fishing results of all 
net types were, on average, satisfactory. The relatively 
low catches of some of the polyamide nets during the 
first experiment were caused by the mesh size of the nets 
being too large for the average size of the herring. 

Table III Firftt experiment 

ng rosul'tu or 

of nct& in 

* twint 


ttaan catofar per Total fish 
Treatment net/day in ktf. per net in 

l.Perlon 263 d/9 




2.Perlon 265 d/9 




J.Dederon 265 <*/9 




A.Dederon 265 d/9 
5.St*elcm 265 <V9 




6.Steelon 265 d/9 




T.bteelon 265 d/9 




e.JCuralon Ha 40/9 




9.Cotton Nm 50/15 

oil coated 



lO.Cotton Jfin 54/15 




ll.Cotton Nm 54/15 

tarred oliro-oop. 



When analysing the catch data per net per day, the 
mean catches of the polyamide nets reached 37 kg, of 
the 'Kuralon' nets 31 kg, and the cotton nets 31 kg. 
While the fish catches of driftnets depend highly on the 
proper choice of the mesh size, it is also important to 
obtain proper correlation between the fish size, the kind 
and thickness of the material and the quality of the 
net treatment. 

The more soft the material, the higher its fish catching 
ability. This explains the high efficiency of untreated 
polyamide driftnets and the efficiency of thin 'Kuralon' 
driftnets. The thicker and stiffer the netting, the lower 
the efficiency. This can be seen from the tables but this 
conclusion is also based on additional observations 
mentioned later in the paper. 

Type and degree of fish damage 

The first experiments with polyamide driftnets in com- 
mercial lugger fisheries were failures. When Polish 


fishermen were given 'Steelon' nets of equal strength to 
the best cotton driftnets, they handed them back after 
only a few trials and reverted to the standard cotton 
nets. This emphasised the need for a detailed study of 
synthetic materials for driftnets as the future of the 
Polish lugger fisheries depend on developing better 
gear than the traditional cotton nets. The damage of 
the netting to the fish caused by the synthetic material, 
was partially responsible for this initial negative reaction 

Table IV - seoond experiment 

Pithing results of nets in I960. Codei S - Steelon, K Kurulon, C- 
Cotton, D - Dederon. 

& twint 

Kind of 

I.oah catch 
per net/day in k w % 

Total fieh per 
not in k ( > 

l.S 265 d/9 




2.D 265 d/9 




3.S 265 d/12 
4.3 265 d/12 
5.S 265 d/12 
6.S 265 d/12 

11 + 2/ : tar 
eaprolaotam Ify' 
oaprol. + 'tar 



7.K Nm 54/15 

not finished 



8.K Nm 54/15 




9. K Nn 40/15 

not finished 



10. K Nm 40/15 




11. K Km 20/9 




12. K NB 20/9 




13.C Nm 54/15 
U.C Nm 54/15 

])luc tar 




of the fishermen. In driftnets, the herring should be 
gilled strongly enough to prevent its falling out while 
the net is still in operation, but not so strongly that 
they are prevented from being easily shaken out during 
hauling. Too flexible a mesh causes damage to the fish 
which diminishes its value, and requires additional work 
from the crew. In this regard, cotton net gives a good 
performance and the tendency is to make nets of syn- 
thetic materials as much as possible similar to cotton 
nets. This can only be done by special physico-chemical 
treatment of the presently available synthetic material. 

During 1959 and 1960, trials were made to define the 
influence of particular materials on the quality of the 
fish captured. The index was decided by taking the aver- 
age percentage of damaged fish taken from each kind 
of gillnet. The results obtained are given in Table V and, 
as can be seen, there is a certain relation between the 
kind of material and the degree of damage to the fish. 
Polyamides, owing to their mechanical properties and 
especially their elasticity, are among the most gripping 
materials and consequently have the greatest percentage 
of damaged fish (more than 25 per cent). 

The use of thicker and stiffer materials diminishes the 
percentage of damaged fish (see items 5 and 8 in Table V). 
The same result is obtained when the nets are treated 
with a stiffening preparation (see items 3, 4, 6, 7 and 9 in 
Table V). The simultaneous increase of the twine dia- 
meter and application of physico-chemical treatment 
reduces the percentage of damaged fish by the polyamide 
nets to that of the cotton nets. 

This applies also to 'Kuralon' nets which, however, 
give a lower percentage of damaged fish in the untreated 
condition than polyamide. It suffices to select a proper 
thickness of 'Kuralon' twine to obtain even better 
average results than with cotton fabrics. 

for different kinds of driftnets. P - 

Porlon. D 

De dor on 

S . Steelon, K Kuralon. 

C - Cotton 


dtuartged fish 

material & 
Twine Number , 

diam am 

1. P 210 d/9 




2. P 265 d/9 




5. D 265 d/9 
4. T) 265 d/9 



27. B 

5. S 265 d/9 




(>. S 265 d/9 




7. r> 26 r . d/9 




8. S 265 d/12 



#- 14.7 

9. S 265 d/12 



+ tar 

" 7-6 

10. S 265 a/1? 



10/< 5.2 

11. 5 265 d/12 


it it 

+ tar 


12. K to 40/9 




15. K Kin 54/15 




14. h ton 54/1? 




15. K Urn 40/15 



16. K Nm 40/15 




It. K Nn 20/9 




18. K Urn 20/9 




19. C Nm 50/15 


oil coated 


?0. JJm' 54/15 


P*r 6,0 

2). C Jfoi 54/15 





When determining the coefficient of damage to fish 
by the netting, we should stress that results may vary 
from ship to ship. For example, highly qualified crews 
may cause less damage by skilful handling of the nets 
during shaking, than inexperienced crews. It is most 
difficult to assess the damage caused by inefficient 
manipulation of the nets during shaking as the effect 
is combined with that of the gripping qualities of the 
material as well as the mesh size in relation to the girth- 
size of the fish. However, some indication can be obtain- 
ed by the number of fish remaining gilled in nets of 
different materials after identical shaking. In Table VI 
the results obtained are reduced to a comparative scale 
with the chromate copper treated cotton nets as control. 
These indices again stress that the average performance 
of the driftnets depends on the accurate choice of the 
material and the mesh size. 

Wearing rate of gillnets after fishing 

After every voyage of comparable length to commercial 
voyages, a visual inspection of the gear was carried out 
by a group composed of skilful quality control inspectors. 
They assessed the necessity of repairs to the nets both 
as to quantity and quality, to restore it to full technical 
efficiency for the next voyage. 

An estimation of the wear was based on the following 
conventional numerical scale used in Poland: 

VI uantity of fiah rem'iininA gilled 


after shaking 



iintr^.atea or onJLy 

dyed.. 2.5 - 5 



1.5 - 2.5 


untr^otc-ci or only 

dyed. 1.2 - 1.8 





chroma te-coiipcr 



S single small holes in the net. 
10 numerous holes of several mesh sizes, single 

25 multidirectional tears of more than 10 meshes 

in length spread over the net, but not to the 

extent that replacement is necessary. 
75 tears and holes such that only parts of the 

netting can be saved to patch others. 

T I'crlon, 1) - Dederon. b - Steelon* K Kuralon. C Cotton 

Totorial & 
Trine Number 


Decree Uenarkv 

1. T 180 d/9 


25 after two exper- 

?. Jj 26 S d/9 

brown dyed 

10 imental voyngei. 

1. D 265 d/9 



4. S 265 d/9 


5 after two 

S y 2^5 d/9 

brown dyed 

10 voyojee* 

6. S 265 d/9 



7. !> 26^ d/12 

cuprolnotom 2> 


H. S 26 b d/12 

" * tar 

9. S i?63 d/12 

eaprolactaa 10 

f 5 

!). S 2t5 d/12 

" 4 tar 

11. K NCJ 40/9 

brown vlynd 


12. K NDJ 54/15 



13. K 2ha 54/15 



U. K I-Jm 40/15 



15. K Un 40/15 
16 K Win 20/9 

brown dyed 


17. K Nm 20/9 



18. C No 50/15 

oil coated 

10 old nets 

19. C Nm 54/15 


r 25 

20. C Nm 54/15 

ditto -t- tar 


oil or tar COP ted 

0.5 - 0.7 

As indicated in Table VII, the wear rate depends on 
the type of material and proper preservative physico- 
chemical treatment considerably reduces net wear. So far 
as the polyamide untreated nets are concerned, a low 
mesh stability resulted in significant mechanical wear 
despite their high initial strength. The treated polyamide 
nets are highly resistant to abrasion. Untreated 'Kura- 
lon' should not be used, since it has many disadvantages 
which can be overcome by proper treatment. 

The following general conclusions may be drawn from 
this part of the experiment: 

(a) Driftnets should be made of materials with a rela- 
tively high strength and sound mesh stability. 

(b) A careful selection of the twine is necessary to 
obtain proper relation between twine strength 
and the forces acting on it during the fishing opera- 

(c) A proper physico-chemical treatment of the net 
materials is of great importance as it reduces 
mechanical wear. 

Effect of mesh arrangement 

The driftnet known in Poland as the Dutch driftnet is a 
basic type commonly used in the North Sea. In the 
main netting the run of the meshes is with the knots in 
the lengthwise direction of the net, but across the knots 
for the top and bottom selvedges. 

In the opinion of many observer-fishermen, this arran- 
gement was best because of the reduced number of fish 
falling out during hauling, and also because the main 
forces during hauling, act "with the knots". It seemed 


advisable to try various arrangements and this was done 
as indicated in Table VIII. 

Tablt VIII ?;tth arrnn.*aentH of the driftnets used 
K normal* S giaBlifiedt St. S tee Ion. K. Kuralon. C Cotton 

twin* jise Twin* eiee Treat- No. 

of main netting of stlveds*9 Ijaterial nent of nets 

1.X oinrolifitd 

265 d?9 

265 d/9 


tarred 53.0 


2.K simplified 

265 d/12 

265 d/12 

' -t. 



3.N Bimplifi.d 

265 d/12 

265 d/12 


turrod 31.3 


4.N une lengthened 






5 N untren^thned 

Km 20/9 



tarred 20.6 


6,N unotrencthened 

Km 40/15 





7.H unatrengthened 

Nm 40/15 



tarred 34.25 


6.S inverted 

Nut 20/9 

Nn 20/12 




9.3 inverted 

Nm 20/9 

Nra 20/12 


tarred 34.3 


10. S inverted 

265 d/12 

265 d/12 


tarred 31.3 


11. N inverted 

Km 50/15 

Km 50/18 


tarred 27.9 


These gillneti were uaed simultaneously with normal driftnets, 

On the basis of these experiments, a simplified arrange- 
ment for thinner materials and a simple unstrengthened 
arrangement of the netting of greater mechanical strength 
has been chosen for industrial manufacture. 

Estimate of crew work 

The amount of crew work differed according to the 
material used. In general, work increased when polya- 
mide and 'Kuralon' driftnets were used, decreased when 
oil-coated cotton gillnets were used. 

The amount of work was estimated directly and 
indirectly by measuring the operation time for each 
type of driftnet, the fatigue rate of the crew and the 
eventual frequency of shifts during removal of the 

In general, the thin untreated polyamide nets, with 
yarn number Nm 50/9 and Td 265/9 or Nm 34/9 are very 
tiring, and in fact are rather unsuitable as a commercial 
fishing gear. 'Kuralon' is better to handle than polyamide, 
especially when coated with a tar substance. 

Based on the experiments, 'Kuralon' twines of Nm 
34/12 have been selected for use in commercial fisheries. 

tl experiments 

Commercial experiments were planned and carried out 
in 1961 and 1962. They consisted in equipping a com- 
mercial vessel with an experimental set composed of 
either 'Kuralon' or cotton and 'Kuralon' driftnets. The 
crew and the skipper fished this set according to typical 
commercial principles and prepared a report after each 

During the two years mentioned, 20 commercial 
voyages were made, during which it was concluded that 
the fishing effectiveness of the 'Kuralon 9 driftnets does 
not differ from that of standard cotton gillnets. The 
wear of 'Kuralon' nets was, on average, 10 times smaller 


compared with cotton. This may not hold true in future 
experiments as some of the cotton nets had been used 
whereas all the 'Kuralon' nets were new. The skipper, as 
well as the crew, indicated that the amount of work 
for the operation of the 'Kuralon' nets is similar to that 
required for the operation of cotton gillnets. 

On these experiments, Polish fishermen have gradually 
started to use 'Kuralon' nets made of Nm 34/12, and 
treated with petroleum tar (coating of 30 per cent.). 

By the end of 1962, more than 2,000 'Kuralon' driftnets 
were in use compared to 8,000 cotton driftnets. From the 
wear data for the year 1962, it transpires that the replace- 
ment for the 'Kuralon' driftnets (losses at sea and normal 
wear) averages nine per cent per year compared to approx- 
imately 50 per cent for cotton nets. 

Our further work will deal with the elaboration of 
driftnets made entirely of synthetic fibres. 

General conclusions 

1 . Of the analysed materials, polyamides as well as 
polyalcohol vinyl ('Kuralon') are suitable as material 
for herring gillnets. 

2. In spite of the many qualities of polyamide (great 
strength, fishability, gripping quality) 'Kuralon' has 
better overall utility for herring driftnets and costs less. 

3. The special treatment required for the polyamides 
and the necessity of a greater thickness of the twines for 
handling limit to some extent their use as herring gillnets. 

4. The simpler treatment of the 'Kuralon' and good 
fishing results indicated that this material is better suited 
for herring driftnets. 

5. The number of fish damaged during shaking 
depends on the quality of the material, the thick- 
ness of the twine and the degree of stiffening. 

6. The mesh arrangement in a gillnet has no direct 
influence on the fishing efficiency of the net. Based on 
observations, however, the best arrangement seems to be 
that normally used in the North Sea herring driftnet 

7. The commercial fishing experiments have confirmed 
the high qualities of the 'Kuralon' twines for driftnets. 

8. On the basis of two years of experiments, 'Kuralon* 
nets (with treated sisal lines) need only one-fifth as much 
replacement as cotton driftnets. Into consideration there 
was taken Polish type of fishing operations, in which 
vessels catch with driftnets through a part of the year and 
with trawls through another one. 

New net materials 

Mr. Pierre Lusyne (FAO) Rapporteur: At the first World 
Fishing Gear Congress in 1957 the characteristics and use of 
several synthetic fibre materials were much discussed because 
of their newness. Since then some have practically dis- 
appeared while others have firmly established themselves, 
such as nylon, polyester, polyethylene, polyvinyl alcohol and 
its various combinations. To choose appropriate material 

for a specific fishing gear is not simple and among the syn- 
thetics now used in fishing, many factors such as wet knot 
strength, elasticity, abrasion resistance, etc., must be carefully 

A new material is polypropylene which belongs to the 
polyolefin group and has many of the characteristics of 
polyethylene which was brand new six years ago, but is now 
well established as trawl twine material. The characteristics 
of polypropylene are fully described in Dr. Klust's paper 
after testing some 40 samples. Polypropylene has the lowest 
density of all materials used for twine manufacture and has 
the same strength in wet or dry condition. Based on breaking 
length it has the same strength as nylon, but will, for the same 
strength, be thicker due to its low density. Klust also 
showed that in the wet knotted condition polypropylene and 
nylon of comparable R tex value have the same breaking 

He found a remarkable difference in extensibility; poly- 
propylene stretched much less than nylon and the load 
extension increase was almost linear. This is very different 
from nylon which has a great extension at low load but with 
increasing load gives a constantly decreasing percentage of 
stretch. Polypropylene, on the other hand, has a high creep 
value when under continued load, so that the permanent 
elongation is higher than that of polyamide. This is an 
important factor as it may affect its suitability for net parts 
where heavy and consistent loads are applied, which then 
cause undesirable mesh changes. 

The paper by Carter and West on the properties of 'Ulstron' 
one of the first polypropylenes used (at least in the fishing 
twines), gives much the same figures as Klust in regard to its 
properties, but refers to other synthetic twines such as 
polyethylene, polyester, nylon, polyvinyl alcohol and some 
of the natural fibres and provides a very useful basis for 

Polypropylene softens at 160- 170 C and should therefore 
not be submitted to high temperatures during stretching and 
heat-setting, although 100 C (and therefore boiling water) 
is quite safe. 

The fact that some papers vary in details of the catching 
efficiency of polypropylene compared to other twines is no 
doubt due to the need for more experience and further adaption 
of gears. Klust quotes Carrothers in giving polypropylene 
a slightly higher catch efficiency than nylon both in gillnets 
and bottom trawls. Carter and West's results are approxi- 
mately the same. The Japanese, however, found poly- 
propylene gillnets to be rather bulky for handling and their 
catching efficiency slightly lower than that of nylon. Poly- 
ethylene, in the meantime, has been fully accepted and is 
now extensively used especially as a trawl net material. 

According to the Japan Chemical Fibres Association 
78,000 tons of synthetic nets and ropes are now in use in 
Japan compared to about 9,000 tons of natural fibre material, 
which shows their highly modern outlook. It is, however, 
remarkable that while the conversion to synthetics is 90 per 
cent for netting, about 80 per cent of the ropes used are still 
of natural fibres. 

Much headway has been made since the last Gear Congress 
in the construction of braided twines. On this point 
Bombeke reports tests which showed that the drag of braided 
nylon twines is lower than that of twisted twines; both showed 
one-half less drag than cotton at a speed of three knots. 
The actual testing was done in an air tunnel and the results 
recalculated to give the drag in water. 

Mohr reports on experiments carried out in Germany to 
use plastic material for creels and pots. While first efforts 

were not quite satisfactory, plastic traps were later made 
which caught as many eels as the usual rattan traps. Such 
plastic traps, though initially more expensive, had a much 
longer useful life. 

Dr. Katsuji Honda (Japan): As a result of continued effort in 
recent years, the strength and quality of the twine made from 
polypropylene had been considerably improved as these 
figures show: 

Multifilament Spun 
yarn yarn 
$.56.0 g/d 3-7 3 -8 g/d 
6-07-0 4-0-4-5 
7-58-0 5-56-5 

5*5 6-5 g/d 


In strength it has been made even stronger than nylon 
and in weathering there would be no further problem. As 
with other fibre materials all kinds of industrial uses can be 
expected of this material but experimentation was still 
required regarding its use as rope. It had been found that 
in strength it was stronger than, or almost as strong as, 
polyvinyl alcohol but not yet as strong as nylon. There are 
various advantages, however, because of its light specific 
gravity, its flexibility and abrasion resistance; and it is expected 
to be adequate for use as rope. For further improvement 
they were studying prices and economic conditions and also 
the question of durability. 

Mr. V. Valdez ( Peru) : There are now 1 ,200 boats operating 
in the Peruvian anchovetta industry. The great richness of 
plankton in that area caused cotton nets to rot quickly but 
synthetic nets do not rot and all the nets now used are of 

Mr. D. Roberts (UK): In trawling some years back, we 
were not too sure about synthetics but I am now 100 per cent 
convinced and we never want to buy anything but synthetics 
for trawling in future. 

Mr. J. Norman (UK): As a District Inspector of Fisheries 
my duties necessarily include enforcement of regulations. 
With the introduction of synthetics it has become difficult to 
distinguish between the four main groups and 1 would like to 
know if there are any simplified methods of identifying them. 

Dr. von Brandt: That problem has been much discussed. 
There are many fibre producers here. Can anyone give an 
answer for the main types of synthetic fibre especially those 
used in trawls ? 

Mr. Eric Kwei (Ghana): In using synthetic nets we found 
that in our tropical waters fish that were gilled died quickly 
and when they were taken out had already begun to putrefy. 
What is the possible remedy ? 

Mr. B. W. Crewdson (UK): The question about mesh sizes 
as interpreted by Government Authorities is a very vexatious 
one. Dealing with synthetics is very different from natural 
fibres. The material itself might not shrink but when put 
into a net form there is very pronounced shrinkage. It is our 
trade custom to measure mesh sizes between net knot centres, 
whereas government authorities measure the mesh opening 
(lumen). Braiding nets is not an exact science. If done by 
hand there is variation. If done by machinery it is a matter of 
relation. If you are A th of an inch below you are in the red 
with the Ministry and if above, with the Skipper. We are 
in a most difficult position. There should be some standard- 
isation which, at the moment, there is not. In America, nets 
are measured before use and they are sealed, but here the mesh 
is supposed to be measured after use and when wet. That is 
the question of mesh size in relation to synthetics and it is very 
serious. Can manufacturers tell us what proportion of 
shrinkage there is in the product they produce? 


Dr. von Brandt: This problem we have also discussed on 
the Permanent Commission. It is very difficult not only for 
synthetics. Specifications of some nets call for mixtures 
which differ from time to time. There is no real answer. 

Mr. W. Hewtead (UK): We can all sympathise with Mr. 
Crewdson, but yarn manufacturers are entitled to sympathy 
too. We know this problem and we are doing all we can to 
combat it. One constructive suggestion we made was that 
there should be one regulation for monofilaments, one for 
staple fibres, and one for multifilament yarn. 

Mr. Harper Gow: Can anyone give details of the United 
States method and say whether it is satisfactory to the net 
maker and the skipper? 

Mr. Lusyne thanked Dr. Honda for clarifying the position 
in Japan regarding the production and properties of poly- 
propylene. For distinguishing different fibres, various means 
were known, such, for instance, that burning nylon gave off 
white smoke and left a residue of hard black quality; other 
fibres reacted differently, but the real tests were chemical and 
they could only thus distinguish the different fibres. 

There were practical methods which had been published 
and re-published. We could suggest that FAO include in 
the Fishing Gear Designs Catalogue some simple methods for 
testing fibres. 

The only practical answer to Mr. Kwei's problem of fish 
rotting in synthetic gillnets was to haul the nets and clear 
them more frequently. 

To Mr. Crewdson's problem he could see no real answer. 
Stretch could arise not only from the material but from the 
knots themselves tightening or giving. There would always 
be movement in the mesh of knotted nets. He thought the 
regulations should state clearly whether, and how many 
meshes should be taken or sampled in different parts of the 
net to decide whether the average mesh size conformed with 
the regulations. The nets did change in use and the regula- 
tions should recognise that. 

Dr. von Brandt: That is also a problem for the Permanent 
Commission and he thought most delegates of the different 
countries would discuss the right regulations for checking 
mesh sizes. 

Mr. Kristjonsson (FAO): We should be discussing here 
mainly the suitability of the different materials for different 
fishing methods and fishing gear. He suggested that poly- 
propylene might advantageously be used in some purse seines 
because of its lightness and capacity for floating in the water. 
This should reduce fouling when making sets in shallow 
water provided the necessary weights were concentrated in the 
purse rings instead of using a leadline. He would like to 
hear other provocative statements about the use of certain 
types of materials for different fishing needs. 

Mr. VaWez (Peru): Our Fishery Institute is considering 
buying polypropylene in order to test it for purse seines. 

Mr. R. K. N. Ocran (Ghana): Our traditional methods 
were to fish with natural fibres but cotton nets rotted after 
two months. After the Hamburg Congress and his own 
experience in Scotland, they tested nets of cotton, 'Kuralon' 
and nylon. 'Kuralon' was very heavy and sank quickly and 
tended to get caught on the rocks. With nylon used in 

shallow water, they had some success, but in deeper water, 
because the net was light and slow to sink, 75 per cent of the 
fish in a nine-months trial of seine nets escaped before they 
could catch them. If they used leadlines the net would sink 
quickly enough but the net would foul. To solve that 
problem they were now trying another net from a different 

Mr. E. F. Gundry (UK): One of the problems of a netmakcr 
is the multiplicity of synthetic fibres. This is handicapping 
the commercial success of netmakers. Some settlement 
should be arrived at as to the ideal materials to be used for 
different fisheries so that netmakers would not have to carry 
so much stock in different lines. This would contribute to 
the economy of the netmaking industry. 

Mr. . Alters Nilssen (Norway) agreed it would be far easier 
for manufacturers to have on*y a few synthetic fibres. Syn- 
thetics certainly floated more and sank slowly, therefore they 
had tried to find out the right kind of dressing to keep purse 
seine nets upright in the water. He did not agree that 
polypropylene would be of any advantage in purse seining. 
On the contrary. On the setting of knots and mesh sizes, 
their experience was that if netmakers used synthetic fibre 
they could know exactly what the shrinkage was. If a net 
was really properly knot set there would be very little variation 
in the mesh size. 

Mr. . A. Schaefers (USA): In experiments of synthetic 
materials tar has reduced the abrasions on the fish caused by 
the material. It has also probably reduced the catchability of 
the nets but they still maintain the durability which is one of 
the main features of synthetics against cotton fibres. 

Mr. Ocran: The difference between fish caught by cotton 
and nylon is that when the fish struggle the fine twine cuts and 
the fish deteriorates before it reaches the consumer so the 
impression in our country is that nylon-caught fish deterior- 
ates more than cotton caught fish. 

Mr. C. Crewdson (UK), on the point of dressing synthetics 
with bitumastics or pitch material, said he had always 
understood the fibres were impervious to bacterial action. 
They did not need dressing to give durability. His firm gave 
bitumastic treatment to synthetic nets merely to give rigidity 
because those nets were so limp that they tended to foul in 

Mr. Schaefers: I have been misunderstood. I was merely 
trying to point out that the treatment of synthetics with tar 
was to reduce abrasion on the fish and not to extend the life 
of the material. 1 agree completely with Mr. Crewdson. 

Mr. Lusyne, in summing up, said that Mr. Gundry's 
problem of limiting the number of materials could not be 
easily met. They could not just say that any particular 
material would make a good net. They had to qualify that by 
saying it is a good net at certain places at certain times. One 
fisherman would succeed with it and another would not. 
Their individuality had to be met. The gear the fisherman 
used was that which best suited his operation. No agreement 
would be possible in the way of manufacturers limiting 
themselves to three or four materials. Furthermore such 
standardisation would mitigate against progress. 

Part 1 Materials for Nets and Ropes 

Section 4 Ropes, Knotless Nets and Monofilaments 

Ropes of Polyethylene Monofilaments 


Polyethylene monofilaments are manufactured by extrusion, 
followed by an orientation process. Two factors have a significant 
effect on the properties of these filaments, i.e., type and quality of 
the polymer used for production and the degree of stretch applied 
at the orientation stage. The properties of monofilaments made from 
three different polyethylene types have been compared with the 
normally given specifications for these raw materials. The influence 
of the stretching percentage on the filament properties, has been 
determined quantitatively. Polyethylene monofilaments with the 
same diameter, but with different properties, were twisted and stran- 
ded into twines and ropes. The mechanical properties of these 
twines and ropes were compared with the filament properties. The 
conclusions arrived at were as follows : ( 1 ) Melt index and specific 
gravity are insufficient specifications for characterising polyethylene 
raw material, suitable for monofilament production. (2) Under 
comparable circumstances ethylene copolymers give stronger 
monofilaments than ethylene homopolymers. (3) The monofilaments 
of highest breaking strength do not give the best net yarns in respect 
of the knot tenacity in netting. (4) For rope making, highly stretched 
monofilaments are desirable. 



La production des monofilaments de polyethylene s'execute en deux 
phases suivies Tune apres Tautre: rextrusionct K orientation. Deux 
facteurs importants infiuencent les proprietes des filaments obtenus: 
le type et la qualite du polymer destine a leur production et le degre 
de Tetirage appliqu6 lors du proccs de Torientation. 

Les proprietes des filaments fabriques a base de trois types 
differents de polyethylene ont etc comparees ayec les specifications 
donnees reguliercment pour ces matures premieres. L'influence de 
Petirage sur les proprietes des filaments a et determinee quantita- 
tivement. Des monofilaments d'un meme diametre, mais avec des 
proprietes differentes ont 6t6 rctordus et assembles en cables ct en 
cordes. Les proprietes mecaniques de ces cables et de ces cordes ont 
et6 comparees avec les proprietes des filaments. 

Les auteurs sont arrives aux conclusions suivantes: ( 1 ) Klndice de 
fusion (melt index) ct la densitS sont insuffisants pour caracteriscr la 
matiere premiere servant de base a la fabrication de monofilaments. 
(2) Dans des circonstances comparable*, les copolymers d'ethyldne 
produisent des monofilaments plus resistants quc les homopolymers 
d'ethylene. (3) Les monofilaments ayant la plus grande resistance 
de rupture ne sont pas les meillcurs pour la confection des filets, par 
rapport a la tenacit au noeud. (4) Pour la fabrication de cordes, 
il est souhaitable quc les monofilaments aient etc soumis a un 
etiragc tres eleve. 


Se fabrican los monofilamentos de polietileno por medio de extru- 
si6n, seguido por un proceso de eslirar. Dos factores licnen un 
efecto significante sobre las propiedades del producto: la clase y 
calidad del polimero y el grade de estiramiento aplicado en la fase 
de orientacibn. Las caracteristicas de los filamentos hechos de tres 
tipos diferentes de polietileno han sido comparadas con las especih- 
caciones usuales para esta materia prima. La influencia del estirar 
sobre las propiedades de los monofilamentos ha sido establecida en 
cantidades determinadas. 

Hilos monofilamento del mismo diametro pero con caracteristica 
diferentes han sido retorcidos y ensamblados en cables y cordonos. 
Las propiedades mecanicas de estos cables y cordones ban aido com- 
paradas con las propiedades del monofilamento. Se Ileg6 a las 
conclusiones siguientes: ( 1) El indice de fusi6n y la gravedad especf- 
fica son insuficientes para caracterizar el polietileno bruto que reuna 
buenas condiciones para la produccibn de monofilamentos. (2) tn 
condiciones andlogas dan monofilamentos mas fuertes los copoli- 
meros que los homopolimeros de etileno. (3) Los monofilamentos 
de mayor resistencia a la rotura no producen los mejores hilos para 
redes rcspecto a la tenacidad de los nudos en los paftos. (4) Enjas 
fabrication de cuerdas convien 


' conviene emplear monofilamentos muy 

C. C. Kloppenburg 

J. Reuter 

IT is well known that monofilaments from polyethylene 
are manufactured by an extrusion technique, followed 
by an orientation stage. In principle one can distinguish 
two different processes. The whole operation can be 
done in a single "in-line" unit or in two separate units, 
the first devoted to extrusion and the second to orienta- 
tion. Fig. 1 gives a comparison of these two manufac- 
turing methods. At the top is single-stage production. 
The polymer is molten in an extruder, forced through a 
multihole die and quenched in warm water. Directly 
after that, filaments are stretched up to eight to ten times 
their original length in boiling water, and then wound 
up under constant tension. This process can also be 
done in a discontinuous way, as is shown below. Here, 
the unstretched filaments are spooled and afterwards 
stretched in a second operation. 

Both methods have advantages and disadvantages in 
production. The in-line method requires fewer pieces of 






F::.AM( NT 

Fig. I. Single-stage and two-stage monofilament production. 


equipment and a simpler attention than the two-unit 
method. The speed of stretching, however, is in point of 
principle limited by the extrusion speed. This is a draw- 
back in regard to output. The final selection of the method 
to be used will depend, among other things, on the 
number and diameter of the monofilaments. The manu- 
facturing process for polyethylene monofilaments may 
be relatively simple but needs to be carefully controlled. 
Different manufacturing conditions can be used and 
they have more or less influence on the properties of the 
oriented monofilaments. But this influence is not always 
clear and some investigations contradict each other. 
Two factors, however, have a significant effect on the 
properties of the end product. These are : type and quality 
of the ethene polymer used for the production and the 
stretching percentage (draw ratio) applied in the orienta- 
tion stage. Both factors are discussed. 

The raw material 

For producing polyethylene monofilaments, mainly three 
types are considered: 

(a) Homopolymer of ethylene, manufactured by 
Ziegler procedure. 

(b) Product obtained by copolymerising ethylene and 
1-butene by Phillips process. 

(c) The newer commercial ethylene copolymer, ob- 
tained by Ziegler process. 

These materials are only specified by melt index and 
specific gravity and sometimes by viscosity data. 

For filament production, the types with specific gravity 
= 0*95 and melt index 0*1 0*3 are recommended. It 
turned out, however, that these two or three polymer 
specifications are insufficient for characterising poly- 
ethylene material suitable for filament production. In 
other words, the suitability of an ethylene homo- or 
copolymer for making monofilaments is not guaranteed 
when the specific gravity and melt index are within the 
above-mentioned limits. This does not only relate to the 
economy of the process but also to the mechanical 
properties of the obtained filaments (Table I). 

From eight different ethylene homopolymer batches, 
all produced according to the Ziegler process, were 
determined: density, melt index and intrinsic viscosity. 
Afterwards, output samples were taken for manufactu- 
ring monofilaments in exactly the same way, using a 
special investigation equipment, and applying a draw 
ratio of 8 : 1 . The final diameter of the filaments amoun- 
ted to 0*38 mm (0*0150 in), corresponding with tex 
108 (tex=weight in grams of 1000 m of monofilament). 
Table I indicates the average values of some typical 
properties of the obtained monofilaments, namely the 
tenacity determined with and without an "overhand 
knot", and the elongation at break. These determinations 
were carried out with a Scott IP4 inclined plane tester. 
Drawing time: 13 sec. The tenacities are expressed in 
grams per tex. Dividing these figures by 9, gives tenacity 
expressed in grams per denier. 


polyathylana Ha 

Raw satariala 


fetch or 

Datarainad apaoifioationa 

Maoftanloal propartUa 




Tenacity (g/ta*) 










19 % 







19 % 







18 % 







21 % 







17 * 







22 H 







19 * 







18 % 

i>iM* mtio 
lJiaiurtr monofilnt 
Tex monofil amenta 

- 6 i 1 """""" 
* 0.30 M (0.0150 in.) 45.2 
- 108 a/km. 



From these data the conclusion is well founded that, 
within the investigated range, there is no correlation 
between density, melt index and intrinsic viscosity on 
the one side and tenacity or breaking elongation on the 
other. Starting with the same type of raw material, there 
are remarkable differences in filament properties depend- 
ing on the polymer batch that has been used. It is inter- 
esting to compare these results with the figures obtained 
by experimenting, in exactly the same way, with ethylene 
copolymers produced according to either the Phillips or 
the Ziegler process (Table II). 

Tabla II. konofilaaanta from athylana oopolvaai-a 
l)raw ratio 8 t 1. Konofilahent diamvUr - 0.38 DM 
(0.0150 in.) 

lav aatariala 


Baton or 

Datarainad apaoifioationa 

Maohanioal propartiaa 




Tanaoity (g/tax) 





normal OT J|Jj* d 

Phillipa oopolyaar 







20 % 







20 % 







. 18 % 







19 % 







22 % 







22 % 







20 % 







21 % 




giaglar oopolyaar 







19 % 







20 % 







23 % 







21 % 







22 % 







17 % 







17 % 







19 % 










Ovtrhond knot 



















Prow Ratio 



Fig. 2. Mono/Moments made from Ziegler-type polyethylene. Homo 
polymer. (Filament diameter 0-41 mm). 


70 I 



Overhand Knot 



Draw Ratio 

Fig. 3. Monofilaments made from the Ziegler-type polyethylene. 
Copolymer. (Filament diameter=0-41 mm). 

Table III. ProprtiB of 
Ziegltr typ poly thy Itnsi 
(0.0161 in.} 

Draw ratio 

Breaking ttnaoity in 



Overhand knot 





6.5 s 1 

7.25t 1 




7.8 t 1 




8 t 1 




6.25s 1 




8.5 i 1 




9 * 1 



14. # 

9.75 1 




10.4 > 1 




11.5 t 1 








6.5 i 1 

7-25* 1 




7.8 t 1 




8 s 1 




8.25s 1 




8.5 s 1 




9 1 



16. # 

9.75s 1 




10.4 s 1 




11.5 s 1 



12. <# 

Here the same picture is seen. The filament tenacities 
may diverge to 20 per cent depending on the polymer 
batch that has been used. It is not possible to predict the 
mechanical properties of the monofilaments from such 
specifications as: density, melt index and viscosity. 
These data are apparently insufficient to describe the 
behaviour of an ethylene polymer for filament production. 
We can only hope that, in the near future, the suppliers 
of the raw material will be able to furnish stricter speci- 
fications; such as: molecular weight distribution and 
data concerning unsaturation and branching. 

The draw ratio 

It is common knowledge that the draw ratio applied in 
the orientation stage influences the elongation and 
tenacity of the resulting monofilaments. As a general 
rule one can say: the higher the stretching percentage, 
the higher the tenacity and the lower the elongation. 

This correlation has been investigated quantitatively 
by extruding monofilaments with different diameters 
and stretching the said monofilaments in such a way that 
the final diameters of the obtained products were the 
same. The applied draw ratios in this investigation 
ranged from 6*5: 1 to 11-5:1. From the resulting mono- 
fils not only the tenacity and elongation were determined, 


but also the knot tenacity. Table 111 gives the results 
obtained with monofilaments produced from a Ziegler- 
type homopolymer and a Ziegler-type copolymer, both 
batches representing an average quality. From these data 
the two graphs, shown in Figs. 2 and 3, have been made. 

Fig. 2 relates to monofilaments made from the 
Ziegler-type homopolymer. On the x-axis is plotted 
the applied draw ratio during the production, and on the 
y-axis the determined tenacity (in grams per tex) and 
elongation of the obtained filaments. As it turns out, 
the tenacity increases to a certain level and the elonga- 
tion decreases when we increase the stretching percentage. 
This is not the case with the knot tenacity which exhibits 
a distinct maximum. It is difficult to explain the occurrence 
of this maximum because a simple "overhand knot", 
regarded physically, is a rather complex whole in which 
among other things, a bending modulus, a torsion mod- 
ulus and a sliding modulus play a part. 

Fig. 3 relates to monofilaments made from the Ziegler- 
type copolymer. The three curves exhibit the same shape 
as in Fig. 2 but are all at a higher level. This finds better 
expression in Fig. 4, where the curves are taken together. 
The drawn lines indicate the results obtained by investi- 
gating the Ziegler copolymer and the dotted lines repre- 
sent the behaviour of the Ziegler homopolymer. The 
knot tenacity curves have different maxima, concerning 
both the draw ratio and the tenacity. 


The foregoing poses the questions as to what raw material 
should be used, and what draw ratio applied to get 
the best products for rope and twine makers. The answer 
may seem rather simple. From the graphs shown, it is 
possible to find the circumstances for getting monofils 
with the highest normal strength. According to general 
opinion, these monofils will automatically yield the best 

ropes and net yarns, and are therefore most desirable. 
To test this the following investigation was carried out 
(Fig. 5). 

Pilomnt Dlam*tr o.+lm.m (o.oiei in) 








Biti! tiggd ^ n * 


to: I 

Draw Ratio 
ICI I2: 

Fig. 4. Monofilaments made from the Ziegler-type polyethylene. 

Homopolymer compared with copolymer. The draw ratios applied to 

the investigation as performed are shown by the thick down lines. 



OTMT latio 

AUMUr 0.41 
(0.0161 in.) 

op jurat 48110 x 3X65 
Trwrl twin* t 481^0 z 3Z70 

3 35 / 


2-3/4 ia. 

Fig. 5. Programme of Investigation. 


Starting from both the Ziegler-type homopolymer 
and the Ziegler-type copolymer, eight different kinds 
of polyethylene monofils were made using four different 
draw ratios during the production, viz. : 6-5 : 1 ; 7-8 : 1 ; 
9 : 1 and 10'4 : 1. The resulting products all had a 
final diameter of 0'41 mm (0-0161 in). To prevent 
confusion, the monofilament types were made in different 
colours. Each filament type was used for making both 
rope yarn, with the construction 4 S 1 10 x 3 Z 65, and 
trawl twine with the construction 4 S 1 50 x 3 Z 70. From 
3 x 35 rope yarns, two types of rope were manufactured: 
rope 1 with a normal lay, and rope II with a hard lay. 
The runnage of the resulting ropes was about 4-5 m/kg, 
corresponding with a circumference of 70 mm (2| in). 

Table IV. Mechanical properties of polyethylene 

monofilaments; diameter 0.41 mm (0.0161 in) 

(average values of 20 determinations) . 


HroakUn; trnth 


knot breaking 







6,'j i 1 








7.0 1 


r/i U)T ^ 


126. V 


4,. 7 




homopol/ i *P 

9." i 1 






11. '< 



10.4 i 1 









6.5 1 









7.6 i 1 









^ie)tr type 


9.0 i 1 

tr inapar- 








10.4 i 1 





"' 7 




Filament properties 

Table IV surveys the mechanical properties of the fila- 
ment types used. 

Not only were the elongation and breaking strength 
determined, but also the "cold flow'* under a permanent 
load of 12 g/tex. 

Fig. 6. Cold flow of polyethylene monofilaments. Permanent load: 

12 g/tex, monofilament diameter: 0-4 J mm (0-0161). The per cent 

elongation is shown in relation to the time in hours. 

Fig. 6 shows remarkable differences among the investi- 
gated monofilaments. The lower the stretching percentage 
during production, the higher the fibre creep afterwards. 
One can realise that the two monofilament types 1 and la, 

which have a creep behaviour as is shown, having been 
stretched to only 550%, cannot be used for practical 
purposes. Further, it should be noted that the copoly- 
mers la, 2a, 3a and 4a show more fibre creep than the 
corresponding homopolymers 1,2,3 and 4. 

Trawl twines 

To judge net yarns, the main requirement should be the 
ability to give a strong net. As the examination of net 
strength is not so simple, the different net yarns were 
compared on the basis of their dry "knot strength", 
using a not too tightly drawn "overhand knot". 

The necessary yarns were all manufactured in the 
same way and on the same machine. To determine 
breaking strength, use was made of a horizontal appa- 
ratus of the pendulum type, constructed so that only the 
maximum load could be determined. The measuring 
accuracy amounted to 1 Ib and the drawing speed to 
80 cm (31 '5 in) per min. To test the yarn, it was wound 
round smooth rollers with a circumference of 19 cm 
(7-5 in). The distance between the rollers amounted to 
50 cm (19-5 in). The yarn always broke either between 
or below the rollers but never on the rollers. Table 
V gives the results found by testing the above-mentioned 
eight different trawl twines. 

Table V. Mechanical properties of polyethylene 

trawl twines (made from 3x4 monofilaments). 

Runnage in m/kg. 





a strength 

Knot breaking 







































































These values are an average of 20 determinations. The 
only reasonable way to correlate these figures with the 
monofilament data, is on the basis of the breaking length. 

From Table VI it appears that the higher the breaking 
length of the monofilaments, the higher the breaking 
length of the net yarns, as would be expected. For the 
netmaker, however, it is far more important to compare 
the breaking length of the monofilaments with the so- 
called knot breaking length of the trawl twines. If this 
thought is followed, it is found that the strongest monofils 
do not give the best net yarns. There exists a maximum 

in the knot strength of thejiet yarns, depending on 
Ac draw ratio applied during the production of the 

Tabl TI. Broking lamrthi. (Trawl 
twin** ooapar*d with BonofilaMnta ) 


Draw ratio 
during th* 
of aonof il* 



Trawl twin** 
with over- 
hand knot 



6.5 t 1 






7.8 i 1 








9.0 i 1 






10.4 > 1 






6.5 i 1 







7.8 j 1 








9.0 t 1 






10.4 t 1 





The samples for the following investigation were made 
as follows: from each of the eight monofilament types, 
rope yarns were manufactured with the construction: 
4 S 110 x 3 Z 65. This work was carried out on one 
machine and the resulting rope yarns were tested in 
the same way as described for the net yarns. The strands 
were all made from 35 rope yarns and manufactured 
under the same circumstances. From these strands 
two types of 2f-in ropes were produced, one with a 
normal lay and one with a somewhat harder lay. 

The testing of the ropes was performed by means of a 
hydraulic breaking strength machine fitted with grips. The 
distance between grips was 50 cm (19*7 in), and the rate 
of movement of the straining head 12-5 cm (4-9 in) per 
min. From the 32 determinations, only three breakings 
occurred in the grips. In these cases, however, no differ- 
ent values were obtained. Elongation was also measured 
under different loads. This was done by marking a 
distance of about 20 cm (7-9 in) on the rope, and meas- 
uring the increase of this length under increasing load. 
In this manner, load-elongation curves of these poly- 
ethylene ropes could be determined. 

Fig. 7 shows four characteristic load-elongation curves 
obtained by testing ropes made from homopolymer 
monofilamcnts. The two lower curves show a normal 
behaviour: increasing elongation with increasing load. 
The two upper curves, however, exhibit a rather strange 
behaviour. Here the load increases till a maximum value 
is attained (by which the rope does not break) and then 
decreases with increasing elongation. These two curves 
are found by testing the yellow and red ropes. From the 
preceding graphs it will be remembered that the yellow 
and red monofilaments got the lowest stretch during 
production and, moreover, showed a high creep when 


Fig. 7. Ropes made from ethylene homopolymer (normal lay). The 
per cent elongation is shown in relation to the load in kg. The draw 
ratio of yellow was 6-5:1, red 7-8: 1, green 9:7, and brown 10-4:1 

exposed to a permanent load for several hours. The 
load-elongation curves of the ethylene copolymer ropes 
exhibit an analogous behaviour, as is shown in Fig. 8. 

The conclusion of these experiments is obvious. To 
avoid undesired creep in the rodes, the monofilament 
producer has to take care that the stretching percentage 
in the orientation stage is not too low. The cold flow of 
monofilaments, when exposed to permanent load, gives 
already some indication of the rope creep to be expected. 

Table VII gives a survey of the data that have been 
determined by testing the four rope types made from 
ethylene homopolymer. Each rope has been tested in 
duplicate and the data show little or no differences. The 
rope figures can be compared with the yarn properties, 
indicated at the left side of the table. From the foregoing 
it is clear that the maximum load does not always corres- 
pond with the breaking load. Further, it can be stated 
that the higher the breaking length of the yarn, the higher 
the breaking length of the corresponding rope. The hard- 
laid ropes show a somewhat lower breaking load than the 
normal laid ones. 


Fig. 8. Ropes made from ethylene copolymer (normal lay). The per cent 

elongation is shown in relation to the load in kg. The draw ratio of 

white was 6-5:1, light yellow 7-8:1, transparent 9:1, black 10-4:1. 

Table Vlll gives the data found by testing ropes made 
from ethylene copolymer. The tendency is the same as 
in Table VII, but the figures are on a higher level. 

Fig. 9 shows that a linear correlation exists between 
the breaking length of the rope yarn and the breaking 
length of the rope. 

continued on page 88 

Table VII Ropea made from ethylene Homopolymer. N normal lay, H - hard lay 

Ropa yarns aada from 
3 x 4 monofilamanta 

Ropes made from 3 x 35 yarns 




break inj 
In kff 




f laye 

per m 

In mm 

in kg 

in kg 



lo IB k| 

























































































22 f 5 




















. _ 






























24,6 , 










71 * 











Table VIII Ropes made from ethylene Copolymer. N - normal lay, H - hard lay 

Rope yarna made from 
3x4 monofilamenta 

Ropea made from 3 at 35 yarns 




in kg 




of laya 
per m 

in mo 

in kg 


in kg 


load in kg 















62 * 

















61 % 








61 % 























66 * 
































71 * 





















70 % 

















66* i 

Tests on Knotless Raschel Netting 


Knotless nets are of increasing interest for fishing. They are made by 
two different methods: the Japanese twisting method and the 
Raschel knitting method. Knotless nets are said to have certain 
advantages in comparison with knotted nets. Some of these pro- 
perties (mesh strength, weight, resistance in water, constancy of 
meshes, and catching efficiency when used for herring driftnets) 
have been tested for fine knotless Raschel netting. As the main 
criterion for the exchangeability of knotted and knotless netting, 
equal mesh strength in wet condition has been adopted. With 
knotless Raschel netting made of the same fibre material, the mesh 
strength, the diameter of the bars and the weight per unit area 
depend to a great extent on the type of construction of twine 
(bars) and connections. The Raschel machines allow a wide range 
of various constructions. On the base of equal mesh strength in 
wet condition, the types of Raschel netting tested in comparison 
with knotted netting, were found to have (a) less weight per unit 
area, (b) bigger diameter of bar, and (c) same towing or current 
resistance in water. The constancy of the mesh size of Raschel 
netting is better than that of knotted netting. For herring drifting 
in the North Sea, the catching efficiency of Raschel netting was 
found to be equal to that of conventional knotted and treated cot- 
ton netting of equal twine (bar) diameter (one mm). (The same was 
found earlier for knotless netting of the Japanese twisting method.) 

Epreuves de filets sans noeuds, type Raschel 


Pour la pgche, les filets sans noeuds offrent un interet croissant. 
Us sont fabriques de deux facons differentes : selon la methode 
japonaise de retordage et selon la methode de tricotage Raschel. 
Les filets sans noeuds sont supposes avoir certains avantages sur 
les filets noues traditionnels. Certaines de lews proprietes (resistance 
des mailles a la rupture, poids, resistance a Pavance dans Peau, 
longueur des mailles et efficacite de capture lorsqu'ils sont employes 
dans les filets maillants pour la peche au hareng) ont etc eprouvees 
pour les filets sans noeuds Raschel, en fil fin. Le principal critere 
de cpmparaison est la force de rupture des mailles en condition 
mouillee. Avec les filets sans noeuds Raschel, constants dans un 
mSme matriau la force de rupture des mailles, le diametre du fil 
entre les connexions et le poids au metre carr dependent du type de 
fabrication de ce fil et des connexions ellesmemes. Les machines 
Raschel pcrmcttent une grande varied de fabrications. Sur la base 
d'egales forces de rupture des mailles, en condition mouillee, les 
types de filets Raschel eprouves avaient en comparaison avec les 
filets noues: (a) un moindre poids au metre carre; (b) un plus grand 


Dr. A. von Brandt 


Institut fur Netz und Materialforschtmg, 

diametre de fil; (c) une meme resistance a 1'avance dans Peau. 
La dimension des ma i lies est plus stable dans le filet sans noeuds de 
type Raschel que dans le filet nou6. Pour la peche au hareng dans la 
Mer du Nord, avec des filets maillants, les filets sans noeuds Raschel 
ont permis des captures egales a eel les effectuees avec des filets 
npues conventionnels de meme dimension de mailles, pour un 
diametre de fil de un mm. (La meme constatation avait et faite 
avec les filets sans noeuds de type retordu). 

Ensayos con redes Raschel sin nudos 


Las redes sin nudo despiertan cada vez mas interes en la pesca; 
se hacen de dos manera distintas: la japonesa de la torsi6n y la 
Raschel del tejido. Las redes sin nudos tienen ciertas ventajas 
sobre las anudadas. De las redes Raschel sin nudos se han ensayado 
algunas propiedades, entre las que estan la robustez de la ma I la, 
peso, resistencia en el agua, firmeza de las mallas y rend im lento de 
pesca cuando se emplean en los artes de deriva arenqueros. Como 
criterio principal de la posi bill dad de intercambiar redes con y sin 
nudos, se ha adoptado la igualdad de robustez de la malla cuando 
esta humeda. Con las redes Raschel sin nudos hechas de la misma 
fibra, la robustez de la malla, el diametro de los hilos entre nudos y 
el peso por unidad de area dependen mucho de la manera de fabri- 
carlas y de las conexiones. Las maquinas Raschel son capaces de 
fabricar muchas clases de red. Basandose en la igualdad de la robus- 
tez de la malla humeda, las redes Raschel ensayadas eran, con 
respecto a las anudadas, (a) menos pesada por unidad de area, (b) de 
mas diametro entre nudos y (c) de la misma resistencia al remolque 
o a corriente de agua. Las mallas de las redes Raschel son mas con- 
stantes que las de las anudadas. En el caso de la pesca a la deriva 
del arenque en el Mar del Norte, la capacidad de captura de las 
redes Raschel era igual que la de redes de algodon tratado y 
anudado normales de hilo del mismo diametro entre nudos (una 
mm). El mismo resultado se habia pbtenido anteriormcnte para 
redes sin nudos hechas con hilos torcidos. 

continual from page 86 



specifications for characterising polyethylene raw material 
suitable for rnonofilament production. 

2. Under comparable circumstances, ethylene copoly- 
mers give stronger monofilaments than ethylene homo- 

3. The knot tenacity of monofilaments shows a maxi- 
mum as function of the draw ratio. 

4. The strongest monofilaments do not give the best 
net-yarns regarding the knot tenacity. 

5. For rope making, highly stretched monofilaments 
are desirable. 

Fig. 9. Freaking length rope versus breaking length rope yarn. 


From the investigations come these conclusions: 

1. Melt index and specific gravity are insufficient 



The monofilaments were manufactured and tested by Nyma 
Rayon Works (Kunstzijdespinnerij NYMA N.V.), Nijmegen, 

The trawl twines and ropss were made by Esbjerg Tovvaerks- 
fabrik A/S, Esbjerg, Denmark, and tested by "Nederlandsche 
Visserij-Proefstation", Utrecht, Netherlands. 

AT present knotted netting is still the main material 
for fishing nets, including modern fishing gear. 
Knotted netting is made by hand braiding or by semi- 
or fully automatic machines. The weaver's knot and the 
reef knot (especially in Asia) are the main knot types 
used in netmaking. Double knots developed from these 
two basic types are also used (v. Brandt 1957). 

Recently so-called knotless netting is becoming 
increasingly important. This interest stems from reports 
that knotless netting is cheaper than knotted netting and 
has certain technical advantages. This development 
makes it necessary to establish testing methods for deter- 
mining the qualities of knotless netting which are impor- 
tant for fishing purposes and to establish rules for substi- 
tuting knotless for knotted netting in fishing gear. 

Manufacturing techniques 

Knotless nets have been well known for a long time. 
Loosely woven fabric as used, for example for Japanese 
minnow nets, can also be regarded as knotless netting. 
Such small meshed material, which cannot be manu- 
factured by knotting, will retain its importance in Asia 
for catching small fish. 

Introduction of chemical fibres for fishing brought the 
idea of welding or glueing synthetic fibres together to 
form netting. Stamping or moulding finished net sheets 
has been tried. Such techniques so far have not produced 
satisfactory fishing nets. 

In 1922, knotless netting made by a twisting technique, 
was introduced in the Japanese fishery. (Nippon Seimo 
1959.) This type of knotless netting is made of twines 
consisting of only two yarns (Fig. la). The connection 
of the twines to form meshes is made by interlacing 
the yarns of two twines, once or several times. According 
to the construction of the joining points, the twines run 
diagonally through the netting or in a zig-zag line. If 
the interlacing of the twines at the joining points is done 
several times, the shape of the mesh may be changed 
from rhombic into hexagonal. This Japanese twisting 
technique stimulated efforts elsewhere to develop pro- 
duction techniques for new types of knotless netting. 

As a result of these efforts, another type of knotless 
netting was introduced into fisheries around 1951. The 
manufacture of this type is based on the Raschel tech- 
nique, well known in curtain-making for at least 100 
years (Fig. Ib). This development has been especially 
promoted in the U.S.A., Belgium, Italy and Germany. 
The Raschel technique of knitting is done by special 
machines (Reichel 1960). 

The bars of the meshes are built up by one or two knit- 
ted strands and one to three additional woofs for 
strengthening (Fig. 2). For each strand two guide bars 
are needed. Usually knitting machines with six to eight 
guide bars are used for fish netting. 

The Raschel machine not only makes the connections 
to form meshes but also knits the mesh bars. Therefore 
the Raschel machine produces per unit time, not a 
certain number of connections but a certain area of 
netting. The size of this area is not influenced by the 

Fig. 1. The two main types of knotless netting used in fishery, 
(a) Japanese twisted type. (/>) Raschel type. 

mesh size, i.e., with the same type of material the pro- 
duction of a certain mesh size needs double the time 
needed for producing a mesh of half that size. Thus 
Raschel machines operate quite differently from knotting 
machines. The knotting machines work with finished 
twines and produce only the knots. Per unit time they 
produce a certain number of knot-rows on which the 


mesh strength can be produced, depending on the type 
of construction. In all cases tested here, the mesh strength 
of Raschel netting is higher than that of the knotted 
netting made of the same fibre material. But it must not 
be forgotten that the construction of the twine (bar) in 
the Raschel netting is completely different (Fig. 4) and 
that the twine number given for the Raschel bar is there- 
fore not directly comparable with the conventional 
twine number. 

Weight of knotless Raschel netting 

In general the trade with netting is based on the weight 
of a net sheet 100 m long and 100 meshes wide, absolutely 
dry, plus an official moisture content which depends on 
the kind of fibre. With decreasing mesh size, the weight 
of a given area of netting increases very quickly because 
the number of mesh bars and knots is increasing. In the 
heavy codends of bottom trawls made of manila, 
double braided, with a mesh size of 70 mm stretched, 
the weight of the knots can be more than 60 per cent 
of the total weight. The share of the weight of the knots 
increases with decreasing mesh size and increasing twine 

Since for the present tests only small samples were 
available, the tables commonly used by netmakers for 
calculating the weight of net sheets could not be used, 
and the net weight had to be newly determined. This 
was done for pieces of netting of different size but of 
32 meshes each made of medium-laid continuous multi- 
filament nylon twines (Fig. 5). 

height of knotted n*tttn 
with 12 mBh vru 
m**litx* (trtched) for 
nylon continuous nultlflli 

- - -- 6 to 210 

Fig. 5. Weight of knotted netting per unit area versus mesh size 

(stretched) for nylon continuous multifilament twines. 

Td 210 x 6 to 210 x 60. 

The mesh size (stretched) was determined with a special 
pressure gauge (1 kg) constructed for fine netting (Florin, 

For the comparison of weights of netting it must be 
taken into account that knotted netting very often and 
knotless netting, especially when made of synthetic 
fibres, are practically always treated during manufacture 
or later with some bonding agent which adds to the 
weight. The data in Fig. 5 are for untreated knotted 

netting and can only be compared with untreated knotless 

The weight of the Raschel netting samples in Table I, 
in comparison with knotted netting of the same mesh 
strength according to Table II, is given in Table III. 

TABLE 111 

Mesh size 

Raschel netting Weight 





210x11 normal 0-5 


210 x 11 normal 



210x11 normal 



210x11 special ( 



210x11 special 



210x11 special 



210x11 super ( 



210x11 super 



210x11 super 














1 -6 









According to this small number of samples at equal 
mesh size and equal mesh strength, of the three construc- 
tions tested, Raschel netting is lighter than knotted 
netting. This means that Raschel netting of the tested 
types gives more net per unit weight than knotted netting 
of the same mesh size and mesh strength. 

Towing resistance of Raschel netting 

The towing or current resistance of netting in water is 
of high interest for all towed or dragged fishing gear, 
as well as for gear set in a current, because the resistance 
and the water pressure wave caused thereby have a 
bearing on construction and power requirements and 
may also influence the catching efficiency of the gears. 
The current of towing resistance depends on mesh size, 
twine diameter (mesh bars) and size and kind of the 
joining points of the meshes. 

The diameter of normal net twines is relatively easy to 
measure. Some values for medium laid, continuous 
multifilament nylon twines are given in Table IV (Klust 

To determine, for comparison, the diameter of the 
mesh bars in Raschel netting is more difficult. Due to the 
construction, the cross-section of the bars in Raschel 
netting is not circular, but more or less rectangular. 
Since it cannot be predicted which side of the bar will 
face the waterflow in a fishing gear, the wider side has to 
be considered. 

The following comparison has been made with the 
same samples of Raschel netting (Table I) versus the 
same corresponding samples of knotted netting (Table II). 
As can be seen in Table I, the diameter of the bars in 
Raschel netting can vary considerably according to the 
structure, even if the fibre material is the same. Due to 
the specific structure, the same twine number or count 
results in a thicker twine or mesh bar in Raschel netting 

















Td. Diameter 

Td. Diameter 








compared with normal twisted net twine in knotted 
netting. The reason is that in the knitted bars of Raschel 
netting the main strands are reversed in direction by 
knitting and appear in the cross-section several times 
(See Fig. 2). The comparison of the diameters of the bars 
of Raschel netting with those of knotted netting of equal 
mesh strength is given in Table V. 

For equal mesh strength the bars of Raschel netting 
are always thicker. This could be a disadvantage of 
knotless Raschel netting in comparison with the knotted 
netting but, as mentioned before, the towing or current 
resistance of netting in water does not depend on the 
diameter of the bars alone, but also on the area and shape 
of the connections. 

Raschel netting 
material structure 
210x11 normal 
21 Ox 11 special 
210 x 11 super 


Diameter Equal to knotted 
(mm) netting material 
1-05 210x18 




1 -00 

The determination of the area of the joining points as 
a criterion for towing resistance is even more difficult. 
The joining points can be projected on a plane and the 
area measured by planimetering (v. Brandt 1958a). 
This is, however, not enough for a comparison of knotted 
and knotless connections because the differences in the 
three-dimensional shape of the joint points is neglected 
and this will also influence the resistance in water. 

The determination of the projected area of netting is 
therefore considered an insufficient guide in comparing 
towing or current resistance of netting. For reliable 
comparative results it has to be measured with samples 
of complete netting (bars and joining points together) 
in a towing tank. For the present studies netting samples 
of 1 m 2 were tested normal to the direction of tow 
(N.N. 1960). Since such towing tests are rather expensive, 
the number of samples tested so far is limited and com- 
prehensive tables for the towing resistance of knotted 
netting of different materials and different mesh sizes 
are not available yet. 3 

For the present comparison the same samples (Table f 
and Table II) have been tested. To keep the number of 
samples small, this comparison was limited to a mesh 
size of 50 mm stretched (No. 4, 5 and 6 in Table I and II). 
The results for a towing speed of 0*6 to 2*0 m/sec (1 '2 to 
3-8 knots) are shown graphically in Figs, 6 to 8. 






Raschel, normal 
knotted, 210x18 



Raschel, special 
knotted, 210x21 



Raschel, super 
knotted, 21 Ox 24 

Mesh size 


of bar 








I -10 




1 -14 



3 The present tests have been made in the towing tank of 
Ingenieurschule, Hamburg, by Oberbaurat BischofT. 




Fig. 6. Towing resistance of samples of I m* of netting of equal mesh 

strength normal to the towing direction. Mesh size 49 mm stretched 

mesh shape square. 2a. Raschel, normal, 210x11, diameter of bar 

1-05 mm. 2b knotted, 2 WxJ8 9 diameter oj bar 0-84 mm. 

In spite of the thicker bars of the Raschel netting, no 
difference in the towing resistance could be found in 
comparison with knotted netting of the same mesh 
strength. Obviously the higher towing resistance of the 
thicker mesh bars of the Raschel netting is just compen- 
sated by the lower resistance of the joining points. As 
regards towing resistance, knotless Raschel netting and 
normal knotted netting are practically equal. 

Constancy of mesh size in Raschel netting 

Mesh-size measurements for comparison have to be 
made with mesh measuring gauges applying defined 
pressure. For fine netting the official pressure gauge of 
the Lake of Constance can be used (Florin 1957). For 
heavier netting, e.g., bottom trawls of deep-sea trawlers, 
a special pressure gauge has been developed for the area 
of ICES and ICNAF which measures the opening 
of the meshes only, excluding the knots (Bohl 1961 a). 
For testing the constancy of mesh size, a representative 
number of meshes in a net sample is measured and the 
standard deviation is determined. The test may be done 
with new or used netting according to the purpose of 
the study. 

For the present study, roundfish bottom trawl codends 
made of 'Perlon' were compared. One was single braided 




F<^. 7. Towing resistance of samples ofl m 2 of netting of equal mesh 

strength normal to the towing direction. Mesh size 50 mm stretched, 

mesh shape square. 3a Raschel, special 2/0x77, bar diameter 1-10 

mm. 3b knotted, 210x21, bar diameter 0-92 mm. 

Raschel netting, with short joining points. Two others 
were of knotted netting. With the knotless codend 
some heavy redfish catches had been made. The results 
are given in Table VI (Bohl 1961b). 

Codend type 



Number of 






Mesh size in average 

121 -3 0-1 mm 
128 -8 0-2 
145 -7 0-2 



The standard deviation for the mesh size is much less 
in the knotless Raschel netting than in knotted netting. 
Of course, the type of the Raschel connections will 
influence the constancy of the mesh size, and other con- 
nection types may give different results. 

Comparative catching efficiency of knotted and knotless 
herring driftnets 

As mentioned, the mesh strength is not always the only 
criterion for the exchangeability of knotted and knotless 
netting. In driftnets used for herring fishing in the North 
Sea the bars of the meshes must have a certain diameter 
(1 mm). A smaller diameter would damage the gilled 


fish. Conventional cotton gillnets (knotted), preserved 
against rotting and stiffened as used in German herring 
drifting, have been compared with knotless Raschel 
netting, normal construction (Fig. 4a), Terlon' 420 X 3, 
also treated to give similar physical properties (Table 


Mesh size (bar) 
Mesh strength: 



Bar, diameter 

Cotton Terlon' 

knotted knotless 

25 -3 mm 24 -9 mm 

6 -Okg 15 -5 kg 
6 -3 kg 14 -2 kg 

1 -0 mm 1 -0 mm 

2 -05 g ] -05 g 

Table Vll shows the mesh strength of the Raschel 
nets was much higher than that of the knotted cotton 
nets. This could not be avoided, because it was necessary 
to have the diameter of the mesh bars no smaller than 
1 mm. The knotless nets were much lighter but this, to a 
certain extent, was due to the difference in treatment with 
bonding and protecting agents. 

For this experiment, 11 knotless driftnets were in- 
serted between conventional knotted cotton nets in the 
same fleet of driftnets. For comparative tests of the 
catching efficiency of certain single driftnets operated 



1.2 M io aW M xt xn 40 

Ff^. 5. Towing resistance of samples of 7m 2 of netting of equal mesh 

strength normal to the towing direction. Mesh size 50 mm stretched, 

mesh shape square. 4a Raschel, super 210 x 11, bar diameter /* 14 mm. 

4b knotted, 210 x 24, bar diameter 1*00 mm. 

with a great number of conventional driftnets in the 
same fleet, the catch in all the nets of the whole fleet has 
to be considered. If a small school hits a fleet of driftnets it 
will fill the nets in the section corresponding to its size 
and leave the rest empty. The resulting uneven distribu- 
tion of catch over a long fleet of nets is quite normal in 
herring drifting. In order to avoid that the nets under 
test do not meet statistically inadequate conditions in a 
limited number of observations, a special observation 
method has been developed (v. Brandt 1955). The number 
of fishes gilled in each single driftnet is counted or 
estimated, and represented graphically. In this way it 
can be seen how the catches vary from net to net in a 
fleet. Where the efficiency of nets under tests differs from 
the conventional nets, the more or Jess smooth curve 
of the diagram will be interrupted. 

This method was used earlier to compare catching 
efficiency of knotless *Manryo'-nets (Japanese twisting 
technique) with conventional knotted cotton driftnets for 
herring in the North Sea. No difference could be found 
(v. Brandt, 1958b). 

For the present comparison of conventional knotted 
cotton nets with knotless Raschel nets (Table VII) the 
results of eight sets have been evaluated. 4 Different 
quantities of herring were caught per set (2'5 to 10*5 
tons). The catching efficiency of the Raschel nets tested 
did not differ from the conventional knotted cotton 

For two of these sets the graphs showing the number 
of gilled fish in each net are given in Fig. 9. The knotless 
Raschel nets are marked by underlining. In the other 
sets the position of the nets under test in the fleet of 
nets was also altered. As can be seen from the graphs 
the quantity of the catch in the knotless nets fits well in 
the catch distribution of all nets in the whole fleet and 
the catching efficiency of both net types can therefore 

be considered equal. This is of interest because knotless 
Raschel nets, due to their lower weight, are easier tc 

4 These experiments have been made with the help of Vereinigte 
GlanzstoffFabriken A.C., Wuppertal-EIberfeld, and Plutte, Koecke 
and Company, Wupperta I- Barmen. The Bremen-Vcgesacker 
Fischerei-Gesellschaft kindly allowed the experiments to be 
conducted on one of their drifters by Dipl. Biol. Koops. 

Fig. 9. Example of graphical evaluation of the catches in a fleet of 
herring driftnets for the determination of the comparative catching 
efficiency of single nets of different types. The tested nets are underlined 


Bohl, H.: 1961 a. Vergleichende Untersuchung Uber die Eignung 
von Druckmessgeraten zur Maschenmessung an Schleppnetzcn. 
Arch.f. Fischereiw. XII, pp. 117-137. 

Bohl, H. : 1961 b. German mesh selection experiments on redfish. 
ICES, Comp.Fish.Committee No. 88. 

v. Brandt, A.: 1955. The efficiency of driftnets. Journ. du Cons. 
Int. pour 1'Explorat. de la Mer, XXI, pp. 8-16. 

v. Brandt, A. : 1958 a. Knotenlose Netze. Protokolle zur Fischerci- 
technik 5, pp. 100-112. 

v. Brandt, A.: 1958 b. Die Fa'ngigkeit knoten loser Heringstreib- 
netzc aus Manryo. Informationen flir die Fischwirtschafl, 5, pp. 192- 

Florin, J.: 1957. Em Messgera't zur einheitlichen Bestimmung 
von Netzmaschenweiten. Schweiz. Fischerei-Ztg. 65, p. 243. 

Klust, G.: I960. Synthetisches Netzmaterial fiir die Kutter- und 
Kustenfischerei. Land-u. hauswirtschaftlicher Auswertungs- u. 
Informationsdienst No. p. 191 

N.N.: I960. Uber Widerstandsmessungen an Schleppnetzcn. 
Dtsch. Seiler-Ztg. 79, pp. 196-197. 

The Nippon Seimo Co. Ltd.: 1959. The knotless net. Modern 
Fishing Gear of the World, pp. 107-109. 

Reichel, H.: 1960. Herstellung und Eigenschaften knotenloser 
Netze. Fischereiforschung 3, No. 10, pp. 3-22. 

van Wijngaarden, J. K. : 1959. Testing methods for net twines and 
nets, etc., Modern Fishing Gear of the World, pp. 75-81. 


Knotless Netting in the Norwegian Fisheries 


The use of knotlcss netting in the Norwegian fisheries has increased 
from 17 tons in 1960 to about 200 tons in 1962. Initially all was 
imported but by 1962 19 machines for the production of knotless 
netting of Raschel type had been installed by Norwegian net manu- 
facturers. Consequently only three per cent of the present consump- 
tion of knotless netting is now imported. High tenacity polyamide 
continuous multifilament yarn is used. Most of this netting has up 
until now been of small mesh size and used in purse seines for 
small herring and sprat. Table II gives denier values of equivalent 
plied (twisted) twines and Raschel; the latter tend to be of appreci- 
ably heavier denier for the same breaking strength, but a much 
smaller proportion of the twine length is used in the interlacings 
than in the case of knotting. At present prices in Norway, purse 
seines made from small-meshed knotless netting are 25 to 30 per 
cent cheaper than if made from knotted netting. However, with 
increasing mesh size, a point is reached where knotted netting can 
be produced more economically on conventional machines. 

Filets sans noeuds dans la ptehe Norvegienne 

l/emploi de filets sans noeuds dans la peche Norvegienne a aug- 
ment* de 17 tonnes en 1960 jusqifa 200 tonnes en 1962. Au dbut, 
tous les filets sans noeuds 6taient importes mais en 1962 les pro- 
ducteurs de filets norvtgiens ont installe 19 machines pour la 
fabrication de ces filets, du type Raschel. De ce fait, trois pour cent 
settlement de 1'actuelle consommation de filets sans noeuds, sont 
importes. Les fils sont fails de polyamide a haute tenacite, en multi- 
filament continu. La majeure partie de ces filets ont des petites 
mailles et sont utilises dans les sennes coulissantes pour le hareng 
et Tesprot. Le Tableau II donne les valeurs en deniers des fils equiva- 
lents, tournes et en tresse Raschel ; le dernier est plus lourd en deniers 
pour le meme force de rupture mais dans les entrelacs des mailles, on 
utilise une longueur inferieure a celle des filets noues. En Norvege 
present, les sennes coulissantes faites de filets sans noeuds, a 
petites mailles sont de 25 a 30 pour cent rnoins chers que les filets 
noues. Cependant, avec des mailles agrandies on parvient a obtenir 
sur des machines conventionelles, des filets noues plus economi- 

Redes sin nudos en la industria pesquera Noruega 


El empleo de redes sin nudos en la pesca Noruega ha aumentado 
desde 17 toneladas en 1960 hasta unas 200 en 1962. Inicialmente se 
importaban todas pero para 1962 los fabricantes noruegos habian 
instalado 19 mdquinas para tejer redes sin nudos del tipo Raschel 
con lo cual solamente el tres por ciento de las redes sin nudos 
empleadas actualmente son de importacidn. Los hilos son de 
poliamidos de varios filamentos continups de gran resistencia. 
Hasta ahora casi todas estas redes han sido de malla pcquena y 
se han dedicado a la construcci6n de artes de cerco de jareta para 
la captura de espadin o arenque pequeflo. La Tabla II da los 
valores denier de hilos torcidos y Raschel equivalentes; los ultimos 
tienden a ser de denier bastante mayor para igual resistencia en la 
rotura, pero se emplea una proportion menor de hilos en las ligadas 
que en el caso de las redes de nudos. Actualmente en Noruega las 
redes de cerco de jareta de malla pcquena sin nudos cuestan de un 
25 a un 30 por ciento menos que las anudadas, pero con el aumento 
del tamano de la malla se llega a un punto en el que las anudadas se 
fabrican mas economicamente en las m&quinas corrientes. 

BECAUSE of very limited local production, knotless 
netting used in Norwegian fisheries since 1960 has 
been mainly imported. Annual consumption of this 
type of netting (about 1 7 tons in 1960) gradually increased 
to almost 200 tons in 1962, as can be seen from Table I. 
By 1962, 19 machines for making knotless netting had 
been installed by Norwegian net manufacturers, and 
imports were gradually reduced to only 6'7 tons in 1962, 



Norvald Mugaas 

Statens Fiskeredsksimport 

about three per cent of the total knotless netting used 
that year. Norwegian knotless netting is mainly made of 
high tenacity polyamide continuous multifilament yarn, 
and most of it has been of the small mesh type used in 
purse seines for small herring and sprat. 

TABLE I Import, production, and consumption of knotless 
netting in Norway 1960-1962 

Imported polyamide knotless 

netting, tons . . . . 15-0 

Knotless netting locally produced 
from imported polyamide 

Total Consumption 

1 -6 









When knotless netting was first introduced at the 
beginning of 1960, experiments were carried out by 
inserting sections of knotless netting in the purse seines 
made traditionally of knotted netting. This type of 
netting has also been used on a smaller scale in purse 
seines for big herring which have a somewhat larger 
mesh size, as well as in shrimp trawls and other gear. 

The knotless netting used is of the warp knitted type, 
produced on Raschel type machines, mainly of six-bar 
patterns. This construction method allows a great variety 
of mesh shapes, all based on the same yarn size, depend- 
ing on the number of interlacings; this in turn influences 
apart from shape of mesh, also the strength and weight 
of netting. With only one or two interlacings at the 
joints, the meshes will be square as with knotted netting; 
by increasing the number of interlacings the mesh be- 
comes hexagonal in shape. The square-shaped mesh 
seems to be the best as it provides equal mesh bars in all 

Experience in making knotless netting indicates that 
the best ratio of strength to weight is got by using filament 
yarn of the same count throughout in each section, i.e., 
by not mixing different yarn counts in one netting 
construction. Basic yarns used for different types of 
netting are standard counts available in polyamide 
yarn, such as 210d, 300d, 420d, 630d and 840 denier. 
Table II gives the Raschel construction denier values 
which can be substituted for common plied twines. The 
indicated total denier is the approximate value and takes 
into account the shrinkage of the yarn due to twisting 
in plied twines and to knitting in Raschel construction 

continued on page 97 

Knotless Fishing Nets on Raschel Equipment 
in Italy 


At the end of 1962 about 20 Raschel looms were in operation in 
Italy. Each machine has a mean capacity of 30,000 kg per year 
The production of knotted nets of polyamide fibre has in the past 
three years, averaged 500 tons per year in Italy. The main reason for 
the rapid introduction of the Raschel method is lower production 
costs due to saving in material and higher operating speed, ranging 
from 350 courses per minute (eight guide-bars) to 600 courses (four 
guide-bars) ; the coarser the yarn, the fewer are the courses in the unit 
length. Depending on the thread counts, the production of 100 kg of 
net on a modified Raschel machine, four guide-bars, 700 meshes, 
takes from three hours (210/90) to 22 hours (210/4). The smaller 
the mesh, the more apparent the advantage of the Raschel method 
becomes; 40 mm is considered the break-even limit, except for 
nets made of high-denier yarns. In Italy, knotless nets are less 
expensive than knotted for almost all mesh dimensions. High 
tenacity polyamide multifilament yarns, bright 210 denier (and 
multiples) are generally used. The best combination of denier/ 
interlacing/machine gauge has not yet been determined and is still 
the subject of research. Run-proof nets are aimed at but not yet 
achieved. Heat-setting is often still used according to the technique 
for knotted nets but equipment for continuous dry setting is being 

Filets>aTis>oeuds>roduits>ur]equipement]Raschel en Italic 

A la fin de 1 962, environ 20 metiers Raschel fonctionnaient en Italic. 
Chaque machine produit, en moyenne, 30.000 kilos par an. La 


Mario Damiani 

Societa Rhodiatoce S.p.A. 

production de filets sans noeuds en fibre polyamide, pendant les 
trois dernieres annees a 6te en moyenne de 500 tonnes par an pour 
r Italic. La raison principale de 1'introduction rapide de la m6thodc 
Raschel est le prix de revient de production plus has. La mdthodc 
Raschel en efiet, permet une economic de materiel et une Vitesse 
d'operation plus elevee, entre 350 et 600 courses par minute; plus 
grossier sera le fil, moins nombreuses seront les courses par unite 
de longueur. Selon le nombre des fils, la production dc 100 kilos de 
filet sur une machine Raschel de type modifte, quatre barreaux de 
guide, 700 mailles, demande de trois heures (210/90) & 22 heures 
(210/4). L'avantage de la mtthode Raschel est plus apparent pour 
les petites mailles; des 40 mm, il devient plus coftteux que pour les 
filets nou6s except&s pour les filets fails de fil de fort denier. En Italic 

continued from page 96 

TABLE II Denier values of equivalent plied twines and Raschel 

Twine No. 

Total Denier 

Raschel Con- 

Total Denier 


in twine 


in Raschel 






















Of the total length of twine used in making a mesh, a 
substantial part goes into the knot in knotted netting 
and this portion increases as the mesh size becomes 
smaller. In Raschel netting a much smaller proportion of 
yarn or twine length is used in interlacings. Therefore, 
the ratio of total denier for plied twine and Raschel 
twine does not represent the correct ratio between the 
total material in the same size panels of knotted and 
knotless construction. Furthermore, owing to the differ- 
ent construction of the joints (knots and interlacings), 
the percentage of twine strength lost in the joints cannot 
be compared (see Table III). 

Production capacity of Raschel machines, compared 
with that of machines for producing conventional knotted 
netting, presents most favourable conditions for pro- 
ducing small mesh knotless netting (as used in purse 
seines) and the bulk of knotless netting used in Norway 
has been for such nets. 

As mesh size increases, a point is reached where knotted 
netting can be produced more economically on conven- 
tional machines. Where production costs for both types 

of netting are the same, the deciding factors must then 
be the serviceability of the netting in operation, such as 
bulk, resistance, etc. As knotless nets are rather new 
in Norwegian fisheries, it is most difficult to secure a 
valid opinion. However, the increasing demand for 
knotless netting shows the great interest created. At 
present prices in Norway, a complete purse seine made 
from small-meshed knotless netting will be approximately 
25 to 30 per cent cheaper than if made from the conven- 
tional type of knotted netting. 

TABLE 111 Weight and strength of comparable knotted and 
knotless netting 

Mesh strengths and weight of panel 600 meshes x 100 metres 

Polyamide knotted netting Polyamide, knotless netting 

(single knots) 
Mesh size: 35-7-5 mm 

Twine No. 


Wet mesh 










































The knotless netting is given preservative or stiffen- 
ing treatment similar to the knotted netting; coal-tar, 
bonding, dyeing, etc., are used depending on the end 
use. Coal-tar increases the mesh strength of knotted 
netting but this effect is less noticeable with knotless 


ks filets sans nocuds sont moins chcrs que les filets noufc pour pros- 
que toutcs les dimensions de mailies. Les fils de polyanude multifila- 
ments de haute tenacite, brillants, de 210 denier (et multiples) sont 
generalemcnt utilises pour la fabrication des filets. La meilleure 

etc determinee et la recherche se poursuit. On essaie de produire 
des filets qui sont indemaillables mais on n'y est pas encore parvenu. 
La fixation par la chaleur est encore utilise* tres souvent suivant la 
technique des filets noues mais Tequipement pour la fixation & sec 
est toujours en cours de developpement. 

Redes sin irados producktas por las 

Raschel en Italia 


A fines de 1962 funcionaban en Italia unos 20 telares Raschel. Cada 
telar ticnc una cajpacidad media de 30.000 kg al afto. En los ultimos 
tres afios se ban fabricado en Italia, por tcrmino medio, 500 tons de 
redes anudadas de fibres de poliamidos. La principal raz6n de la 
rapida propagaci6n del metodo Raschel es que los costos de produc- 
tion son mas bajos debido a un ahorro de material y a una mayor 
velocidad de funcionamiento que va de 350 puntos por minuto 
(ocho barras-guia) a 600 puntos por minuto (cuarto barras-guia). 
Cuanto mas grueso es el hilo menos puntos hay que dar poJ 
unidad de longitud. Segun el numero del hilo, la producci6n de 
100 kg de red en una maquina Raschel modificada de cuatro barras- 
guia, 700 mallas, tarda de tres horas (210/90) a 22 horas (210/4). Las 
ventajas del metodo Raschel son mayores cuanto mas pequefla es 
la malla, considerandose la de 40 mm como el limite, excepto en el 
caso de redes hechas de hilo de mas denier. En Italia las redes sin 
nudos son mas baratas que las anudadas en casi todas las dimen- 
siones de malla. Generalmente, se emplean poliamidos de varies 
filamentos de 210 denier (y multiples). Todavia no se ha detcrminado 
la mejor combinaci6n de denier/ligamcnto/y calibre de la maquina, 
por lo que continuan los estudios. Tampoco se ha conseguido 
fabricar redes en las que no se corran los nudos. Todavia se usa con 
frecuencia la fijacion termica segun la tecnica para las redes 
anudadas, pero se fabrican maquinas para la fijacidn continua en 

FROM West Germany the Raschel method of manu- 
facturing knotless fishing nets spread quickly to 
other European countries. In Italy lately, and especially 
last year, all the most important net manufacturers have 
added Raschel looms to the conventional knotting 

At the end of 1962, about 20 Raschel looms were in 

Most of these looms have six guide-bars (also four 
and eight), 124-inch and 139-inch width (also 100-inch), 
while gauges range between 18 and 28 needles per two in. 
The most popular type is 24 gauge. 

For a country like Italy, where the production of 
knotted nets (polyamide fibre) has averaged 500,000 kg 
per year in the last three years, this number of Raschel 
looms is undoubtedly high considering the production 
capacity of these machines on either light or heavy nets, 
a machine has a mean capacity of 30,000 kg per year. 

The reason for this new trend is economic; production 
costs will be lower with a considerable saving of material 
by eliminating knots. The weight of knots is often out of 
proportion to the weight of net. Elimination of knots 
becomes more important as meshes grow smaller and 
the threads become coarser. In addition, towing speed 
can be noticeably raised. 

Actual knitting speed ranges, roughly, from 600 
courses (four guide-bars) to 350 courses per minute 
(eight guide-bars). 

The quantity of net produced in the unit of time (most 
important as nets are sold by weight) depends also upon 
the number of courses, the width of the machine and 


the denier of the yarn used. The coarser the yarn, the 
fewer are the courses in the unit length; the runnage per 
hour for 840 den (8-9 courses per cm) is about twice that 
of 210 den (15-16 courses per cm), while the weight of 
the net produced is three times as much (Table 1). 

Table I Production of 100 kg of net on a modified Raschel machine, 
four guide-bars, 28 gg, 700 meshes (thread counts are expressed in 
deniers, as is customary for the twines used in knotted fishing nets) 

(210x6)- 16,, 

(210x27) =5,, 
(210x90) -3,, 

There are two advantages: in material and in proces- 
sing. Firstly, for Raschel machines either flat or low- 
twist yarns are used, whereas for knotted net machinery 
higher priced twine is required. Average price ratio of 
ply yarns (on beams) versus twine is 1/1*2. Secondly, 
Raschel looms are equipped with warp beams on which 
a great length of yarn is wound ; this means that they can 
be operated for a much longer time than conventional 
machines, the performance of which is limited because 
of low bobbin capacity. 

The advantage of the Raschel method over the conven- 
tional one becomes more apparent the smaller the mesh. 
Actually, in the first case the yield, which does not depend 
upon the mesh size, is inversely proportional to the 
number of courses per cm; in the second case the yield 
increases with the size of the mesh. Admittedly 40 mm is 
the limit beyond which the knotting machines become 
more economical. It is clear, however, that this limit 
increases considerably in the case of nets made of high 
denier yarns. 

In Italy, knotless nets are less expensive than others 
for almost all mesh dimensions, as indicated in Fig. 1 
which shows the price in lire per kilo of the two types 
of commercial nets on the basis of mesh size and weight 
of the net. 



Mg.l. Frio* llrw'lcg of two tyiws of ooimaroial 

Of the rit. 

a? 5000 



Q __ 

.... . . s /// p 

u 4000 



/^v^ 4 " 6 " 9 




.- ^^21*24 




i -. Prom48to90 




30 40 ^0 

mcsbiizt: mm 

In the manufacture of knotless nets high tenacity 
polyamide multifilament yarns bright 210 den (and its 
multiples 420 and 630 den) and 840 den (and its multiples 
up to 8,400 den) are generally used. For very fine nets 
also 100 den finds some use. 

The most important characteristics of these yarns are 
shown in Table II (the figures refer to Rhodiatoce 
*Nailon' 1 yarns). 


Denier Tenacity Extension Breaking 3-pIy yarns Knot 
g/den at break length/ breaking strength 

% km length/km % 

210 7-5 22 67 59 68 

840 8-2 16 73 56 55 

Singles are generally twistless or low-twist (130-160 
turns/metre). As a rule the twist for ply yarns ranges 
from 150 to 110 turns/metre. Usually the yarns are 
supplied on knitting beams. The choice of the denier is 
dependent on the type of net desired and on the gauge of 
the loom. 

Table 111 shows the relation existing between these 
data. (The figures refer to a six guide-bar machine.) 

Construction of knotless nets does not differ substan- 
tially from that adopted in other countries. Nets are 
composed of many threads forming the mesh sides, 
which are interlaced at regular intervals at "cross-over 
points" at the apex of the mesh. Threads correspond 
to twines in the same way as cross-over points corres- 
pond to knots in the knotted nets. 

1 'Nailon' is the registered trademark of Socict& Rhodiatoce 
S.p.A. in Italy. 

In the most common version, a thread is formed by 
three ends; two laid-in threads (1 and 3) and one (2) 
looped, entwined together. The laid-in threads are in 
an almost rectilineal position, the looped threads (2) 
follow a more complicated path (Fig. 2). 

Owing to this construction, the task of bearing the 
net load is mostly carried by laid-in threads, the length 
of which is about one-fourth that of the looped thread 
(2). In general, the looped threads (2) may be lower of 
denier than the other two. Yet, as the exact value of the 
possible reduction in count has not been established so 
far, this technique is not adopted in Italy at present. 

At the mesh apex there are different types of joints, 
depending upon the way the yarns are interlaced. The 
more complex the structure of these joints, the stronger 
and more durable they are, according to whether only 
the looped threads or also the laid-in threads are en- 
twined and depending upon the number of binding 
points (generally two to four). 

In Fig. 3 the most common stitches used in Italy are 

It has not been established as yet which are the most 
suitable constructions for maximum mesh wet-strength. 
To avoid damaging the catch, round threads are being 
developed, but this is only a minor point. 




Fig. 2. 

Max. denier 









Fig. 3. 


gauge Needles width 100 in width 124 in 

per in needles meshes needles meshes 

8 4 400 200 496 248 

12 6 600 300 744 372 

14 7 700 350 868 434 

16 8 800 400 992 496 

18 9 900 450 1116 558 

24 12 1200 600 1488 744 

28 14 1400 700 1736 868 

32 16 1600 800 1984 992 

Fig. 4. 

width 139 in 












The best combination of dcnicr/interlacing/machinc 
gauge has not yet been found and research is being carried 
out in this direction. The field is still too new. For exam- 
ple, it has already been seen that interlacing A (and almost 
all new machines are equipped with the glider chains for 
this interlacing) does not give very satisfactory results. 
In particular the threads have a tendency to ladder 
when some ends break. For this reason several net 
manufacturers favour stitch B. 

The system, followed by some manufacturers, of letting 
yarns (2) pass from one mesh to another (Fig. 4) does 
not seem to produce much stronger joints but makes 
them more resistant to laddering. Absolutely run-proof 

nets have not yet been produced. Actual experience, 
though limited, has shown, however, that heat-setting and 
impregnation of the nets ensure satisfactory performance 
in this respect. Often heat-setting is still carried out 
according to the technique used for knotted nets, i.e., 
by wrapping the net around a steel cylinder and setting 
it in either boiling water or steam. Equipment for con- 
tinuous dry setting by means of hot air is, however, 
being developed. 

Table IV gives figures of the outstanding characteristics 
of several types of knotless nets with type A joint made 
from 'Nailon' yarn. For two of them a direct comparison 
is established with corresponding knotted nets. 





Moat M! denier 



dry k*- < 





dryU) wet 

dry (3Jtet 



T. C. 



fcg I 




7 kg 


420 420 



8,3 94* 





16 1 14 

7 6 


13 -* 12 




12,4 8tf 




14 t 12 

10 8 


14 -^11 

840 840 



16 94* 





30 I 29 

13 12 


23 - 21 




30 90* 





24 1 22 

23 19 


21 -* 19 

2520 3360 





125 4 97 

48 47 

^. Table IV 

93 * 83 

Tha email figures rafar to knot tad nata corraapondfng to tha knotlaaa nits considered. 

CAPTION: - T Ufd-in threads 
C looped threads 
CR braakfng load 
I . breaking length 

(1) 1 x 10 aaahaa 

(2) Msh'sfreclnens: 20 ct x 1 aesh : 4 lengthwise ; - crosswise 

(3) according to drawing 

(4) Mnlng 100 it th prlc* of th* knotUis ntt. 


Resistance a la Rupture de Filets sans Noeuds 

La construction de filets sans noeuds du type Raschel s'est de" velop- 
p6e en Europe depuis la guerre. Un des avantages de ces filets sur 
les filets nou6s de type traditionnel est le manque de stability des 
noeuds de ces derniers. Les filets peuvent etre construits sur des 
machines Raschel utilises par Industrie textile depuis de nom- 
breuses ann6es, Pour les filets de pdche les barres des mailles sont 
composes d'un ou plusieurs composants tricot6s, support6s par des 
composants entrelac&s et il existe diff6rentes fa$ons de faire la 
connexion des mailles. Jusqu'ici les filets sans noeuds ont une 
resistance a la rupture plus faible dans le sens transversal que dans 
le sens longitudinal. A la suite d'une etude faite en Italic, un nouveau 
type de filet sans noeuds a 6t6 d6velopp6 dont la force de rupture est 
a peu pits ggale dans Unites les directions. La communication 
decrit la construction de ce filet dont les composants simples se 
divisent a chaque intersection de mailles, suivant une sequence 
sinueuse pendant laquelle, non seulement le nombre de composants 
formant les barres des mailles mais aussi le fil des barres, varient 
ce qui fait que les forces s'exerc^uit sur le filet sont ^parties sur 
tous les fils. Les r&sultats des dpreuves montrent que pour le type 
normal de filet sans noeud perd de 15 a 20 pour cent de resistance 
transversale tandis que la nouvelle construction de filet ne perd que 
4,5 pour cent en comparaison avec la direction longitudinale. 

Breaking strength of knot less webbing 


The construction of knotless net of the Raschel type has developed 
in Europe since the war. One of the advantages of knotless nets over 
the traditional knotted types lies in the knot-fastness. The nets can 
be made on modern Raschel machines used for many years in the 
textile industry. For fishing nets the mesh bars are composed of 
one or more knitted components supported by intertwining com- 
ponents and there are various ways of making the mesh connections. 
In most cases the webbing has a lower strength when pulled across 
the connections. Resulting from a study made in Italy, a new type 
of knotless net was developed which has almost equal strength in 
all directions. The paper describes the construction of such netting 
whereby the number of straight components making up the bars 
divide at each mesh connection according to a regular sequence. 
The knitted components as well as the straight furthermore change 
from bar row at regular intervals, resulting in a thorough spread 
of the components as knitting progresses. Test results showed that 
the usual type of knotless webbing had 1 5 to 21 per cent less breaking 
strength across the connections than with the connections. The new 
type of webbing loses only 4*5 per cent when tested in the across 
the connections direction. 

Resistencia a la rotura de redes sin nudos 


La fabricaci6n de redes sin nudos de tipo Raschel comenzo en 
Europa despufc de la guerra; con ellas se ha vencido el grave 
inconveniente del deslizamiento o corrimiento de nudos que ocurre 
en las tradicionales. Pueden fabricarse en las mquinas Raschel que 
cmplea la industria textil desde hace varios aflos. Para las redes de 
pesca las mallas las forman uno o mas componentes trenzados 
sustentados por componentes entrelazados y existen diversas 
maneras de hacer las conexiones de las mallas. Hasta ahora los 
pafios sin nudos tenian una resistencia a la rotura mas debil en el 
sentido transversal que en el longitudinal. Como resultado de un 
estudio realizado en Italia se fabrica un nuevo tipo de redes sin 
nudos en el que la resistencia es casi igual en todas las direcciones. 
Describe la poncncia la fabricaci6n de estas redes en las que los 
componentes senciUos se dividen en cada interseccidn de mallas 
segun una serie sinuosa en la que el numero de componentes que 
forman las mallas, asf como el hilo de cstas, varian de manera aue 
las fuerzas a que est* sometida la red se reparten por todos los 
hilos. Los resultados de los ensayos son que la clase normal de 
redes sin nudos tienen de 15 a 21 por ciento menos resistencia trans- 
versal, en tanto que con la manera nueva de fabncar solo se pierde el 
cuatro por ciento de resistencia cuando la red se somete a esfuerzos 


Francesco Pianaroli 

LA fabrication des filets sans nocuds de type Raschel 
s'est dlveloppie en Europe depuis la fin de la guerre. 
Ces filets se sont rvils tris utiles par suite du manque 
de stability des noeuds des filets traditionnels fabriquis 
avec des fils en fibres synth6tiques. Un autre avantage 
des filets sans noeuds est la dimension constante des 
mailles. Ces filets pouvant etre tricots sur des metiers 
Raschel normaux, leur emploi dans le domaine de la 
pfiche s'est rapidement diveloppi. 

Les filets sans noeuds de type Raschel sont fabriqu6s 
de la facon suivante: les fils tricotds en chainette, s'entre- 
lacent des distances dtermines pour former les mailles. 
Les fils tricotis servant de base peuvent etre renforc6s 
par rinsertion d'un ou plusieurs fils simples, suivant 
diflterentes techniques. II existe aussi plusieurs mithodes 
de connexion des mailles et parmi les plus typiques, on 
peut citer celle propose aux U.S.A. par M. Henry 
Goldsmith et celles utilises par quelques fabricants 
europdens et japonais, depuis 1955. 

Aujourd'hui, la qualitl des filets de peche produits dans 
le monde est assez satisfaisante bien que I'expdrience ait 
dmontr qu'il dtait souhaitable d'augmenter encore 
les caractSristiques de resistance et de durfe. En Italic, 
des essais effectuis en Laboratoire ont confirm6 que par 
la recherche, on pouvait encore amliorer la resistance 
des entrelacements. 

D'une manure gdn&rale, la resistance des filets sans 
noeuds & la rupture par suite de traction transversale est 
de beaucoup inftrieure & la resistance & la traction longi- 
tudinale. En outre, la tension n'est pas 6galement distri- 
bu6e sur les fils composants, dans les barres des mailles 
aussi bien que sans les connexions. 

Les etudes effectives avaient pour but: 

(a) d'augmenter la resistance des filets par une 
meilleure distribution des forces; 

(b) d'uniformiser la resistance des fils eiementaires 
dans les entrelacements afin de lutter aussi bien 
contre les tractions longitudinales que transversales ; 

(c) de creer une suite de points d'interchangement des 
fils uniformement rdpandus dans le filet et avec 
la plus grande densite possible, ce qui augmentera la 
resistance & la rupture du filet par suite de forces 
transversales exercees sur les entrelacements. 

De trfcs bons r&ultats ont ete obtenus par la mdthode 
que nous decrivons ici dans ses grandes lignes. Les 
filets sont constants par des metiers "Raschel" normaux 
et les barres des mailles sont consdtu6cs par des fils sim- 
ples et des fils tricots en chatnette, les premiers s'entre- 
laoant et se croisant dans les seconds de telle fa$on que 


lorsquc le filet est soumis & ure force, la tension est 
rtpartic uniformemcnt sur tous les fils composants. 

Pour obtenir ce rfsultat, dcs paires multiples de 
"rangs" formant les maillcs dans le sens de la construc- 
tion du filet ont M entrelac&s de maniire & ce que les 
fils puisscnt s'entrelacer et se Sparer et ce, aussi bien 
pour les fils tricotfe en chainettc que pour les fils simples. 

Ainsi, dans une maillc les deux barres convergentes de 
gauche et de droite ferment une demi-maille; dans 
chaquc barrc, les fils composants continuent conjointe- 
ment chacun avec sa sequence sinueuse typique, jusqu'4 
un noeud du premier rang; & ce moment on proofcde 
& un fchange multiple parmi les aiguilles des groupes de 
fils voisins, en chainettc, de telle sorte qu'apris s'etre 
plusieurs fois croiss pour constituer 1'entrelacement, 
les fils passeiit au rang suivant, dans la "file" voisine 
respectivement droite et gauche, pour former la barre 
inverse correspondante d'un nouveau rang. En meme 
temps, les fils simples de chaque barre, apris s'Stre 
crois6s dans Fentrelacement, se divisent en deux groupes 
ingaux, les uns passant dans le rang suivant & la "file" 
de droit et les autres dans celle de gauche pour former, 
avec les fils tricotfe, les barres d'un nouveau rang. 
Successivement, dans les entrelacements du deuxiime 
rang, la mme division des fils simples se reproduit mais 
les groupes s'invertissent tous les deux rangs. En meme 
temps, au deuxiime rang les groupes de fils tricotls en 
chainctte, de deux barres voisines font un double 
exchange sur les aiguilles, chacun d'eux restant & la 
mme file, pour former le troisifcme rang. Les entrelace- 
ments du troisiime rang seront done une repetition des 
mouvements cites pour le premier rang, mais le nombre 
de .fils de chaque file sera interverti. Le sch&ma des 
mouvements des fils est reprsent dans la figure 1. 

Les filets construits selon cette mfthode ont donn6 des 
r&ultats tris satisfaisants en ce qui concerne la resistance 
& la rupture centre les forces de toutes directions. Le 
petit tableau ci-dessous donne les rsultats obtenus sur 
dififerents types de filets examines. Le filet nou utilise 
tait fabrique en fibre polyamide 66 210d/9, de 7 g/d de 
tenacite et comportait 1000 mailles de largeur sur un m 
de hauteur et pesait 0,700 kg. A une tnacit6 de 7 g/d, 
la charge thforique de rupture de chaque fil aurait dfi 
Stre 210x9x0,007=13,330 kg mais les valeurs trouvfes 
pour les charges de rupture longitudinale et transversale 
etaient de 8 kg et 7,200 kg respectivement. Cette dimi- 
nution de resistance est due & la perte de 40 pour cent 
dans les noeuds. 

Des essais ont t6 effectuds sur quelques types de filets 
sans noeuds, fabriqus en Italic et dans d'autres pays 
d'Europe. Sur des sections de filet de 1000 mailles 
sur 1 m, pesant chacune entre 690 et 720 grammes, les 
charges de rupture ont t tris inferieures & celles des 
filets nouls et accusaient une perte de 15 21 pour cent 
dans le sens transversal. 

Des essais sur un filet sans noeuds construit d'apres le 
syst&me d&rit plus haut out montri que la resistance & 
la rupture sous des forces transvcrsales et longitudinales 
6tait tris sup6rieure & celle des autres constructions. 

Fig. 1. Repartition des fils composants dans la formation de\ mailles. 

Type dc filet 

Filet nou&, traditionnel poids: 
0,7 kg/1 000 mailles haut. 1m 

Filet sans noeuds, en gnral 

Filet sans noeuds construit d'apres 
le systeme 6nonc6 

Charge de 



en kg/fil 


Charge de 

en kg fil 



Le r^sultat le plus rcmarquable est qu'avec la methode 
de fabrication d^crite, la resistance ^ la rupture est 
augment^e, bien que la resistance de certaines barres 
soit moindre que celle des autres constructions. Ceci 
a t obtenu en proportionnant les fils simples et tricots 
composant les barres des mailles et en faisant participer 
plus ftroitement les fils simples dans la formation des 


Monofilaments in Fishing 


Synthetic fibres became widely used in the fishing industry several 
years ago, but mainly in the form of twines made of continuous 
imiltifilaments (in the range of 2-25 denier/filament). During the 
last few years a major breakthrough has been witnessed in the use 
of monofilaments, i.e., filaments of 50-1,000 denier. Before mono- 
filaments could make any dramatic impact on the traditional 
market, polymers had to be produced giving fibres with the follow- 
ing characteristics: (a) sufficient flexibility, (b) high tenacity (c) 
capability of manufacture at high speed and (d) availability at 
commercially attractive prices. The bending moment of a circular 
monofil varies in proportion to the fourth power of the diameter. 
Shapes of mononl cross-sections are now mainly circular, but 
rectangular as well as ribbon-like cross-sections have been used in an 
endeavour to increase flexibility and firmness of knot. However, 
it seems that to be effective in this way the cross-section must be 
extended to a ribbon-like form, but then other factors, notably 
abrasion resistance, can become critical. In netting, the wet knotted 
strength is of major importance, as well as the load/extension 
characteristics. These values are given in tables for monofilaments 
of polyvinylidene chloride, nylon, polyethylene and polypropylene, 
as well as changes in tenacity at varying temperatures. High tenaci- 
ties can be achieved through stretching during spinning, but at the 
expense of extensibility, and a favourable balance must be struck 
between these properties. From experiments it appears that p. load of 
up to two g/den is commonly applied to monofilaments during 
normal trawling operations, and the extensibility at such loads is 
expressed in graphs in this paper. Test values are also given for 
other properties such as creep, elastic recovery and impact strength, 
stiffness, knot stability, abrasion resistance, towing resistance, rot- 
proof characteristics and resistance to sunlight. The use of mono- 
filaments in certain types of fishing gears is discussed, such as in 
Danish seines, wing trawls, midwater trawls, deep-sea trawls, 
shrimp nets, lobster pots, salmon trapnets and longlines, as well as 
the use of monofilaments for ropes. 

Monofilaments dans la pgche 


Depuis quelques annees les fibres synthetiques sont bien connues 
dans 1 Industrie des peches, principalement sous forme de fils 
retordus en filaments continus Centre 2-25 denier/filament). Au 
cours de ces dernieres annexes F utilisation des monofilaments c'est- 
a-dire des filaments de 50-1.000 deniers s'est dvelopp6e consider- 
ablement. Pour que les monofilaments deviennent acceptables 
par le march* traditionnel, il fallait des polymers donnant des 
fibres ayant les caracteristiques suivantes: (a) flexibility suffisante; 
(b) haute resistance a la rupture; (c) possibility de fabrication 
acceieree; (d) disponibilite a des prix interessants. Le moment de 
flechissement d'un monofil circulaire varie en proportion a la 
4eme puissance de son diam&re. Les coupes des monofilaments 
sont normalement circulaires mais des formes allant du rectangle 
jusqu'au ruban ont et utilisees au cours d'essais pour accroitre la 
flexibilite et la fermete des noeuds. II semble que pour ces deux 
caracteristiques, les fils en forme de ruban sont les plus efficaces 
mais alors d'autres facteurs deviennent critiques, notamment la 
resistance a 1'abrasion. La force de rupture du filet noue mouille 
est de la plus grande importance de meme que les caracteristiques de 
force/extension. Ces valeurs sont donnees dans des tableaux pour 
des monofilaments de polyvinyliddne chloride, nylon, polyethylene 
et polypropylene ainsi que les variations de resistance a la rupture a 
des temperatures differentes. De hautes forces de rupture peuvent 
6tre obtenues par etirage pendant le filage mais au detriment de 
1'extension de sorte qu'un 6quilibre favorable doit fctre trouve entre 
ces proprietees. Les r6sultats des experiences ont montre qu'une 
force de deux g/den est nonnalement applique aux monofilaments 
pendant des operations normalcs de chalutage et 1'extensibilite sous 
de telles forces est exprim6e dans un graphique. Sont donnees 
egalement les valeurs determiners au cours des experiences pour les 
autres proprietes des monofilaments telles que: reprise eiastique, 
resistance aux chocs, raideur, stabilite des noeuds, resistance a 
Tabrasion, resistance a 1'avance, caracteristiques de poumssement 
et resistance aux rayons solaires. L'utilisation des monofilaments 
dans certains types d'cngins de pfiche tels que les sennes danoises, 
les differcnts types de chalut, les filets a crevettes, ies nasses, les 
trappes pour saumon, les palangres aussi bien qu en cordene est 
traitec dans cette communication. 


D. F. C. Ede 

Resin Products Ltd 

W. Henstead 

Celanese Ltd 

Los monofilamentos en la pesca 


La industria pesquera emplea mucho desde hace varies aftos fibras 
sinteticas, principalmente en la forma de hilos hechos de multi- 
filamentos continuos (en la gama de 2-25 denier/filamento). En los 
ultimos anos se hand hecho prodigiosos adelantos en la fabricaci6n 
de monofilamentos, y actualmente se obtienen de 50 a 1 .000 denier. 
Antes de que estos monofilamentos pudieran entrar en el mcrcado 
traditional, tuvieron que producirse polimeros que dieran fibras con 
las siguientes caracteri sticas : (a) suficicnte flcxibilidad, (b) gran 
tenacidad, (c) capacidad de manufactura a gran vclocidad y, (d) 
disponibilidad a precios interesantes. El momenta de flexibn de un 
monofilamento circular varia en proporci6n con la cuarta potencia 
del diametro. Las formas de las secciones transversales de los mono- 
filamentos son principalmente circulares, pero tambien se emplean 
rectangulares y como cintas, para tratar de incrementar la flexibilidad 
y firmeza del nudo. Sin embargo, para que sea eficaz, la secci6n 
transversal debe ser como una cinta, pero entonces entran en juego 
otros factores, principalmente la resistencia a la abrasion, que 
pueden llegar a ser criticos. En los paftos para redes la resistencia 
del nudo humedo es de la mayor importancia, y tambien lo son las 
caracteristicas de carga y estiramiento. Estos indices se dan en 
tablas para monofilamentos de cloruro de pplivinilideno, nylon, 
polietilcno y polipropileno, asi como los cambios en la tenacidad a 
diyersas temperatures. Puede lograrse gran tenacidad estirando 
mientras se teje pero a expensas de la extensibilidad, por lo que hay 
que encontrar un equilibno favorable entre estas propiedades. De los 
experimentos se deduce que una carga hasta de 2 g/den se aplica nor- 
malmente a los monofilamentos durante la pesca a) arrastrc; a exten- 
sibilidad a tales cargas se expresa graficamente en esta comuni- 
cacidn. Tambien se dan valores obtenidos en ensayos de otras 
propiedades como deslizamiento, recuperacidn elastica y resis- 
tencia a los impactos, rigidez, estabilidad del nudo, resistencia a la 
abrasi6n y al remolque, resistencia a la podredumbre y a la luz del 
sol. Se examina el cmplco de monofilamentos en artes de pesca 
como los de arrastre danses, pernadas de artes de arrastrc, artes 
flotantes, artes para la pesca de gran altura, redes camaroneras, 
nasas langosteras, trampas y palangres salmoneros, y el empleo de 
monofilamentos para cuerdas. 

WE deal in this paper with markets and applications 
held traditionally by hard fibres, manila, hemp and 
sisal, and soft fibres, linen and cotton. Over a period of 
years the industry had built itself up to a high degree of 
efficiency with stability in price. 

Developments in fibre-forming polymers began to 
break new ground in the post-war period, nylon being 
first as a fibre possessing exceptionally high tenacity. 
Other properties were excellent impact and energy 
absorbing characteristics, together with resistance to 


These fibres, which were already established in apparel 
uses, possessed excellent flexibility due to their fineness 
(a range of 5-25 denier/filament). The logical develop- 
ment was the use of multifilament yarns in place of soft 
fibres, linen and cotton, followed by the introduction of 
monofilamcnt yarns. These monofilaments were in the 
range 50-1,000 denier/filament. 

Having made the distinction between the multifila- 
ments and monofilaments, we follow the progress of 
monofilament materials in relationship to other syn- 
thetic yarns. Before monofilaments could make any 
dramatic impact on traditional markets, polymers had to 
be produced, giving fibres having these characteristics: 

(a) Sufficient flexibility to be capable of yarn, strand 
and net production on conventional machinery. 

(b) A high tenacity. 

(c) The capability of manufacture at high speed. 

(d) Availability at a commercially attractive price. 

Nylon monofilaments 

Earliest monofilaments were made from nylon and were 
soon used for very fine gillnets in Northern European 
lakes. They had the great advantage of high strength for 
small filament diameter and were relatively invisible in 
the water. Further development in larger diameter 
filaments was limited, firstly, by rapid increase in stiffness 
and, secondly by, commercial considerations, which 
tended to favour the promotion of multifilament 
yarns and staple fibres. More recently, in Germany, 
where nylon has reached a high level of acceptance, 
nylon monofilaments have been used in large rope 
structures alongside nylon multifilaments. The former 
confer on the rope a very high degree of abrasion resist- 
ance, whilst the multifilaments bear the major load. 

It will be seen that the basic stiffness of nylon tends to 
limit its use as a fibre except in fine diameter or in large 
rope structures where flexibility is not so important. 
To some extent these remarks apply also to polypropy- 
lene, particularly when aiming for tenacities required by 
everyday applications. 

Polyolefin monofilaments 

It will be appreciated from the foregoing that a more 
widespread replacement of traditional fibres by mono- 
filaments could occur when a sufficiently flexible filament 
in the higher denier range had been developed. Such a 
fibre did, in fact, emerge in the earlier days of the poly- 
olefins. The earliest fibres produced were spun from 
conventional (low density) polyethylene and gave fila- 
ments of high flexibility but very low tenacity (in the 
range 1-0 to 1-5 gpd). With such a low tenacity there 
was little chance of use in cordage applications, and it 
was not until linear polyethylene had been spun that 
high tenacity filaments were possible. These gave tenaci- 
ties of the order of 5-6 gpd but were rather less flexible 
than those produced from conventional polyethylene. 
It was possible, however, to produce right from the 
outset a complete range of twines and rope yarns, with 
the exception of very fine twines where the multifilament 
yarns still held their own. 


By 1957/1958, nylon multifilament had secured a very 
prominent position in the netting field and was in wide- 
spread use in gillnets of all types, for purse seine nets, 
principally in Norway, and for herring driftnetting. All 
these end uses required excellent twine flexibility for effec- 
tive fishing; nylon, and to a lesser extent the polyester 
fibres, provided this in ample measure. Although some 
synthetic ropes were in existence, their acceptance at 
this time in the fishing industry was not high and current 
usage was confined more to pure marine fields where 
nylon towing ropes and hawsers had proved very effective. 

During the last few years, major breakthroughs in 
net and rope have been witnessed much of it associated 
with the widespread use of monofilaments. 

There has also been a pronounced narrowing of the 
price gap between the natural fibres and the man-made 
fibres. This review is intended to convey in straight- 
forward terms to the twine maker, netmaker and trawler- 
owner, the considerations which a yarn producer has in 
mind when marketing a yarn intended for the cordage 
market. The technical properties of the yarns, allied 
to production aspects, are outlined in the following 
comparison of monofilament properties and references 
made to the economic considerations in some end uses. 

Monofilament deniers, shape of cross-section, etc. 

These usually fall within the range 100-4,000 den, the 
common ones in commercial use in fishing at present 
being between 100 and 1,000 den per filament (11-110 
tex). The shapes of cross-sections are mainly circular 
but, in certain instances, rectangular cross-sections have 
been used with a width : depth ratio of approximately 
3:1. More recently we have seen experiments with 
ribbon-like cross-sections, e.g., having a width : depth 
ratio of approximately 50 : 1, and also combinations 
of circular and ribbon-type cross-sections, e.g., round 
sections spaced at intervals across the cross-section of a 
continuous width of ribbon-like monofils. 

Much work has been done on flat cross-sections with a 
view to increasing both the flexibility of twines and the 
firmness of the knots. In the former, the bending moment 
is reduced as a result of the thinner axis in one direction 
and increased flexibility results (the bending moment of a 
circular monofil varies in proportion to the fourth power 
of the diameter). In our experience, we have found little 
advantage in these respects, unless the cross-section is 
extended to a ribbon-like form and, in such cases, other 
factors, notably abrasion resistance, can become critical. 


The strength of yams or monofilaments is often quoted 
as a selling point and discussed in terms which suggest 
that this figure alone determines the strength of net, 
twine or rope made from same. In actual use, in a fishing 
net for instance, it would seem that the wet knot strength 
of the twine is a more proper indication of strength. 
Bearing in mind the vast range of twines and twine con- 
structions in use, it falls upon the yarn manufacturer to 
devise his own standards. These are based on laboratory 
and practical tests and aim at an optimum relationship 

between tenacity and extensibility for the particular 
end uses. 

(a) Tenacity 

Table I gives a general indication of the tenacity of 
monofils used in twines and ropes: 

Polyvinylidcne chloride fibres 2 g/den 

Nylon 5_ 7 g/dcn 

Polyethylene 5-0-7-5 g/den 

Polypropylene 58-0 g/den 

(In the case of nylon there is a loss in tenacity when wet) 

Fig. 1 shows typical load/extension curves for these 
fibres (ref. "Textile World", 1962). 









FIG. 2. 

20 o/ 

Fig. 2 illustrates the effect on tenacity of various stretch 
ratios applied to 'Courlene' high density polyethylene 
monofilaments during spinning. 

(b) Effect of heat 

The tenacity of thermoplastic monofilaments is also 
affected by heat but most values quoted relate to tests 
carried out under standard conditions at 20C. Table II 

illustrates typical changes in tenacity (g/den) of some 
monofilaments at varying temperatures thus: 










(Tests carried out in water) 


The above figures apply to monofils of approximately 
0-010 in diam which are commercially available. It 
will be appreciated that higher tenacities can be readily 
achieved. In the case of such monofils, other considera- 
tions, e.g., stiffness, susceptibility to fibrillation, give 
rise to problems during processing which have not yet 
been finally overcome. 

(c) Knot tenacities 

These are to a large extent dependent upon the tenacity/ 
extensibility relationship as determined during spinning 
of the monofilaments. As the extensibility decreases, 
the monofilaments become more brittle to a point 
beyond which the increased tenacity obtained by extra 
stretching is more than offset by the consequent reduc- 
tion in knot strength. (Fig. 3 (Lonsdale 1957).) 



(d) Load-bearing characteristics 

These are best considered in relation to the load/extension 
curves (Fig. 1). From experiments, it appears that a load 
of up to 2 g/den is applied to monofilaments during 
normal trawling operations and an idea of the amount 
of extensibility at such loads can be obtained from these 

There is also the effect of creep under continuous 
loading to be considered. This varies between types of 
fibre and also quite substantially between grades of 
polymers produced by manufacturers. Some monofila- 
ments are criticised on account of creep and Table III 
shows test results obtained recently for the various 
monofilaments (outlined in Table II) when subjected to 
continuous loadings. Fig. 4 shows the time scale which 
coincides approximately with a normal trawling cycle of 

(e) Elastic recovery 

Extensibility at working loads is largely recoverable in 
the case of most synthetic fibres. The monofilaments 
considered in Fig. 4 were used in illustration of this 
property (Fig. 5). 


(f) Impact strength 

This is the amount of energy necessary to rupture the 
yarns. Tests were carried out on a ballistic tester at 
20 C C and 65 per cent R.H. as shown in Table III thus: 

2-7 xJO* 
3-1 xlO 8 
4-2 xlO* 

'Courlene X3' 



Polyvinylidcnc chloride 

Stiffness or flexibility 

This is important when considering the winding or 
processing of monofilaments. Recovery from creasing 
is perhaps the best means of comparison and relative 
values obtained on the 0-010 in diam monofils under 
consideration are as follows in Table IV: 

Courlene X3' 



Polyvinylidene chloride 





From this table, polyvinylidene chloride might be con- 
sidered to be the most difficult monofilament to process 
but the bending modulus (resistance to bending) for this 
monofilament is low compared with the others. 

The crystallinity of fibres is also of importance with 
regard to recovery from deformation. Table V (Nichols 
1954 and others), gives an indication of values for 
normal monofilaments. In general, the high crystallinity 
is associated with less recovery from large bending 

'Courlene X3' 



Polyvinylidene chloride 


Knot stability 

The main properties which determine the ease of 
knotting are stiffness, diameter and shape of cross- 



section. The crystallinity gives a guide to whether the 
monofilaments, once knotted, will tend to become loose. 
From these considerations it will be appreciated that 
polyethylene has advantages in this respect. 

With nylon, the use of double knots, or heat-setting 
of knots, is commonly practised to overcome deficiencies 
in knot stability. 

Laboratory tests have not shown rectangular cross- 
sections to be beneficial unless the width : depth axis 
ratio is at least 10 : 1. 

Monofilaments which are substantially round in cross- 
section, but having a serrated outline, have also proved 
disappointing from a knot stability point of view. 

Abrasion resistance 

The results of abrasion resistance testing are known to be 
unreliable when relating them to actual usage and 
technologists have long been reluctant to be definite 
on this subject. A large variety of tests has been devised 
over the years and, for comparison, we quote results in 
Table VI obtained by a reciprocating motion of the yarn 
under 500 g load, across a shot-blasted 'Dural' cylinder 
of 3J in diam. 

Arbitrary units 

'Courlene X3' (-009 in diam) 

Polypropylene (-011 in diam) 

Nylon (-010 in diam) 

Polyvinylidene chloride ( -01 1 in diam) 

830 den nylon multi- (2 tpi) 
filament yarn 

The same tests carried out in water showed the polyvinylidene 
chloride monofils to be almost the equal of the nylon monofils. 

These results help to illustrate the effect of denier/ 
filament, although multifilament yarns can be much 
improved by judicious use of twist. Twines made from 
monofilaments generally require less twist than compar- 
able twines made from multifilament yarns, thereby 
avoiding loss of strength due to overtwisting. 

The twist used in the make up of the twine has a direct 
bearing on abrasion resistance, and a point occurs where 
twine strength is adversely affected as twist increases 
(Fig. 6 (Shimozaki 1957)). 

/NC flfC .0 VE, A Y - >Q GAAMS 

~ * 


lO IO 2 




outdoors in temperate climates for many years. A 
recent comparison of monofiiaments is given below: 

\ V 

MVfflONSHtP ifTtfflff Tftf IHlAKlNG STHCHtTH MO 

fiti/Upf* Of TWIST3 QIVtN TO Tltf/Nft <..<. IflOOO. N v^ 

*_Purs t *4MONoriUMiNrs PKQOUCCQ BY oirre*iNr . 


Over the past few years, we have carried out many 
practical trials on abrasion resistance, incorporating 
varying diameters and cross-sections. The results, in- 
fluenced by other considerations, have in general borne 
out the textile fibre producer's well-known fact, i.e., 
the coarser the fibre, the better the abrasion resistance. 

Resistance to drag 

There has been evidence in the past (Scharfe 1957) that 
nets made from continuous filament yarns offer approxi- 
mately six per cent less resistance to towing than com- 
parable manila nets, due to the smoother surface of the 
twine and also to the fact that a thinner twine could be 
used. Monofilament twines could be considered even 
better than multifilament twines due to the lower number 
of fibres in the twine. Both these types of twine appear 
to possess the same advantages over natural twines, and 
trawler owners have confirmed a five per cent fuel saving 
when changing over from natural fibre nets to 'Courlene' 
nets over a trial period of several months. 

Rot-proof characteristics 

Synthetic monofiiaments have excellent resistance to 
sea water. Nylon absorbs some water, with consequent 
swelling of the fibre and reduction in breaking load when 
wet (Lonsdale 1957). 

It is now common practice not to dry synthetic nets. 
Preservatives are also regarded as unnecessary unless 
used to aid knot stability or abrasion resistance. We 
have had experience of polyethylene nets which have 
lost none of their properties after immersion in sea water 
for one year. 

Resistance to sunlight 

This can vary between different families of polymers and 
similar polymers made by different manufacturers. The 
thicker the monofilament the twine or the rope the 
better, due to greater resistance in depth to penetration 
by ultra-violet rays. Dark colours are helpful in lessening 
damage by sunlight. The yarn producer also has access 
to a wide range of additives (anti-oxidants), some of 
which may be usefully incorporated. 

Test sites are located all over the world for comparison 
of outdoor tests and laboratory tests. Polyethylene has 
been developed to the point where it can safely be used 

Polyvinylidene chloride . . 

(clear yarn -01 1 in diam) 

(clear yarn -0075 in diam) 

(clear yarn -0075 in diam) 
'Courlene X3' 

(clear yarn -0075 in diam) 

Use of monofiiaments 

% breaking % extensibility 
load retained retained 
76 89 




Reference to the physical properties described 
facilitate an appreciation of the following end uses. 

Polypropylene monofiiaments are not yet in wide- 
spread use for nets. The monofiiaments are stiffer and 
more difficult to process when making twines or braiding 
into nets. Ways of overcoming this problem are: 

(a) The use of finer monofiiaments. 

(b) Applying less stretch in spinning, which results 
in monofiiaments which are more manageable 
but lower in strength. 

(c) Variation of cross-section. 

All these approaches have certain disadvantages in 
economy, strength, or abrasion resistance. The develop- 
ments over the next few years will be interesting. 


Where first-class durability and abrasion resistance is 
required, stout monofiiaments have been proved extremely 
suitable. Polyolefin monofiiaments, because of their 
low specific gravity, tend to remain clear of the sea bed, 
thus avoiding excessive abrasion, whilst the mouth of the 
trawl can be maintained in the correct position with 
fewer floats. 

Danish seine nets. One of the earliest successes occur- 
red in this type of fishing, principally in Scotland. 
Within a very short space of time penetration was high, 
and today more than 90 per cent of these nets are made 
from polyethylene. The material technically was found 
to be excellent and the economics decidedly favourable. 
A service life of several years is not uncommon. 

Wing trawls. As with Danish seine nets, a very large 
number of monofilament wing trawls is now in use and 
these are universally accepted as an economic replace- 
ment for natural fibres. 

Midwater trawls. In this area of fishing, which is largely 
at an experimental stage in the United Kingdom, a 
number of nets has been made from both multifilament 
and monofilament yarns. All have been successful whilst 
the type most likely to be used will be the net providing 
minimum drag for a given strength. 

Deep-sea trawls. Deep-sea trawl fishing offers per- 
haps the best potential in terms of fibre usage and the 

continued on page 108 

Nylon Monofilament in the Viet-Nam Fisheries 


Viet-Namcse fishermen have for centuries used different types of 
gear on the many fishing grounds surrounding the peninsula. 
Different types of gillnets constitute the main equipment used and, 
white these have varied little in design, the fishermen have over 
recent years adopted the use of synthetic materials. The Viet-Nam 
Fisheries Directorate introduced multifilamcnts to the fishermen, 
who, of their own accord, changed of er to monofilament for the 
construction of their nets. Some of the advantages attributed by 
the fishermen to nylon monofilament nets are: they do not retain 
spiny shells (limilus sp.) and other impurities drifting around in the 
water, and easier to work; spiny fish especially are easier to remove 
from the nets; less damage is caused when the nets become snagged 
on reefs; the meshes are largely open for true gilling nets; for day- 
light fishing the nylon monofilament is nearly invisible, while 
multifilaments give a bright reflection, Monofilaments are further- 
more cheaper by half. Disadvantages as compared with multi- 
filament are: that the netting is more bulky and tends to blow 
around during operations; monofilament is more difficult to braid 
and tends to become stiff at low temperatures: Of an estimated 
11,000 gillnets in operation in the Viet-Nam fisheries, about 8,000 
are of monofilament as compared with only 160 multifilament nets. 

L'Utffltatkw de monofflaments de nylon dans fes pecfaes Vietnamiennes 



Fisheries Directorate, Viet-Nam 


Les pteheurs Victnamiens ont depuis des siecles utilise" divers 
types d'engins pour opercr sur les differents lieux de pfche situes 

tout autour de la peninsule. Les differents types de filets maillants 
constituant F6quipemcnt principal ont peu vari6 dans leur con- 
struction et, au cours des dernieres annees les pecheurs ont utilise 
des mat&iaux synthetiques. L'Office des Pfiches Vietnamien a 
introduit les fils synth6tiques multifilaments mais les pteheurs, de 
leur propre initiative les ont remplac6s par des monofilaments pour 
la constructive leurs filets. Quelques uns des avantages attribue*s 
par les pteheurs aux filets en nylon monofilament sont les suivants: 
ils ne retiennent pas les impuretes derivant dans 1'eau et sont plus 
faciles a travail ler; les poissons dpineux en particulier, sont plus 
faciles a ctegager des filets; les filets sont moins endommag&s 
lorsqu'ils sont accroches dans les coraux; Touverture des mailles 

continued from page 107 

United Kingdom industry provides good facilities for 
experimentation. Penetration, however, has been slow, 
not because of technical inferiority or unsuitability, but 
due to the unfavourable economic parity ratio arising 
from the high incidence of gear loss. Modern detection 
methods are already helping to minimise this. 

The rate of usage will rapidly increase as the effects of 
the improved economic life are realised and particularly 
now that a replacement net in synthetics can be purchased 
at no more than twice the traditional net. 

Shrimp nets and lobster pots. The ease of cleaning 
and long life are particular features of monofilament 
nets for this type of fishing. 

Salmon trapnets. Cleanliness, rot-proof characteristics 
and long life are important. Protection against scaven- 
gers, such as seals, has been afforded by monofilament 
nets compared with cotton nets. 

Longline fishing. Monofilament lines and snoods are 
renowned for their relative freedom from tangling and 
cleanliness in use. The floating characteristics of poly- 
olefin monofilaments ensure that snoods tend to rise 
in the water and be clear of the sea bed. 


The last five years have seen a progressive attack on the 
whole field of activity where hard fibres have held the 
market so completely for so long. Whilst the mono- 
filaments had much to commend them at the start of the 
development phase, knowledge of the possible polymer 


deficiencies manifest in fibres compelled caution. If a 
filament were subject to high stress at an elevated 
temperature for a long period of time, it was known 
that an irrecoverable yield would result. The question 
was, therefore, do such combinations occur in practice? 
A comprehensive testing programme was necessary to 
test this and other possible deficiencies in the material, 
covering end uses from large marine mooring hawsers 
to longlines in Norway. These field trawls have shown 
that such a combination of conditions rarely happens in 

For example, 65 in circ-three strand polyethylene ropes 
have stood the test of two years continued use as first 
ropes ashore on a 20,000-ton tanker, and are still in use. 
By comparison, a hard fibre rope is normally replaced 
every nine to twelve months. 

It is evident that the fishing industry provides an oppor- 
tunity for the widespread usage of polyolefin mono- 
filaments, since the question of temperature sensitivity 
does not arise. A wide range of end use applications has 
confirmed the basic suitability of these materials. 

Quarter ropes and mooring ropes for all types of 
trawlers, together with headropes for seine nets, are all 
in regular use. More recently, successful trials have been 
carried out with weighted polyethylene ropes for use as 
seine warps. 


Textile World. Man-made Fibre Chart. 1962. 

Lonsdale, J. E. Nylon in Fishing Nets. 1957. 

Nichols, J. B. J. Appl. Phys. 25. 1954. 

Shimozaki, Yoshinori. 1957. Characteristics of synthetic twines 
used for fishing nets and ropes in Japan. 

Sch&rfe, J. Experiments to decrease towing resistance of trawl 

eat plus constante; pour la p6che pendant la jounce, le nylon 
monofilament cat piesque invisible tandis quc Ics multifilaments 
donnent ties reflets brillants. DC plus les monofilaments sent de 
moiti6 moins chcrs. Par centre, compares a oeux en multifilaments, 
les filets monofilaments sont plus volumineux et susceptibles d'etre 
gonfles et chasses par le vent pendant les operations; le monofila- 
ment est phis difficile a travailkr et devient raide a basse tempera- 
utre. DCS 11.000 filets maillants en operation dans les pecheries 
vietnamiennes, environ 8.000 sont en monofilaments et seulement 
160 en multifilaments. 

El empfco de monofllainentos de nykm en las pesquerias de Vlet-Nam 


Los Pescadores vietnameses emplean desde hace siglos diversas 
clases de artes en Jos muchos caladeros que rodean la peninsula. 1 
equipo principal lo constituyen diversas clases de redes de enmalle y 
aunque sus formas ban cambiado muy poco, en los ultimo* aftos 
se emplean materiales sintdticos. La Direcci6n de Pesca de Vietnam 
les ofreci6 multifilamentos, pero los Pescadores prefiercn emplear 
monofilamentos para construir sus redes, porque entre las ventajas 
que les atribuycn estan : menos peligro de enredarse y manipulacibn 
mas facil; es mas sencillo descnmallar los peces, particularmentc 
los de aletas duras; menos averias cuando las redes se enganchan en 
las rocas; la malla es mas constante para los artes verdaderamente 
agallcros; los monofilamentos de nylon son casi invisibles de dia, 
en tanto que los multifilamentos dan reflejos brillantes; los mono- 
filamentos cuestan la mitad, aproximadamente. Los inconvenientes 
son: las redes son mas voluminosas; los monofilamentos son mas 
dificiles de trenzar y se endurecen cuando la temperatura es baja. 
De las 11 .000 redes de enmalle usadas en Viet-Nam, unas 8.000 son 
de monofilamentos y s61o unas 160 de multifilamentos. 

RICE and fish are basic foods in Southern Viet-Nam. 
Along its 1,800 km of coastline a great variety of 
gears are used because of the many different fishing 
conditions and fish species. 

Beach seines, fish traps, pair trawls, gillnets of different 
operational types, and the usual variety of hookline gear 
have developed. Of these gears, gillnets, and especially 
driftnets, are perhaps the most important. Whilst 
traditionally made of ramie and cotton, synthetic 
materials have been in use for some time. 

In 1958 the Fisheries Directorate imported some multi- 
filament nylon gillnets of three inches (four fingers) stret- 
ched mesh size for use on the east coast where similar 
cotton and ramie nets were used. The fishermen at first 
were very reluctant to use them. However, a few fisher- 
men did insert a few such nets in their own string of nets, 
to compare the catch efficiency. The excellent results 
completely broke down the fishermen's prejudice- 
catches were up to 300 per cent higher than in traditional 
nets. Demands for synthetic twines quickly surpassed 
the supply available. 

Nylon monofilament had for some time been in use 
for the construction of snoods in different hook gears 
as well as for handlines. Some fishermen, unable to 
obtain multifilament nylon for gillnets, turned to the 
light monofilament, being convinced that even mono- 
filament nylon was better than the natural fibre previously 

From the beginning, these monofilament nets obtained 
at least as good results as the multifilament nets and, by 
introducing small adjustments to mesh size and hanging, 
the fishermen very soon obtained even better results 
with monofilament nets than with multifilament nets. 
Thereafter the demand for monofilament of various 
diameters greatly increased.' The nets are used with 

local-made plastic floats and tight-fitting leads; both 
are available in several sizes to fit the mesh size and type 
of operation (Fig. 1). 

Fig. 1. Monofilament net with plastic floats and lead sinkers. 

The Directorate of Fisheries has to date not yet 
conducted suitable comparative fishing experiments to 
ascertain whether the monofilament nets do in fact catch 
more. The fishermen on the east coast are convinced 
that these nets have a higher catch efficiency because of 
their transparency. This, on the other hand, does not 
justify the preference for monofilament of fishermen 
operating on the west coast where the water is very 

Visibility of monofilament nylon nets is not the only 
feature governing the preference of fishermen, and it 
may be useful to record some of the advantages and 
disadvantages attributed by fishermen to these materials. 

Advantages of monofilament nets 

(a) The twine is relatively stiff and very smooth and 
does not retain spiny shells (limilus sp.) and other 
impurities drifting around in the water. Multi- 
filament, owing to its high flexibility and very fine 
denier, picks up such trash very easily so that 
cleaning nets is much more difficult. 

(b) Gilled and entangled fish, especially spiny fish, 
appear to be much more quickly released from 
monofilament than from multifilament nets. 

(c) Some of the fishing is done near coral reefs and, 
when the nets become snagged on coral, mono- 
filament nets are released much more quickly and 
with less damage. 


(d) The fishermen believe that, owing to the stiffness 
of the yarn, the meshes remain largely open. 

(e) The transparency of monofilament nets reduces 
their visibility in the water, particularly in daytime, 
whereas multifilament nets give a brilliant reflec- 
tion which may scare the fish away. This would 
seem to be especially the case round the edges of 
tears, so that the hole in the net is very visible 
to Hie fish. During daytime monofilament gillnet 
fishing is more efficient than multifilament nylon 
nets. However, the fishermen remark that multi- 
filament nets are particularly good in catching the 
Stromateus sp. as the nets entangle the fishes. 

(f) The monofilament nets have been found to catch 
at least as much fish as the multifilament ones and 
are cheaper by half. The reason for the lower cost 
of monofilament twines is that multifilament 
must still be imported whereas nylon monofilaments 
are manufactured in Saigon by six local firms. 

Disadvantages of monofilament twines 
(a) One disadvantage of monofilament nets is bulkiness. 
Fishermen combat this by forcing the netting in the 
forward fish-hold and weighting it down (Fig. 2). 

Fig. 2. Storage of nets on board. 

(b) The netting during shooting and hauling tends to 
blow around more than the multifilament nets 
and this can slow down the operation. This has 
largely been overcome by adapting the shooting 
and hauling techniques to this condition. 

(c) Monofilament does not braid as easily as multi- 
filament because of its stiffness; on the other hand, 
no serious difficulty has been experienced as regards 

continued on page III 

Fig. 3. Joining of twine between knots when mending. 

Fig. 4. Mending the nets. 


Monofilament Gillnets in Freshwater Experiment 
and Practice 


Experimental studies on the comparative importance of the 
properties of gillnet materials have shown that in clear water the 
visibility is the most important factor. Net materials should have 
low visibility, giving little or no contrast to the background and 
be transparent; synthetic monofilaments yield the best catches 
The softness of the netting was found to be of only secondary 
importance. The diameter of the netting material influences the 
catch efficiency only inasmuch as thicker twines are more visible 
than thinner ones. The elasticity appears to have hardly any in- 
fluence on the size of the fish caught while the visibility does appear 
to have a selective effect. The breaking strength is only of impor- 
tance for large fish and operational considerations, because small 
fish such as perch and roach cannot produce the force required to 
damage commercial gillnets. The hanging determines the shape of 
the meshes, the distribution of the forces in the net and the looseness 
in the netting. The framing lines may influence the catch efficiency 
by their visibility. The amount of floats and sinkers depends on 
the operational conditions, the current and the amount of tension 
required in the netting. 

Filets maillants de monofllament dans la pche continental 


Des etudes experimentales sur I'importance comparative des 
proprietes des materiaux pour filets maillants, ont montre que la 
visibilite est le facteur le plus important pour la peche en eaux 
claires. Les mat&riaux de filets doivent avoir unc faible visibilite 
contrastant peu ou pas tout avec le milieu et doivent tre trans- 
parents; dans ces conditions, les monofilaments synthetiques pro- 
duisent les meilleures captures. La mollesse des filets est d' importance 
secondaire. Le diametre du materiel des filets a une influence sur 
refficacite de capture seulement dans la mesure ou les fils epais 
sont plus visibles que les fils fins. L'elesticite semble avoir peu 
d' influence sur la grandeur des poissons captures tandis que la 
visibility a un remarquable cffet selectif. La force de rupture n'est 
importante que pour les grands poissons et pour des considerations 
cToperation puisque les petils poissons comme la perche et le 
garden n'ont pas assez de force pour en ommager les filets com- 
merciaux. Le taux d 'assemblage determine la forme des mailles, 
la distribution des forces dans le filet et le relachement du filet. 
La visibility des ligncs encadrantes peut influer sur Pefficacite de 
la capture. Le nombre de flotteurs et le poids dependent des con- 
ditions d'operations, du courant et de la tension ndcessaire dans le 


R. Steinberg 

Institut fur Netz und Material- 
forschung, Hamburg 

Redes de monofilamentos para la pesca de enmalle en agua dulce 


Los estudios experimentales de la importancia relativa de las pro- 
piedades de los materiales para la fabricacidn de redes de enmalle 
ban demostrado que en agua clara la visibilidad es el factor mas 
importante. Los materiales para redes deben ser casi invisibles, 
crear muy poco o ningun contraste con el fondo y ser transparentes. 
Los monofilamentos sinteticos dan las mayores captures. Se demos- 
tr6 que s61o tenia importancia secundria que el material fuera 
blando. El diametro de los hilos s61o influye en el rendimiento de 
la pesca en que los de mayor diametro son mas visibles que los de 
menor diametro. Parece ser que la elasticidad apenas tiene efecto 
en la talla de los peces capturados y que la visibilidad tiene un 
efecto selective. La resistencia a la rotura s61o tiene importancia 
para los peces peces grandes y en lo que a la manipulacibn se refiere, 
ya que peces pequeftos como la carpa no tienen la fuerza necesaria 
para ayeriar las redes de enmalle comerciales. La mancra de armar, 
es decir, los metros de pafio por mctros de rclinga, determina la 
forma de las mallas, la distribuci6n de las fuerzas en la red y si los 
pafios quedan flojos. Las relingas de las cuarteladas pueden influir 
en el rendimiento de pesca a causa de su visibilidad. La cantidad 
de flotadores y plomos depende de las condiciones de trabajo, la 
corriente y la intensidad de la tensibn necesaria en los paftos. 

IN spite of relatively high cost, gillnets are of great 
importance. By adopting proper mesh size only fish 
of the desired range are caught, while undersized fish 

continued from page 110 

knot slippage as the fishermen make the nets with 
double knots and also mend between knots as 
shown in Figs. 3 and 4. The great majority of 
nets in Viet-Nam are braided by hand by the fisher- 
men and in practice it has been found that this 
cannot be done well with monofilaments thicker 
than 0.90mm diam. 

Monofilament seems to be more sensitive to the lower 
temperatures and nets made from this material used in the 
cooler regions and during the winter season tend to 
become stiff thereby becoming even more bulky out of 
the water and less flexible during operations. Water 
temperatures vary from 22C to 29C the year round. 

Future of monofilament gillnets 

Use of nylon monofilament for gillnets is progressing 

very rapidly in Viet-Nam. No accurate figures of the 
total nets in use are available but a general estimate 
shows that some 1 1 ,000 gillnets are in use. The number 
of monofilament and multifilament synthetic fibre nets 
in use over the period 1958/62 is given in the table: 

1958 1959 1960 1961 1962 
Multifilament gillnets 5 50 80 140 160 

Monofilament gillnets 2 30 1000 3000 8000 

Gillnet fishermen in Viet-Nam normally work on a 
share-on-catch basis and the popularity of monofilament 
nets is such that the best fishermen engage themselves 
only with owners of motorised boats using monofilament 
gillnets. The sail boats, many of which still use natural 
fibres, are experiencing difficulties in obtaining good 
crews and, furthermore, operate at economic disadvan- 
tage to motorised craft. 




g. 5 Tht twinning fore* of roaoh 
in relation to ! 

190 tOO ISO MO iSO 400 430 

A greater number of older and larger fish seems to be 
caught with low visibility nets, indicating a more careful 
approach towards gillnets than younger specimens. This 
influence of the visibility on size of fish, however, varies 
according to the fish species; it appears stronger for 
perch than roach. The elasticity of the net materials is of 
importance for the gilling process and for retaining 
gilled fish in the meshes, but this has hardly been studied 
so far. 




sS I.Pd/tlWtn* 

/ ^y 2J1yln 210 denx2 

3. Poly a mid monofilomtrt 0.25 


4 Pdytthylen* mtnolllQmtnt 0.25 
5. Cotton Nm 140/6 

mo/o///omrnf 0.25 

10 12 14 16 18 20 22 24 26 28 30% 

Fig. 6 Lo4-longation ourres of nt twin of 0.25 disaster. 

Breaking strength Because of the fineness of the net 
materials, breaking strength is of high importance. 
Nowadays most gillnets are made of machine-braided 
netting, which process already requires a certain minimum 
breaking strength and the strength required for handling 
and operation must also be considered. Selection of net 
material is, therefore, a compromise between the higher 
breaking strength needed for lengthy trouble-free opera- 
tion and the most desirable diameter for minimum 
visibility. The strength of the fish only needs considera- 
tion where it concerns large-sized species. High tenacity 
materials, therefore, are essential for gillnets. 

Constructional properties of gillnets which influence 
the catch efficiency are: mesh size, hanging, framing 
lines, floats and sinkers and height of the net. 


Merit size With true gilling nets the mesh size delimits 
the size of the fish caught so that such gillnets are very 
selective, catching only fish of a distinct size. Smaller and 
larger fish may, however, be entangled if the net is loosely 
hung. 1 30 This selectivity of gillnets enables the taking 
of only those specimens from fish stock which, by their 
size, represent a good market value and are mature so 
that their loss is of no serious consequence for the preser- 
vation of the stock. 

The selection of the optimum mesh size requires a 
sound knowledge of the composition of the fish stock 
and must be determined for each water body in accor- 
dance with that knowledge and their needs. 

Hanging While hanging determines the shape of the 
meshes and the distribution of the forces in the net, it 
may also influence the selectivity. 1 It would seem that 
for fish with a narrow and high cross-section the mesh 
shape should be stretched in vertical direction while for 
flatfish the meshes should be stretched horizontally; 
the latter, however, gives very tight hanging. The hang- 
ing defines thf looseness of the netting and very often 
it is desirable to have loose netting so that fish can be 
entangled as well as gilled. This looseness can also be 
obtained by attaching connecting twines between 
floatline and leadline which prevent the netting from 
stretching fully. 

Framing lines Gillnets used in freshwater fishing are 
normally provided with a complete frame of lines, 
bulky enough to handle and strong enough to withstand 
shooting and hauling. 

The visibility of the framing lines may have an effect 
on the catch efficiency. 8 26 30 Experiments with nets 
framed with differently coloured lines showed that those 
with green lines caught 61 perch and 153 roach while 
the white-lined nets caught under the same conditions 
22 perch and 37 roach respectively. 

Floats and sinkers Floats and sinkers, to achieve the 
desired vertical position of the net in the water, must be 
distributed uniformly. A large number of small floats 
is usually preferable to a small number of large floats 
and for submerged nets the total buoyancy must not 
produce undue tension 1 in the netting. 

The shape and colour of the floats should not contrast 
unduly with the water or background, otherwise they 
may affect the catching efficiency in the same way as 
the framing lines. 

Net height Bottom-set gillnets in inland fishing are 
usually of small height and experiments showed that an 
increase beyond 1*0 to 1*5 m does not normally result 
in better catches. 

Duration of fishing Catch efficiency of gillnets decrea- 
ses when the fishing time is increased and also with an 
increase in the number of fish gilled. In experiments in 
the Great Slave Lake 17 it was found that, by extending 
the duration of a set from 24 to 48 hours, the catch 

continued on page 115 

Etudes sur le Freinage et L'Usure des Fils de 

La fabrique de fils et cordcs "Cousin Freres", France, precede a des 
etudes constantes jx>ur amdliorer ses produits. Lorsque les rtsultats 
obtenus sont d'une importance technologique generate, elle consulte 
des Instituts in dependants en ce cas Tlnstitut de Mecanique des 
Fluides de Lille et le Laboratoire Federal Suisse de Saint Gall 
arm que ceux-ci cffectuent des epreuves de controle, le resultat est 
alors porte a la connaissance des pfccheurs, des constructeurs de 
filets et des clients en general. La presente communication traite de 
trois de ces essais. Les deux premiers conccrnent la trainee compara- 
tive de fils de filets; au cours de ces essais il a ete constate qu'a 
une vitesse dc trois noeuds, la trainee du coton etait sup6rieure de 
50 pour cent a celle du nylon en monofilament continu d'un dia- 
metre equivalent et avait trois fois plus de resistance a la traine, 
pour d'egales forces de rupture. Les fils en nylon tresse de meme 
diametre que les fils en nylon cable avaient une resistance a la 
trainee legerement inferieure. Le troisicme essai destine a eprouver 
la resistance a 1'abrasion des fils a montrd que le nylon cable avait 
sept fois plus de resistance que le manille et le sisal a sec mais que 
mouilie, il n 'etait que tres peu superieur au sisal. D'un autre 
cdte la resistance a 1'abrasion du nylon tresse est cinq fois plus 
grande que celle du nylon cable et sa resistance a 1'abrasion est la 
m&me sous les deux conditions. Les fils de nylon tresse ont done un 
gros avantage pour la construction des chaluts par suite de leur 
haute resistance a 1'abrasion et leur tres basse resistance al a trainee. 

Study on the drag and abrasion of fishing twines 


The net and rope factory of Cousin Freres, France, carries out 


Maurice Bombeke 

Establissements Cousin Fr&res 

continuous studies aimed at improving their products. From time 
to time when results are obtained which are of general technological 
importance, independent institutes, in this case, Institute de Mecan- 
ique des Fluides de Lille and Laboratoire Federal Suisse de Saint 
Gall, are requested to carry out conclusive tests, the results of which 
are then brought to the notice of fishermen, net-makers and clients 
in general. The present paper concerns three tests : the first regarding 
the comparative drag of twines when made into nets, during which 
it was found that at three knots the drag of cotton twines was SO per 
cent higher than that of nylon continuous multifilament of equiva- 
lent diameter, and about three times higher resistance when com- 
pared on equal strength basis. Braided nylon twines of comparative 
diameter offer slightly less resistance than cabled nylon twines. 
The second experiment on the abrasive resistance of twines showed 
that cabled nylon had up to seven times higher abrasion resistance 
than manila and sisal in the dry condition but only slightly more 
resistance than sisal in the wet condition. On the other hand, the 
abrasive resistance of braided nylon was five times higher than that 
of cabled nylon, and its resistance is almost the same in dry and 
wet condition. Braided nylon twines have, therefore, great advantages 
for use in trawls because of high abrasive resistance and low drag. 

continued from page 1 14 

increased only very little. Similar results were obtained 
in Northern German Lakes. Gillnets should be lifted 
and cleared at least once a day and even at much shorter 
intervals under certain conditions, such as in the tropics 
or where predators are abundant. 


1 Baranow, J. 1. : Theorie und Berechnung dcr Fischfanggerate. 
Moskau/Leningrad. 1939. 

2 v. Brandt, A. : Bekommen wir unfaulbare Netzgarne. Fisch. 
Ztg., 42, 301-302. 1939. 

3 v. Brandt, A.: Arbeitsmethoden der Netzforschung. Stuttgart 

4 v. Brandt, A.: Netzweichheit und Netzharte. Arch.f.Fischerei- 
wissenschaft, 1, 173-181. 1949. 

6 v. Brandt, A. : Unsichtbare Netze zum Forellenfang. Allg. 
Fisch.Ztg., 76, 300-301. 1951. 

6 v. Brandt, A.: Erfahrungen mit Platil-Netzen beim Fang von 
Edelfischen. Fischwirt, 2, 210-213. 1952. 

7 v. Brandt, A.: Kiemennetze aus Perlondraht. Schweiz.Fi.-Ztg. 
62, 98. 1954. 

8 v. Brandt, A. : The Efficiency of Drift-Nets. Joum.du Cons., 

y v. Brandt, A. and G. Klust: Neue Faserstoffe fur die Fischerei. 
Prot.z.Fischereitechnik, 3, 249-300, 1955. 

10 v. Brandt, A. and R. Liepoldt: Ergebnisse von Fangversuchen 
im Wolfgangsee mit Stellnetzen aus Baumwolle und Perlon. Oest. 
Fischerei, 8, 93-97. 1955. 

IJ v. Brandt, A.: Net Materials of Synthetic Fibres. FAO 
Fisheries Bulletin X, 182-210. 1957. 

12 v. Brandt, A. and P. J. G. Carrothers: Test Methods for 
Fishing Gear Materials, (submitted to II. Fishing Gear Congress). 

1 3 Einsele, W. : Uber den vom 7. bis 12. Oktober 1 957 in Hamburg 
abgehaltenen Internationalcn Fanggerate-Kongress. Oest. Fischerei, 
10. 121-126. 1957. 

1 4 Geisel B P : 3 Jahre Erfahrungen mit Platilnetzen im Laacher 
Sec. Fischwirt, 3, 310-312. 1953. . u . , 

IS Herter, K.: Die Fischdressuren und ihre smnesphysiolo- 
gischen Grundlagen. Berlin 1953. 

16 Kajewski, G.: Untersuchungen zur Auswirkung der Sicht- 
barkeit monofiler synthetischer Fischereigcspinste auf die Reaktion 
von Fischen. Fischereiforschung, 1. 1958. 

17 Kennedy, W. A.: The Relationship of Fishing Effect by 
Gillnets to the Interval between lifts. Jo urn. of the Fish. Res. 
Board of Canada, 8, 264-274. 1951. 

18 Klust, G. and Kohnke, E.: Stellnetze. Fischwirt, 3, 237-241, 
268-273. 1953. 

1 Mohr, H. : Note on the Behaviour of Herring in a Round 
Tank. Comp. Fish. Com., No. 87, Int. Counc. Explor. Sea, CM. 
Kopenhagen. 1961. 

20 Molin, G.: Natmontering och fisklighet. Svensk Fiskeri 
Tidskrift, 66, 30-31. 1957. 

21 N.N.: Perlon-Drahtnetze haben sich be with rt. Fisch win, 4, 
44-45. 1954. 

22 N.N.: Nachrichten aus Oberbayern. Fischwirt, 5, 185. 1955. 
2 'N.N,: Erfolgreiche monofile Stellnetze. Fischwirt, 5, 351. 


2 N.N.: Nochmals Maranennetze aus Kunststoffdrfthten. Fisch- 
wirt, 6, 102. 1956. 

2 * N.N.: Summary Report of the Joint Scientific Meeting of the 
International Commission for Northwest Atlantic Fisheries. 
Lisbon. 1957. 

26 Nomura, M., Mitsugi, S. and R. Nahano: On the Horizontal 
distribution of Catch Influenced by the Seam of Net in Sardine 
Drift Nets. Tokai Reg. Fish. Res. Lab., Collected Reprints, 245-248. 

27 Niimann, W.: Untersuchungen Uber die Berechtigung einer 
verstftrkten Schwebnetzfischerei im Bodenscc. Allg. Fish.-Ztg., 
86, 403-404, 427-428. 1957. 

28 Riedel, O.: Contribution to the Experimental Determination 
of the Selection Parameter of Gillnets. (unpublished). 

2 g Schrader, K.: Kiemennetze und Spiegelnetze aus Platil. 
Fischwirt, 4, 104-106. 1954. 

' Steinberg, R.: Die F&ngigkeit von Kiemennetzen fUr Barsch 
und Pldtze in Abhftngigkeit von den Eigenschaften des Netz- 
materials, der Netzkonstruktion und der Reaktion der Fische. 
Arch.f.Fischereiwisscnschaft, XII, 173-230. 1962. 

"Wagler, C.: Die Bewirtschaftung der Coregonenseen. Int. 
Rev.ges.Hydrobiologie, 37. 1938. 

22 Wigutoff, N. B.: Pacific Salmon Drift GUI Netting. Fish and 
Wildlife Service,Fisch.Leaflet 386, Washington. 1951. 


CM to rcsMtmcto y Q6fl0ute cte UkM para 


La fftbrica de redes y cordelerfa Cousin Frires, Francia, realiza 
continuamente estudios para mejorar sus productos y de vez en 
cuando, los resultados obtenidos son de importancia tecno!6gica 
general, solicita de institutes independientes, en este caso el Institut 
de Mecanique des Fhiides de Lila y el Laboratoire Federate Suisse 
de Saint Galle que realiccn ensayos definitives cuyos resultados se 
comunican a Pescadores, fabricantes de redes y clientela general. 
Esta ponencia alude a tres ensayos: el primero sobre la resistencia 
oomparativa de hilos cuando se hacen las redes. Durante este 
se observd que a tres nudos de velocidad, la resistencia de los 
hilos de algod6n es un 50 por ciento mayor que la de los filamentos 
continues de nylon del mismp diametro y unas tres veccs mayor a 
igualdad de robustez. Los hilos de nylon trenzados ofrecen algo 
menos resistencia que los torcidos de diametro analogo. El segundo 
experimento, sobre la resistencia al desgastc, demostr6 que el 
nylon torcido ofrecfa seite veces mas resistencia al desgastc que el 
abaci de Manila y el sisal estando secos, pero s61o un poco mas 
que el sisal estando humedo. Por otro lado, la resistancia al des- 
gaste del nylon trenzado es cinco veces mayor que la del torcido y la 
resistencia a la traccibn casi igual estando seco que humedo. Se 
deduce de ello que los hilos de nylon trenzado ofrecen grandes 
ventajas en los artes de arrastre por su gran resistencia al desgaste y 
pequena a la tracci6n. 

DANS le but dc perfectionner leurs produits, les 
ftablissements Cousin Frires de Wervicq-Sud, 
France, font constamrhent des 6tudes sur les matiriaux 
utilises ct les mithodes de fabrication. De temps en 
temps lorsque des risultats intiressants sont obtenus, 
des 6preuves concluantes sont excutes dans des con- 
ditions contrdl&s par les Instituts reconnus; les risultats 
de telles ipreuves sont alors port6s & la connaissance des 
pccheurs, des fabricants de filets et des clients en general 
La prsente communication traite de trois de cespreuves. 

Mesure de la trainee de fib de pfiche 

Le but de cette Stude faite par I'lnstitut M6canique de 
Lille, dtait de connaitre la force employer pour tracter 
deux fils cables de meme force la rupture : Tun en coton 
et 1'autre en nylon. 

Les fils & essayer etaient de cinq types: 

(a) fil de coton, diamitre approximatif 1 mm 

(b) fil de coton, diamitre approximatif 1*8 mm 

(c) fil de nylon, diamitre approximatif 0'9 mm, cab!6 

(d) fil de nylon, diamitre approximatif 1 mm, tresse* 

(e) fil de nylon, diamitre approximatif 1 mm, tressS 

Pour les essais, chaque type de fil est monte" sur un 
cadre en bois profil de une mitre de cote", fix rigide- 
mcnt aux mats verticaux d'une balance airodynamique. 

Les fils sont months verticalement et horizontalement, 
de fa<?on i obtenir des mailles carries de 20 mm de cot6 
environ. II y a ainsi, pour chaque type de fil 98 mitres de 
fil essay*. 

Les essais ont 6t6 effectuds dans la Soufflcrie Horizon- 
tale de Tlnstitut de Mecanique des Fluides de Lille, de 
diam&tre 2*40 mitres en veine guide*e. 

Trainee des fib dam I 9 air 

Les resultats des essais effectuds en soufflerie sont con- 
si gnds dans les tableaux suivants ; les colonnes A donnent 
la pression dynamique en mm eau; les colonnes B 
donnent Tefifort de traindc en kg (Tableau I). 


Courbet -f(q) 

q mm d'eou 

50 100 ISO 

Planche 1. Trainee des fils depeche dans Vair. 

Trainee des fib dans 1'eau de mer 

A partir des essais effectu6s dans 1'air, nous calculons 
reffort de trainee s'exergant dans 1'eau de mer sur 100 
mitres de fil de chaque type, pour des vitesses d'avance- 
ment evaluees en noeuds. 


Pour un meme nombre de Reynolds: R on 


determine d'abord la vitesse du courant d'air corres- 
pondant 4 une vitesse d'avancement dans 1'eau. Comme 
le diamitre D est constant, on doit avoir: 


V V 
air= eau 
v v 

.. . , 7 v air 

V air= V eau x 

v eau 

Les valeurs de la viscosite" cinematique v sont: 

Air 4 15: t;=0'15 stokes 

Eau de mer & 4 : v=0'01 5 stokes 

Ce qui donne: V (air) =10 V (eau) 

A une vitesse d'avancement dans Feau de quatre 
noeuds par exemple, correspond une vitesse du courant 
d'air de 40 noeuds, soit 20*55 m/sec, qui dquivaut & une 

Pour une vitesse donnfe, on icrit ensuite, que les 
coefficients de trainee dans Fair et dans Teau sont 

Fx Fx 

P a V z air "~p a V 2 eau 

p eau V eau 2 

Ce qui donne: Fx (eau)=Fx (air)x r x.. . 

P air V air 

Les valeurs de la masse sp&ifique p sont : 

Air i 15: P =l-226 kg/m^ 

Eau de mer 4 4: P= 1-026 kg/m 3 

d'otl: Fx (cau)= Fx (air)= 

=8-37 Fx (air) 

1-026 1 
1-226* llf 

Dans 1'exemple choisi, on lit sur la P1.2 la valeur Fx 
(air) correspondant & q=26'4 mm d'eau, soil Fx (air)= 
2-350 kg pour le fil tress6 nylon traitement Wercord. 
Pour une vitesse d'avancement dans Feau de duatre 
noeuds, on a done, un effort de trainee: 

Fx (eau)=8-37x 2-35= 19-620 kg 

Les trainees ainsi obtenues dans Feau de mer sont 
donnies dans le tableau cidessous, et portees en courbes 

Tableau I 

Nylon trM* traiUment WERGORD 
d 1 mm 

Nylon trM4 d 1 mm 


B A 



0,695 M r > 



1,790 1*75 



2,720 ?*M 



3,650 3 9 .2 






5,580 57.40 



6,700 67,60 



7,880 76 



8.9SO M* 20 



9,980 97 2 



11,220 % 108 10 



12,100 11WK> 



13,820 1 32^0 



15,050 145 


Coton dft 1 yyini 

Nylon de 0,9 mm 

Colon de 1 ,8 mm * 



~ A 


A B 



. 73,80 


12,75 2.980 
28,10 6,595 
38,30 8,800 
55,30 12,490 
66,40 14,825 
82,20 18,225 
98,00 21,625 
109,30 23,940 
122,30 26,660 





TABLEAU II Effort de trainee en kg sur 100 m de fil 

Vitesse de 09 mm 

1 noeud 1,275 
2 noeuds 5,150 
3 noeuds 11,675 
noeuds 20,700 

de 1 mm 


Coton Wercord 
de 1*8 mm de 1 mm 

3,270 1,255 
13,00 5,010 
29,40 11,370 
52,00 19,650 

de 1 mm 





Courbes F x f(V M notudi) 

tn kg 


Planche 2. Trainee des fils de pechc dans reau. 


Les caracteristiques des torons qui constituent le 
fil de nylon "cabte" de diamitre 0*9 mm sont identiques 
celles des torons du "tress machine'*. La force de 
trainee 6tant directement proportionnelle & la surface 
frontale du fil, on peut, & partir des risultats obtenus 
pour le 0-9 mm, calculer les efforts de train6e s'exer$ant 
sur le fil de nylon "cbl6" de diamitre un mm. Ces 
r6sultats portSs en courbes & la planche deux sont & 
1'avantage du nylon "tress" qui, dans 1'eau subit un 
effort de trainee tegferement interieur 4 I'effort que 
subirait un fil de nylon "cabte" de mme diamitre. 

L'essentiel de cette tude 6tait de trouver quelle itait 
la difference de trainee existant entre les cables et les 
tresses: la conclusion est que la tresse nylon offrant 
moins de resistance & la traction que le cftbli nylon, 
son emploi est vivement recommand pour les chaluts 

Tenue a 1'abrasion des tresses et cAbKs nylon pour filets de 

(Etude faite par le Laboratoire F6d6ral Suisse de Saint 


Articles essay6s:l bobine de tresse en Nylon 4840/3 
1 bobine de cdbli-Nylon carte Rouge 
502-rtf. 270 

1 pelote de manille trois fils 8/10 
1 pelote de sisal trois fils 8/10 
1 pelote de ficelle CH-D trois fils 8/10 

Conditions d'essai: 

Le materiel est climatisd & 65 pour cent d'humidit 

relative et 20C. 

Mode d'examen: 

Des prouvettes prleves de chacun des 6chantillons 
sont tressdes dans une plaque pcrfordc. Les boucles 


sortaot du c6t6 supe'rieur sont frott&s par I'tiiment 
frottant plat de 1'appareil Schiefer jusqu'au moment de 
la destruction de la premiere boucle. La pcrtc de poids 
moycnnc, calculi par 1,000 tours, est determined en 
mme temps. Les deux dchantillons de nylon sont pes6s 
apits les premiers 2,000 tours, ceux de manille et de 
ficelle CH-D apris les premiers 500 tours et I'&hantillon 
de sisal apris les premiers 250 tours. 

Afin de determiner les pertes de poids des eprouvettcs 
mouillces, tress&s dans les trous du disque perfore et 
puis abrasccs pour atteindre le nombre de tours indiqud. 
Ensuite elles sont sichies, climatisies et pesies. 

Les essais ont donne" les resultats suivants: 

Mb S flta K/10 

U flb />U . . 

ilk CH-D 3 U IJIO 




5100 I 


Cette itude ddmontre la grande resistance a 1'usure de 
la tresse nylon, elle a determine Femploi de la tresse 
nylon dans la construction, notamment, des chaluts et 
des grandes seines tournantes. 

En France, Femploi de la tresse nylon pour les chaluts 
est maintenant g6nral; les chaluts de grande p6che 
portugais, espagnols et italiens sont igalement fabrique*s 
avec la tresse nylon. 

Discussion: Materials for 

Lines, Ropes and 


Dr. J. Reuter (Netherlands) Rapporteur: Most synthetic 
fibres are produced by extrusion with orientation or stretching 
afterwards. As a result of this, textiles, net twines and lines 
and ropes are often made of the same stretched fibre. It 
seemed strange that different articles used for different 
ourposes can te made from the same type fibre and still 
have for different purposes, optimal and good properties. 

By experiments, he and C. C. Kloppenburg had tried to 
find out what fibre type of polyethylene was the best for 
special purposes specifically for (a) a strong trawl twine 
which remained strong when knotted; and (b) a strong rope 
in the sense of having the highest breaking strength. 

E ; ght different types of monofilaments based on poly- 
ethylene made respectively out of homopolymer and 
copolymer (with different orientation) were produced. From 
all these monofilaments trawl twine was made and tested 
on breaking strength and knot breaking strength. For both 
types of polyethylene, the filament that gave the highest 
breaking strength as twine did not give the highest knot 
breaking strength. 

Ropes were also made from those eight types of filaments 
in normal and hard lay. As an outcome of these tests it 


could be said that the type of filament that gave the highest 
knot breaking strength of trawl twine was the same that 
gave the highest rope breaking strength; but the highest knot 
breaking strength of the twine must be made from a different 
type of filament. This meant that special filaments of 
polyethylene had to be made for either ropes or nets. For 
ropes they must be made to give breaking strength and for 
nets, knot breaking strength. 

On knotless nets: It is necessary to point out strongly 
that there are two types of knotless nets: (a) the Japanese 
type made by a special way of twisting and (b) the Raschel 
type made by knitting. 

The importance of this last type is pointed out by Mugaas 
and by Damiani. Mugaas shows that in the last two years 
in Norway the use of this type of netting has increased 
from 17 to 200 tons because in the small mesh sizes the 
knotless nets are 20 to 30 per cent cheaper than the knotted 
ones. Damiani also shows that in Italy the Raschel machine- 
made netting is cheaper for smaller mesh sizes but above 
a certain size the old knotted ones are less expensive. 

Dr. von Brandt's paper on testing Raschel knotless nets 
correlates the properties of this netting with fishing experi- 
ments made with the nets, whereas Hamuro deals only with 
experiments to improve a fishing net by using knotless 
netting in parts of the trawl. Their work shows that people 
are seeking to discover for what type of fisheries these knotless 
nets are better or cheaper than the old ones. It is quite 
certain that in one or two years' time certain fisheries will 
change over nearly totally to the knotless type of netting. 

Dr. von Brandt showed that the construction of the "twine" 
and of the joins is rather complicated. But some uniformity, 
will be the result of increased testing by discovering the best 
methods for these joins. A year ago this mesh strength in 
the depth direction of the netting was different to that of the 
length direction. It is in fact already claimed now that the 
knot strength of this type of netting can be equalled in both 
directions. Pianaroli states this, too. These differences in 
mesh strength are admitted by von Brandt and I wonder 
how much, in consequence of this, new joins with equal 
strength in both directions, the comparison of knotted to 
knotless nets based on mesh strength will change. This new 
basis will also change the weight comparison based on the 
respective mesh breaking strength. Although these two 
methods of comparison are somewha,t doubtful they do 
indicate that the knotless nets have a ittle advantage over 
knotted ones. 

The total resistance of this type of netting shows no 
difference compared with knotted nets of equal mesh- 
breaking strength. Used in herring gillnets in the North Sear 
the catching capacity of knotless nets was neither higher no, 
lower than that of customary knotted gillnets. 

It would seem that below a certain mesh size knotless 
Raschel nets are absolutely cheaper. Due to a new technique 
of making the joins, the mesh breaking strength in both 
directions can be made equal and, for that reason, comparison 
with knotted nets has to be put on a totally new basis. In 
other words a lot of experimenting and testing has to be done 
all over again. It is a pity that not much is published in the 
same way for comparing Japanese twisted knotless nets, 
especially in relation to physical properties and catching 

On monofilaments: Synthetic fibres for making twines, 
lines, ropes and nets can be made in different thicknesses with 
few exceptions. The thicker filaments are often preferred 
because one thinks that they are more useful and better 
adapted for certain tasks. The difference of thickness is the 

only reason why one fibre is called multifilament and the other 

Multiftlaments are mostly produced in thicknesses of 2 to 
25 denier per filament and monofilaments in thicknesses of 
50 up to 1,000 denier and even thicker. Even ropes made of 
filaments 4 mm thick are available. 

This difference of filament thickness creates certain 
properties which make the fibres better adapted for certain 
work but there are technical reasons which make it sometimes 
difficult to use monofilaments. Their very thickness creates 
properties not liked by fishermen. 

In their paper Henstcad and Ede review these different 
properties of monofilaments as made of different materials 
polyamide and polyolefin. The properties considered, to 
mention only a few are: (a) tenacity, (b) effect of heat, (c) 
non tenacity, (d) degree of flexibility, (e) abrasion resistance. 

In practice these monofilaments are made and used as a 
single monofil, as twine or as rope, and are used in different 
kinds of fisheries. Experience was required to discover in 
which fisheries a special part of the netting could be made 
best of monofilaments or multifilament products. There are 
many fisheries in which monofilaments have played an 
enormous role, for instance in Viet-Nam. Filaments made 
there are cheaper than imported multifilament and twine but 
technical advantages also accounted for their fishermen 
preferring monofiiament nets. These advantages are less 
fouling, easier to work with, less entangling and catching 
not less but sometimes more fish than knotted nets. These 
properties compensated for the bulkiness and other dis- 
advantages of such monofiiament nets. 

Steinberg lists the properties of monofiiament in comparison 
with other types of nets as (1) visibility, (2) softness, (3) 
diameter, (4) elasticity and (5) breaking strength. 

In this complex matter experiments are needed to find out 
what is worth while. Monofilaments are used not only as 
netting material but to produce twine and rope. Mostly such 
products are made out of olefin monofilaments because of 
their low specific gravity I know of only one type of rope 
made of monofile polyamide. 

We stand at the beginning of a period in which fibres and 
filaments will be produced for special purposes. To adopt 
monofilaments to the special needs of fishing involves a 
tremendous amount of practical experiments. 

Dr. C. C. Kloppenburg (Netherlands) explained that the 
colouring pigments had been added to the material tested 
only for identification purposes and exercised no influence 
on strength. 

Mr. Masao Kobayashi (Japan) expressed pleasure that 
knotless nets were featured in the Congress: his company 
had developed twisted knotless nets for many years. They 
were used for practically all types of fishing in Japan and the 
annual consumption was some 1,600 tons a year. There had 
not been sufficient time to study the papers on Raschel nets 
in comparison with the Japanese twisted nets, but they would 
do so and submit results. Both the twisted net and the Raschel 
net were different from the conventional knotted net. Accord- 
ingly when such knotless nets were used for purse seines, set 
nets and trawl nets the way of assembling the knotless netting 
should differ a little from that of knotted nets. 

A suitable method of assembling knotless nets was fully 
studied in Japan as was shown in Hamuro's paper and it was 
used in every type of fishing net except a gillnet. His firm 
belief was that the knotless fishing net was most advantageous. 
Dr. von Brandt: I would be very interested to compare 
types of Japanese and Raschel nets. There are big variations 
in the different types: both in making and in the joins. 

Mr. V. Valder (Peru), on behalf of Julie Castille, gave 
details'of Peru's purse seine fishery in methods and equipment. 
Nylon' netting, introduced 10 years ago, had undoubtedly 

contributed greatly to the meteoric growth and development 
of the industry. Knotless webbing had only recently been 
effectively introduced through Raschel knotless webbing 
becoming available at the end of 1961. Some earlier supplies 
had been too heavily tarred and been awkward to handle and 
easily breakable. Latest figures showed that 75 purse seines 
of knotless webbing were in use in a total fleet of 1,200 craft. 
A survey made in April 1963 by the Marine Resources 
Research Institute listed these advantages: (1) lower price 
because of lesser weight, (2) non-tangling of baby fish making 
operations easier, (3) less friction on the boat and bottom 
during hauling, thus promoting durability. A company had 
been formed and was beginning large-scale manufacture of 
knotless anchovetta nets in a factory containing ten Raschel 
machines of 960 mesh width. It was believed development 
in use would exceed Norway's growth and sales in excess 
of 400 tons were anticipated for 1964. This local manu- 
facture was likely to preclude overseas importation. 

Mr. D. L. WortfleM (UK): We have a method which we 
find very useful for comparison purposes between knotted 
and knotless nets. If you take the ratio of the wet mesh- 
strength to the weight of a given area of netting, say 1,000,000 
meshes (1,000 by 1,000 square), this can be used for both 
knotted and knotless nets and this allows for the amount of 
twine used in the knot or in the junction of the net. If this 
parameter K, we call it 1 over K, is plotted against the mesh 
size you can find the break-even point as far as the strength is 
concerned, for knotted as compared with knotless netting. 
Then, by allowing for production costs, this should be very 
useful for finding when knotless nets can replace knotted 

Mr. P. Lusyne pointed out that there were at least four 
different types of Raschel netting and they should know more 
about the joins and also more about the strains of the legs of 
the meshes themselves. Whenever they had mesh breaks it 
would be useful to know whether they were in the knot or in 
the legs of the mesh. It would be helpful also to have some 
project for finding which types of knot or netting gave the 
greater strength and the greater elasticity. Another point of 
interest was the breaking strength according to the direction 
of the pull. Much depended here on the type of connection 
between each mesh and they needed to know the different 
effects of the inner lines crossing or weaving. 

Arising from a query from Mr. M. Praet as to the meaning 
of the terms "normal, special and super 1 ' in Dr. von Brandt's 
paper on testing Raschel nets, Dr. von Brandt explained that 
they had been adopted instead of (a), (b), (c) simply to dis- 
tinguish three different constructions of material so that they 
could see what happened. 

Mr. Helmut Panzer (Germany) supplier of the material 
gave a technical explanation of their choice of three types of 
typical Raschel nets which were already on the market. 
The most interesting point was the join (see Fig. 4 in Dr. 
von Brandt's paper this section). 

Mr. Kristjonsson: Should the term "monofiiament" not be 
reserved for one filament used alone and not for two or more 
filaments twisted together? They should talk about heavy 
multifilaments when using heavy denier for making ropes 
as is now being done. 

Dr. Renter, summing up, said he considered the most 
important subject was testing; the testing of knotted net in 
relation to knotless netting. He had tried to get from a fac- 
tory making Raschel nets a certain twine but as yet had not 
received it. A new system had been devised and people 
wished to keep it for themselves. It was impossible to repair 
knotless nets without using knots. Knots had to be used. 
As to terminology, any change would be difficult because 
custom was involved. 





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Top quality, precision-made MOMOI NETTING is made to 
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Ako, Hyogo-ken, Japan 
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10 reasons why Samson Braided Ropes' 
help you catch more fish faster 


The big, value-conscious fishing industries of the West 
Coast of the United States and Canada have proven Samson 
Braided Ropes are today's most advanced lines for every fish- 
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thoroughly. We will be happy to send literature. Write today, 
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Samson Ropes are patented and engineered; backed by most 
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available (in ropes only 21/8" diameter) up to 120,000 pounds! 


Up to 25% stronger than any other 
rope of same size, material. Example: 
W Samson Braided Nylon Rope with 
splice, 8000 Ibs. breaking strength. 
V4" Twisted Nylon rope without 
splice, 6500 Ibs. Samson rope is made 
with latest synthetic fibers. 
*Note: Identical ropes are produced 
in Canada by our associated corn- 
pany, Canada Ropes Ltd., under trade 
name ''Buccaneer Brand." 


Engineered to correct degree of stretch 
required by specific jobs. Some shock- 
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stretch than other ropes of same size 
and material! 


You can make faster splices, stronger 
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twisted rope of same material, Trial 
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Sold under registered trademarks: Samson 2-in-1 and Buccaneer 2-in-1 Braided Ropes. For further information write to: 



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Wear is spread over 40 to 50% of 
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Samson Braided Ropes run smooth in 
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Holds freely-suspended loads without 
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far better than with twisted rope. 40 
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Thanks to the highest skill and best workmanship as applied by Japan* s 
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first and 
foremost for 

fry I on) 



Polyvinyl chloride fiber 

Tevinye, Tevix 


Whichever you choose, you get: 

* Easy handling 

* Non-absorbency and -shrinkage 

* Ideal undersea shape-forming 

* Abrasion resistance 

* Corrosion- and 
chemicals-proof properties 

Otok.' Tokyo, JMI 
Cto Addrw* "TWIN OSAKA 




If you had to choose between 'good' and 
'excellent' - which would it be? 
PERLOlSTof course! Ae an expert 
you will quickly appreciate the reasons: 
PERLON le reeietantto abraeion. 
It doee not rot A PERLON net 
ie lighter - therefore eaeier 
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Section 5 Stem Trawling Page 

The Stern Trawlera Decade's Development in Trawl Handling . . . . Conrad Birkhoff 147 

Some Small Stern Trawlers E. C. B. Corlett 153 

Ross Daring Experiment Dennis Roberts 158 

Discussion on Stern Trawling 160 

Section 6 Bottom Trawling 

Some of the General Engineering Principles of Trawl Gear Design P. R. Crewe 165 

Some Comparative Fishing Experiments in Trawl Design W. Dickson 181 

Development of an Improved Otter Trawl Gear Chikamasa Hamuro 191 

Towing Power, Towing Speed and Size of Bull Trawl Chikamasa Hamuro 199 

Suggestions for Improved Heavy Trawl Gear Eldon Nichols 204 

Development of Soviet Trawling Techniques . . . . . . . . . . A. J. Treschev 206 

Double-Rig Shrimp Beam Trawling . . . . . . . . . . J. Verhoest and A. Maton 209 

Discussion on Bottom Trawling . . . . . . . . . . . . . . . . . . ..218 

Section 7 Midwater Trawling 

One-Boat Midwater Trawling from Germany /. Scharfe 221 

Universal One-Boat Midwater and Bottom Trawl . . . . S, Okonski 229 

Two-Boat Midwater Trawling for Herring with Bigger Boats R. Steinberg 235 

Development of the Cobb Pelagic TrawlA Progress Report Richard L. McNeely 240 

Underwater Telemeters for Midwater Trawls and Purse Seines Chikamasa Hamuro and Kenji Ishii 248 

Reaction of Herring to Fishing Gear Revealed by Echo Sounding H. Mohr 253 

Discussion on Midwater Trawling 257 

Section 8 Gillnetting, Longlining and Traps 

King Crab Pot Fishing in Alaska R. F. Allen 263 

Les Madragues Atlantique et Sicilienne Vlto Foder 271 

Eel Traps made of Plastic H. Mohr 211 

Types of Philippine Fish Corrals (Traps) . . Arsenio N. Roldan Jr. and Santos B. Rasalan 279 

A New Fish Trap used in Philippine Waters Santos B. Rasalan 282 

Dropline Fishing in Deep Water Ronald Powell 287 

Discussion on Gillnetting, Longlining and Traps 291 


Section 9 Parse Seining 

Recent Developments in Icelandic Herring Purse Seining Jakob Jakobsson 294 

Sonar Instruction Courses for Fishermen G. Vestnes 306 

Discussion on Purse Seining 310 

Section 10 Deck Machinery 

The Application of Hydraulic Power to Fishing Gear D.W.Lerch 314 

Advances in Centralised Control and Automation H.E.H. Pain 338 

Driftnet Hauler for Salmon Fishing Chihiro Miyazaki 347 

Mechanization of Driftnet Fishing Operations P. A. Kuraptsev 352 

The Complex Mechanization of Beach Seining 5. 5. Torban 355 

Discussion on Deck Machinery 361 

Section 11 Fish Detection 

A Comprehensive Echo-Sounder for Distant-Water Trawlers G. H. Ellis, P. R. Hopkin and 

R. W. G. Haslett 363 

Sector-Scanning Sonar for Fisheries Purposes D. G. Tucker and V. G. Welsby 367 

A New Sonar System for Marine Research Purposes T. S. Gerhardsen 371 

Detection et Localisation des Banes de Poissons Robert Lenier 376 

Echo-Detection of Tuna M inoru Nishimura 382 

Echo-Sounder Measurement of Tuna Longline Depth Kyotaro Kawaguchi, Masakatsu Hirano and 

Minoru Nishimura 385 
A 200 Kc/80 Dual Frequency Echo-Sounder for Aimed Midwater Shrimp Trawling Masakatsu 

Hirano and Takashi Noda 388 

Ddtecteur de Poisson "Explorator" J.Fontaine 396 

Bio-Acoustical Detection of Fish Possibilities and Future Aspects . . . . G. Freytag 400 

Study of Acoustical Characteristics of Fish E. V. Shishkova 404 

Frequency Analysts of Marine Sounds . . Tomiju Hashimoto and Yoshinobu Maniwa 410 

Identifying Pacific Coast Fishes from Echo-Sounder Recordings E. A. Best 41 3 

Echo-Sounding through Ice . . Tomiju Hashimoto, Yoshinobu Maniwa, Osamu Omoto and 

Hidekuni Noda 415 

Discussion on Fish Detection 417 

Section 12 fleet Operations 

Japanese Motbership and Fleet Operations for Salmon, Crab, Longlining and Tuna Hiroshi 

Tominaga (2), Masatake Neo, Nippon Suisan Kaisha Ltd., and Goro Okabe 423 

Las Pesquerias Espanolas Austro-Atlanticas V. Paz-Andrade 438 

Discussion on Fleet Operations 445 

Advertisement Section 

Trade announcements relating mainly to fishing vessel gear and equipment. 447-468 


Part 2 Balk Fish Catching 

Section 5 Stern Trawling 

The Sterntrawler A Decade's Development 
in Trawl Handling 


The development of big sterntrawlers started by investigating the 
possibilities of processing the catch at sea. It was found that produc- 
tion lines inevitably crossed on a large sidetrawler and that the 
freeboard with shelter decks became too high for handling fishing 
gear. These difficulties disappeared when operating the gear from 
the stern. The problem of lifting the heavy gear on board at the 
stern was overcome by building a ramp over which the gear could 
be hauled without the time-consuming splitting of the codend to 
bring the catch on board. The first big sterntrawlers were factory- 
ships, such as the converted Fairfree, which led to improved types 
such as the Fairtry, Puschkin and Majakowsky class. Fishing 
conditions and the size of the vessel must be given due consideration 
in the design and construction of sterntrawlers. Matters which are of 
minor importance on large vessels may be of critical importance on 
smaller vessels. On the other hand, as smaller vessels normally 
work under better sea conditions, some problems can be dispensed 
with. Apart from more space for fish processing, sterntrawlers 
now have advantages over the sidetrawler in faster and almost fully 
mechanised handling of the trawl. The paper then describes the 
various types which have been realised to date and discusses ways 
and means for arranging the deck and winches for operation of the 
trawls over the stern. The future of fully mechanised sterntrawling 
is brought forward and the author points to the possibility of pro- 
viding vessels, even as small as 26 m in length, with a stern ramp and 
tilting or fixed bipod posts to haul the gear. Summing up, the im- 
portance of close co-operation between the shipbuilder and gear 
technologist towards further development of sterntrawling is 

Le chalutier & pche arrtere Comment il a revolutionne la manipu- 
lation de chalut au cours de ces dix dernieres annees 


La construction des grands chalutiers & peche arriere resulte des 
recherches sur la possibility de traiter les prises en pleine mer, 
a bord du bateau. Sur les grands chalutiers pechant par le cote les 
operations de traitement amenent inSvitablement un croisement des 
chaines de production et la hauteur de franc-bord de ces bateaux 
devient excessive et rend difficile la manipulation de 1 'en gin d* 
p&che. Cette difficult6 disparait lorsque Tengin est manipute par 
rarriere. Le probleme que posait le relevage par Farriere d'un 
lourd engin de p6che a etc resolu par la construction d'une rampe 
sur laquelle Tengin peut fctre releve, Svitant ainsi la perte de temps 
provoquee par les remplissages et v id ages successifs du cul de 
chalut. Les premiers chalutiers a peche arriere furent des navires- 
usines tels que le navire transforme Fairfree sur lequel furent bases 
les projets relatifs a des types perfectionn6s tels que les bateaux 
de la classe de Fairtry, Puschkin, Majakowsky. Les conditions de 
p&che, les dimensions du bateau ainsi que sa distribution interieure 
doivcnt faire 1'objet d^tudes attentives avant d'etablir les plans du 
bateau et de proceder a sa construction car des details d'une 
importance relative dans un grand bateau peuyent avoir une 
importance capitate dans d'autres bateaux de dimensions plus 
reduites; de plus, ces derniers travaillant normalement dans des 
conditions de mer plus favorables, certains problemes ne se posent 
pas. L'installation de la rampe a rarriere offre non seulement plus 
d'espace pour le traitement de la prise, mais a egalement Tavantage, 
par rapport aux bateaux pechant par le cdt, de permettre une 
mecamsation presque complete de la manipulation de rengin. 
L'auteur decrit plusieurs modeles construits jusqu'a present et traite 
des criteres et des systemes d'amdnagemcnt du pont etd 'installation 
des treuils qui manipulent les chaluts par rarriere. 11 examine les 
perspectives futures et decrit egalement un chalutier a rampe 
arriere completement mdcanise, avec une rampe arriere et un mat 
bipode basculant ou fixe pour hisscr ie filet (cette mecanisation 
pouvant equiper egalement des bateaux n'ayant que 26 m de long) 
L'auteur termine en mettant en relief 1'importance de la collabora- 
tion entre le constructeur de bateaux et le spccialiste d'engms de 


Conrad Birkhoff 

Naval Architect, Hamburg. 

peche en vue du perfectionnement ult&rieur du chalutage par 

El arrastrero con rampa a pope como ha evoludonado la 
lacion del arte en una decada 


La construccibn de grandes arrastreros con rampa a popa se debe 
a la investigaci6n de las posibilidades de elaborar la capture a 
bordo. Se obscrv6 que al instalar maquinaria elaboradora en 
un gran arrastrero que pesca por el costado era inevitable 
que las cadenas de produccibn se cruzasen y que el francobprdo 
adquiriese demasiada altura para manipular el arte con facilidad. 
Esta ultima dificultad desaparecia cuando el arte se manipulaba por 
la popa. El problema de meter a bordo por la popa el pesado arte 
se solucion6 construyendp una rampa por la que la captura se 
podia izar de una vez a cubierta, evitando asi la lenta tarea de llenar y 
vaciar el saco repetidas veces. Los primcros arrastreros grandes 
con rampa a popa fueron buques fabrica como el transformado 
Fairfree, en el que se basaron modelos mejorados como los de la 
clase Fairtry, Puschkin, y Majakowsky. 

Las condiciones de pesca, las dimensiones del barco y su dis- 
tribucton interna tienen que ser objeto de cuidadoso estudio antes de 
preparar los pianos de formas y proceder a la construccidn, ya que 
detalles de poca importancia en un barco grandc pueden ser vitales en 
otro pequeflo; ademas, los barcos pequefios, por trabajar en con- 
diciones de mar mas benignas, no plantean ciertos problemas. 

El arrastre por la popa no s61o deja ms cspacio para la elabora- 
ci6n de la captura, sino que tiene la gran ventaja con respecto al 
arrastre por el costado de que permite mecanizar casi por completo 
la manipulaci6n del arte. El autor describe varios modelos construi- 
dos hasta la fecha y examina los modos y recursos para la dis- 
tribuci6n de la cubierta y de las maquinillas que manipulan las 
redes por la popa. 

Al examinar las posibilidades futures el autor describia un 
arrastrero con rampa a popa totalmentc mecanizado y un barco de 
solamente 26 metres de eslora con rampa a popa y pbrtico bascul- 
ante a fijo para izar el arte. Termina poniendo de relieve la impor- 
tancia que tiene que el constructor de barcos y el especialista en 
materiales de pesca cooperen en el pcrfeccionamiento ulterior de 
la pesca al arrastre por la popa. 

original ideas which led to the development of 
A stern trawling were combined with the need for a traw- 
ler with a large processing deck. A conveyor belt system 
could not very satisfactorily be installed on a sidetrawler: 
production lines inevitably crossed. Furthermore, the 
high deck of a sidetrawler with an additional proces- 
sing deck rendered the handling of gear more difficult. 


Even such alternatives as the semi-shelter or trunkdeck 
ship could not fully solve the problem. 

All these disadvantages are avoided if the catch is 
hauled in at the stern. Moreover, by this method various 
simplifications in trawling can be achieved by mechani- 
sation. So the hauling-up ramp was thought of; as used 
on whalers. Such a ramp also offered the possibility of 
pulling in the entire codend in one operation. The 
laborious splitting of bags with larger catches could 
therefore be avoided and much valuable time saved. 

There were, however, numerous sceptics who at once 
saw a danger of damaging the fish. Furthermore, it 
was also feared that difficulties could arise in hauling up 
large catches, as in fact was at first the case. 

Now, after 10 years of progress, the actual possibilities 
can be considered in a clearer light. At the same time 
we realise a certain change in the problems. It is therefore 
interesting to recall some of the stages in this development. 

The big factarytrawler 

As early as 1945 a patent was applied for on the Ger- 
man side for a trawler with a stern-ramp. Similar 
efforts were subsequently evident in Great Britain and 
it was Chr. Salvesen & Coy., Ltd., there, which mostly 
specialised in whaling that had the courage to build the 
first experimental vessel in this line. The experiments 
with the converted vessels, Oriana and Fairfree, led to the 
construction of the new factorytrawler, Fairtry /, and 
later on to Fairtry II and ///. Alongside this, in Germany, 
a fleet % of 20 ships of the Puschkin-type factorytrawler 
were developed for the U.S.S.R. The experience gained 
with these vessels led to various new series being con- 
structed in the U.S.S.R., Poland and in Eastern Germany 
among others, the well-known Majakowsky Class and, 
on account of the special conditions prevailing in the 
tropics, the Tropic Type. The size of the vessel justified 
equipping the upper deck with a large number of technical 
aids for gear handling. Mobile warp trolley-rollers were, 
for example, developed on the Puschkin type to simplify 
handling the gear. The Majakowsky Class was fitted 
with swinging console gallows to meet requirements 
during trawling as well as hoisting the trawl doors. 

The crew required for the complete processing of the 

Fig. L Factorytrawler type (Fairtry, Puschkin, Majakowsky) with 
normal diesel engine plant, voluminous superstructures^ sufficient deck 
space for the trawl but separated navigation bridge B and trawl- 
bridge A. 

fish made large Uving quarters a necessity, which in 
turn required considerable superstructure. Consequently 
the navigation and the trawl bridge had to be separated 


so as to give an adequate view over the stern. In spite 
of the large superstructure, sufficient deck space could 
be kept free on this big vessel for unimpeded handling 
of nets (Fig. 1). 

On newer German factorytrawlcrs, powered with 
diesel electric drive, the superstructure was eased 
forward on top of the forecastle so as to provide the 
argest possible trawldeck. If the normal diesel motor 

Fig. 2. Factorytrawler type (Heinrich, Meins, Heinrich Kern, Hans 
Bockler) with diesel electric plant. Superstructures at fore-ship, long 
deck space and combined bridge A and accommodation in forecastle 


unit required a division of the hold fore and aft of the 
engine-room, by diesel electric drive, the hold could then 
be installed in the most convenient place, as far as trim 
was concerned, at midships (Fig. 2). Disadvantages soon 
arose, however, because of the position of the bridge 
and trawl winch, which were too far forward, disadvan, 
tages which were apparent from the high acceleration 
when pitching and from the poorer view to aft, to the 
ramp and gallows. Moreover, the living quarters of the 
crew were pushed still further forward, further in fact 
than a sterntrawler, according to its very structure- 

With a normal engine plant, the optimum design of 
the vessel made it advisable for the engineroom to be 
sited aft and the bridge at midships with an aft connected 
enginecasing. Consequently, a trawler with superstruc- 
ture on one side was proposed, following the model of 

Fig. 3. Factorytrawler type (Colonel Pleven II) with semi-side super' 
structure amidships and combined console bridge A. 

an aircraft carrier, with a combined bridge for navigation 
and trawling purposes hanging over the trawldeck 
(Fig. 3). Another solution with the same purpose in 
mind is the trawler with superstructure on both sides 
and a combined ferryboat type of bridge. This had 
already been suggested as a possible alternative several 

years ago but had not been realised before 1962 (Fig. 4). 


Fig. 4. Factorytrawier type (Troms0y I, Lofottral I, II) with two Re- 
ward superstructures and combined ferryboat-like bridge A. 

The partial lengthening of the actual trawldeck aft 
was meant to serve for laying on deck the long trawl 
wings but on a rolling ship the floats, shackles and wing- 
legs became hooked up and entangled during the process 
of handling the gear if both wings were very close to- 
gether. Since the trawl is V-shaped, the deck lent itself 
to a corresponding division. With side passages for 
operating the netwings and laying them out on the deck, 
the customary midship superstructure, desirable also 
from the point of view of the construction, was made 
possible. Behind this deckhouse the trawlwinch is 
installed, which now comes as near as possible to the 
gallows. In this way the trawling gear is "split" as early 
as possible during the hauling-up operation with the 
long wings on both sides (Fig. 5). 

Fig. 5. Factorytrawler type (Narwal) with diesel engine plant at aft- 
V-shaped trawldeck and combined bridge amidships A. 

In this case, with ships of over 40 m in length, it is 
possible to install the engineroom aft behind the fish 
hold which is preferable to all other possibilities. Due 
to that arrangement, even for the diesel electric trawler, 
a more practical distribution is achieved because cable 
lengths are shorter, control is better and one bulkhead 
is saved ; more practical, that is, if on the other hand good 
trim conditions can be effected in the ship through skilful 
arrangement of the tanks. 

The deep-sea trawler 

Medium-sized sterntrawlers, i.e., midwater and deep-sea 
trawlers, require a relatively simple and particularly 
stout deck equipment to be able to compare the new 
system of handling the gear favourably with the previous 
type of sidetrawler. Since the deck area of these vessels is 

limited, an especially well thought-out balance between 
naval architects' interests and those concerning the pure 
fishing technicalities had to be struck. 

The desired compromise solution provided a deck 
area aft for hauling up the cod-end and some fairly 
wide gangways leading forward to the foreship. At 
first the trawl winch was built round on three sides, set 
into the superstructure, and the bobbin groundrope was 
hauled over the tiawl wings, which were dragged up 
behind the winch. Disadvantages due to the accumula- 

Fig. 6. Deep-sea trawler (Stanislava, Mare de Labrador, Juan de 
Urbieta) engine at aft, combined bridge amidships A and sideward 
tracks for the trawl wings B. 

tion of the fishing gear, and particularly the trawl wings, 
behind the winch, could be avoided by extending the 
gangways at the side of the winch as far as possible 
towards the fore part of the ship. 

Efforts to handle the Pareja gear, of about twice the 
bottom gear length in comparison, originally suggested 
this course (Fig. 6). 

By using additional warping heads or similar drums on 
the trawl winch the job of releasing the warps from the 
trawlboards was saved, with a gain in time of about 
five minutes. At the time, the trawlboards were pressed 
flat against the gallows by this action so that work could 
be carried on without any danger. 

Such warping heads or gilson drums offer the most 
varied combination of winch arrangement : 

(a) On the same shaft. 

(b) Gilson drums on the counter shaft. 

(c) Gilson drums on a separate winch or winches, 
serving perhaps a dual purpose as ice or unloading 

(d) Combined trawl warp, gilson and hauling drums 
separated on each side of the ship. 

Solution (b) is contemplated here especially for 
smaller vessels. 

With medium-sized vessels good possibilities were 
presented for comparison with the previous type of ship, 
the sidetrawler. The sum total of all the direct and in- 
direct advantages, which clearly outnumber the disadvan- 
tages (including, among other things, higher building 
costs), only now become fully obvious. The most vital 
advantage was the shorter time required for handling 
the nets; this lengthened the active fishing time, which 
in turn produced on an average a 20 per cent profit in 
terms of the valuable "trawl on the bottom" time and a 
correspondingly better result in the catch. 


This revolution and change to the new type of vessel 
brought with it also a change of the crews from the old 
ships to the new ones, which enabled the shipowners of 
the latter to have a good selection for crewing the vessels, 
so the technical advantage was further strengthened by 
these personal influences. Crew problems on the older 
type of vessel compelled shipowners in several countries 
who were still holding back, to invest in this new solution. 

Different bask conditions for near-water and coastal 

All the disadvantages which are of little consequence 
with distant-water trawlers, for example factorytrawlers 
and deep-sea trawlers, are of greater significance with 
smaller vessels. For this near-water type of vessel the 
construction methods must be adapted to the general 
weather conditions prevailing on the particular fishing 
grounds. Often these small fishing boats work in rela- 
tively calm waters and this therefore permits solutions 
which would be impossible for rough seas. 

In recent years two different ideas have been offered 
for this type of craft. One was the type without a ramp, 
built with a bipod post which in some cases was tiltable. 
The shipowner's fear of possible damage to the fish on 
the ramp gave place to the idea of splitting the bag on 
sterntrawlers too. Having relatively calm sea, only the 
wings, with groundrope and headline, are hoisted over 
the bulwark aft. Then the bipod post hoists the bag part 
of the codend up out of the water and swings it out over 
the deck-ponds. On the backward swing of a tiltable 
bipod post, or by means of an additional codend derrick, 
the codend is again shot into the water so that the fish 

Fig. 7. Near-water sterntrawler without ramp but bipod post with 
fallow consoles and codend derrick. 

Fig. 8. Near-water sterntrawler with turnabte gantry. 

can be swilled quickly back again. This can be accom- 
plished in rythmical succession in a really short time 
(Figs. 7 and 8). The larger the catch, however, the greater 

the time-saving, which the use of a ramp has over this 

The sterntrawler with ramp, which is basically intended 
to save hauling time, goes a completely different way. 
Even if the average catch is relatively small, this vessel, 
without any further disadvantages, is better prepared to 
handle the larger catches in seasonal periods, too. The 
installation of a short ramp is possible even on ships of 
up to 28 to 30 m. The codend is guided by the gentle 
curving of the ramp in this case and is brought up without 
being bounced along against the stern. Damage to the 
fish by friction is impossible due to the special construc- 
tion of the codend. 

The codend is hauled up on deck in the main direction 
of the ramp by means of a hoisting block and emptied 
out by the tilting tackle on the fixed bipod post. A codend 
derrick, of some 4 m in length and pointing aft, takes 
care that the gear is lowered into the water behind the 
swirl of the ship's wake. 

A ramp for hauling up the voluminous fore part of 
the gear and also occasionally used for hauling the whole 
codend, was considered as a very promising alternative 
solution. Indeed in this case, through the combined work 
of the tilting tackle on the bipod posts and the shooting 
tackle on the codend derrick, an operations area compar- 
able with that employing the swinging posts, and possibly 
even more favourably sited, could be created ; this would 
also permit a similar division of the contents of the codend 
(Fig. 9). The strong installation of the solidly built-in 


Fig. 9. Near-water sterntrawler with bipod gallon post, fixed codend 
derrick working with both blocks. 

bipod posts and codend derrick guarantees safe working 
in all weather conditions. The longer trawl tunnel 
or lengthening piece, which might perhaps be necessary, 
could be compensated many times by the advantage of 
having a ramp available and consequently a better control 
of the gear. If, in addition, it is possible to tip the con- 
tents of the codend out under deck, the gear is again 
ready for shooting only a few seconds later, even in 
cases of very large catches. The significance of this in 
shoal fishing should be considered. The extra cost of 
protecting the working deck could, in the majority of 
cases, be more than compensated. This solution, after 
all, permits even the most sceptical shipowner to re- 
consider his prejudice without any disadvantage com- 
pared with his more progressive colleagues. 

This system of tipping out the entire codend can finally 
be put into effect on ships of up to 26 m in length, 

taking into account the length of the ramp as a usable 
extension to the deck. It is possible to combine the cover 
of the filling hatch with the ramp as a swinging-table 
type of ramp. To achieve the required security, this 
flap can even act as a bulwark when lifted up aft. The 
codend is hauled up only on this flat type of ramp and, 
after the codline is opened, is emptied out by the tilting 
of the flap. With a suitable device for opening the codend 
a little more forward and a holding device at the extremity, 
even the expensive bipod posts can be spared on such a 
type of craft (Fig. 10). 

Fig. 10. Sterntrawler with turnable ramp without bipod post. 

Where the side spaces are long enough this deck equip- 
ment, developed for the single towing trawler, offers the 
possibility also of handling a Pareja trawl, as already 
mentioned in connection with midwater trawlers. If 
one compares the usual practice in Pareja fishing as we 
know it at present with these new possibilities, there are 
even greater advantages than with the handling of one- 
boat trawls. The tremendous simplification of the 
manual work by extensive mechanisation produces a 
saving in time which is about 30 per cent higher than 
with hand-laboured vessels. Consequently, with this 
30 per cent longer active fishing time, a corresponding 
increase in the results can be expected (Fig. 1 1). 

Towards fully automatic trawling 

Although school fishing is now an almost fully automated 
business, using light and electricity effects as well as 
pumps, is relatively complicated to introduce into net 
fishing methods. The purse seine can be handled auto- 
matically, particularly when one thinks of pumping 
out the purse in school fishing. These possibilities cannot 
be directly transferred to trawl fishing. 

The trawlnet is always working in a water current, 
i.e., pulled in a constant direction. Also the instances 
are too few when one can really count on constant size 

of fish in the catch. Finally, any possible automation 
must also be capable of working in different weather 
conditions. With these basic conditions and require- 
ments in mind one can develop a similar thought in 
terms of trawling. 

A development such as this goes forward step by step. 
When the movement of the trawlboards at the stern 
were found to be too violent when the ship was pitching, 
guideways and fixed attachments were designed. When 
the handling of the wings on the aft deck was found to be 
too awkward on a heavily rolling ship, long side alleys for 
splitting and laying the wings on deck in one pull were 
devised. (Fig. 12a). If the de-shackling of the trawl- 
boards and going round them with pennants proves too 
troublesome and awkward, we can install a pair of swing- 
ing arms, perhaps in the form of guide-rails, to allow 
pulling the sweeplines or legs up to the foreship by 
using trolley rollers. In this way, the laying of the trawl 
gear on deck is fully automatic (Fig. 12b). The last bit 
of hard work in handling the nets, which requires six men 
on board, will then be spared. With a saving in manpower 
there is likewise a saving in accommodation on board 
and the extra cost of installing the swinging arms and 
guide-rails on deck will be more than offset by this 

The flap-type ramp makes it possible to empty the 
codend with the tilting tackle and this leads to the final 
problem, which is the connection between the hoisting 
tackle and the splitting strop on the codend. The length 
of this line has to be adjusted according to the length 
of the available gear deck. If this last piece of manual 
work were not so simple, some automatic facility would 
have to be found here too. 

These suggestions are not final solutions but examples 
which will surely, in the future, lead to still other related 

Trawl gear and fishery ships 

Net and gear constructors will no doubt agree that the 
sterntrawler is a new development in naval architects 9 
construction in which the shipbuilder has taken a lot 
of trouble to adapt his product to suit the gear construc- 
tors 9 work. What could and can now be done by the 
netmakers to meet the efforts of the naval architect? 

Ten years' practical experience with the new type of 
vessel has brought with it some alterations in the trawl 
gear for sterntrawlers, which should not be forgotten. 
First of all, the codend was adapted to the requirements 

Continued at foot of page 153 

Fig. 11. Pareja sterntrawler with sidewards tracks for the trawl wings, 


flu 12aondb 12a (bottom) Today's version of a partial aaomat^perated tterntrawkr. 12b (top) Future version of a complete automat- 
rigs, tai mu*. v operated sterntrawter. 


Some Small Stern Trawlers 


The paper describes some problems arising from stem trawling 
operations with small vessels. Bringing the gear on board requires 
a long deck space so that the smaller the vessel is, the more difficult 
the problem. A number of solutions have been proposed, such as 
cantilever deck houses with a winch installed far forward, or a 
deck arrangement as on an aircraft carrier with casings all on one 
side. On small ships of, say, less than 100 ft, such constructional 
changes cannot help as the available areas on deck are too small 
at the outset. The mouth of the net must then be brought to the 
stern and held there while the codend is lifted over. The use of a 
tillable gantry is only one of several ways of doing this, but has 
proved to be very effective in use. The Unigan arrangement, as 
used on the Universal Star, has incorporated this method. Such 
gantries have been fitted in vessels up to 210 ft in length and have 
been developed for vessels as small as 50 ft. The functions of the 
tiltable gantry during shooting and hauling operations are described 
in this paper, and some of the fundamental construction problems 
concerning stern trawler construction are discussed and suggestions 
given towards their solution. 

Quelques petits chalutiers a peche arriere 

La communication decrit quelques probtemes d&coulant du chalu- 
tage par Tamere avec les petits bateaux. Pour rcmonter le chalut 
bord, le pont doit avoir un espace assez long, c'est pourquoi plus 


E. C. B. Corlett 

Burness, Corlett & Partners Ltd. 

le bateau sera petit, plus le probleme sera difficile a resoudre. DC 
nombreuses solutions ont 6t6 proposees tclles que: superstructures 
en cantilever de chaque c6te du bateau avec treuil install^ tout a 
fait 1'avant, ou bien arrangement de pont comme sur les porte- 
avions avec des revfttements d'un c6te seulement. Sur de petits 
bateaux (jusqu'& 32 m) des constructions de ce type nc sont pas 
tres efficaces car 1'aire du pont est trop petite. L'ouverture du met 
doit dtre halee justqu'& Farriere du bateau et maintenue la pendant 
que le sac est montt sur le pont. L'utilisation d'un portique 
basculant n'est qu'une des nombreuses maineres de lever le sac et 
a prouv& son efficacite dans la pratique. Le syst&ne Unigan utilis* 

Continued fro/n page 151 

of a ramp. It was fitted with longitudinal or diagonal 
ropes as reinforcement and these absorb the whole pull, 
and it was divided into two or three longitudinal combs 
by vertical walls or seams. The latter improvement made 
the codend flat and itiore flexible for the curve in the 
ramp. The flat-lying position was often further improved 
by floats fastened at the seams, which made rolling 
impossible. The underside was fitted with hides and 
chafing ropes running lengthwise and close together. 
For emptying a heavily filled codend, a second opening 
was made at an appropriate distance from the original 
codline; for instance, by a ring-splitting connection. 
Indeed, this latter idea was subsequently used for 
separating and exchanging complete codends as necessary 
for transferring the catch or replacing damaged codends, 

Besides the alterations to the codend, the saving of 
one "false headline" together with one kite on the herring 
gears should be especially mentioned as this easily 
becomes entangled in the wake behind the ship when 
shooting. In order to maintain the same uplift a special 
float (an otter with wing-planes) was used, which is 
mounted just before the remaining kite. 

On rocky bottoms the advantage of shorter lower 
wings had already become evident due to the fact that 
fewer stones are collected and so it turned out to be 
practical, when hoisting the trawl wings along the deck, 
to shorten the upper wings slightly too. The scaring 
effect of the longer legs compensated for the loss in 
wing length. Depending upon the conditions of the deck, 
a slight shortening of the legs is possible without catch 
reduction if, in this way, a more practical and satisfactory 
constructional distribution can be achieved. 

If occasionally, in spite of the ramp which is there, 
one wants to bring the contents of the codend on deck 
by splitting bags, a longer tunnel or lengthencr is required 
in order to have sufficient room in the net for swilling 
the fish back. 

With still further mechanisation or full automation 
in the handling of the trawl, a certain adaption of the 
sweeplines and trawlboards may become necessary. In 
the case quoted, the length of the sweeplines would have 
to be restricted to the length that could be pulled in on 
board : some 80 per cent of the ship's length. 

The laying out on deck of the entire trawl gear may 
render it advisable to divide the net with ringstrop 
connections to allow for replacing individual sections 
of netting in the event of damage. 

These requirements and possibilities are mentioned 
only as examples of how the nctmaker can adapt his 
work to the arrangement of the new vessel types. The 
more the shipbuilder is prepared to mechanise or 
automate the handling of the trawl, the more the net- 
maker has a duty to develop a trawl gear suitable for 
such automation. The advantage in design which is 
then achieved may well compensate for certain losses in 
catch capacity. Nevertheless, it is the task of the gear 
constructor to find such an adaption without vital loss 
to the catch results. 

The development of the simplest automation and the 
production of trawl gear which has the maximum effect 
will, in future, call for a closer co-ordination between the 
constructors of fishing gear and those of fishing vessels. 
On behalf of my colleagues in the shipbuilding industry, 
I believe I can especially stress that we are prepared to 
do our very best to achieve this end. 


sur T Universal Star cst base sur ccttc roethode. Cos portiques ont 
etc months sur des bateaux dc 70 m de long et developpes pour des 
petite bateaux de 16 m. Lcurs fonctions pendant la p6che sont 
dtaites dans la prtscnte communication. Quelques uns des probl- 
emes fondamentaux relatifs a la construction des chalutiers pechant 
par Fan-fere sont posts et des solutions sont suggerees. 


Describe el autor los problemas que plantea la pescaal arrastre por la 
popa con foarcos pequenos. Para meter el arte a bordo se necesita una 
cubierta larga, de manera que cuanto mas pequeflo es el barco mas 
difidleselproblema. Se ban propuesto varias soluciones entre ellas 
al puente de mando cantilever con la maquinilla instalada muy a 
proa o una cubierta como la de los porta-aviones con las supere- 
stmcturas en un lado. En barcos pequenos de, por ejemplo, 
menos de 35 m, no se pueden adoptar estos cambios en las estruc- 
turas porque, ya para comenzar, las superficies disponibles en 
cubierta son demasiado pequeftas. Debido a ello, la boca del arte 
se tiene que Uevar a la popa mientras se iza el copo. El portico 
basculante es una de las diversas maneras de lograr esta finalidad 
y ha resultado excelente en la practica. El sistema unigan empleado 
en el Universal Star empka este procedimiento. Estos porticos se 
ban montado en barcos hasta de 64 m de eslora y se ban construido 
para otros de 15 m nada mas. El funcionamiento del portico 
basculante mientras se cala o se recoge el arte se describe en esta 
comunicacidn, en la que tambien se examinan algunos problemas 
ftindamentales relativos al proyecto y construction de arrastreros 
con rampa a popa y se hacen sugerencias para solusionarlos. 

THIS paper describes some stern trawling develop- 
ments in small trawlers which present quite different 
problems from those in larger ships. In all fishing 
vessels rising wages and operating costs throw emphasis 
upon crew economies and obtaining high utilisation. 
In larger ships, crew savings possible with stern trawling 
are not as important as with small ships and the pre- 
dominant reasons for adopting stern trawling may be 
quite separate; for example, increased space in the ship 
for processing plant, increased trawling capacity, the 
difficulty of side fishing in large ships with considerable 
freeboard, etc. However, in small ships where perhaps 
three to fifteen men are carried, a saving of three or four 
men in say 130 ft. (40 m) ships and one man in the smal- 
lest ships is of real economic importance. 

Stern fishing allows a ship to fish in worse conditions, 
the ship being aligned better with the weather when 
shooting and hauling and the crew having much greater 
protection at all times. Furthermore, experience is that 
the hull design required for practical reasons in stern 
trawlers seems to produce ships with greater sea-keeping 
capacity than normal side trawlers with their low free- 
board amidships. Moreover, the stern trawler with 
mechanized systems can handle gear more quickly and it 
is difficult to install comparable systems in a side trawler. 
As a result, because of better weather ability and quicker 
gear handling, the ship can actually fish for a longer 
time on a given voyage than can a side trawler. These 
arguments are particularly applicable to small ships 
ranging from say 40 down to 1 5 m. But there are difficul- 
ties in adopting stern fishing with these smaller ships 
which are not met with in bigger ones, due mainly to 
the inherently limited fore and aft length of the fishing 
deck and to sheer lack of space elsewhere. 


Basic factors affecting stern fbUog 

However the gear is brought aboard over the stern, 
warps must be led forward. Big ships can use the net 
as an extension of the warps and haul the fish bag over 
the stern via a ramp, the whole net coming on board 
before the fish bag. Clearly this requires space, length 
in particular, and is not really possible with small ships, 
say under 130 ft (40 m) in length, unless the arrangement 
of the whole ship is subordinated to the requirement. 
A number of layouts have been proposed and indeed 
some have been built using cantilever deck houses with 
a tunnel leading to a forward winch installed at the 
break of the forecastle. Others have used compact 
high-speed machinery installed aft with side funnels and 
casings and the net hauled over the top of the engine 
room. Alternatively, it is possible to arrange the ship 
in aircraft carrier fashion with casings to one side and 
still more variations are possible, but generally all these 
arrangements lead to an awkward overall arrangement 
of a small ship. 

If the engine room is forward, Fig. 1 shows a typical 

Fig. I. See Text. 

layout with cantilever deckhouses, while, if the engine 
room is arranged aft, Fig. 2 shows another typical 
arrangement. Again there must be cantilever deckhouses 
and the net and warps led between twin casings. An 
advantage is that the centre of pressure of lateral 

Fig. 2. See Text. 

windage of the above-water profile is aft and this helps 
with handling problems but the arrangement is only 
possible if the machinery is of small height and hence of 
high revolutions and small volume. 

A good example of this type of ship is shown in 
Fig. 3 which shows a recently constructed trawler. Here 
the main machinery consists of twin engines rated at 
788 total continuous bhp at 1,400 engine rpm 

returned to the water. The vessel is then given a kick 
ahead driving the remaining fish back into the codend 
which is then lifted aboard the vessel as before. Fig. 4 
shows a typical outline arrangement of a Unigan trawler 

Fig. 3. Detail of newly built trawler. 

geared together on to a constant speed controllable 
pitch screw with a constant speed trawl winch generator 
driven off the forward end of the gearbox. The arrange- 
ment is very convenient but, as mentioned, is dependent 
upon the use of rather specialised machinery. 

Up to a point, the smaller the ship the more difficult 
is this problem but in very small ships which, anyway 
usually fit high-speed engines, the problem eases again 
as such engines go under the deck and it is not difficult 
to arrange exhausts and casings to suit. 

The Unigan tillable gantry solution was developed to 
avoid these difficulties, the concept being that during 
hauling only the mouth of the net should be brought over 
the stern and held there while the codend would be lifted 
on board over the stern. This has many advantages, 
being simple, quick and convenient, requiring a minimum 
of space and manpower and avoiding the length taken 
up by a ramp, although the inherent lack of space still 
poses some problems. For example, arrangements must 
be made for bringing the whole net on board for repairs 
but generally this can be handled by bringing the codend 
on board and leading it forward on strops from the trawl 
winch. Access to an underdeck net store, if fitted, can 
be a difficult minor problem. 

The Unigan arrangement was first developed on the 
well-known Universal Star, has since been fitted to some 
twenty vessels and is being fitted to several others either 
ordered or under construction. Generally speaking, 
the system uses a strongly constructed hinging portal 
frame gallows from which the trawl warps are led and 
the gear towed. For example, in a 100-ft trawler where 
the warp breaking strain is 14 tons, the gantry will weigh 
approximately 1-1 J tons and will be designed to move a 
gilson block load of 1-5 tons, although much heavier 
loads can be handled by hauling in the codend with the 
gantry in an upright position. In the case of an extremely 
heavy catch it is sometimes desirable to bring the fish 
aboard in two or more operations. This double-bagging 
procedure is accompanied by tightening the half-bag 
becket by means of the gilson wire so splitting the catch. 
The codend is then brought aboard in the usual manner. 
After the fish have been deposited on the deck the codline 
is re-tied, the half-bag becket loosened and the codend 

Fig. 4. Outline arrangement of Unigan trawler. 

with the engine room forward and showing the lead of 
gear from the trawl winch. 

The gear has been fitted in trawlers up to 210 ft (64 m) 
in length and has been developed for ships as small as 
50 ft (15 m). However, the advantages of using the 
tillable gantry vary with the size of the vessel. A net of 
considerable size can be handled with the gantry on a 
small but relatively powerful boat without difficulty, 
whereas such a net cannot be manhandled by a small 
crew. An example of such a net would be: 

ft in 


Headline 63 19*0 

Square 13 4-0 

Baitings 25 7-5 

Top wing 24 6 7-4 

Lower wing 45 13-6 

with a belly of 300 9-1 

Indeed, in day fishing where gutting and heading is 
not carried out, such a boat of 50-55 ft (15-17 m) 
length and say 250 hp arranged with grouped propulsion 
steering, gantry and winch controls, etc., might be opera- 
ted by two men in active trawling. The saving is consider- 
able as a heavily designed vessel of this size with adequate 
power can be made equivalent in fishing ability to a nor- 
mal side trawler of some 15-20 ft (4-5-6 m) greater 
length which may well have 6-7 men in its crew. One 
might say with some confidence that for approximately 
equal fishing capacity such ships can halve the number 
of crew often required by larger conventional vessels 
while, if registered under Part IV of the Merchant 
Shipping Act, they can be inside the 25 tons registered 

At the other end of the size range the main attraction 
is that the catch is lifted rather than dragged aboard. 
With a heavy bag the damage that may be inflicted on 
the catch by ramp trawlers when the codend is dragged 


up the ramp is eliminated and this may be a factor in the 
value of the catch when destined for the normal white 
fish market. It would seem likely that a gantry which is 
really simply added to the normal ramp system will be- 
come more and more popular in such ships as, although 
derricks and cranes can carry out the same function, they 
cannot do it with the same efficiency and economy. The 
operation being primarily a transfer of weight fore and 
aft, it is best done with a system with only the one rele- 
vant degree of freedom. The gantry can be positioned to 
facilitate shooting the gear and also handling heavy 
gear such as trawl boards, etc. It must be remembered 
however that sometimes in ramp stern trawlers the 
quality of the catch is not a consideration, as it is under- 
stood that a number of large stern trawlers are fishing 
mainly for fishmeal production. 

Factors in stem f idling vessel design 

The design of small stern fishing trawlers offers many 
difficulties. Problems of accommodating the gear in 
the ship, especially are not simple and the hull design in 
itself needs to be sophisticated. Fundamentally the ship 
shoots and hauls her gear up-wind or down-wind. 
There are advantages, providing the weather is not too 
severe, in hauling down-wind as the gear streams naturally 
behind the vessel and little or no steerage way is required 
for the ship. This basic difference from the methods used 
in side trawlers is one of the factors responsible, of course, 
for the great improvement in the comfort and safety of 
the crew, commonly experienced with stern trawlers. 
However, if hauling headed into the wind, if the sea and 
wind are not in the same direction or there is a confused 
sea running, it may be difficult to prevent the ship's 
head blowing off. Furthermore, if a lone stern trawler 
is fishing in company with a number of side trawlers it 
may not be possible to shoot and haul before the wind 
and the ship may have to conform with the general trend, 
i.e., with the wind abeam. However, this is a temporary 
factor and the time will come when the odd side trawler 
will have to conform when fishing with stern trawlers. 
Ability to control the ship's head being critical, therefore, 
the ship must be designed with deep sections and good 
immersion aft. 

The pitching centre must be as far aft as possible as 
otherwise, although the ship as a whole may be sea 
kindly and although the gear may fish very well, it may 
be impossible to handle gear on the aft deck or fantail 
due to its amplitude of motion. At the same time the 
bow must have good immersion and a hard forefoot in 
order to assist in preventing the ship's head blowing off 
and lowering the steering effort required to keep the ship 
on the intended course when hauling. As a result a large 
rake of keel is an anomaly, unlike side trawlers where it 
may be an advantage and necessary for steering when 

Windage forward, both in terms of area and drag co- 
efficient, must be kept to a minimum for the same reasons 
generally speaking, therefore, the stern trawler will tend 
towards vessels of deep draft, low rake of keel, low block 


coefficient with a deeply immersed and hard forefoot 
profile with flat sections aft, carefully blended, however, 
into deep Vees to prevent slamming. If the pitching 
centre is far enough aft slamming does not occur with 
such sections, providing they are well immersed. Deck- 
house and rigging detail, etc. must be as clean as possible 
and funnels avoided if the owners will permit this. 

The stern must be so designed as to avoid entanglement 
of fishing with sterngear especially in the difficult stage 
of getting the codend on board. As a result it is desirable 
to fit the propeller and rudder well forward which is, 
of course, assisted by the low block coefficient inherent 
in stem trawlers. 

Added protection for the propeller may be given by 
the fitting of a steering nozzle as has been done on the 
majority of the larger stern trawlers. Other advantages 
of a steering nozzle are an improvement in trawling 
pull, good steering capabilities at low speeds and the 
ability to use relatively high powers in conjunction with 
high rpm, without undue loss of efficiency. At the 
same time the introduction of a nozzle would appear to 
improve the pitching characteristics of the vessel. It is 
questionable whether the ideal layout of a stern trawler 
may not be obtained with twin screws as in the Universal 
Star but with engine rooms aft. 

The twin screws naturally lend themselves to this type 
of layout as relatively small prime movers are inherent 
which may be installed aft and placed so that their 
casings are brought up the sides of the ship, leaving the 
fishing deck clear in between. There is much to be said 
for this layout which gives unequalled controllability 
in all phases of the shooting, trawling and hauling 
operation. It is not necessarily more expensive as, with 
half the power on each propeller, high-speed machinery 
in an altogether lighter range of engines may be used 
which, although they can be robust and conservatively 
stressed, are often cheap per horsepower and can offset 
the greater cost of steel and sterngear. Twin rudders are, 
however, a sine qua non, and a single rudder would be 
far from satisfactory. Prejudice against twin propeller 
propulsion does not seem reasonable in the case of stern 
trawlers though it may be for side trawlers. 

Adequate ballast capacity is essential in a stern trawler, 
perhaps more so than with any side trawler. It is not 
enough to bring the stern down in order to give adequate 
immersion aft, because in such a case the draft at the 
bow may still be so light that it will blow off. Neither 
is it enough to bring the bow down far enough to give 
grip without ensuring at the same time that the stern is 
deep enough to bring the pitching centre suitably far 
aft. Fundamentally, a smallish stern trawler should be 
designed to be able to maintain substantially constant 
draft at all phases of her operations. This is not difficult 
to achieve. 

Recent designs have shown that in most cases ballast- 
ing can keep the draft and trim within a few inches of the 
design target throughout all phases of the voyage. In 
this respect, an owner should not be allowed to dictate 
the quantity of ballast in such a ship. This is a delicate 

matter falling inside the jurisdiction of the designer who 
should not allow himself to be deprived of the necessary 

How Unigan operates 

For shooting, the gantry is set up with the trawl laid out 
on the after deck, codend and body arranged over the 
stern ramp. Trawl boards are in a stowed position port 
and starboard, between the gantry legs and outer bul- 
warks. The boards are stowed in a fore and aft position 
just forward of the gantry pivot position and are attached 
to the outer bulwarks by short wire strops clipped to the 
forward edges of the boards. 

The sweep wires are led from the winch under foot 
rollers at the base of the gantry and over the hanging 
blocks attached to the outboard sides of the gantry legs. 
The danleno bobbins and butterflies are attached to 
the sweep wires in the usual fashion and are lifted to the 
hanging blocks. 

When ready to shoot the gantry is swung aft and the 
headline legs and towlegs are attached to the butterflies 
The codend is pushed over the stern and the drag from 
the water streams the net bellies after it. The quarter 
ropes are rove through special sheaves on the inside of 
the gantry legs and these, with the aid of the winch 
warping heads, are used to lift the footrope and heavy 
bobbins over the stern. In the case of light gear the drag 
from the net in the water is sufficient to stream the entire 
wings, footrope and headline over the specially shaped 
stern hump without manual effort. 

This stern hump, or dwarf vee-shaped bulwark, is a 
recent innovation and supersedes the relatively more 
expensive hydraulically operated stern flap as fitted to 
the Universal Star and some subsequent vessels. 

The stern hump gives added protection to the crew 
and facilitates the shooting and hauling of the gear. 

With the entire net in the water, the warps are slacked 
until the strop links engage the Kelly eyes attached to 
the trawl board backstrops. At this point the gantry is 
swung forward and the warp slackened off. The G-link 
assembly can then be attached to the trawl boards, the 
warp taken in and the boards lifted to the hanging 
blocks. The strops holding the boards to the bulwarks 
can now be released and the gantry swung aft to allow 
them to drop clear of the stern when the winch brakes are 
released. The requisite amount of warp is let out in the 
usual manner. 

The operation of hauling is in reverse sequence to 
that of shooting until the danleno bobbins are brought 
up to the hanging blocks. At this point the quarter 
ropes are detached from the butterflies and, by means of 
ropes rove through the gantry quarter sheaves, are used 
to bring the mouth of the net over the stern. A messenger 
rope, leading from the winch warping end and passing 
through a gilson block at the top of the gantry, is then 
attached to the pork line and the codend heaved up to 
the gilson block. The gantry can then be swung forward 
to deposit the fish in the deck ponds. The entire operation 
from the time the trawl boards break the water to deposi- 

ting the fish on deck usually takes approximately 10 

Nothing, however, is static and in some cases it has 
been felt that towing from the gantry leads to an unduly 
heavy structure which in turn, of course, is reflected in 
the size and general cost of the hydraulic gear. An alterna- 
tive arrangement has been produced. In this the gear is 
towed from gallow posts and the gantry is used to take 
the otterboards aft from the stowed position. Hanging 
blocks from the top of the gantry are used to lift the gear 
over the stern. As no towing and, in particular, no 
quartering loads come on the gantry it may be of much 
lighter construction, its design being governed only by 
the gilson block load. When the boards are brought up 
to the block on the gallows posts they are secured by dog 
chains, the G-link assembly undipped from the board 
and the sweeps hauled in until the danlenos come up to 
the blocks (Fig. 5). This variation on the basic gear has 

Fig. 5. A board brought up to the block on the gallows post. 

worked well and is now afloat in two or three ships. It 
is not quite as flexible as the original Universal Star 
type gear but, on the other hand, performs well at a 
lower initial cost. The tiltable gantry gear may be modi- 
fied without much difficulty to purse seining and a com- 
bination gear, which can stern trawl and purse seine 
without any fundamental alteration to the ship, is under 

Continued at foot of page 15* 

Ross Daring Experiment 

ROM Tftwkrs Limited are presently completing a 99-ft sterntrawter 
with a high degree of automation and extensive use of centralized 
controls. The vessel will have a 450 hp, 1,800 rpm engine situated 
alt and a flsh-hokl foreward of 4,800 ft 3 capacity. Vessel, gear and 
cww are planned to cope with an average catch of one ton per day. 
The crew consists of skipper and four deckhands. All engine room 
functions, including servicing, bilge pumps, water supply, heating, 
etc, will be fully controlled from the bridge in the same manner as 
airplane engines are controlled and serviced from the cockpit. 
The vessel will operate a normal wing trawl which will be taken on 
deck over a stern roller. Hauling and shooting operations are fully 
mechanized and controlled by the skipper from the bridge with the 
exception of lifting the codend over the stern. A conventional 
sidetrawler of comparable size would need more than ten men on 
deck alone. 


Dennis Roberts 

Ross Trawlers Ltd. 

L'Experkoce ROM Daring 

La "Ross Trawlers Limited" achieve actuellement un chalutier 
pftchant par I'arrttre de 32 m qui aura un haut degre d'automation 
et sur lequel les contrdles seront centralists. Le bateau equipe 
de machines de 450 cv, 1.800 tpm situees a 1'arriere, a une cale 
d'une capacit^ de 135 m 3 . Lc bateau, les engins et l'quipage sont 
privus pour une capture moyenne de un tonne par jour. L'equipage 
est compose d'un patron de pcche et de quatre matclots. Toutes 
les fonctions des machines tellcs quc: service, pompe de cale, 
distribution d'eau, chauffage, etc. seront entierement contrdlees de 
la timonerie, de la meme maniere que les moteurs d'avion, contrdles 
et alimentes de la cabine de pilotage. Les operations de peche s' 
effectucront avec un chalut & grande ouverture qui sera remonte 
par I'arriere, par-dessus un rouleau. Les operations de chalutage, 
entierement mecanisees, seront contrdlees par le patron depuis la 
timonerie a 1'exception du relevage du sac. Pour un meme travail, 
un chalutier traditionnel pechant par le c6t6 aura besoin de 10 
homines sur le pont. 

Ross Daring Un experimento 

Se completa para la empresa Ross Trawlers Ltd. un arrastrero para 
pescar por la popa de 99 pies de eslora muy automatizado y en el 
que se emplean mucho los mandos centralizados. Tendra a popa 
un motor de 450 hp a 1 .800 rpm situado y a proa una bodega de 
pescado de 4.800 pies cubicos de capacidad. Esta proyectado para 
pescar una tonelada diaria con una tripulacidn formada por el 
patr6n y cuadro marineros. Todas las actividades de la sala de 
maquinas, incluidos engrase, accionamiento de las bombas de 
sentina, suministro de agua, calefacci6n etc., se controlaran desde 
el puente de la misma manera que los motores de un aeroplano se 
regulan y atienden desde la cabina. Tendra el barcp un arte de 
arrastre normal que se metera a bordo sobre un rodillo colocado 
en la popa. Las maniobras de largar e izar son totalmente mecanicas 
y las hace el patr6n desde el puente, excepcibn hecha del izado 
del saco sobre la popa. Un arrastrero que pescara por el costado de 
dimensiones analogas necesitaria mas de 10 hombres en la cubierta. 

Continued from page 157. 

development. This must, however, use the normal 
Unigan as a basis. 

The system described is one of several; nevertheless it 
is relatively widely adopted for small and medium-size 
trawlers. Initial teething troubles were not connected 
with the gear itself but arose from the differences in 
technique required in adapting from side to stern trawling 
and also from the twin screw arrangement of the Universal 
Star. In particular, headline breakages were traced to 
turning too sharply at the end of a steering leg and inad- 
vertently collapsing the trawl. The extremely small 
turning circle of the Universal Star was largely respon- 

The ultimate, of course, in small stern trawler design 
is complete mechanization of fishing operation procedure 
allied to centralized engine control in the wheelhouse. 

This has, to a fair extent, been accomplished by Unigan 
and further developments include an operating bay at the 
after end of the wheelhouse with grouped remote con- 
trols for gantry and winch. The reduction in crew due 
to the increased mechanization improves the economics 
of the operation and, at the same time, the better working 


conditions attract good grade fishermen, with a conse- 
quent increase in efficiency. 

For Northern waters all gutting should be done under 
cover away from the elements and on even relatively 
small stern trawlers of say 100 ft (30 m) a shelter deck 
can be arranged to give this protection. This means that 
only one or two men are needed for a short time on the 
weather working deck during shooting and hauling. 

Stern trawling has come to stay, and possibly the sole 
remaining barrier it has to break through is the reluctance 
of many owners to change over from their relatively 
new but technically obsolescent side trawlers to the more 
comfortable and safe stern trawlers that are now avail- 
able. This is understandable and reasonable but the time 
is coming when the move has to be made. A develop- 
ment which may help is an arrangement for converting 
side trawlers into stern trawlers with a high degree of 
automation and crew protection at reasonable expense 
and without major structural alterations. This has been 
done on a trial ship which has proved successful and 
perhaps this development may form the transition phase 
generally needed. Certainly no one who has seen gear 
handled aboard a small stem trawler under difficult 
conditions can doubt that in the end the method will 
replace side trawling. 

BRITAIN has already launched her first 
vJT stem-trawler, employing automation, aimed to cut 
the cost of fishing. From Cochrane of Selby the vessel 
of 99 ft overall will be operating in the North Sea by 
October 1963. Ross Daring will be crewed by a skipper 
and four men. 

The intention is to carry only enough men to handle 
the catch. If a vessel requires more men to work the 
gear than to handle the catch, then the design is wrong 
and it should not be followed. 

Man or machine is the basic problem. Factories 
ashore have installed machines, whilst fishing vessels up 
to now have preserved the man. Each fishing community 
has a different problem, a different point of view, and it 
is impossible for all of us to move forward at the same 
pace and along the same lines of thought. One thing is 
certain . . . the more successful a community, the more 
conservative will be its thought, and the slower its 
development. We must be careful to avoid this lethargy, 
that is why automation in fishing has been so long 

In recent years, fishermen have attained a higher 
standard of living and are enjoying more security, so 
much so that the younger men are not bothering to 
learn their job and hence are very expensive to carry. 
When men price themselves out of the market, the oppor- 
tunity comes for the machine to replace the man. 

This situation has already developed in the North 
Sea. Too many expensive men are employed working 
the ship, many more than are required, to handle the 
catch. The fault lies in the design of the sidetrawler. 
Whatever manning scale is attempted, it is a fact that 
the conventional trawler requires one man for every 
10 ft of length. If we can re-design to make this ratio 
1 to 20, then something useful will have been achieved. 

Catching capacity 

A trawler's capability should not be judged by the number 
of men aboard but by overall length. The minimum 

length required to operate in the North Sea all the year 
round is thought to be 99 ft. This length of vessel is 
capable of catching an average of approximately one 
ton offish per day; therefore the crew should be no more 
than the number required to handle the catch comfortably 
and that is the skipper with four good fishermen. 

It is acknowledged that there are sidetrawlers employ- 
ing only this number of men, but they are very small 
vessels pulling a very light gear. The size of gear suitable 
for a 99 ft vessel requires ten men to handle if operated 
from the side on a conventional trawler. 

As half this number only is being employed, then the 
sterntrawler must have a high degree of automation to 
simplify the work for these men. 

The point can be made here that it is not necessary to 
go to sterntrawling in the North Sea if the crew numbers 
are not being reduced accordingly. 

Are engineers necessary? 

The managing director, travelling in his luxury car, 
does not have a mechanic on the back seat. Then why 
should a fairly elementary diesel engine of this vessel 
have two unproductive men to watch it hour by hour? 
It is nonsense to say it must be so, in a year when men 
are being rocketted into space! It has not happened 
before because there has been no demand for such an 
engine. It will be commonplace in five years* time. 
Control of engines, services, bilge pumps, water supply, 
heating, etc., all can be exercised from the bridge. 

Integrated control 

Control of winches must be integrated with all relevant 
information obtained from bridge instruments. At 
some point, whether bottom or midwater trawling, the 
speed of tow must be adjusted, the length of warp altered, 
perhaps the angle of inclination of trawl doors. It is 
quite wrong that the control of the winch is on the deck 
and that a man must don oilskins and get wet every time 
the skipper requires a warp adjustment. This is the 

Fig. 1. Single-screw steel motor sterntrawler for operation in the North Sea area. Dimensions: 85ft LBP x 23ft Bmld x 12 ft 6 in Bmld. 
KEY: 1. Stern roller. 2. Guide rollers, P. A S. 3. Bobbin trays. 4. Steps to bobbin trays. 5. Gallows, P. A S. 6. Hatch to bobbin store. 7. Mast 
house. 8. Derrick. 9. Fish pounds. 10. Warping winch. 11. Trawl winch, P. A S. 12. Gutting room. 13. Tray. 14. Gutting trough. 15. Washing 
trough. 16. Chute tofishroom. 17. Trawl lead. 18. Bobbin lead. 


point where automation does the fisherman a real service. 
It is now a fact that the skipper can bring the trawl doors 
up into the gallows by the touch of a button. 

Will fishermen accept these exciting new vessels? 
Yes, certainly. Very few labour-saving devices have been 
introduced into the working of the trawl; those that 
have, have indeed been welcomed. It is the aim to 
improve the standard of living of fishermen still further. 
Five good men sharing 7,000 are better off than ten 
sharing 10,000 (these figures are an example and not a 

Fishing gear 

The economics of this vessel have been worked out on 
using traditional trawls. If there is an improved method 
of catching fish, then it will be welcomed. Research 
into improving trawls is going on and, until something 
more useful is developed, a wing trawl will be used. 

Unlike many stern trawlers, the Ross Daring has no 
stern chute because of the risk of swamping the fishing 
deck in winter conditions. The gear is taken over a wide 
roller across the transom and the bobbins are contained 
on a sloping tray or trough. Codends are taken over the 
quarter, as with Canadian stern draggers. 

Both port and starboard winches are hydraulic and 
controlled from the bridge, so the skipper is able to 
bring the wings up to the winches and the bobbins on to 
the tray. Control is then taken over locally by the deck 
crew who have a small hydraulic capstan employing two 
whipping drums, mounted centrally, to swing-in the 
codends, etc. 

The fish is sorted and shot into the gutting and washing 
house. From there, after processing, it is transported 
forward down a chute into the fish-room where it is 
packed away in ice in the traditional manner. The 
fish-room is forward and has a capacity of 4,800 ft 8 . 
Hatches are only removed for unloading. 

The engine room is conveniently situated beneath the 
fishing deck and astern of the fish-hold. The engine is a 
Ruston Paxman 8 R.P.H.C.M. 450 shp at 1,200 rpm. 

The experience obtained from Ross Daring compared 
with that from the new American sterntrawler Narra- 
gansett will be awaited with great interest. From these 
enterprises the development of commercial fisheries 
moves forward into the enlightened age of automation. 
The sooner we see the end of fishermen slogging away 
on an open deck, the better. 

Discussion on 
Stern Trawling 

Mir. Robert F. Allen (USA) rapporteur: Since the last 
FAO Fishing Gear Congress, the principle of stern trawling 
has been applied on vessels large and small throughout the 
world. The basic principle has now been accepted and the 
papers this year deal with the refinements and mechanization 
of the system. Mr. Birkhoff has summarized the various 

types of sterntrawler designs in use today, classifying them as 
factory ships, deep sea trawlers or smaller vessels. Further 
distinction is made in the deck arrangement and the location 
of the navigational bridge. Of particular interest in the 
three papers presented this year on trawling are the deck 
arrangements and the handling gear for small coastal vessels. 
Both Mr. Corlett and Mr. Birkhoff describe the use of the 
tillable or turntable gantry to extend the effective working 
deck area of the vessel. Mr. Dennis Roberts 9 new vessel, 
The Ross Daring, will however use a system of lifting the cod 
end with a boom similar to that used on the Canadian and 
American Pacific Coast system. A similar attempt at design- 
ing and building a small mechanized trawler has been 
accomplished in the United States. This vessel, the 
Narragansett, has a stern ramp and employs a large drum 
for winding on the wings so as to minimize the requirement 
for deck space. 

A designer who is contemplating a new sterntrawler could 
do well to review the considerations of sterntrawler design 
that Mr. Corlett has proposed. Since the vessel is only part 
of the fishing system, considerations should also be given to 
the design of the gear to be hauled. Mr. Birkhoff, in his 
paper, suggests some modifications which have been made to 
the trawl nets which have been found of advantage in stern 
trawling. This is a subject in itself and should be discussed 
at greater length. 

The subject of automation and centralized control, although 
applied to all vessels, is extremely important in stern trawling. 
Mr. Roberts hopes to reduce his complement on the Ross 
Daring to five. A great effort has also been made on the 
Narragansett to automate the deck machinery and to centralize 
the control in the pilot house. This should also reduce the 
complement to a bare minimum. Commander Pain's 
paper suggests that not only the operational functions, but 
the maintenance functions of caring for the vessel's machinery 
can be accomplished from the bridge, thus wholly eliminating 
the necessity for engineers on the vessel. Tt would be 
appropriate to hear from the participants about their exper- 
iences on automation, as we all know that i t is the emergencies 
rather than the normal operations which will ultimately 
determine the extent to which men can be eliminated from 
the vessel. 

Dr. I. H. Heinsohn (Germany): My firm has built some 
fifteen stern trawlers of medium size operating in Northern 
waters for Germany, Norwegian and British owners. They 
are mainly from 180 ft bp up to 220 ft and they work mainly 
on wet fish. The latest are small service factory vessels. 
Referring to Corlett's paper on the Unigan System he endorsed 
most of the points, but he would point out in relation to the 
gantry that the old fairy tale was repeated that fish hauled 
over the ramp suffered damage. This was sheer nonsense. 
Tn contrast it would be impossible, in the case of heavy 
fishing, to lift thirty tons of fish by the gantry and in that 
case what would happen to the fish at the bottom, with 30 
tons of fish on top of it ? Every practical consideration was 
against the allegation of damage and every trawler operator 
would agree to that. 

He also could not agree that the Unigan gantry was better 
and quicker than the ramp. This was true only in the case of 
very small trawlers. In most fisheries, trawling caused dam- 
age to the nets and when they were damaged they required 
space for laying out and repair so that it was always better 
to stretch the net out; therefore, even on smaller vessels, 
emphasis should be laid on the need for giving as much space 
as possible. Still, he agreed that for very small vessels the 


gantry was worth its money but on bigger vessels it was not. 

He congratulated the Ross Daring experiment as it was 
really promising for near-water fishing. They had to save 
man power and every mechanical device should be tried to 
work with fewer men. 

On the bigger trawlers much could be said for twin screws, 
if they were properly designed that is with big propellers. 
They gave better efficiency when towing and also gave more 
reserve power which should be of advantage when shooting 
complicated gear. If, however, twin screws were installed 
there should be also twin rudders. A single rudder was no 
good with twin screws and slowed trawling speeds. 

Birkhoff's paper raised the question of mechanization as 
did other papers. On this point he had to pour some water 
into the wine. On paper, everything worked very well but 
as soon as the skipper had fouled gear, you had to start 
working by hand or, if the warp had parted you had to make 
the gear ready again and that meant hard manual work. 
Mechanical devices were normally installed for performing 
any one purpose. They could not be very flexible so that if 
anything happened out of the scope of the design you are 
in trouble. On this point he had worked out just what an 
ordinary trawler would require in the way of full mechaniza- 
tion. To adequately handle all the needs of shooting and 
hauling a net on a big sterntrawler at least seven winches 
would be needed besides the trawl winch. Each winch 
should have a hauling power of between one and three tons 
depending on the size of the net. Such winches cost anything 
between 300 and 1,000 so you could easily run up to 
anything between 5,000 or 6,000 for winches. The question 
is whether it is worthwhile? Even if you have mechanized 
your system, in the case of fouling gear you have still to work 
something on the warp drums and you must, in any case have 
a few men. 

Further, a few men must be on board to operate the 
mechanized equipment. He suggested that you could not 
work with less than two men on each gallows which makes 
four, one man supervising them, and at least one man in 
control of the winches and supervising all the gadgets. If 
they had so many winches and gadgets to operate it could 
easily lead to a mishap and involve accidents. So, at least 
under German law, he had some doubt whether full mechan- 
ization would be allowed, because control could easily get 
out of hand: in any case somebody had to watch and if any- 
body got caught in the wire, he could be drawn into the drum 
and a severe accident happen. 

On the other hand the normal practice was for the men 
handling the gear to do gutting as well; therefore he could not 
see that, with big gear, they could get down to only two or 
three men by mechanization. To haul the gear only by 
mechanical means on a big vessel they would need a slipdeck 
of from 30 to 40 metres in length and with the hauling winch 
placed as far forward as possible. To prevent the danlenos 
and bobbins rolling and banging about under the weather 
conditions of the Northern waters, there would have to be 
preventive structures together with a trolley way running 
along the coaming little as he liked that idea. With the 
headline being shorter than the footrope, there would have 
to be a U-shaped guard to control the bobbins. All these 
points needed consideration if full mechanization was being 

If the gear became fast on the ground, a side trawler was 
better able to manoeuvre free than a stern trawler as it could 
swing and go over the gear more quickly. Quick changes, 
of course, were sometimes necessary for various reasons and, 
therefore, emphasis should be laid on the need for the aft 

trawl blocks being able to allow the warps up to 90 per 
cent horizontal clearance on each side without fouling else- 
where. The shape of the stem and the position of the stern 
blocks demanded care in guarding the trawl boards from 
swinging and banging. These, again, would require moving 
devices and one strong recommendation to all designers was 
to eliminate as much as possible everything that moved 
because they involved attention tmd maintenance. All of 
these things gave food for further thought. We should not 
keep our heads in the clouds too high regarding mechanization, 
automation and so on. It was necessary to keep your feet 
on the ground. 

Mr. W. W. Carlson (USA): My company are the builders 
and operators of the Narragansett and other vessels. If we 
ever heard of all the problems referred to by the last speaker 
we would probably have been afraid to build that vessel. 
However, in our ignorance, we went ahead and built it and 
have operated it very successfully. We do not have seven 
winches and we operate everything from the pilot house; 
two men on deck with reasonable intelligence had no trouble 
in getting the net over and getting it back. I am no fisherman 
and I have done it myself. We operate quite well, do not 
tear our nets and have reasonably good catches. The boat 
has been really successful. 

Mr. Harper Gow: Your point is that automation on your 
vessel can be achieved with much less equipment than would 
be required on larger vessels. 

Mr. D. L. Alverson (USA): There is a difference between 
stern trawling and stern ramp trawling. Simple stern trawling 
came from the Mediterranean and has been practised in the 
U.S.A. since the inception of trawling there. But stern ramp 
trawling has only recently developed. On the Pacific Coast 
80 ft to 130 ft vessels have worked and continue to work 
successfully with four men and sometimes only three men 
aboard. Some of their trips range up to 20 days in length. 
One vessel of 140 ft has only four men. This is a matter in 
which historic development has to be recognised and the psy- 
chology of the fishermen themselves; you must also consider 
the economics involved and how the catch is shared among 
the fishermen. Our West Coast trawlers have changed 
somewhat in the last four years. Most of the vessels now use 
a drum and roll on the net; this is very close to the stern and 
when the codend is reached it is swung round on the small 
gantry and lifted over on the side of the vessel. The 
Narragansett has a small ramp to bring the codend right up 
into the vessel. 

Mr. Conrad Blrkhoff (Germany): As a naval architect I 
agree with the views of Corlett and Roberts that the use of 
the ramp in small ships has limits. Under certain conditions 
this ramp is not absolutely necessary and sometimes not even 
possible. But the conditions of today might alter and open 
possibilities for later use. It has been proved that well 
constructed ramps do not cause any damage to fish and that 
hauling big catches up the ramp saved much time. He 
thought, therefore, they should always, if possible, consider 
the construction of a ramp which permitted hauling the gear 
as far as the gallows and, when necessary, made it possible 
to haul the entire trawl gear onto the deck for mending or 

The codend suffered less by sliding over the ramp than by 
almost vertical hoisting which might involve a swinging of 
the bag in rough seas. This idea was involved in Coriett's 


new bulwark form. He thought, however, that the hauling 
during the different phases of the gantry movements could 
easily be accomplished by a combination of two blocks on a 
fixed framework formed by a bipod post with gallow consoles 
at the posts and an outrigger codend derrick aft. This 
construction would be cheaper and more robust for being 
fixed. The recently developed gantry nose had the same 
function as the codend derrick before but involved some 
additional costs. 

He was not, however, against the idea of a tillable gantry 
but it should be contemplated in relation to total building 
costs. He recommended, at any rate, a fixed construction of 
gallows which was alluded to as an innovation in Corlett's 
paper. It would be a further step ahead if the gallows were 
built so that the trawl doors could be located close to the 
gallow posts. It might then also be possible to exclude from 
the tasks of the gantry the hauling of the codend by building 
a normal fixed bipod post. For the remaining tasks during 
the shooting operations such as moving the codend (and 
possibly the quarter ropes) and facilitating the shooting in the 
case of the bobbin gear hoisted on deck, a bipod outrigger 
derrick leading from the gallow consoles would then be the 
lightest, cleanest and most adequate solution. Such a 
gantry would be useful for accomplishing these various tasks 
and last but not least for the alternative employment of the 
ship in different fishing methods. For the normal trawl 
fishery he could, however, see no reason why the fixed 
gallows should be abandoned. 

Corlett's arguments on the necessity of the lateral under- 
water and above-water area of the ship should be underlined. 
Those ideas were conclusive in relation to the bulbous bows 
for this type of vessel and they reduced pitching. 

Automation of the remaining phases of handling the net 
was more possible on bigger ships than little ones. But the 
relative economy of a reduced crew was of greater importance 
cm smaller vessels so that due regard should be given to the 
idea especially for such vessels. Mr. Roberts* paper showed 
that the technical ideas had been well adapted to small 
vessels in rough seas. The possibilities of a bobbin track for 
various bobbin gears without imposing restrictions on further 
gear should be contemplated in detail. The swinging of the 
codend over the side near the propeller might be dangerous 
in rough seas because of movement in that area. 

His view was that the basic character of trawl gear had 
remained unchanged for years and had not even been affected 
considerably by pelagic nets. But we should be able to cope 
with all these assumptions in coming years. He therefore 
suggested the automatic positioning of the trawl doors which 
did not need pennants any more, if the further hauling of 
the net could be done by counter pulling the sweeplines by 
means of gillson blocks. Apart from the question of the 
crew required for gutting, the most dangerous part of the 
work would then be done automatically even when the 
gear became bigger and heavier in the future. If improved 
gutting machines did become available in the future then a big 
saving of crew would be achieved. 

Mr. Dennis Roberts (UK): I had hoped some speakers 
would criticise the Ross Daring plan but most speakers had 
welcomed it. Dr. Heinsohn had advised them to keep their 
feet on die ground. He would say that he was a practical 
fisherman with 25 years experience at sea. It had always been 
his dream to build a trawler so simply handled that he could 
take his wife and children with him and go out and fish. He 
was very pleased to learn about the Narragansett. The big 
difference between that vessel and the Ross Daring was that 


the Ross Daring took their gear over a transom stern. When 
he had visited the Canadian Pacific Coast and saw their vessels, 
the fishermen told him they did not think their type of vessel 
was suitable for fishing in the North Sea. He thought a 
100 ft vessel with a ramp working in winds of force 7 or 8 
would be swamped. A transom stern would be much safer. 
A lot of his ideas were based on experience with 60 ft Canadian 

Mr. E. C. B. Corlett (UK): The gantry system is essentially 
for smaller trawlers and he had been surprised that it had been 
used in Germany on some of their large trawlers. He was 
very interested in designing small vessels and the plan of the 
Ross Daring impressed him very much and he wished Mr. 
Roberts good luck. If, however, he caught more than one 
ton of fish per day he might find difficulty in handling it. 
If he caught ten tons a day he would be in difficulties. His 
firm had recently designed and completed a trawler almost 
identical in size length 91 ft, beam 23 ft, depth 12 ft 3 in., 
and draught aft under 10ft almost identical it carried 12 
crew and was also highly automated. It was fishing in the 
rough waters north of Norway and was catching ten tons of 
fish per day, every day. Three or four men simply could not 
cope in those conditions with catches of that size. 

On the question of the gantry it had to be built and stressed 
so that it would take a full drawing load up to 90 degrees off 
the towing line. It was expensive to provide that, but, in 
fact, by far the biggest load on the gantry came under those 
conditions. That was the reason for adopting the simpler 
arrangement for towing from the gallows post. This system 
worked very well, although it did have a slight disadvantage 
for handling the boards. It was also a good deal cheaper 
and the gantry could be built lighter and the hydraulics 
involved were less expensive. Mr. Roberts had made a 
good point regarding the ramp on smaller trawlers. It was 
most important they should have sufficient buoyancy and 
protection for the men. The smaller trawler he had mentioned 
had very powerful split winches seven tons pull each. 
Fixed gantries were more or less standard practice on larger 
trawlers but on smaller trawlers they provided a good deal 
of windage and iced up in the Arctic. Remotely controlled 
winches were interesting possibilities. They had provided 
one which was worked by an operator who had a whole 
deckhouse between himself and the winch. He had been 
very sceptical about this at first but it did work very well. 
Remote control of winches did help the design of stern 
trawlers a great deal and he thought the practice would 
spread even if full automation was not possible. 

Mr. C. Maurin (France): We have had experience of a 
gantry based on a German patent. The system originally 
used turned out to be fairly difficult particularly so far as 
handling the trawl boards was concerned. The hauling was 
fairly difficult and use of the gantry in heavy seas was danger- 
ous. The catwalk for supervising the hauling was too close 
to the sea and in heavy seas created some risk. An alteration 
was made whereby the catwalk was raised. This made it 
possible to bring the trawl boards in quite easily. Finally the 
system was improved by bringing the wings in up to the level 
of the winch which meant that the central part of the trawl 
could be brought up to the deck directly. At times the trawl 
came on to the deck in heavy seas and there was risk from 
movement. In later French ships the net has been wedged 
alongside the deckhouse. The disadvantages arose from 
heavy seas. On the Tallinn an active rudder was used which 
meant that they could manoeuvre better in a current This 

active steering required a strong motor for working it. In 
the case of fouling it was easier to get clear on a side trawler 
but in a stern trawler there was available far greater speed in 
manoeuvring and speed in handling and with the modification 
made, the size of the crew could be cut down compared with a 
side trawler. On a stern trawler men had greater protection. 
So far as damage to the fish being concerned because 
of the slope of the ramp, he had never observed any damage 
of this kind. On the contrary, he found that the fish came in 
very good condition. Another advantage was that where 
fishing was done in calm waters and without wind the trawl 
when shot from a stern trawler, went deep into the water. In 
a side trawler the trawl sometimes went down incorrectly and 
there was a risk of trawl boards crossing, but working from a 
stern trawler that difficulty was overcome. The Tallinn, with 
the alterations now made, was proving very satisfactory. 
They now had working from Lorient two medium tonnage 
ships working with a gantry system and the owners of those 
ships could give a very satisfactory account of their results. 

Mr. F. Dorville (France): The two vessels referred to have a 
sheltered control deck and the Unigan System does give the 
advantages described. By its use you can place the whole of 
the trawl in a very good position for strapping, backing and 
also bringing in the trawl boards without danger. The 
crews work under better conditions. We did have some small 
difficulties in the first place when the boat was rolling and 
this resulted in the bag swinging about but this was overcome 
by a fairly simple operation in placing two legs between the 
gantry which cut down the swinging as the bag came aboard. 
I can confirm that these two ships are operating very success- 
fully. We can thus say that the Unigan system, because of 
its simplicity and automation and concentration on the stern 
of the ship, can be applied on various types of ships. We had 
two trawlers with covered sterns. Another has a working 
area in the bow and there is a conveyor which brings the fish 
forward to the working area. Both on the 15 to 20 or 45 to 
50 metre length of craft that we have, the system is very 
flexible and at the same time gives very useful mechanization 
which takes a lot of work off the crew. 

Mr. Jean Frechet (Canada): Over the last three years 
Canada has been very active in investigating the best trawler 
design for small craft and their mechanization. They con- 
sidered stern trawling was still in its infancy and many prob- 
lems had still to be overcome. One related to the multiplicity 
of designs for operating the trawl over the stern. They felt 
that stern trawling should be fully operative over the stern 
and that the deck layout and equipment should relate to that. 
These problems would be solved over the next decade. With 
a view to that end, some small trawlers had been built and 
would be followed by others so that they could learn by 
experience and their mistakes should not be too big. They 
now felt safe in going further forward. 

Dr. G. C. Trout (UK): For investigations at Lowestoft 
we are considering a small trawler of some 35 metres in length. 
After his trip on the Fairtry he had been so impressed by the 
quality of the fish that they considered going in for a stern 
ramp although the vessel was of such small length. He 
would like more information about the freeboard. He 
understood the Universal Star had a very low freeboard and 
it might be that that resulted in her being a wet ship and 
therefore on consideration he would not advocate a ramp. 
Mr. Birkhoff's design, although not yet built, had a freeboard 
of about three metres or more and that might be why there 

was no unanimity of opinion about a stern ramp in a small 
ship. He was extremely heartened to see the design of the 
Narragansett which looked a delightful ship and he was glad 
to know it worked well. 

Mr. D. J. Doust (UK): The papers presented by Birkhoff 
Corlett and Roberts are of particular interest, since they 
show the interdependence of the fishing gear and handling 
problems with the requirements of good design. For economic 
success, the design of vessels for a particular type of fishing 
must properly be those in which all the essential technical 
factors are fully integrated. What sometimes appears to be 
an advantageous innovation from one point of view, may, 
however, incur another more serious disadvantage resulting 
in an overall loss of earning capacity, or even complete failure. 
This factor is clearly brought out in Corlett's discussion of the 
design requirements of smaller stern-fishing vessels which 
require a pitching axis well aft of amidships to avoid excessive 
stern motions and accelerations. Failure to provide for this 
feature of the design nullifies the advantages of stern-fishing 
and improved fishing gear, purely on account of the physical 
inability of the personnel to use the new techniques in adverse 
weather conditions. 

With the transition from side to stern trawling and the 
attendant multiplicity of internal and deck arrangements, 
more attention has to be given to this requirement of minimum 
motion and acceleration at the stern. Machinery and gear- 
handling layouts which favour such a stern trawler design 
are more likely to become economically successful, mainly 
due to their ability to make more use of the time spent on 
the fishing grounds. From the point of view of the vessel's 
hull shape, the type of transverse sections which can be 
incorporated in such designs is largely governed by the 
relative positions of engine and fish rooms. Diesel-electric 
machinery for the larger deep-sea trawlers would seem to 
offer the maximum flexibility in this respect, since the full 
amidships portion of the hull can be utilised to store the catch, 
thus minimizing the problem of change of trim with increasing 
fish load as the voyage progresses. 

The requirement of minimum motion of the stern generally 
favours an asymmetric hull shape, with an LCB position 
about 4 per cent aft of amidships BP, although there are 
possibly further advantages if the LCB position could be 
moved even further aft. Vessels with machinery aft, as shown 
in Birkhoff's figures 3, 4, 5 and 6, and Corlett's figures 2 and 3, 
rather lend themselves to the more favourable hull shapes 
on this account, although somewhat increasing the fish- 
handling problems on board with the forward fish rooms. 
There would, however, on balance, appear to be more 
favourable overall design characteristics for the smaller 
stern trawlers, by adopting an aft position of engine room, 
with the fish rooms below the working deck amidships. 

Mr. W. Dickson (UK): The development of stern trawling 
raises the question of the types of nets required by them. 
The separate winches used for the sweeplines make it much 
easier to haul double or triple lines and he did not see why 
they should necessarily have danlenos or bobbins. Provided 
the sweeps were fairly long, once the doors were disconnected, 
the double wire could be hauled to the sweep wire winch. 
Regarding Mr. Alverson's comment about winding the net 
on the drum, they could not get the whole trawl onto the 
drum because of the heavy ground ropes but it might be 
possible to get up to say 20 feet of the wings and that would 
save 20 feet of working deck. It was not really necessary to 
use kites and all those complicated things that could go 

wrong when shooting from the stern. 

Did anyone tealiy have evidence that those kites did scare 
the fiih down in to the net? In the absence of that, they might 
juft as well get the headline height by re-designing the net. 
There was also the possibility of easier handling of the trawl 
boards because the shooting was straightforward. One 
could then go in for more refinement in the doors and be 
sorer that they would go away all square than with side 
trawlers. They thus had the option of securing greater 
headline height and greater spread and even a bit of both. 
One usually had to pay for those advantages by either increased 
damage or increased drag. For practical purposes they had 
to choose. They could go to sea with two or three different 
nets suitable for fishing but not more. At the moment they 
did not really know how much headline height was best. 
For design purposes they had to have a figure: 12 or 15 feet 
and that height would be determined by what they could find 
out about fish behaviour. There were two approaches to 
this: (1) either direct observation by camera, TV, netzsonde 
equipment or observation by submarine and that was a task 
for research workers; (2) the other way was just to try it and 
see: make a net and compare it with a standard Granton 
trawl To do this one must have accurate measurements on 
the fish caught, haul by haul, and measure at least a decent 
sample. Some of this could be done by research vessels but 
it would be a great mistake to think it could all be done by 
them. There should be more co-operation between com** 
mercial craft in the industry and the research workers in 
doing that work. 

Asked by Mr. Harper Gow for a clearer view because he, 
(Mr. Gow), had the impression that the higher the net the 
better, Mr. Dickson said: I do not know the real answer. 
There is not much doubt that if you make a net bigger you are 
likely to catch more fish but do you make it by bigger height 
or by shaping and spreading the net? The bigger you make 
the headline height the more you cut down the spread. You 
have to look at the height of the fish off the bottom and 
ascertain their concentration and do this at different times of 
the year and design a net to suit average conditions the year 

round what proportions go into the height, what into the 
spread and what the taper should be. If you have a high 
headline you have to have a big taper. You must work out 
the average for a reasonable headline height for the whole year. 

Mr. Robert F. Alton (rapporteur) summing up said: There 
still seems to be many differences in the net handling methods 
on large European trawlers and the discussion has reflected 
these differences. Strong opinions were expressed by three 
of the delegates that experience shows a ramp does not 
damage the fish if properly designed. This seems to be a 
rather conclusive result. There is still discussion, however, 
and differences of opinion on the method of hoisting the codend 
and positioning the trawl bag. The minimum deck space 
available on smaller vessels is a determining factor in designing 
the gear for hoisting the codend aboard. Ramps for this 
type of vessel are not generally considered practical because 
of the loss in deck space and the hazard of taking sea on board. 
Freeboard is a big consideration in the seaworthiness of a 
ramp design, but this particular problem is more acute on 
the smaller vessels where freeboard is limited. 

Although the aim of designers is full automation of the 
fishing operation, a major deterrent in achieving this end is 
the extenuating circumstances which are encountered when 
the gear is stuck or ripped, or when unusually large catches are 
encountered. Mr. Corlett has pointed out that there is a 
great difference in the number of men required on an auto- 
mated vessel catching one ton per day compared with vessels 
catching ten tons per day. This would indicate that great 
strides could be made in reducing crew if automatic machines 
were available to process the larger catches. 

In consideration of the comfort of the crew and the safety 
to the fishermen, Mr. Doust's comments on hull form with 
regard to pitching should be well noted by naval architects. 

Lest we forget that the fish are still caught in the nets, Mr. 
Dickson reminds us that more experiments are indeed 
necessary to provide the most effective gear and to this end 
let us strive for more co-operation between commercial craft 
in the industry and the research workers 


Put 2 Bulk Fish Catching 

Section 6 Bottom Trawling 

Some of the General Engineering Principles 
of Trawl Gear Design 


In 1959 the White Fish Authority, with financial support from the 
British Trawlers' Federation and H.M. Government, through 
contract, began an extended investigation to develop a distant water 
trawl having as its specific features: (a) increased headline height; 
(b) increased netmouth width; (c) sound structural design and (d) 
improvement in handling. This paper reports on the hydrodynamic 
studies which formed a part of this project and is well illustrated with 
some 39 figures. Details of experiments, special equipment used, 
observations and results concerning the behaviour of the warps, 
otter board forces and moments, otter board stability, width and 
height of the mouth of the net, and drag of the net are given. A set 
of approximate equations from which warp curvature can be esti- 
mated with sufficient accuracy was developed. It was found that 
when measurements of ship speed and engine r.p.m. are available, a 
value of towing pull can readily be deduced. Experiments indicated 
that aerofoil type forces can be determined from measurements of 
a model in a wind tunnel, a ground being placed adjacent to the 
otter board to represent the seabed. The quantities measured are 
side-force, drag, lift, pitching moment and yawing moment. A 
method was developed for calculating headline height from a 
knowledge of the characteristics of the headline lifters and other 
factors. The author believes the objectives set forth were accom- 
plished and that a basic theory has been provided from which 
gears can be designed to meet certain specifications. He also states 
that close co-ordination between gear engineering design and fish 
behaviour projects is necessary. 

Quelques princlpes du genie i 


nt le dessin de chalut 

En 1959, la "White Fish Authority" travaillant sous contrat, avec 
1'aide financiere de la "British Trawlers' Federation*' et du Gouverne- 
ment, a commence une investigation poussee, pour le ddveloppc- 
ment des chaluts hauturiers. Les buts de cette recherche etaient: 
(a) acroitre la hauteur de Fouverture verticale; (b) accroltre 1'ouver- 
ture du filet; (c) 6tablir des principes de base de construction et (d) 
ameliorer Foperation de 1'engin. La communication decrit les 
etudes hydrodynamiques qui ont constitue une partie de cc projet 
et contient 39 illustrations. Sont donnes ggalement, des details sur 
les epreuves, 1'equipement special utilise, les observations et resulta 
obtenus concernant la configuration des funes, les mpuvcments 
et forces des panneaux, leur stabilite, 1'ouverture horizontal^ et 
verticale du filet et la resistance & la trainc de 1'engin. Des equations 
approximatives ont M developpees & partir desquelles on peut 
estimer avec assez de precision la courbure des funes pendant 
1'operation. II a etc trouvd que, lorsque la vitesse dans 1'eau et les 
tpm de la machine sont connus, la valeur de la force de touage 
peut tre facilement calculfe. Les experiences ont indiqut que les 
forces de type aerodynamique, peuvent dtre ddterminees par des 
mesures effectuees sur un modele dans un tunnel aerodynamique, 
lorsqu'on a pris soin de placer un fond pres du panneau de chalut 
pour reprfeenter le fond de la mer. Les valeurs mesurees pendant 
les experiences etaient: force laterale, resistance a la trainc, force 
d'elevation, importance du tangage et importance d'embardage. 
Une mthode fut developpdc pour calculer Touverture verticale & 
partir de la connaissance des caracteristiques des elevateurs et 
autres facteurs. L'auteur croit que les objectifs ont etc atteints, 
qu'une theorie de base a etc deveioppee et permettra de dessiner 
des engins pouvant satisfaire certaines conditions spfcifiques. 
Une coordination etroitc entre le dessin technique des engins et 
les projets de comportement du poisson est necessaire. 

Prlndplos de mecanica general de las fornas del arte de amstre 


En 1959 la White Fish Authority, con la ayuda financiera de 
gobierno y de la Federaci6n de Armadores de Arrastreros de la 
Gran Bretafia, inici6 una amplia investigation para encontrar un 
arte de arrastre para la petca de gran altura que tuviera como 


P. R. Crcwe 

Westland Aircraft Ltd. 
(Saunders-Roe Division) 

caracteristicas especfficas : (a) mayor altura de la relinga de corchos ; 
(b) mayor abertura horizontal; (c) buenas formas estructuratot, y 
(d) mas facilidad de manipulacidn. Esta comunicacion da cuenta de 
los estudios que formaron parte de este proyecto. Esta muy bien 
ilustrada con 39 figs. Se dan detalles de los experimentos, el 
material especial empleado, observaciones y resultados relativos 
al comportamiento de los cables, fuerzas, movimientos y estabilidad 
de las puertas, altura y anchura de la boca y resistencia de la red. 
Se encontraron ecuaciones aproximadas con las cualcs la curvatura 
del cable puede estimarse con exactitud suficiente. Se observe que 
cuando se conocen la velocidad del barco y las r.p.m. del motor se 
puede deducir fAcilmente el valor del tiro de remolque. Los experi- 
mentos indican que las fuerzas de tipo aerodinamico pueden deter- 
minarse a partir de medidas de un modelo en un ttinel de viento, 
colocandosc un suelo junto a la puerta para representar el fondo del 
mar. Los valores medidos son: fuerza lateral, arrastre, fuerza 
ascensional, momento de cabeceo y memento de guiiiada. Se 
encontr6 un procedimiento para calcular la altura de la relinga 
de corchos bas&ndose en el conocimiento de las caracteristicas de 
sus elevadores y do otros factores. El autor cree que se alcanzaron 
las finalidades propuestas y que se ha formulado una teoria fun- 
damental con la cual se pueden proyectar artes que satisfagan 
determinadas especificaciones. Afirma que es necesario coordinar 
los aspectos mccanicos de las formas de los artes y los proyectos 
de comportamiento de los peces. 

1. FIRMS that engage in aircraft design almost 
always have aerodynamics departments, and wind 
tunnels for testing models. These departments are 
concerned with the airflow round the craft and the way it 
affects powering requirements, stability and other such 
behaviour. One difference between the Westland Aircraft 
Company and other British aircraft contractors is, 
however, that its Saunders-Roe Division also has a 
hydrodynamics department, and towing tanks. These 
undertake corresponding work with regard to craft and 
equipment that move on or through the water. In recent 
years they have made a number of special investigations 
in connection, for example, with a large cargo submarine, 
yachts and, currently, trawl gear. 

Trawl gear work commenced at Saunders-Roe 
with limited investigations for one or two firms, and in 
particular for the Lord Line Company, Hull. In 1959 
the White Fish Authority, with financial support from 
the British Trawlers' Federation and H.M. Government, 


gave a contract for an extended investigation leading to 
the development of a distant-water trawl having as its 
specified features: 

(a) Increased headline height. 

(b) Increased net mouth width. 

(c) Sound structural design. 

(d) Improvement in handling. 

These are purely engineering objectives, which could 
be achieved without reference to problems of fish beha- 
viour, and consideration of the latter was not required. 
However the available information on fish reactions was 
borne in mind. This in the main comprised two observa- 

(i) The practical experience of the industry that catch 
is increased as the otter boards are moved further 
and further ahead of the net, using long sweeplines 
or legs. 

(ii) Indications of fisheries laboratory experiments 
that fish do not appear to react to gear until it is 
extremely close to them, unless they can see it. 

The present paper seeks to demonstrate that the Com- 
pany's know-how and facilities in hydrodynamics, 
model testing, structural engineering, instrumentation 
design and electronic equipment have proved most 
suitable for establishing or confirming the general engin- 
eering principles on which good trawl gear design depends 
and for designing the trawl gear specified. 


Consider the schematic trawl gear shown in Fig. 1. 
Logically one should perhaps start with the behaviour 
of the net, and work forward to the trawler, but in 
practice the performance of the gear depends upon the 

Fig. L Typical bottom trawl layout reproduced by permission of the 
Gourock Rope Co., Ltd. 

amount of warp out, the speed of tow, the depth of 
water and the type of sea bottom, so that in analysing 
performance it is convenient to start at the ship and 
work aft. 


The instruments used for obtaining the data reported 
on in this paper are described in Nicholfs paper in 
Section XIV in which also appears Dickson's paper 
"Performance of the Granton Trawl". 

2-1 The behaviour of warps 

The warps take up shapes under tow that are in general 
curved both in planform and elevation as shown in 
Fig. 2, These curves depend upon warplength, water 


Fig. 2. Warp curvature. 

depth, warp diameter, warp weight in water per fathom 
of length, towing speed, tension in the warp, and lateral 
distance or spread between the lower ends of the warps. 
The warp shape varies with towing speed because of the 
hydrodynamic water force that is generated by moving the 
warp through the water, and the magnitude of this water 
force depends upon the extent to which the warp vibrates. 

Special electronic acoustic instruments shown in Fig. 3 
have been employed for measuring the spread between 
the lower ends of the warps directly. The bodies are 
towed by wires which are attached to the warps, 
about 25 fm ahead of the otter boards. The conical 
tails, which stabilise the bodies directionally, are based 
on Kelvin Hughes direct reading current meter practice. 
However it is undesirable to need to attach this type of 
equipment to the trawl for every instrumented haul, 
and in any case other measurements are necessary to 
determine the effect of warp vibration. It has, therefore, 
been found best to employ an instrument which measures 
the "divergence" angle between the warps, just aft of 
the block, and the "declination" angle of the warps to 
the horizontal, as shown in Fig. 4. The instrument is 
simple to install, once the warps are in the block, and 
systematic readings with it have been obtained for nearly 
all the test hauls made as part of the investigation. 

A set of approximate equations has been developed 
from which warp curvature can be estimated with 

Fig. 3. Electronic Instrument for measuring spread near the otter boards, (a) (on left) Mark II Spreadmeters, the instrument on the right it 
ready for attaching to the trawl, (b) (centre) Mark II recorder shown withdrawn from the pressure base, (c) (on right) Mark II Electronic Unit 


Fig. 4. Divergence and Declination Meter on warps immediately 
aft of towing block. 

sufficient accuracy. Four of these equations, taken alone, 
provide an adequate account of curvature in a vertical 
plane. Theoretical estimates for the declination angle, 
appropriate to different amplitudes of warp vibration, 
are compared with experimental values given by the 
instrument, and the effective amplitude of the vibration 
is deduced, as illustrated in Fig. 5. In the case shown, 
the "vibration factor" of id is appropriate to a non- 
vibrating condition, and thus gives an upper limit for 
achievable values of warp declination angle. The vibra- 
tion is generally of such a magnitude as to increase the 
hydrodynamic forces by about 50 per cent compared 
with a non-vibrating wire of the same dimensions 
(e.g., a vibration factor of id as compared with id), 
but a rather greater effect can occur in deep water or in 
conditions that encourage vibration. The vibration is 
believed to be due mainly to the wire causing large eddies 
which are shed alternately from either side, as illustrated 
in Fig. 6. However, ship vibration and disturbances 


ff +ilN-' 



(WARP DIAMETER - <) __. * ,. * 






w y^ 


* ^ TK 











x . x- :^ 







^ B 





Fig. 5. Variation of warp declination with speed. 





Fig. 6. Vortex street behind a wire rope. 

The estimates of warp behaviour require a knowledge 
of the tension in the warps* The tension just aft of the 
block can be calculated from a knowledge of the trawler 
propeHer characteristics and hull form. This has been 
confirmed by attaching load cells to the warps a little 
ahead of the btock, as shown in Fig. 7. Fig. 8 shows 

Fig. 7. Load cells on warps ahead of the block. 

variation, with ship speed, of towing pull available, and 
compares points deduced from load cell measurements 
on a gear of Granton type with curves estimated from 
propeller thrust and ship resistance calculations. The 
agreement, which is good, is further supported by direct 


ft.V.tftNMT HOLT" 

Fig. 8. Comparison of predicted towing pulls with measurements of 

warp tension. The figures across the graph indicate the ship's speed 

in knots. 

measurements of propeller thrust, obtained from a 
Michcll thrust meter mounted on the propeller shaft 
(see Fig. 9). All results are converted to still air conditions 
since the atmospheric wind exerts an appreciable force 
on the ship superstructure, which may help or hinder 
its motion through the water. 


It may be seen from Figs. 8 or 9 that when measure- 
ments of ship speed and engine r.p.m. are available, a 
value of towing pull can readily be deduced. If, for a 
given trawl gear r.p.m. is reduced, ship speed and towing 
pull required tend to decrease almost along a straight 
line, which cuts the towing pull axis at a small positive 
value, as shown in Fig. 10. This is because headline 




I I 

Fig. 9. Comparison of predicted towing pulls with measurements of 
propeller thrust. Figures across the graph show ship's speed in knots. 

height generally increases as speed reduces, for otherwise 
towing pull required would vary more nearly as the 
square of speed. (Further consideration of this point is 
given near the end of the paper.) It follows that if checks 
of towing pull required are satisfactorily established by 
direct measurement, at one speed and r.p.m., the speeds 
appropriate to other r.p.m.'s can be estimated from a 
knowledge of the ship's propeller characteristics. The 
tension in one warp is, of course, nearly enough given by 
(total towing pull)/2 (cosine of warp declination angle), 
which is approximately equal to (towing pull)/ 1 -85, in 
most practical cases. 



Fig. 10. Towing pull requirements. Figures across show ship's speed 
in knots. 

The tension does not reduce to zero as speed is reduced 
to zero since it has to continue holding the warps taut, 
which can theoretically be shown to require a value 
equal to (depth of water) x (weight of warp in water 
per unit length); e.g., a 3-25 in circumference warp 
weighing 8*5 Ib per fm, requires a tension of 850 Ib in 
100 fm of water. When the gear is under way, the tension 
at the bottom of the warp is less than that at the top by 

the above quantity together with the component of 
force parallel to the warp that is produced by the water 
flowing past it. This has been confirmed by measuring 
tension both ahead of the block and just ahead of the 
otter boards. The load cell designed and built for the 
latter purpose is shown in Fig. 11, and the differences 

Fig. 11. Load cells for measuring forces in wires. (Top) Load cell 

dismantled. (Bottom) Load cells in place as seen from gallows before 


in experimental tension between the top and bottom of 
the warp are compared with theoretical estimates in 
Fig. 12. The component of water force parallel to the 
warp is given nearly enough by: 

ip(*ds)(l-689V k )*C T , 

where p is water density in slugs per cubic foot, d and s 
are warp diameter and length in feet, and V k is speed in 
knots. The coefficient C T is due partly to skin friction, 
and also to water pressure on the strands of the cable. 

Let us now return to the general performance charac- 
teristics of the warp. Once the tension (or towing pull) 
and the effective magnitude of vibration have been 
established, as illustrated in Figs. 8 and 5 respectively, 
curves of warp length required to give any desired angle 
of the warp to the vertical, at its attachment to the otter 
board, can be calculated. A digital electronic computer 
was employed for this purpose, a sample set of results 

being shown in Fig. 13. These refer to fishing with 275 
fm of warp aft, in various depths of water, and at various 
ship speeds. The weight of warp in water in Ib per fm, 
w, can be given any suitable value when using this dia- 






CT * '01* 


o s 10 is ao as 


Fig. 12. Warp tension differences. 

gram. It should however be noted that the results only 
apply if the drag of the gear is proportional to w. To 
obtain specific results, for a case with a drag that is 
characteristic of distant-water trawls, a value for w for 


r- GEAR EXCLUDING WARPS - 471 wVfc , Lt. -/--. 





Fig. 13. Typical warp length] water depth curves. The vertical scale 

gives the ratio of warp length to water depth and the figures across, 

the towing speed in knots. 


8$ f appropriate to a 3-25 in circumference warp, may 
be considered. 

1 Each line corresponds to a different constant value of 
vertical upward pull of the warp on the otter board 
bracket, the appropriate values being given adjacent to 
the lines. Experimental fishing conditions for a 3-25 in 
warp are shown by points. In these cases the warplength 
to water depth ratio used was about 3-5 irrespective of 
speed. Some variation with speed should give a rather 
better performance, as will now be discussed. To keep a 
given up-pull on the board, the warplength to depth 
ratio should be varied with speed along one of the 
constant up-pull lines, as for example by the variation 
marked A- A. If so much warp is paid out that values of 
length to water depth lying above line B-B occur, then 
there is no up-pull, the lower end of the warp lies on the 
seabed and the otter board comes down on its bracket. 
For a normal otter board weight in water of about 
0'75 ton, the board will be pulled off the bottom when 
the vertical up-pull significantly exceeds 0-75 ton. This 
occurs for warplength to depth ratios lying below the 
line C-C, say. If otter board behaviour depended only 
on water forces arising from its motion through the water, 
the up-pull would be required to vary as speed squared 
to keep the board upright as speed varied. This would 
demand an almost constant warplength to depth ratio, 
irrespective of speed, as shown by the line D-D. Due to 
the presence of weight and buoyancy forces, that do not 
vary with speed, the variation required in practice will 
be between that of line D-D and the values indicated by 
an appropriate line of constant up-pull. 

It is of interest to compare the theoretical estimates of 
required variation of warplength to depth, with trawler 
skippers' "rule of thumb" practice. This is done in 
Fig. 14 for varying depths and characteristic trawling 


\ AAPS|4,000 X SPEEP, KNOTS) L, 
I ANP VlfclUTON FACTOR /4 .4 . 

i KMOTj 

oo aoo 


THEORY, I 7 * CIRC. MM. RP| . 1 KMC 

-ifc - aio ivin vWdSAL^ 



THEORY, rente. u- '/a KNOT* 

| Of fclflft Li. APPHOd). 


KAN Til, 

Vfrtrtft DEPTH , FATHOM* 

fig. 14. Comparison between theoretical variations of ratio of warp 

length to water depth against water depth with values used by practical 



speeds. It will be seen that the agreement is good, as 
would be expected from the very extensive practical 
experience upon which such rules of thumb are based. 
A characteristic value for the weight in water of a distant- 
water otter board is 1,700 Ib so that the lengths of warp 
used in practice leave about 300 Ib of the board weight 
to press on the seabed. This reserve permits a reasonable 
variation of speed and water depth to occur, at a given 
length of warp aft, without the board being pulled off 
the seabed. 

The behaviour of the warp in vertical projection has 
been satisfactorily explained, assuming that, both with 
and without vibration, and apart from buoyancy, the 
resultant water force nearly enough lies in a plane con- 
taining the direction of motion and the straight line 
joining the ends of the warp, and is perpendicular to the 
latter straight line, i.e., it is as Nj in Fig. 15a. 




** / \ 






OF N a . 


Fig. 15a. Components of water force acting perpendicular to a wire. 

Returning to curvature in plan, it would appear, from 
the experimental data so far available, that a "normal" 
water force, N 2 say, also defined as in Fig. 15a, may act 
in addition. For example Fig. 15b gives ratios of theore- 
tical spread at the otter boards, assuming N 2 to be zero, 
to the value which would be obtained from the divergence 
meter if the warps were assumed to be straight in plan- 
form. In the cases given values of about 1-15 are appro- 
priate to representative towing speeds. The corresponding 
electronic spreadmeter readings agree rather well with 
the estimates, as shown by the test points obtained with 
that instrument. In other cases not shown here, experi- 
mental factors as high as 1*4 have been obtained. 
Investigation of this matter continues and if it is found 
that N 2 zero theoretical values do not lie sufficiently 
close to experiment the theory will be elaborated accord- 
ingly. N t would be non-zero if the "boundary layer" 

1 1 


I f 

' I 



j i 


At M 

tOINCt At 



1 * " 





T ! 


I ' 



I i 



r - 

*"Ste~ . 






- 1 i .. . 

'1 4 



^ 1 is 


-^ - 

* 1 












j j 

90 30 


Fig. 15b. Warp planform spread factors. 

of the flow past the warp separated from the surface 
of the wire at different longitudinal stations on the top 
and bottom surfaces respectively, as illustrated in Fig. 
1 5a. This might occur due to the variation in hydrostatic 
pressure from the top to bottom of a transverse cross- 
section of the wire, which arises from the density of 
water, since this pressure change is of the same order as 
the hydrodynamic pressures caused by the motion of 
the wire through the water. Again the recently published 
reference 7, of 15th February, 1963, describes how 
stranded electrical cables, in air, can have this type of 
asymmetrical separation, due to the lay of the strands 
presenting different profiles to the fluid, at opposite 
ends of a diameter of the cable. 

In addition to the above possibilities, the lay of a 
stranded cable whose axis is inclined to the direction of 
motion is known to generate a hydrodynamic force, in 
the N 2 direction, by causing fluid to swirl round the 
wire (see Fig. 16). The magnitude of this component 





react ON FORMftftD 




JO- ft AFT WAR* 




of N t is not likely to be very large, however, and would 
be expected to make the planform curvatures of the two 
warps different from one another, without appreciably 
affecting the spread at the lower end relative to the 
divergence angle. Asymmetry in the planform shape of 
the two warps, arising from the swirl component of N t 
would be eliminated by using handed warps. Asymmetrical 
boundary layer separation might be prevented by encasing 
the warps in a smooth plastic sheathing. It would be 
of academic interest to make experiments under these 
conditions. The relatively small spread factor of about 
1-15, that has been measured with traditional laid warps 
(see Fig. 15b), is an advantage in practice since only a 
small correction has to be made to the readings obtained 
directly from the divergence meter, in order to obtain 
the spread of the otter boards. 

When a gear is turning, the spread at the lower end 
can greatly exceed 1 -4 times the value given by the angle 
between the warps at the block. This has been confirmed 
by the electronic spreadmeter, and an extreme example 
is provided by the technique of turning the ship at such 
a high speed that the warps cross, and yet the boards 
remain well spread. 

2-2 Otter board forces and moments 

The mathematical evaluation of otter board performance 
is a complicated matter. In essence they behave like small 
aspect ratio aerofoils but they operate at considerably 
greater attitudes and over a considerably greater range 
of angles of heel than is usual for the latter (see Figs. 1 7 
and 18). Also the otter board may have appreciable 
tilt so that it runs on its heel rather than with the sole 
plate equally in contact with the seabed all along its 
length (Fig. 19). 



Fig. 16. Circulation forces due to warp lay. 

Fig. 17. Typical hydrodynamic side force and drag coefficients for 
flat rectangular otter boards of aspect ratio^I/2. 


(ntMn.Tt PO* A CAMMftf* ftOAftO) 


ft0 HMLtB 



TftMML miO 

Fig. 18. Variations of otter board heel with speed. Full-scale tests. 











Fig. 19. Some measured variations of tilt with speed, standard otter 
board. Full-scale tests. 

The large change in heel with towing speed, at a given 
warplength and depth of water, is perhaps the most 
immediately striking characteristic of the results shown 
in Figs. 17 to 19. From the left-hand side of Fig. 18 it 
will be observed that, when using a traditional warp- 
length aft, a "standard" flat distant water otter board 
was almost upright at the towing speed of about 3*5 
knots, at which it was customarily used. From the right- 
hand side of Fig. 18, a change of towing speed of one 
knot requires a change in warplength of about IS per 
cent in order to preserve a given angle of heel. This is, 
of course, consistent with rule of thumb trawling prac- 

ment but the change in warplength necessary to maintain 
a given heel can also be deduced from theoretical curves 
such as those of Fig. 13. 

Referring to Fig. 17, the proximity of the seabed alters 
the effective aspect ratio, defined as (heights/surface 
area, as is the case also with an aerofoil, but, in addition, 
so-called "ground shear" forces arise due to seabed 
material piling up against the lower face of the board 
and sliding along it, or due to the board catching on the 
bottom and proceeding in a jerky fashion. 

The "aerofoil type*' forces can be determined from 
measurements of a model in a wind tunnel, a "ground" 
being placed adjacent to the otter board to represent the 
seabed (see Fig. 20). The quantities measured are side- 
force, drag, lift, "pitching moment" and "yawing 
moment", defined as in this figure. 


80* 10* -10' -10* 


Fig. 21. The effect of heel on the hydrodynamic forces acting on a 
flat rectangular otter board of aspect ratio==]/2. 

Typical model test results for an unheeled otter board 
are shown in Fig. 17, while Figs. 21 and 22 refer to the 
effects of heel and tilt. 

When presenting these figures, the customary fluid 
dynamic coefficients have been used, instead of giving 
forces in Ib or kg. Thus for example sideforce coefficient 

^ sideforce, Y Ib, , . A , . A r 

c y = T g (1.539 y ) e p 1S dcnsit y f watcr 

in slugs per cubic foot ( P is nearly enough 1-0 for salt 
water), SB is the area of one side of the board, projected 
on a plane through its leading and trailing edges, and 
V k is towing speed in knots. Similar definitions give the 
drag coefficient C D and the coefficient of upward vertical 
force, C L , which is near enough zero at zero heel (see 
Fig. 21). By employing this type of presentation, and 
making the usual assumption that the coefficients are 
near enough independent of speed and of the size (but 
not the proportions) of the board, the forces appropriate 









fig. 20. 



wind tunnel tests on model otter board*. 



IO 2O 3O 4O O IO 2O 3O 4O 


Fig. 22. The effect of tilt on the hydrodynamic forces acting on aflat rectangular otter board of aspect ratio === 1/2. 

to any chosen speed and size can rapidly be deduced. 
In practice the drag coefficient CD shows more tendency 
to vary with size and speed than do CY and CL, but to 
first order, constancy can still be assumed. 

From Fig. 21 it will be seen that maximum sideforce 
occurs when the board is heeled inwards (i.e., -ve heel 
with bracket towards the seabed). A heel of about 10 
is appropriate to a practical attitude in the 30 to 40 
range. This maximum sideforce is associated with a 
rather larger drag than occurs at the positive angle of 
heel of the same absolute magnitude but for overall 
force efficiency the warp should be long enough to allow 
the board to be heeled a little inwards. This does not 
necessarily increase the pressure of the board on the 
seabed. The water force acts nearly perpendicular to 
the board surface so that while lengthening the warp redu- 
ces the warp up-pull, heeling-in gives a compensating 
increase in vertical hydrodynamic force. 

From Fig. 22, a small amount of tilt does not greatly 
affect the force efficiency but the sole plate should be 
kept at 5, or less, to the horizontal. As might be 
expected from theoretical considerations concerning the 
effective aspect ratio of an aerofoil adjacent to a ground, 
nose-up tilt reduces sideforce more than nose-down 
tilt. The latter can lead to digging in however. 

The centre of pressure at which the resultant hydro- 
dynamic force acts can be determined from the measured 
quantities, including yawing moment, and is as illustrated 
in Fig. 23. 






O 4 

O 3 



IO 2O IO 4O 


Fig. 23. Variation of centre of pressure with attitude: rectangular 

flat otter boards of aspect ratio 4=7/2 with ground effect (from model 

otter board tests at zero heel). 

Influence of camber 

So far the performance characteristics of flat otter boards 
have mainly been discussed. The considerable increase 
in sideforce that can be obtained by giving a board 
curvature (i.e., camber) in longitudinal section is shown 
by Fig. 24. It will be seen that if a moderate camber is 




O IO 20 3O 40 50 


Fig. 24. The effect of simple camber on the hydrodynamic sideforce 
and drag of aspect ratio^l/2 otter boards. 

used, the drag penalty is small and the sideforce to 
drag ratio is thus appreciably improved. These wind 
tunnel results have been supported by full-scale measure- 
ments. By contrast, neither our tests of oval boards nor 
other tests which have been published show such a large 
advantage. Furthermore, an oval board occupies a 
space behind the gallows which can accommodate a 
rectangular board of about 25 per cent more area, and this 
alone appears to be enough to counterbalance the rather 
greater sideforce coefficient provided by the oval board 
as compared with a flat board. Referring again to Fig. 24, 
it will, however, be seen that in order to achieve maximum 
Values of sideforce coefficient it is necessary to tow the 
board at an attitude of about 30, as compared with the 
40 or more that is usual with flat boards. The implica- 
tions of this will be discussed in connection with the 
stability of otter boards. 

Figs. 17, 21, 22, 23 and 24 have been concerned with 
hydrodynamic forces arising from flow of water past 
the boards. Further attention to "ground shear" forces 
will now be given. It is impossible to determine 
whether a model simulation of the latter is reliable 
without first measuring them full scale. Therefore, this 
has been done, employing the specially instrumented 
otter boards shown in Fig. 25. Five load cells, of the 

Fig. 25. Instrumented otter board with covers removed to show load 

cells for measuring ground forces in three directions and cut-outs 

for tilt, heel and attitude recorders. 

type used in the warps at the otter boards but of smaller 
dimensions and capacity, are employed to measure the 
ground forces that act on the sole plate in directions 
parallel to the plate, and sideways and upwards perpen- 
dicular to the plate (see Fig. 26). Some results are shown 











Fig. 26. Full-scale otter board ground force measurement instruments 
(location and action). 

in Fig. 27. It will be seen that the vertical component 
can be about 30 per cent of the otter board weight in 
water, while the "sideways" component can be about 
50 per cent of the weight, and as such can provide a 
considerable part of the overall. spreading action of the 
board. As might be expected, the ground shear appears 
to be more significant on muddy bottoms than on hard 
bottoms. However if the board is made rather heavy 
in water, so as to get a purchase on the mud, it can sink 
in to about one-quarter of its overall height, thus tending 
to offset the gain in ground shear by a loss in hydro- 
dynamic spreading force. It is tentatively concluded that 
when designing boards to achieve ground shear spreading 
effects, careful attention must be paid to the type of 
grounds on which it is intended to use them. 


s 4 / \ f 







Fig. 27. Full-scale otter board ground force measurements. 

2-3 Otter board stability 

To design an otter board properly requires considera- 
tion, not only of the spreading force and drag that will 
act on it, but also of its stability. Static stability is 
concerned with its ability to tow steadily at constant 
values of spread and constant angles of attitude, heel 
and tilt. Dynamic stability is concerned in particular 
with three conditions of unsteady motion. 

(a) When a board is shot, it must take up a series of 
positions, as warp is paid out, that will bring it to a 
reasonable spread and nearly unheeled condition 
on the seabed. When the warps are blocked up 
they will then be level. 

(b) When a warp is shortened, the board should pull 
off the seabed without heeling or tilting excessively, 
so that it will continue to provide spread and can 
go down on the seabed again without falling over 
on its back. 

(c) If the board is disturbed during steady towing, as 
for example by being deflected or heeled over due 
to an obstacle being encountered, it must auto- 
matically return to its condition of steady motion. 

The six equations of force and moment equilibrium of 
an otter board have been formulated, from which the 
bracket and backstrop locations necessary to provide a 
desired attitude and angle of heel, at arbitrarily chosen 
steady towing speeds, spreads and sweep tensions and 
directions, can be determined. The calculated positions 
are confirmed by tests on small models in our towing 


tank. The same equations have been used to calculate 
behaviour when pulling a board off the seabed, and in 
shooting. The latter can demand wire attachment 
positions that differ from those that would give maximum 
force efficiency in towing. Some compromise is then 
necessary. Model tests of shooting behaviour have been 
made and help to clarify full-scale observations. It 
must be emphasised that the increased force efficiency 
of a cambered board arises from its forward surface being 
at a smaller angle to the direction of motion than in the 
case of a flat board, thus allowing the water to flow round 
it more smoothly. The effect is reinforced by the smaller 
attitude, 30 as compared with 40, at which the cambered 
board behaves best when towing. This reduction in 
required towing angle, which, at least to some extent, 
will be a feature of any board having high force efficiency, 
tends to give too small an angle when shooting or hauling. 
This can however be overcome by keeping more way 
on the board. Prototype devices have also been used 
with some success for reducing the attitude of the board, 
relative to the warp and backstrop, as soon as it contacts 
the seabed. 

Stability in steady tow can be studied in terms of force 
data such as those of Fig. 17. It will be observed for 
example that if a standard otter board, fishing at an 
attitude of about 40 is disturbed so as to decrease spread, 
then due to its inward motion, effective attitude is 
increased, spreading force reduces and the tendency to 
decrease spread is encouraged. Such a disturbance 
applied to a board which is fishing at attitudes below the 
stall should, on the contrary, rapidly be damped out, 
due to the positive slope of sideforce against attitude 
in that region. 

The warp and otter board force and moment equations 
that have been elaborated are equally useful for designing 
bottom gear and midwater trawl gear. The most suitable 
aspect ratios (height 2 /area) for the different applications 
are being investigated. 

2-4 Width of mouth of net 

Next attention should be turned to the relationships 
between bridle tension and angle, board spread, spread 
of the net mouth, and drag of the gear. Here, the term 
"bridle" will be taken to mean the total wire system 
between the wing end of the net, and the otter board, 
including legs, cables or sweep wires, and otter board 

The headline and ground rope of the net take up 
planform shapes that are near enough catenaries with 
linear extensions forward. This is reasonable bearing in 
mind that the aft pull on these wires, due to the drag of 
the net and its appendages, such as floats and ground 
rope bobbins, will to a first approximation, be constant 
per unit length of wire except in so far as it is concentrated 
due to local grouping of the appendages. Confirmatory 
experimental evidence has been provided by scratch 
marks on the bobbins, full-scale, and by photographs 
of tank models, such as illustrated in Fig. 31. 

Formulae have therefore been developed on the above 
basis, which relate: 

Fig. 31. Model net in towing tank. 

ground rope planform angle at wing end, 8 g . 

headline planform angle at wing end, 8 h . 

width between ends of headline, 2y h> 

width between ends of ground rope, 2y g . 

and ratio of ground rope width to headline width, 

These quantities are defined in Fig. 28. Various headline 
ground rope and leg lengths, and various proportions 
of straight to catenary length for the ground rope, 
defined by a quantity //, have been assumed, the headline 
being taken as a pure catenary. 







Fig. 28. Definition of plan shape parameters used in Figs. 29-34. 


to 20 10 

40 iO 


Fig. 29. Headline and ground rope planform calculation results. 

Some results are given in Figs. 29 and 30 together 
with confirmatory test results obtained with small 
towing tank models, such as that of Fig. 31. The model 
test results given in the upper part of Fig. 29 refer to a 



J h (DIOftElt) 

Fig. 30. Headline and ground rope planform calculation results. 


Oranton type of gear, and indicate that the ground rope 
ahead of the bobbins is nearly straight. Full-scale 
scratch marks have shown this also. When a gear is 
spread wider, // tends to zero however. The lower pan 
of the figure indicates that when long legs are used the 
wing end spreads at the headline and ground rope can 
differ significantly, especially at the higher spread angles. 
The nature of the dependence of y g /yh on leg length is 
illustrated by Fig. 30. 

A development of the theory has been used to calcu- 
late the corresponding spread at the forward end of the 
leg and cable system, and the planform angle of the cable 
at the backstrop, taking account of the effect of water 
forces in curving the leg and cable wires in planform. 
To perform this calculation it is, however, necessary to 
know the ratio of tensions in the head and toe legs. Load 
cells mounted in the legs of full-scale gears have given 
suitable values for this ratio. Some calculated results 
for spread at the backstrops of the otter boards, in 
relation to the spread of the groundrope and headline 
at the wing end, are given in Fig. 32. The effect of 


* y t 



Fig. 32. Calculated headline and ground rope spreads for a particular 


changing // will be seen. In practice the headline also 
ceases to be a pure catenary, at large spreading angles, due 
to constraints applied by the wings, or selvedge ropes. 
Fig. 33 compares ground ropes spread against wing end 
angle with towing tank model results. In this case a /* 
of zero was found to be adequate. Fig. 34 gives estimated 
otter board backstrop planform angle against otter board 
spread for a given net and leg length. 

It will be seen from Fig. 35 that the otter board 
spreading force required to provide a given spread of a 
gear of known drag can now be determined from the 
warp and bridle theories and otter board data that have 
been mentioned in this paper. In general allowance must 
be made for board heel. 

2-5 Drag of the net 

It remains to relate net drag to its specification and mouth 
height and spread. Both model and full-scale data have 




to 20 


Fig. 33. Calculated ground rope spread with ground rope plan angle 
for a particular trawl. 



Fig. 34. Calculated otter board backstrop plan angles with board 
spread for a particular trawl. 

been obtained on this, and theoretical estimates have 
also been made, based upon tank model measurements, of 
the forces on simple plane specimens of net webbing. 
Some results are illustrated by Figs. 36 to 39. 

Fig. 36 shows the variation of the resistance of a plane 
net specimen having a constant characteristic setting 
angle y of 30, when fl, the angle of the plane of the 
webbing to the direction of motion, varies between 
and 90. Also shown are curves of theoretical and partly 
empirical formulae for estimating the resistance. 

As in the case of otter board forces, it is convenient 
to use webbing force coefficients, rather than forces in Ib 
of kg. "Twine coefficients*' are based on a "nominal 
frontal area 49 of solid material in a mesh, 2ad, where a is 




Attfi 2 ^ J a -AMUMED 




I tg. 35. Simple illustration of otter board spreading force require- 





90 4O SO 60 


Fig. 36. Resistance of plane net specimens. 

bar length between knot centres, and d is the twine dia- 
meter. To obtain the drag per square foot of webbing, 
it is necessary to multiply by d/a sin q> cos (p. If the area 
of webbing is calculated at a standard setting angle of 
45 say, then </> would be taken at this value in the multi- 
plier, but not elsewhere. 

It will be seen that at attitudes, 0, above about 30, 
a simple theory applies quite well. This assumes that the 
water speeds up so that the same volume passes through 
the area of the netting as would pass in the absence of the 
netting, the bar and knot components of the webbing 
having the same drag as would occur if they were each 
tested separately in a stream of such increased speed. 
At smaller attitudes the effective velocity of the stream 
is reduced due to water being deflected round the sides 
of the specimen. Sir Geoffrey Taylor has produced a 
theory for wire meshes of high solidity (i.e., small ratio 
of frontal area of holes to total frontal area) at 9=90. 
An attempt has been made to generalise this theory to 
small conditions, at which the effective solidity of net 

webbing should be high. As will be seen, the peak 
resistance given by the generalised theory lies close to 
the dashed line empirical curve drawn through the experi- 
mental points, but at smaller and larger 0's, it underesti- 
mates drag. Finally a simple expression, similar to that 
appropriate to large conditions, can be derived for drag 
at zero 0, assuming that the speed of flow of water past 
the webbing is unaffected by the presence of the webbing, 
and that the knots have a negligible effect. This agrees 
well with experiment, for setting angles less than about 
40, which should generally bracket the practical range. 
At large setting angles interference between longitu- 
dinally adjacent webbing components is large, and redu- 
ced forces are obtained. 

It has been concluded that it is best to employ the 
simple theories at high 6 and zero 0, and develop an 
empirical formula based on experiment for the "transi- 
tion" region lying between the two, such as that shown 
by a chain dashed line in Fig. 36. 

The sideforce and lift provided by plane net specimens 
that have the axes of the meshes located asymmetrically 
relative to the stream have also been studied in detail. 

Analogous methods have been applied to complete 
nets, and some full-scale drag measurements are com- 
pared with a partly empirical formula in Fig. 37. This 
case refers to a net of varying mouth area, due to varying 
headline height. It will be seen that there is a considerable 
component of drag even at very small mouth area, 
together with a component that increases fairly slowly, 





r ci 
s 1^ u 







D - 3'/2 
















O 100 200 3OO 40O SOO 4OO 



37. Variation of drag of tut and net appendages with mouth area. 


atfd almost linearly, with mouth area. It is believed 
that when a net mouth is opened so wide that setting 
angles of 45 or more occur, the drag may cease to 
increase any further but such large setting angles are 
unlikely to be acceptable for structural reasons. The 



TWINE DIA. .-<>' 


i^ ^^^ 

1 ". J 


AT a FT./ SEC.) \/ 







o i a i 4 s 


Fig. 38. The resistance of a simple conical net of fixed frontal area 
(Sounders Roe Tank Tests). 

data in Fig. 37 all refer to a single towing speed. If a 
simple conical net of fixed mouth area is towed at varying 
speeds, the resistance increases almost as the square of 
speed, as shown in Fig. 38. A practical net tends to 
reduce headline height as speed increases, however, 











s AT 

















o J a 4 s 


Fig. 39, The resistance of a practical bottom trawl net with frontal 
' " * area reducing as speed increases. 


and the effect is to give an increase in drag that is much 
more nearly linear with speed (see Fig. 39). This has 
already been mentioned in connection with Fig. 10. 

Headline height can, of course, be increased by increas- 
ing the number of headline floats or by using dynamic 
lifters. A method has been developed for calculating 
headline height from a knowledge of the characteristics 
of the headline lifters, the details of the leg system, and 
the design of the net wings. This makes it possible to 
provide a bottom trawl of fixed mouth spread but with 
any required height, up to say, 20 ft., without greatly 
increasing towing power requirements. 


In this paper it has only been possible to give a limited 
descriptive and selective account of the extensive 
theoretical and experimental investigations that have been 
made into the fluid dynamic and engineering principles 
on which the scientific design of trawl gear depends. 

The author claims that the requirements for a distant- 
water trawl, given at the beginning of the paper, have been 
met, and at the same time that a basic theory has been 
provided from which gears having other requirements 
can be designed. Furthermore the theory can be used to 
analyse and predict the mechanical and structural pro- 
perties of other proposed or existing gears. However, it 
would be disingenuous to suggest that the achievement 
of given engineering objectives will necessarily catch 
more fish. 

It would appear that a close co-ordination between 
gear engineering design and fish behaviour projects is 
necessary to achieve the type of gear that is efficient from 
an engineering point of view, and also takes adequate 
account of fish behaviour. For example, determination 
of an optimum relationship between size of otter board 
and size of net may not depend merely upon finding 
which arrangements require relatively low towing powers, 
and suit the nature of the seabed at the fishing grounds 
under consideration. 

This paper is published with the permission of the 
Westland Aircraft Company, the White Fish Authority 
and the British Trawlers Federation, in whose employ- 
ment and for whom the work was undertaken. The 
responsibility for any statement of fact or opinion is, 
however, solely the author's. 


Yakovlev, A.I. : Fundamentals of the calculation of the hydro- 
dynamics of otter boards. Translated from Trans. Inst. Mar. Fish. 
U.S.S.R., Vol. 30, pp. 61-76, 1955. 

Walderhaug, H. Aa. : and Akre, A. : First IF report on otter board 
tests. The Norwegian Ship Model Experiment Tank. Presented at 
the IF meeting in The Hague, November, 1962. 

Frey, K. and Sohle, H.: Model experiments with various forms 
of otter boards, Schiffbau, Schiffahrt and Hafenvau 35, No. 4 
(1934), A.C.S.l.L. Translation No. 991. 

Matrosov, I. R.: Increasing the stability and spreading effect of 
trawl otter boards by the use of a multiple-slotted shape with efficient 
moments effects. From "Rybnoe Khozyaystvo", No. 9, September 
1958, PRC/3667. 

Miyake, Y.r On the plane nets I. 1. Resistance of plane nets in 
water. Journal of the Imperial Fisheries Institute, Vol. XXIII, 
No. 2. 1927. 

continued on page 181 

Some Comparative Fishing Experiments in Trawl Design 


A series of fishing tests was made to compare the relative merits of 
a large lightweight wing trawl and a smaller and stronger otter 
trawl, both nets suitable for a 180-hp ship. At first the comparisons 
were made with the wing trawl skimming just clear of the ground 
and the otter trawl in close contact with it. In presenting the results, 
the author describes fully the methods he considers necessary for 
the treatment of comparative fishing data. Though in some areas 
and at some times the big wing trawl skimming the bottom did 
very well, it was at other places and times outfished by the small 
trawl in close contact with the bottom, even for species such as 
whiting. A second series of tests was done with the wing trawl also 
in close contact with the bottom. It now outfished the small trawl 
steadily but suffered repeated damage when the small trawl did not. 
The general conclusion is drawn that the wing trawl is about the 
biggest (70-ft headline, 85-ft footrope) and the thinnest twine 
(210/30 nylon) that can reasonably be used by that power of ship. 
A case is made for carrying on board both a large wing trawl and a 
small strong otter trawl for use on hard ground. Another series of 
experiments was done to find the best mesh size for the square, 
wings and belly of the net. The conclusion was drawn that, in the 
northern North Sea, fishing for haddock and whiting, more of the 
net should be in 31 inch mesh (89 mm), i.e., smaller mesh than is 
present commercial practice, particularly along the sides. 

Quelques essais de p6che comparative de chalut 

Une seiie d'6preuves ont 6t6 faitcs pour comparer la valeur relative 
d'un grand chalut 16ger a haute ouverture et d'un petit chalut 
de plus forte construction, tous deux de dimensions convcnant a 
un bateau de 180 c.v. Les premieres pcches comparatives furent 
exScutdes avec le chalut a grande ouverture effleurant le fond et le 
petit chalut en plein contact avec le fond. En prdsentant les r6sultats, 
1'auteur d&rrit les mthodes qu'il consider^ comme n6cessaires 
pour 6 valuer 1'information obtenue par des pSches comparatives. 
Bien que dans certains lieux et en certaines occasions le chalut a 
grande ouverture effleurant le fond pdchait tr6s bien, le petit chalut 
qui pechait en contact avec le fond capturait plus de poisson en 
d'autres lieux et occasions, meme pour des especes comme le merlan. 
Dans une seconde sdrie d'dpreuves, le chalut a grande ouverture 
oprait aussi en contact avec le fond. II capturait alors constam- 
ment plus de poisson que le petit chalut mais 6tait endommagd 
tres souvent tandis que le petit chalut ne Tdtait pas. On peut con- 
clure qu'un filet de 21 m de ralingue superieure et 26 m de ralingue 
infdrieure, construit de fil le plus fin (210/30 nylon), est le plus 
grand chalut qu'on peut utiliser raisonnablement pour ce type de 
bateau. II est conseille d'avoir a bord d'un chalutier de ce type, up 
chalut leger a grande ouverture pour les bons fonds et un petit 
chalut robuste pour les fonds durs. Une autre s6rie d'exp6riences 
6tait effectude pour determiner la meilleure grandeur de mailles 
pour les diffferentes parties du chalut. La conclusion fut que, dans 
le nord de la Mer du Nord, pour la peche de merlans et d'&glefins, 
plus de filet en mailles de 3,5 pouces (89 mm) devra dtre construit, 
c'est-i-dire avec des mailles plus petites que celles utilisees actuelle- 
ment dans la peche commerciale. 

Ensayos de pesca para comparer foraws de artes de arrastre 

S realiz6 una serie de ensayos para comparar las ventajas e incon- 
venientes de un arte grande de pernadas muy altas y otro mas 


W. Dickson 

Marine Laboratory, Aberdeen 

pequefto y robusto de puertas, ambos a prop6sito para un barco 
con motor de 180 hp. Al principle, las comparaciones se hicieron 
con el arte de pernadas altas pasando por encima del fondo y el 
de puertas en contacto con 61. Al presentar los resultados el autor 
describe con toda clase de pormenores los m&odos que considera 
necesarios para el tratamiento de los datos de pesca comparativa. 
Aunque en algunos caladeros en diversas ocasiones el arte de perna- 
das muy altas que pasaba por encima del fondo dio bucnos resultados, 
en otro lugares y mementos el arte pequeAo en contacto con el 
fondo pescaba mas, incluso especies como la merluza americana. 
Se realizd una segunda serie de ensayos con el arte de pernadas muy 
altas en contacto con el fondo. 1 resultado fue que pesco mas 
el arte de puertas pequefto pero sufri6 repetidamente averias que 
el otro no tuvo. La conclusi6n general es que el arte de pernadas 
muy altas es el mayor (relinga superior de 70 pies e inferior de 
85 pies) y el de hilos mas fines (nylon de 210/30) que puede emplear 
un barco de esa fuerza. Se dan razones convincentes para llevar 
a bordo un arte de pernadas muy altas y otro mas robusto y pequefto 
de puertas para emplearlo en fondos sucios. En otra serie de experi- 
mentos se determind el tamaflo de mall as mas conveniente para 
la visera, pernadas y vientre del arte, llegandose a la conclusibn de 
que, en el septentridn del mar del Norte cuando se pesca eglefino y 
merluza americana, una mayor proporc|6n de la red deberfa ser 
de malla de 3,5 pulgadas (89 mm), es decir, mas pequefia que en la 
actualidad, particularmente en las secciones laterales. 

SIMPLY to make trawling gear bigger ought to increase 
its catching rate, but there are attendant disadvan- 
tages. The towing drag is increased, and there are extra 
problems in handling gear and increased liability to 
damage. The first is a hydrodynamic problem, the 
second a practical fisherman's problem and the third 
concerns strength of materials but also requires a know- 
ledge of the seabed. Damage to netting does not 
only arise from its interaction with the seabed, it also 
arises from internal localized stresses resulting from 
inadequate design in the shaping of the net. A gain in 
fishing dimensions is often sought by the use of thinner 
twine, but unless stronger material is used a mere 10 per 

continued from page 180 

Taylor, G. 1. : Air resistance of a fiat plate of very porous material. 
Aeronautical Research Council reports and memoranda, R. and M. 
No. 2236. 1948. 

Curing long-span conductor oscillation. Engineering, 15 February, 
pp. 250, 251. 1963. 


The author would like to give the fullest acknowledgment 
to the representatives of the White Fish Authority, 
the scientific personnel of the Fisheries Laboratories, 

and the many people in the fishing industry who have 
provided information and assistance. The full-scale 
development tests of gear, forming an important part of 
the investigation, have been made on the Fisheries Re- 
search Vessels, F.R.V. Explorer and R.V. Ernest Holt. 
Finally, the extensive published investigations of earlier 
workers in the field, especially the Japanese, Russians 
and Germans, have been used as starting points for some 
of the work described here. 


cent advantage in twine diameter incurs a 20 per cent 
loss of strength. The stronger synthetics allow a useful 
strength gain with thinner twine and as important a 
mitigating factor as their increased wet knotted strength 
is the elasticity and ability to withstand shock loads, 
possessed by some of them. Increasing the mesh size 
is another way of keeping the drag down, but this may 
lead to poorer shepherding in the forward parts of the 
trawl and more escapes in the after parts. 

This paper describes some experimental attempts to 
determine optimum net size, twine size and mesh size by 
comparing the catches of different trawls. Optimum size 
had of course to take into account the power of the 
trawler, in this case 180 hp. 

(1) The purpose of comparing a small strong trawl 
against a much larger lightweight one was to 
assess the catch gain using a big net and to assess 
the factors limiting its use. 

(2) The purpose of comparing nets, identical except 
for the different mesh sizes in their forward parts, 
was to narrow the range of choice in this most 
important feature of net design, even if only for 
one or two important species in localized areas. 

Comparisons between two nets are like a series of 
football matches. Replays at different times and at 
different places lead to different results, and past form 
is no guarantee of the results of the next match. When 
doing comparative fishing tests on a research ship, 
where the attempt is made to measure every fish or else to 
sub-sample accurately, a great deal of data is accumu- 
lated. The condensing and presentation of this in meaning- 
ful form is not easy and, oddly enough, considering the 
large amount of data to start with, there rarely seems to 
be enough processed material at the finish to be certain 
of the conclusions that can be drawn from it. When 
comparative fishing with one boat only, the variability 
in catch between hauls demands a large number of 
replicates, which are seldom achieved! In short, most 
answers provided by comparative fishing are provisional. 

Any net catches a biased sample from the sea. It will 
be better at the capture of some species than of others 
and quite apart from codend mesh size, it may tend to 
take bigger rather than small fish or vice-versa. In tests 
like this the codend mesh size is made small enough to 
take everything of interest. Alternatively, the codend is 
fitted with a small meshed cover. The objective of this 
type of comparative fishing experiment is to discover 
the nature and degree of the main bias and to look for 
explanations of it. 

Wing trawl and otter trawl 

The Vinge or wing trawl with its forked wing ends is 
used primarily by the continental cutters as a herring 
trawl and it was as such that it was first used on our 
research vessels, Clupea and Mara, though it soon be- 
came obvious to ourselves and others that it had poten- 
tialities for demersal species as well. It is a big and 
rather complicated net of lightweight material. When 
rigged for herring trawling, the net floats (i.e., flotation 


outweighs the sinkers and altogether the net has positive 
buoyancy). The long danleno poles are weighted to 
hold the bottom, while the footrope rides clear of it, 
save for the loosely hung bights of chain, which must be 
just polished at the bottom of the bight. When fishing 
for whiting and haddock, the danleno poles are seldom 

It soon becomes obvious that some fish are escaping 
under the footrope; the temptation is to bring it close to 
the ground. The dilemma is spotlighted in Fig. 1, taken 

Fig. L Haddock on stony bottom, unfishable by light trawls. 

from a flash camera mounted on the trawl headline. 
Haddock arc very often as close to the bottom as this 
and their chance of escaping under a footrope skimming 
one foot clear of the bottom is good. Put the light foot- 
rope of a lightweight wing trawl on to such a bottom, 
and the net will be severely damaged. 

In the first series of tests the wing trawl (Fig. 2) was 
kept buoyant, while the otter trawl (on the left of Fig. 3) 
was in close contact with the bottom. The tests were 
confined to daylight and to good ground. The nets were 
tested at the same engine power so that the big net was 
towed slightly slower in the ratio 3-4 to 3-6 knots. 
Headline heights and spreads are given later. 

Method of presentation 

(a) Circles are used to illustrate species composition. 
The overall area of the circle represents the total number 
of fish caught on an hourly basis and it is segmented 
by proportion into species. Note that this does not give 
the relative commercial value of the catches, because a 
small fish counts as a unit here the same as a big one, 
and some species have a greater market value than 
others. Some of the more interesting examples are shown 
in Fig. 4. The circles give the average catching rate over 
a period of several days fishing in each area. 

(b) Histograms are used to show the number of fish 
of each species caught in each one-hour haul. Looking 
at these gives some visual idea of the quality of an experi- 
ment. It shows how many hauls were made, whether 
the gear was changed over reasonably often, the varia- 
bility from haul to haul and whether the average catching 
rate and species composition as shown by the circles is 
likely to have been biased by one or more hauls greatly 

HEADLINE- loft (30fl>loft.+3oFt,) 




f r 

Skimming tht (jround 
Ntt Buoijont' 

2t ih Plojtic Floor* 

m. Plosh'c 

orrtr Btord 
f tf.t/h 5Ff.3i n . 


Rig ustd oh Cofchij ground 

Fig. 2. Wing trawl. 



All aooyi/lb. Manilla 
txctpf" inn. Mtsh loud./lb. 




Ofr. 3in, 

. Cokk 

5ft DM Ltno 


Fig. 3. Small otter trawls, with different mesh sizes in their forward parts. 

larger than the others. Histograms are shown in Figs. 
5, 6 and 7. For convenience in presentation, their heights 
are shown logarithmically, which means that the net 
which is doing better is doing even better than appears at 
a glance. 

(c) Length composition curves show how the catch of 
each species is divided up into the numbers appearing 
at each centimetre length. This can be done for every 
haul, but results are usually summed for groups of hauls 

CLYDE - January 1959. 


Fig. 5. Histograms, haddock and whiting, Clyde, January 7959. Plain 
column indicates wing trawl. Black column otter trawl. 


Cruden Bay 
Sept. 1959. 

Cruden Bay 
Oct. 1959. 


O Wing Trawl 
Otter Trawl 

Fig. 7. Histograms, whiting only Cruden Bay, September and October, 


in one area at one time. There are two ways of presenting 
length composition curves each having advantages. The 
length composition can be shown proportionally for 
every 10,000 fish caught with each trawl and this is the 
best way of showing the bias in sampling between the 
two trawls irrespective of quantity caught, or it can be 

BUCHAN DEEPS - Early Sept. 1959. 
Whiting - left. Haddock - right. 

Mid-Sept. 1959. 

Whiting - left 
Haddock - right 

tor Trawl 

Fig. 6. Histograms, haddock and whiting, Buchan Deeps, early and mid-September, 1959. 



U flit Wkariour 
MM M* and tton th otter 
ia MweMi 
f i.hing grounds 


Offer TKOW! 
32U fish 

F#. 4. Oi/c/r composition. Wing trawl buoyant. Otter trawl on 

shown on the basis of the hourly catching rate, which 
shows not whether one trawl caught proportionally 
more big ones, but whether it actually caught more 
big ones. The number of graphs is too large to present 
them all here. All showed bias in the same general 
direction and a selected few are shown in Figs. 8 and 9. 

EHscusskHi of the test results 

The wing trawl, skimming clear of the ground, has a 
more restricted range of species appearing amongst 
its catch. Although it can at times outfish a small otter 
trawl, particularly for whiting but to a lesser extent for 
haddock also, it does not always do so and is sometimes 
quite heavily outfished even for these species, presumably 
when they are in close proximity to the bottom. This is 
the essential point to be taken from Fig. 4. 

The length composition curves (Fig. 8) show that the 
wing trawl tends to take proportionally more of the 
bigger whiting and the same was true for haddock to a 
lesser extent. The curves in Fig. 9 show that even when 
outfished in the Clyde the wing trawl took nearly as 
many large whiting and when it outfished the otter 
trawl in the Buchan Deeps, it took no more small ones. 
The bias is noticeable even in the sampling of the baby 




Fig. 8. Length composition per 10,000 fish whiting, Clyde, Buchan 
Deeps and Cruden Bay, 1959. 


k. * 


.1 Firth o.' Clyde J. in. 1059 
r J ! ' , l. 

fish Ltnqth - cm 

Fig. 9. Length composition per one hour fishing, whiting, Clyde and 
Buchan Deeps, 1959 

brood. There were at this stage of the experiment a 
number of possible explanations, some of which are 
ruled out later. They may be sought in differences in 
fish availability to the nets, differences in avoidance or 
differences in mesh selection. 

(i) The smaller fish may keep closer to the ground 
thus being more likely to escape under the ground- 
rope of the wing trawl. 

(ii) If the bigger ones swim higher up, they may be 
more liable to capture by the wing trawl with its 
considerably greater headline height, in the ratio 
of about 8 ft as against 5 ft. 

(iii) The wing trawl has a greater headline spread than 
the otter trawl in the ratio 35 to 25 ft, so it is 
possible that the bigger and faster-swimming 
fish have a better chance of being shepherded 
from the region of the wings into the path of 
the bag, while the smaller ones are more likely to 
pass through the wings. The thinner twine used 


|il\ **J< 

rfr Slock 

r **** . / 

r 60 3Vfn. /I-.3 


itfr Slock 

(n *k ft. * cafr * nil ft.) 


ao jo 


i. ?Ustk Float*. 

7fhv Calk 

. 70. 5wa// hard ground otter trawl. 

in the wing trawl might enhance such an effect, 
(iv) The bigger faster-swimming ones may find it 

easier to avoid the smaller net. 
(v) Jt may be that more small ones succeed in escaping 

from the lengthy bag of the wing trawl before 

reaching the codend. 

A new test 

It seemed that something more might be learned if the 
two nets could be fished, both with negative buoyancy 
and with the same groundrope firmly on the bottom. 
Since the wing trawl is quite vulnerable and ought to be 
kept clear of the ground, it was decided that both nets 
should be rigged with a groundrope comprising 14 in 
rubber bobbins in the bosom and 12 in rubber bobbins in 
the bunts. (See Fig. 2 for the details of the large trawl 
and Fig. 10 for the details of the small trawl.) Note that, 
in this series of tests on rougher ground, the forked wing 
ends of the big trawl were sewn up to make the net less 
liable to snag on the bottom, while the wing ends of the 
small net were opened out. 

The ground chosen for this comparison was catchy 
in places without being really rough. In all 28 hauls were 
made on this ground with each net, and on five the large 
trawl was seriously damaged. Working on a commercial 
basis, it would not have been possible to carry on using 
it on that ground, and it was only possible in this instance 
by having net riggers ashore mending steadily. The small 
trawl was scarcely damaged. It is now suspected that, 

even without the net snagging on the bottom, sudden 
change of shape of the groundrope as it meets an obstruc- 
tion may burst the belly of a lightweight net, even though 
the bobbins later surmount the obstacle; and indeed, 
when they do surmount it and rush to re-assume an 

Fig. 11. Elongation of the meshes where the lower wing Joins the belly 


Btnachit Jlrounu ^tpt.l 
J. All Plat Fith 

'A "^ 

Groundrope Approaching JJnag 

Qrouhdrope Utrikea 
Snag and Distorts* 

Fig. 12. Groundrope meeting obstacle. 

even towing position, the resulting localized stresses may 
cause further tearing. Fig. 11 although photographed 
on a heavy Granton trawl, shows the elongation under 
localized tension of the meshes behind the groundrope 
quarter (position A in Fig. 12) such as occurs in most 
nets. Suppose now the groundrope encounters an obstacle 
at X; the groundrope rapidly changes shape and A moves 
rapidly forward increasing the localized stress. It is clear 
that a lightweight net is more liable to rupture than a 
stronger one, that nylon with its ability to withstand 
shock loads is a useful material in such situations and that 
prestressing, due to inadequate design in such an area 
of net liable to be shock loaded, is a source of weakness. 
Means of relaxing these localized tensions have been 
discussed elsewhere. (Dickson 1961, Garner 1962.) 

Further discussion of test results 

The species composition, the catch histograms and the 
length distribution curves for the new tests with the 
groundropes of both trawls firmly on the bottom are 
shown in Figs. 13, 14 and IS. 

The species composition for the large wing trawl is 
now quite different from what it was before. Flatfish 
are now making a significant contribution to the catch. 
The catch histograms show a favourable result for the 
large trawl on this occasion, but remember it was at a 
cost in net damage that would have been prohibitive 

The bias in the length distribution curves is in the 



Fig. 13. Histograms, whiting, haddock and flatfish, Benachie Ground, 
September, 1961, plain white column is wing trawl and black is otter 


I*fti Otter Trawl 
240 fi eh/hour. 

Rights Wing Trawl 
735 fih/hr. 

Fig. 14. Catch composition: both trawls rigged with the same bobbin 
groundrope, Benachie Ground, September, 1961. 

same direction as when the wing trawl was buoyant. 
The possible reasons previously given for this can now be 
reviewed. No. i is ruled out because there is now no 
reason to suppose that small fish escape more readily 
under the groundrope of the wing trawl as the ground- 
rope was the same for both nets. No. ii seems unlikely 
as a general reason since the same bias appears to be 
present in the case of flatfish, which are less likely to be 
stacked so that big ones are more susceptible to capture 
by a high headline net. 

g -. 


I - 



Fig. 15. Length composition per 10,000 fish, whiting and flatfish, 
Benachie Ground, September, 1961. 

The general conclusion drawn from the whole series of 
tests is that, while a large lightweight wing trawl can be 
very effective at times, its use even when rigged with 
bobbins is limited to fairly good ground. It seems a 
sensible practice to equip ships with both kinds of bottom 
trawl or at least to hold them ashore in readiness against 
the seasons and the fisheries where each type is required. 
There is no such thing as a wonder trawl which is a 
winner in all circumstances. 

If two such nets are used it is necessary to make the 
nets in synthetics so that the one does not rot while the 
other is in prolonged service. This at least is now the 
practice adopted for our research trawlers. The tests 
also set limits to the sizes of twine that it was found 
reasonable to use in the large and small nets towed by 
a ship of 180 hp. The small net, made in 210/60 nylon 
twine and with 210/108 nylon in the front part of the 
belly, is a very strong net suitable for using on unknown 
rough ground. The trawl is then rigged with bobbins of 
course. The big wing trawl was found to be strong enough 
for herring trawling with the top and lower wings, the 
square, batings and belly all made in 210/30 nylon, 
but when working for demersal species it was found better 
practice to make the lower wings and belly in 210/45 

The otter boards, suitable for that size of ship, only 
spread the headline of the wing trawl to half the headline 
length; it follows that there is not much point in making 
the net any bigger. 

Having thus reached acceptable upper and lower sizes 
for the nets and the twine of which they should be made 
(Figs. 1, 2 and 3), the next thing to look at was how the 
mesh size should be distributed in the plan of a net. 

Mesh size distribution 

The usual practice in North Sea nets is to grade the 
mesh size from 5i or 5i in in the wings down to the 

regulation size for the codend. The grading is usually 
done in the belly. With hand braiding it is still a common 
practice to change the mesh size in several steps Without 
putting in any gaining meshes across the join. As the 
tendency towards machine made netting grows, so does 
the tendency to have as few mesh sizes in the net as 
possible, and this necessitates gaining meshes in the 
joining rows. In the ports where haddock and whiting 
are the most important species sought, it is a common 
practice to have the square and upper body in 3i in 
mesh and to grade the mesh size in the lower belly only. 
Thus apart from the top wings, the roof of the net is in 
small mesh while much of the side formed by the lower 
wings and belly is still in large mesh. 

With haddock and whiting as the species sought, the 
tests were to try and find out whether it was advantageous 
to carry the small mesh further forward and to include 
the sides of the net. On the other hand it might be possible 
to have the square, top and lower wings in much larger 
mesh. To try this a net was made with these parts in 
11 in mesh; but, so that it would fish at about the same 
dimensions as the 5i in mesh net, the twine size of these 
parts was also doubled in diameter. This makes a very 
strong net. The specifications of the 3 in mesh and the 

firth pf ci/4 toroh 1959. 
l Ortany April 1959. 


. 17. Length composition per one hour fishing, haddock and whiting, 
, East Orkney and North Minch, 1*59, Ji to., 51 in, and 11 in 
i , mesh nets. ' 


Firth of Clyde - March 1959. 


UK A" MoK Nit. 

East Orkney- April 1959 


5 imj 

North Minch - July 1959. 

Fig. 16. Histograms, haddock and whiting, Clyde, East Orkney and North Minch, 1959. In the lower histogram the bottom left and right refer 

to whiting and the centre to haddock. 

1 1 in mesh net are given in Fig. 3 along with that of the 
5 1 in mesh net. As compared with the 5^ in and the 
1 1 in, the 3| in mesh net loses a little in headline spread, 
when being towed. It probably loses a little in speed 
too because of the increased net drag. 

Discussion of mesh size results 

The catch histograms are shown in Fig. 16 and the length 
composition curves in Fig. 17. These curves are lumped 
together from others, all of which showed the same 
general bias though taken in different areas. The sur- 
prising thing is how little difference there seems to be 
between the catching rates of the three nets for small 
fish and how the gain in carrying the sides and roof of the 
net forward in 3| in mesh is for the larger faster-swim- 
ming fish. Even some marketable fish can, of course, 
pass through a 3| in mesh net. The selection factor 
(defined as the ratio of fish length to mesh lumen length 


at which 50 per cent of the fish escape through the meshes 
of a codend and 50 per cent do not) may be taken as 3-3 
for haddock and 3-7 for whiting. Allowing for the size 
of the knot, the lumen size of 3| in mesh is 82 mm, 
so that haddock as big as 27 cm and whiting as big as 
30 cm could be expected to escape; similarly for S{ in 
mesh the haddock and whiting sizes are 44 cm and 49 cm. 
Again an explanation of the length composition curves 
is uncertain but the bias is in the wrong direction for it 
to have been due to mesh selection in the forward parts 
of the trawl. Nor does difference in availability seem 
likely with the fishing dimensions of the three trawls so 
similar. This leaves difference in avoidance as the most 
likely explanation, possibly the bigger faster-swimming 
fish putting on a spurt to carry them out through the 
square and bunts of the large mesh nets. 

continued on page 191 

Development of an Improved Otter Trawl Gear 


Experiments have shown that the traditional trawl gear has several 
shortcomings. The trawlboards tend to dig unevenly into the 
bottom, following a zigzag course, thus causing constant changes 
in the net opening; the net shows bulges during operation which 
causes gilling of the fish; the waterflow inside the net is not evenly 
distributed, and is much slower than the towing speed, which causes 
turbulence. Studies of the net's behaviour at relatively high towing 
speeds led to an improvement in the design of the gear. In this 
paper the author gives details of the construction of a four-seam 
high-opening trawl and of the concave trawlboards employed. 
Experiments conducted with this gear showed that the difference 
between the waterflow inside and outside the net was reduced to 
0.3 kn. at a towing speed of 4.5 kn. This design has now been 
adopted by trawlers in the East China Sea. 

Etude du nouveau dessin d'un engin de chalut a plateaux 

Les essais ont montr6 qu'il existe plusieurs defauts aux engins 
traditionnels du chalut. Trop souvent les plateaux s'enterrent au 
fond provoquant une course instable et des changements frequents 
dans 1'ouverture du chalut ; le filet se bombe et les poissons s'emmail- 
lent. Le cours de 1'eau & 1'interieur du chalut n'est pas uniforme 
et beaucoup moins rapide que la v61ocit& du remorquage ce qui 
entraine la turbulence. Apres 6tude du comportement du chalut 
6 des vitesses relativement elevees, on a dessin de meilleurs engins. 
L'auteur donne des d6tails sur la construction d'un chalut & quatie 
coutures et a grande ouverture, ainsi que sur les plateaux de forme 
creuse employes. Les essais efiectuds avec cet engin ont ddmontre, 
que la difference entre le cours de Peau ttnterieur et & Fexterieur 
du chalut a 6t6 reduite & 0.3 noeuds, & une vlocit& de remorquage 
de 2.5 noeuds. Ce nouveau dessin a 6t6 adopt des maintenant, 
pour les chalutiers de la mer orientale de la Chine. 

Fabricacion de mejores artes de arrastre de puertas 

Los experimentos han demostrado que los artes de arrastre tradi- 
cionales tienen varios inconveniences; las puertas tienden a 
enterrarse en el fondo y causan cambios constantes en la abertura 
de la red; los paftos se comban durante la pesca y los peces se 
enmallan; el flujo de agua dentro del arte no es uniforme y es 
mucho mas lento que la velocidad de remolque, causando turbulen- 
cia. Como resultado de un estudio del comportamiento del arte a 
velocidades de remolque relativamente elevadas, se han proyectado 
artes mejores. El autor da detalles de la construcci6n de un arte de 
cuatro relingas de contorno, y gran abertura en altura, asi como 
de las puertas c6ncavas empleadas. Los experimentos realizados 
con este material demostraron que la diferencia entre el flujo de 
agua dentro y fuera de la red se redujo a 0.3 nudos a una velocidad 


Chikamasa Hamuro 

Fishing Boat Laboratory, Tokyo 

dc remolque de 4.5 nudos. Este arte lo emplean ya los arrastreros 
que pescan en el mar del este de la China. 

OTTER trawling and two-boat trawling are widely 
used in Japan and bottom trawling accounts for a 
major share of the total catch of commercial fishing. 

The design and operation of the otter trawl introduced 
in 1904 has undergone many changes and improvements. 
The nets are made of synthetic twines, and V-D gear has 
replaced the traditional leg connection between net and 
board but, until recently, very little has been done to 
improve the general design and structure of the net 

The Fishing Boat Laboratory undertook a rational 
study of otter trawls and by February 1961 had completed 
the design and construction of a new modified trawler 
gear. This gear was tested from the newly-constructed 
No. 50 Akebono-Maru, 1,500 tons, 2,000 hp (Fig 1), of 
the Nichiro Fisheries Company and has since been 
fishing better than the old gear both on the large stern 
trawlers and medium-size side trawlers operating in the 
Atlantic and the Bering and East China Seas. 

Defects of conventional otter trawls 

The general shape of the trawl in action is such that it 

continued from page 190 

Whatever the reason, it does seem to be profitable, at 
least over this size range of fish, to carry the 3 in mesh 
fairly well forward in the top and sides of the trawl. The 
limitations on how far forward to bring it may be set 
by the amount of mending that has to be done and it 
seems likely that part of the belly at least should be in 
larger mesh to allow the bottom rubbish to fall through. 
The design currently favoured, and used by us for a 
North Sea hard ground otter trawl, is of the style shown 
in Fig. 10, though it need not be so small. As experiments 
along these lines are continuing, the design is not a final 
one but it synthesizes what has been learned to date. It is 
necessary to remember the limitations of these tests; 
they were done in daylight only; they cover only two 
species in a limited range of areas and not exhaustively 
even there. 

A knowledge of fish behaviour, however acquired, 
helps in the design of trawls, but so does the fishing of 
trawls whose operational dimensions are known help 
in the understanding of fish behaviour, always provided 
that the trouble is taken to analyse the catch. The design- 
ing of nets with greater headline height, greater spread, 
etc., is a barren occupation unless the designs are put to 
the test of fish catching and put to the test in such a way 
that conclusions can be drawn. 


Dickson, W.: Trawl performance a study relating models to 
commercial trawls. Department of Agriculture and Fisheries for 
Scotland, Marine Research Series, No. 1, pp. 30-31. 1961. 

Garner, J. : The problem of excess load at the quarters. World 
Fishing, Vol II, June, pp. 67-72. 1962. 


Ffr. /. No. 50 Akebono-Mam. 

cannot readily be predicted by mathematical formulae. 
Nor can all the various factors which together compose 
its catching efficiency as yet be identified and defined or 
evaluated mathematically. No part of the gear, for 
instance, should interfere with the entrance of fish till 
caught in the codend; yet little is known of the precise 
reaction of the fish when it detects the gear either by 
sight, noise, smell or feeling. From long observations 
and experimenting with the gear, the author defined the 
trawl boards, bridles and the shape of the net as detri- 
mental to efficient operation of the gear. 

Trawl boards These are dragged at a certain angle of 
attack over the bottom and should follow a straight 
course. This is, however, not always the case. Referring 
to Fig 2, every time the board cuts deeper into the sea- 
bottom the angle of attack forces the board outward in 
the direction of D. The warp tension eventually 
overcomes this ploughing effect, and the board moves 
back and beyond its original track so that the board 
follows a zigzag course. The zigzag movement is increased 
if the point of attachment of the warp is below the centre 
of the board since this causes the board to tilt out and 
plough deeper. Other important factors are the weight 
of the board and the width of the shoe. 



Fig. 2. Course of trawl boards. 

Fig. 3. Headline-height changes with conventional-type trawl board 
during a four-hour haul. 

With the two trawl boards so operating, the distance 
between the wing tips of the net is continuously changing. 
A narrowing of the net opening results in a relative 
increase of the headline height and these fluctuations can 
be clearly seen in Fig 3 which represent the net height 


changes of a conventional type trawl during a four-hour 
haul. The conventional height/length relation of 1 :2 for 
trawl boards is not very efficient and, furthermore, flat 
plates do not give the best shearing power; all of whicljt 
results in oversized boards. 

Bridles With the single bridle method, the legs from 
the wings are joined at the bridle or to a relatively small 
danleno. It is better to take full advantage of the board 
height to allow the wings to rise as high as possible, 
although this means using two bridles instead of one. 
The connection of board to net with two lines, further- 
more, helps to steady vertically the board in action. For 
this purpose it is better to have a large value for the 
ratio H/L of the boards. 

Shape of the net Measurements have shown that 
conventional nets, during operation, take a bulky shape 
in the forebody with a pronounced narrowing in the 
after body (Fig 4). This billowing of the front part 

Fig. 4. Shape of conventional net in operation. 

Fig. 5. Two-boat trawl being towed at high speed with too great 
distance between boats. 

of the net is caused by choking up of water which can- 
not flow freely through the net. Under such circumstances 
the waterflow to the codend is disturbed and causes gilling 
in the tapered body of the net. To avoid such gilling the 
mesh size is usually decreased which in turn causes even 
more resistance and turmoil which impedes capture. 

Traditionally, bottom trawls are made of an upper and 
lower part laced together at the sides. With such structure 
it is difficult to distribute the waterflow evenly through 
all parts of the net and the rear part is therefore flattened 

When a net is dragged at low speed, the net releases 
through its meshes an amount of water which is propor- 
tional to the area of the net opening and the towing 
speed; the waterflow inside the net is then the same as the 
towing speed. However, this is not the case at higher 
towing speeds. Measurements with flowmeters showed 
that in a 36-8m headline trawl towed at 3-5 kn speed, 
the waterflow had decreased to 2*7 kn already at a 
distance of only 2 m behind the footrope. 

When the volume of water entering the net is greater 
than that which can be released by its webbing, a turmoil 
is started which at high speeds spills some water back 
through the net mouth, possibly taking with it some of 
the already caught fish. Pressure builds up inside the net 
which can prevent fish from entering and tends to 

frighten strong swimmers away from the net entrance. 
The above is similar to what happens when a plankton 
net is towed too fast. To conclude, differences between 
waterflow in the net and the towing speed must be a 
minimum in a well-balanced trawl; the measurements 
performed on the conventional otter trawl showed a 
difference of 0.8 kn which points to poor water release. 

Comparison with two-boat trawl 

Since the net is towed by two vessels, the net opening is 
practically constant (both in horizontal and vertical 
direction) and can be regulated to the shape for which 
it was designed. 

With this type of net the length to breadth relation is 
much bigger so that there is better water release. How- 
ever, when the distance between the boats is exaggerated 
at high towing speeds, the net shows the same defects as 
those of the otter trawl (Fig 5). The vertical con- 
figuration of the webbing is much better than with the 
otter trawl. 

Measurements of the waterflow inside a two-boat net 
gave 1.9 kn when towed at 2.3 kn. When large-mesh 
side panels were inserted at the waterflow inside increased 

Leg rope 

Pendant wire 

Ground rope 

Current -^ 

Fig. 6 

Fig. 6. Pattern of water-flow in conventional otter board. 

to 1.94 kn at a towing speed of 2 kn, practically 
equalizing the waterflow inside and outside the net. 
Furthermore, the large-mesh side panels gave good 

This indicates that a four-seam construction method 
for two-boat trawls has advantages over the conventional 
two-seam belly and back structure. 

Improved otter trawl 

Based on the experiments, a new four-seam net was de- 
signed and constructed and operated with concave 
boards of a special design. 





VWtr flow 
(itsidt tfctntr 

Fig. 7. Water-flow characteristics of old-type trawl, two-boat trawl 
and new-type net. 

Otter boards The boards were constructed with H to 
L ratio of 1/1.12 which had been found to give a good 
stabilizing effect during operation. They were equipped 
with flat shoes to avoid ploughing and to minimise the 
friction with the seabottom so that the boards would 
keep a more stable course in relation to the towing 
direction. Construction details of the new trawl boards 
are given in Figs 8, 9 and 10. The boards were made of 
steel and weighed 1200 kg in air. 

Net The net body was designed to be about six times 
as long as the fishing width of the net at the lower bosom, 
to achieve the required water release. The present net 
has a total length of 75.6 m (252 ft) from wing tip to 

Fig. 8. New-type otter board 

end of codend; the bottom net from footrope bosom 
to end of codend is 40 m (133 ft) and the headline is 
68.33 m (228 ft). It was estimated that with 250 kg 
buoyancy the headline height at the wing tips would be 
5 m and at the bosom 8 m. An elevator (Fig 1 1) was 
mounted on each wing tip during some of the experiments 
to evaluate its effect. 
To achieve a smooth flow of water inside the net and 


Fi. P. Construction details of new trawl boards. 

to avoid bulging, the mesh-size of the side panels was 
larger than that of the adjacent webbing of the top and 
bottom webbings, and differed by 36 mm at the opening, 
scaling down to 6 mm at the codend. 

The whole net was made of polyethylene twine of 200 
denier filaments of suitable sizes (Fig 12), to obtain 
lightness and lift. Except for the wing tips and selvages, 
all the webbing was knotless. 

Assembly The net was connected to the boards by 
legs of 120 m (400 ft) length, giving an estimated net 
opening at the wings of 26 m (87 ft), with a distance 

Fig. 10 

Fig. 10. Pictorial view of trawl board. 

of about 70 m (233 ft) between the boards. Each leg 
passes through the stopper rings of strops attached to 
the board and during shooting it is stopped by the 
shackle joining the leg to the pennant (Fig 13). The 
upper strop is 5 m and the lower is 4 m long, giving a 

To otttr board 

upptr leg 

To otter board 

lower leg 

Fig. 11. Hydro-dynamic elevator. 

Fig. 13. Connection of net to boards. 


Fig. 12 Design of new 
four- seam otter trawl. 

Headline: Wirt rope 14mm 

footrope : 


18mm fll 
11 20mm f $ 

Seamllne Polyethylene rope 29 mm 

Codlint = Polyethylene rope 16 mm 

Webbing : Polyethylene 200 denier 










Figs. 14)15. Shooting and hauling sequences. 

difference of 1 m between the upper and lower connec- 
tions of board and net. The shooting and hauling 
sequence is illustrated in Figs 14 and 15. 

Experimental results To ascertain the operational 
behaviour of the gear the following automatic measuring 
instruments were used: 

recording net-height meter 
depth meter 
water flow meter 

Both V-D and two-leg connections were used to 
investigate the effect on net height. During a third 
series, hydrodynamic elevators were attached to the 

with one leg 

with two legs 

with two legs 

Cm 20 <<& CD 

Fig. 16. Headline-height measured by recording net-height meter. 

wing tips. Numerical values obtained during the experi- 
ments are given in Tables I, II and III. 

The horizontal net opening at the wings was calculated 
from the observed warp angles obtained, and was found 
to be slightly above 26 m, though decreasing somewhat 
at increased towing speeds. At this opening the side 
contourlines of the four-seam net make an angle of only 
9 to the direction of movement, as compared to 20 
for the conventional two-seam type trawl (Fig 18). 

The double-leg connection gave about 2 m higher 
net height at the wing tips and at the bosom than the 
V-D connection. With the elevators, which give a lift 

5 r 

*. I0 






Fig. 17. Height of components of trawl nets at various towing speeds' 

1. Height of headline bosom (with two legs and elevator) 

2. Height of headline bosom (with two legs) 

3. Height of wing headline bosom (with legs) 

4. Height of upper net in codend. 

5. Height of wing end (with two legs and elevator) 

6. Height of wing end (with single leg) 

Table I. Data obtained with self-recording net-height meter, depth meter, net - against - water - speed meter, tension meter etc. 



V-D rigged 
Ho. 1 

double legs 
Ho. 2 

double legs with elevators 
Ho. 3 

i i 

2 1 1 









Humber of 

1 leg 

2 legs 

2 legs 

Buoyancy of 

300 kgs 

300 kgs 

300 kgs 

Elevator for 
iring net 








weignt or 

386+90-476 kgs 

386+90-476 kgs 

386+90-476 kgs 

Sea depth 

102 n 











Length of warp 

300 m 

310 m 

360 m 

per minute 








170 o 




Towing speed 

3-1 *t 












Warp angle 
Included angle 


+ 6 

+ 7 




+ 8 






+ll w 





+ 5 








6, 8 







Depression angle 












. right 












warp tension 











Height or net 
vin*r end 

3 m 







Centre of the 
net mouth 

10 m 












7 A 





Water flow speed 
ineid nat 

See Table II 

of about 35-40 kgs at 4.5 kn, when wing tips were raised 
to 5 m and the bosom to 1 1 m fishing height (Table II). 

Table II Towing apad at 4.5 knots 


luab.r . 

Of lagB ^ ^ 

Haight of 
and of wing 

Baight of 
of boaon 

Haight of 

Raduotion of 
flow apaad 
inaida nat 

1 lag 


8 m 



2 laga 


10 m 



2 logs 

with a la vat or 
for tha vine 


11 n 


0.3 knots 

Total tuoyano* waa up to 300 kga. Tha floats (Polyathylana) 
woro 24 o in diaattar, alla^ad to withstand vatar praaaura 
of 40 k/o. 

By observation of chafing on the belly, it was found that 
at a distance of about 6-7 m beyond the footrope the 
net operated off the bottom, so that the top part of the 
codend operated more or less on the same height as the 
headline bosom. This is an important feature, especially 
for trawls operating on rough bottom. 

Waterflow Measurements of the waterflow inside the 
net, obtained during the experiments, are given in Table 
III where the values obtained in a conventional two-seam 
net are given for comparison. From the table it is clear 
that the difference of waterflow inside the net to the 
towing speed, is only 0.3 kn at a speed of 4.5 kn. Figs 19 
and 20 show the recording of the waterflow in both cases. 

Tabla III Bolativo watar flow inaida and out id* not* 

^^^^a^nd of aat 
ZtaM ^**^H V ^^ 

Tha naw typa aat 
(4-saaa aat) 

ConTaatioaal typo not 
(2-aaaa aat) 

Outaida tha aat 

(4.33kt)(4.4kt) (4.54*t) 
2.22m/a 2.26/a 2.34/a 

1.35V* 1- 6 TV. 1.86V. 

Inaida tha aat 

2.07/a 2.1 / 2.17/a 

1.19/s 1.16V* l-21"/ 


0.15/a 0.16a/a 0.17/a 

Oa6a/, 0.49V* 6 5/ 

"\ Kind of iwl 

low typo Mt Coavantioaal trawl nat 

(4-MM aat) (2-ssaa aat) 

It*as v ^- 


Vatcr flow 

(1) -P..* 



4. 54 



3.62 kot 

4*5 knot 

outsida not 






1.86 a/a 

Vatsr flow 

(2) apaad 






1.21 a/. 

3.2 knot 

inaida Mi 

(1) - (2) 

aaad vaftttotioa 






0.65 / 

1.3 taaot 

of tbo flow 

iaaida Us aat 


Old type net 

Secton view of the net 

New type net 

Plan view 

Section view 

Plan view 

Fig. IS Comparison of shapes of old 

Operation Although the net was much longer than 
any net previously used from the No. 50 Akebone-Maru 
no difficulties were experienced in operating the gear' 
either in shooting or hauling. However, measurements 
performed and observation of the net after hauling 
indicated the importance of adjusting the warp length 
for the angular difference between the ship's course and 
the towing direction. This is especially important on 
stern trawlers where the distance between the stern 
rollers is large. 

Om JO 

and new-type trawl nets in operation. 

Kg 21 shows the angle made by the warps when towing 
with the tide on starboard, ft is clear that the warp 
distance from the roller to the otter board is longer for 
the portside and that this warp should therefore be 

wl! I?" *? additl nal length ' equal to D sin : 
where D is the distance between the stern rollers, and 

e is the angle of the ship's course to the towing direction 
For the No. 50 Akebone-Maru, D was 11.10 m and 
during the experiments values as high as 1 m were found 
I Of D sin 9. 



Tfcwinj Speed 
Figs. 19)20. Difference in flow speed inside and outside new and old-type nets. 

Towing Power, Towing Speed and Size of Bull Trawl 


The engine power of Japanese two-boat trawlers has increased 
from 160 to 340 hp in the last 15 years but, due to the use of fixed 
blade propellers, the available engine power was not fully utilised 
in towing. Experiments were carried put with controllable pitch 
propellers and it was found that the towing power could be increased 
from 35 to 75 hp. In order to utilise the increased towing power, 
a high-opening trawl was designed and constructed and during 
comparable fishing operations it was found that, although the 
weight of the catch of the new gear was of the same order as that 
of the traditional gear, the fish were generally of a larger size and 
better value. 


Chikamasa Hamuro 

Fishing Boat Laboratory, Tokyo 

Puissance, vitesse de touage et dimensions do chalut a deux bateaux 


La puissance des machines des chalutiers a deux bateaux Japonais a 
augment^ au cours des 15 dernteres annexes et est passee de 160 
a 340 c.v. mais 6tant donne qu'ils employaient des helices a pas 
fixe, cette puissance n'&tait pas utilisee pleinement dans le chalutage. 
Des experiences ont 6te conduites avec des helices & pas variable 
et on a trouv que la puissance de touage pouvait ainsi passer de 
35 a 75 c.v. Pour profiler pleinement de cette force de touage 
supplemental, un chalut a grande ouverture a 6t6 dessine et 
construit et pendant des operations de pdche comparatives, on a 
constate que les captures effectives avec le nouveau chalut etaient 
egales poids a celles effectives avec le chalut traditionnel, mais que 
les poissons captures etaient generalement plus grands et d'une 
valeur superieure. 

Fuerza y velocidad de remolque y dimensioned del arte 


La potencia de los motores de las parejas Japonesas ha aumentado 
de 1 60 a 340 hp en los ultimos 1 5 aftos, pero por causa del empleo de 
helice de palas fijas la fuerza disponible no se ha aprovechado por 
completo en el remolque. Se realizaron experimentos con pro- 
pulsores de paso controlablc y se demostr6 que la potencia de 
remolque podia aumentarse de 35 a 75 h.p. Para aprovechar plena- 
mente la mayor fuerza de remolque se proyect6 y construy6 un 
arte de mucha abertura en altura y durante actividades de pesca 
comparativa se demostrb que aunque la captura total del material 
modificado era del mismo peso que la del tradicional, el pescado era 
en general de mayor tamaflo y mas valioso. 

THHE engine power of the two-boat trawlers in Japan has 
JL increased from 160 to 340 hp in the last 15 years. 
The main object was to obtain higher speed to minimise 
the time spent in steaming from port to the fishing 
grounds and back, as well as to speed up the move from 
one fishing ground to another. 

Due to the use of fixed-blade propellers, the available 
towing power did not increase proportionally to the 
increase in engine power so that the fishing gear has 
remained almost unchanged. 

The Fishing Boat Laboratory tested a pitch pro- 
peller which increased the effective controllable propeller 
power while trawling from 35 hp to 75 hp. 

Vessel and propeller 

The vessels used during the experiments were the No. 23 
and 25 Yamada Maru, both having 98 tons displacement 
with diesels of 340 hp at 385 r.p.m. Manganese bronze 
controllable-pitch propellers were used, three-blade. 
1,600 mm diameter, 7,500 cm 2 developed area. 

Continued from page 198 

Fig. 21. Difference (A-C) in warp length from roller to otterboards 
when towing with tide on starboard. 

Fig. 22. Net distorted when towing with tidejon starboard. 

When towed with equal lengths of warp in the above 
case, the weatherside wing was found to contain seaweed 
stuck in the meshes while practically none was apparent 
in the leeside wing. This pointed to a distorted net 
during towing, as illustrated in Fig 22. During subse- 
quent hauls, the lee warp was lengthened by the amount 
D sin o according to the drift angle, and seaweed then 
got caught equally in both wings. The angle 6 changes 
as the drift changes and is not constant throughout the 
haul. One way of avoiding constant changes in warp 
length on stern trawlers would be to centre the top rollers 
(towing blocks) at a point from where the warps are 
paid out. However, this would adversely affect the 
opening width of the net. 

The above trawl gear is now being used in commercial 
fishing by big stern trawlers in the Bering Sea and the 
Atlantic, and also by medium-size trawlers in the East 
China Sea, and is giving ISO to 200 per cent better results 
than the conventionally used trawls. 


Such propellers are normally for vessels of the Yamada 
Mam class. 

Summary of the experiments 

Th'e vessels were connected stern to stern with a 209 m 
long manila hawser. Towing was done of varying 
degrees of pitch and r.p.m. of the propeller (Fig. 1). 

Fig. L Operational method during towing tests. 

The towed vessel kept a constant pitch on the propeller 
but changed the propeller revolutions to allow the towing 
vessel to attain the various speeds required for obtaining 
the necessary data. The instruments included specially 
constructed dynamometers of 2*5 to 5 tons, special logs 
designed and constructed at the Fishing Boat Laboratory, 
and strain gauges for accurate measuring of the propeller 
shaft torque. 
The measurements taken included: 

(a) Warp tension (pull). 

(b) The vessel's speed. 

(c) The torque of the propeller's shaft. 

(d) The exhaust gas temperature of the main engine. 

(e) The mean pressure for each cylinder. 

(f) The fuel consumption. 

All instruments were installed on the towing vessel 
which issued instructions as to speed, etc., to the towed 
vessel. The relations were defined between propeller 
pitch, propeller speed, fuel consumption, torque and 
ship's speed. 

Result of the towing tests 

The results are summarised in Fig. 2. The results calcu- 
lated for towing speeds of 2| to 3 knots are also shown 
in Fig. 3 in another way. The maximum towing pull 
normally utilised was extracted from the data provided 
in the graph and was taken for the following conditions : 

(a) For a maximum exhaust temperature of the main 
cylinder during operations of 350C. 

(b) For a maximum mean pressure of the main 
cylinder of 5*2 kg/cm 2 . 

(c) For a minimum fuel consumption. 

The towing power under the above conditions was 
measured and the results are given in Table I. 



2-5 knots 
3-0 knots 
3-5 knots 


3-5 tons 
3-4 tons 
3-3 tons 


Pitch Revolutions 



353 r.p.m. 262 P.S. 

Design of the trawl gear 

The normal power on two-boat trawlers is 320 to 340 hp 
and most vessels have fixed pitch propellers. As the 
pitch has normally been selected for free running, power 
available for towing is therefore relatively small. Tests 
have shown that at 2-3 knots the pull is about two tons; 
this means that towing power is around 35 hp. By 

Fig. 4. Traditional manila two-boat trawl. 













Test results for 2.5, 5 5.5 knots 

r.p.m. of Main Engine 



using a controllable-pitch propeller, the towing power 
was increased to 75 hp. 

Fig. 4 gives the design of the traditional type of two- 
boat trawl. A new type of trawl was designed to have a 
minimum of resistance during towing at high speeds. 
The headline height was found to be about 5 m at 3 knots. 
Construction details are given in Fig. 5. An elevator, 
Fig. 6, is attached at each wing for assisting the wing to 
reach its maximum height. 

Measurements were taken of warp tension and headline 
height; these, together with the data previously given, 
are shown in Table II. 

Towing Speed 
2-8 knots 3*5 knots 


3-3 kg/cm 8 
27-5 kg/hr 
2-2 tons 
4-9 m 

4-7 kg/cm 8 
49-0 kg/hr 
2-8 tons 
4-5 m 

Table II. 

Propeller pitch 
Propeller r.p.m. 
Temperature of the exhaust 
Mean pressure 
Consumption of fuel 
Distance between the boats 
Warp tension 
Net height 

Result of fishing operations 

Experimental fishing was carried out on the normal 
fishing grounds in the Yellow Sea during the beginning 
of September, 1962. 

The new gear and the traditional gear were towed 
alternately by a pair of trawlers. The catches of large- 
and middle-sized yellow croaker were 150 per cent 
higher than with the traditional trawl; the catches of 
small fish and bottom species were, however, less. The 
total weight of the catch was approximately the same but, 
the market price for the bigger fish was higher. 

This new net has been designed for high opening and 
high-speed towing. If the fish population is composed 
mainly of bottom fish, the design of the net should be 
altered to obtain a wider opening at the cost of a lower 
height, conserving the high-speed feature of the trawl. 
In any case, any new design must incorporate total use 
of the full thrust available from the engine having a 
controllable-pitch blade propeller. 

Fig. 5. Design of improved two-boaf trawL 

Fig. 6. Elevator used during experiments. 


Suggestions for Improved Heavy Trawl Gear 


The author who is an outsider to the fishing industry, presents his 
general suggestions for the improvement of heavy bottom trawl 
gear as a stimulus for fisheries experts to work out practical solu- 
tions to the advantage of both the trawling industry and the sub- 
marine cable companies. His proposed modifications aim at 
decreasing the towing resistance and the risk of fouling the gear on 
obstacles on the bottom. The main suggestions are: To modify 
the bracket or bridle arrangement of the otter boards to fend off 
objects which otherwise would be hooked or, even better, to operate 
the otter boards off the bottom, attaching them directly to the 
leadline, and by using a different rig and different designs and/or 
other materials (light plastic) for the boards; also to eliminate 
bottom-touching sweeplines. To enhance catching efficiency, 
sound or electricity should be utilised to guide the fish towards the 
net opening. To carry out such gear development projects efficiently 
and quickly, the establishment of integrated "systems development" 
teams on national, or even international, scale is suggested rather 
than widely scattered individuals or small groups working without 
co-ordination. If the design objectives include the avoidance of 
damage to submarine cables, the international communications 
firms might be willing to assist. 

Suggestions pour modifier le chalut lourd 

L'autcur qui n'est pas un spicialiste dans I'industrie des peches, 
soumet dans cette etude certaines idees pour Amelioration des 
chaluts lourds de fond, pour lesquels les experts en matiere de 
ptehe pourront peut-fctre trouvcr des solutions pratiques interessantes 
pour 1 Industrie des peches et les compagnies de cables telephoni- 
ques sous-marins. Les modifications proposees tendent a reduire la 
resistance de 1'engin et les risques d accrochage aux obstacles du 
'fond. Ces suggestions comprennent la modification des brides ou 
arrangement d'une chatne sur les panneaux de tellc sorte qu'il ne 
soit pas possible d'attraper des objets avec le panneau ou mieux, 
d'operer de telle facon que les panneaux soient au-dessus du fond 
en les attachant directement a la ligne de fond ou encore par la 
modification du dessin de 1'engin et en utilisant pour les panneaux, 
les nouvelles matieres comme le plastique, surtout pour empecher 
que les bras ne tratnent sur le fond. Pour ameliprer la capture, 
pleine utilisation devra fctre faite du son et de I'dlectricitd pour guider 
les poissons vers Pouverture de 1'engin. Pour 1'execution rapide et 
efficace de ces projets, des "systemes de devcloppcment" etablis 
par des quipes nationales ou Internationales seront certainement 
plus efficients que des efforts individuels ou de pet its groups travail- 
lant sans coordination. Les compagnies Internationales de com- 
munications sont prttes a aider a la realisation de ces projets si les 
travaux permettent en m&me temps d'eviter les dommages causes 
aux cftbtes sous-marins. 

ejorar artes de arrastre pesados 


El autor, que es ajeno a la industria pesquera, hace sugerencias 
generates para mejorar los pesados artes de arrastre de fondo con 
obfeto de estimular a los especialistas pesqueros a encontrar 
soiuciones practicas que redunden en beneficio de la industria de 
la pesca al arrastre y de las compafiias explotadoras de cables tele- 
grancos submarinos. Propone, en sus modificationes, reducir la 
resistencia al remolque y d resfo de que el arte se enganche en 
obstaculos del fondo. Las princtpales sugerencias son: modificar 
el brazo o el enganche del pie de gallo de las puertas para evitar 
obstaculos en los que podrian engancharse o, lo que seria todavia 
meior, separar las puertas del fondo sujetandolas directamente a la 
relinga de ptomos, emptear diversos herrajes, formas o materiales 
(plasticos Ugeros) y eliminar las malletas que tocan el fondo. Para 
incremcntar el rendimiento de pesca deberian emplearse sonidos o 
corrientes ettctricas que guien a los peces hacia la abertura del 
arte. Con objeto de que estos proyectos puedan llevarse a la prac- 
tica rapida y eficazmente, convendria crear grupos de "estudios de 
sistemas" en escala national o internacional, en vez de tener indivi- 
duos o grupos pequenos aislados que trabajan sin coordinaci6n. 
Si una de las finalidades del proyecto es evitar causar averias a los 
cables submarinos, las empresas internaa'onales que los explotan 
es posible que esten dispuestas a ayudar. 


Eldon Nichols 

Cables Division, American Telephone & 
Telegraph Co. 

WHY has heavy trawling gear been improved so little 
in recent years? Important advances have been 
made in the design of large trawlers and in techniques 
for handling and processing the catch but trawling gear 
has undergone only minor refinements. The present 
trawl works quite well on relatively smooth bottom at 
moderate depths (the conditions for which it was designed) 
but today much trawling is done on uneven and rough 
bottom at depths ranging to more than 300 fm. Under 
these conditions, hang-ups and fouling are common 
and the width and height of the mouth vary erratically 
as the otter boards lurch and heave. Drag is excessive, 
limiting the size of the net and the trawling speed and 
increasing fuel consumption. 

This situation came to attention during studies of the 
causes of damage to submarine cables and some consid- 
eration was given to it. The results are presented here 
with the hope that a look at the subject from an out- 
sider's point of view may provoke interest among fisheries 
people and lead to effective action. 

Need for basic changes 

Till now most changes have involved only replacements 
and adjustments in one or two specific components, 
e.g., the substitution of heavy bobbins for rollers, and 
not all have been very beneficial. In fact, matters are often 
made worse by attaching weights to doors and reducing 
scope to shorten the hauling-in time. This increases the 
heaving of doors by sea and swell. 

It seems clear that the design of a fundamentally new 
trawl is indicated, rather than the mere substitution of 
new otter boards, floats, or other items. Although 
substantial development work will be required to deter- 
mine details of a new trawl, it may be useful to suggest a 
general line of approach. 

Preview of a new trawl 

First, the basic geometry of the trawl needs investigation. 
In the present structure the vertical component of the 
towing force acting on the warps is opposed and wasted 
by the weight and tilt of the otter boards. If the warps were 
attached to the headline, they would lift it, eliminating, 
or at least reducing, the need for floats. With fewer or 
smaller floats there would be less drag on the headline. 

This would permit the use of smaller otter boards which 
would give further reduction in drag. 

These changes would eliminate the groundrope legs 
and this should reduce hang-ups to less than a third of 
the present number. It may be objected that the otter 
boards must be separated from the wings by legs or 
sweeplines in order to scare fish into the net. This theory 
seems to be questionable when trawling is done in deep 
and turbid water, at high latitudes, or at night. Under 
these conditions the light is undoubtedly too dim for 
the otter boards and lines to be visible for more than a 
few feet. And if sound or pressure waves, rather than 
light, is thought to be the agent that scares the fish, a 
more efficient frightening or guiding device than the 
present otter boards and sweeplines could surely be 
designed. In recent experiments in the U.S.A., increases 
in catches were obtained by passing electric currents 
through the water near the mouth of the net. This 
appears to be a much more promising method than the 
use of sound. 

Improvement in the method of attachment is particu- 
larly important. The forward end of the present type 
of otter board, together with the bracket and the warp, 
form a hook that will foul almost any object in its path. 
This hook can be eliminated by the use of a bridle 
or bracket attached to the top and bottom corners of the 
leading edge of the door in such a way as to fend off 
objects encountered. This, of course, is only a refinement 
of the chain bridle, long in use in small trawls. This 
change would interfere with the use of the "G" link, 
Kelly's eye and stopper for the quick disconnection of 
the otter boards when hauling in. However, with the 
otter boards attached directly to the ends of the headline, 
as will be the case with the groundrope sweeplines 
eliminated, disconnection would not be necessary. 

It is also proposed that the otter boards be removed 
from the bottom and made "free swimming". This would 
do away with the extreme variations in spreading force 
which now occur due to the shoe intermittently scraping 
up ridges of sediment and jumping over them. Also 
eliminated would be the variations in spreading force, 
due to the sudden changes in effective aspect ratio 
whenever the otter board leaves the bottom or returns to it. 

The new otter board should have a little positive 
buoyancy which in the past has been unattainable due 
to the quick waterlogging of the wooden planks under 
pressure. Now it is feasible to make strong, light hydro- 
foils of a rigid plastic foam, such as polyurethane, 
covered with fibreglass. The very smooth surface, 
free from iron braces and bolts, should give a substantial 
increase in lift-drag ratio. The foamed plastic core should 
be either tapered in density from top to bottom, or 
slightly weighted at the bottom, to make it seek a vertical 
position in the water. This will correct the tendency of 
the present otter boards to fall flat when launched, when 
veering or when the warp goes slack for any reason, 
such as the heaving of the trawler in the sea. 

Second, ways should be found to make better use of 
synthetic fibres in nets. Fishermen have been under- 

standably reluctant to make full use of them due to 
their higher cost and to practical difficulties in tying 
them. But if these faults can be overcome, their low 
friction, light weight and high strength should pay 
dividends in reduced drag and maintenance. 

Third, new inventions and ideas should be reviewed 
to see if they fit into the general framework of trawl gear 
improvement, or if any of them are important enough to 
warrant a change in this framework; e.g., a new headline 
float intended to give more lift with less drag would be 
examined as a possible replacement for the current 
types. On the other hand, an invention such as the recent 
Canadian one, in which a hydrofoil equipped with a 
hydrostatic control always seeks a preset depth, would 
be considered as a possible way to eliminate headline 

Further study may show that some of the design features 
just proposed are not practical but the point is that 
trawls of novel design are possible, that there are new 
ideas and materials waiting to be put to work and that 
little is being done about this situation, although even 
a minor improvement involving so many trawlers would 
have substantial value. 

Organising for development 

If it is agreed that the development of a better deep-water 
trawl is possible and desirable, the next question is: 
How can it be accomplished? Much of the rapid techno- 
logical progress in modern industry is due to the employ- 
ment of integrated "systems development" teams capable 
of carrying projects all the way through to commercial 
operation, rather than widely scattered individuals or 
small groups working without co-ordination. Fisheries 
people must find ways to employ this powerful method. 

A development team capable of handling the project 
under discussion should include high-grade fishing 
gear technologists, expert designers and makers of nets 
and gear, outstanding trawler officers and an able and 
experienced leader. Also, there should be arrangements 
for occasional consultation with specialists in such fields 
as hydrodynamics, marine biology, plastics and elec- 

In any of several countries, such a team could be 
assembled from the competent people now engaged in 
fisheries work. A board with members drawn from the 
government fisheries department, universities and manu- 
facturing and fisheries companies could establish policies, 
select the key members of the team and provide general 
co-ordination. The funds required to finance the under- 
taking might W contributed jointly by the companies 
and the government. They should not be much larger 
than those now being spent by the several organisations 
working separately but the results could be much more 

Another attractive possibility is the formation of a joint 
enterprise in which several countries would co-operate, 
perhaps under FAO leadership. In this case, the financial 
contribution from each party would surely be within reach. 


Development of Soviet Trawling Techniques 


Soviet development is toward active high-sea fisheries. The principal 
gear is the bottom trawl. Five stages of its development since 1 878 are 
shown in sketches. Most modern trawling is done at 1 50 to 250 m, 
at speeds up to 3*5-5-0 knots for periods of 1 to 3 hours, 
with bottom gear of these general specifications: 60 m long; 25 to 
40 m horizontal opening; up to 3 m vertical opening. The Novator in 
1948, followed by the Pushkin-class vessels, led to the modern 
1,900 hp sterntrawler with completely mechanised processing 
facilities. Continuing experience dictates changes such as smaller 
vertical opening of the net for plaice; larger vertical opening and 
greater speed for herring; and variations in vertical and horizontal 
opening for cod and haddock. Soviet trawlers use oval, slotted 
otter boards which are more durable, lighter and have 19 per cent 
more spreading force than ordinary otter boards of comparable 
size. Midwater trawling was first tried in the 1920's but is still in 
the developmental stage. North Atlantic midwater trawling has 
occasionally produced more than 20 tons in 30 minutes' trawling. 
A scheme of controlling the net-filling rate is diagrammed. A 
transmitter enclosed in a ball attached to the headline is connected 
with a transducer installed in the codend; the transmitter sends 
signals to a receiver in the wheelhouse. Only some gear parameters 
can be studied with present methods and instruments. The author 
diagrams what he believes is the most advanced installation for 
gear trials. In this, two endless steel ropes are attached to a pon- 
toon mounted on two floats. The gear to be tested is secured to the 
ropes; the pontoon serves as a platform for cameras, lights and 
other instruments to register changes in parameters as the gear is 
towed at various speeds across a natural basin which has no current 
and is sheltered from the wind. The method permits testing full- 
scale gear and one-half and quarter-scale models. The author des- 
cribes another special instrument, an under-water dynampgraph 
which can continuously record, for 90 minutes, the stresses in rope 
and wire to 350 m depth. Another instrument is sketched which 
records the net's vertical opening by means of an expandable 
tank transmitting pressure variations through a liquid-filled hose 
to a recorder. Fish behaviour and gear catchability studies use the 
usual aqualungs, bathyscopes and underwater equipment but, in 
addition, the special submarine Severyanka permits long-term 
observations and photography. 

Dtotoppement des techniques de chalutage sovtetique 


Le dcveloppement de la peche sovietique est ax sur la peche 
hauturiere. L'engin principal est le chalut de fond. Les cinq stades 
de dcveloppement depuis 1878 sont racontes par des illustrations. 
Le chalutage moderne est gen6ralement pratique 1 de 150 a 250 m 
de profondeur, & des vitesses de touage allant jusqu'a 4-5 noeuds, par 
periodcs de 1 a 2 heures, avec des chaluts de fond ayant les 
caractcristiques suivantes: 60 m de long, 25 a 40 m d'ouverture 
horizontal et jusqu'fc 3 m d'ouverture verticale. Le Novateur en 
1948, puis les chalutiers de la classe Pouchkine ont conduit aux 
chalutiers a peche arriere moderaes de 1900 c.v., entierement 
mecanises avec possibilit6 de trailer le poisson. En prolongeant 
rexp&ience, il est apparu que certaines transformations devaient 
fttre introduites, a savoir: ouverture verticale moins grande pour 
les chaluts a plies, ouverture verticale plus grande et vitesse acceteree 
pour les harengs, variations dans Pouverture verticale et hori- 
zontale pour le cabillaud et 1'eglefin. Les chalutiers sovietiques 
utilisent des panneaux ovales, munis de fentcs, qui sont plus 
rdsistants, plus legers et ont une force laterale supdrieure de 19 pour 
cent a celle des panneaux ordtnaires de dimensions comparables. 
Le chalutage, pelagique fut commence en 1920 mais est encore 


A. I. Treschev 

Fishing Technique Laboratory, 

a un stade de dcveloppement. Dans TAtlantique du Nord il a 
parfois produit des captures de plus de 20 tonnes pour une demi- 
heure de chalutage. Un diagramme represente un systeme de con- 
trdle de la vitesse de remplissage du chalut. Un 6metteur place dans 
un globe est attache aux ralingues superieures et relie a un radio- 
imcttcur installe sur le sac; les 6chos recus du sac sont transmis 
a un r&epteur situe dans la timonerie. Les param&tres des engins 
de peche pouvant Stre etudiSs par la pr6sente methode sont limites. 
Des diagrammes de 1'installation que 1'auteur suppose Stre la 
plus avancee pour faire des 6preuves d'engins de peche, sont 
donnds dans la communication. Cette installation comprend 
deux cordes d'acier continues, attachees a un ponton montc sur 
deux flotteurs; les cordes passent par 1'engin a prouver; le ponton 
porte les cameras, lampes et autres instruments pour enregistrer 
les changements des param&tres de 1'engin qui est toue a des vitesses 
differentes, dans un bassin nature! n'ayant pas de courant et a 
1'abri du vent. La methode a permis de faire des essais a 1'echelle 
du modele, et a des echelles reduites de moitie et du quart. La 
communication ctecrit un autre instrument special, un dynamo- 
graphe sous-marin, qui peut enregistrer continuellement pendant 
90 minutes les tensions exercees sur les cordes jusqu'a une profon- 
deur de 350 m. Un autre instrument enregistre I'ouverture verticale 
du filet au moyen d'un reservoir sensible a la pression qui transmet 
a Tenregistreur les variations de pression par rintermidiaire d'un 
tube plein d'un liquide. Pour les Etudes du comportement du 
poisson et de I'efficacit6 de capture des engins, des bouteilles d' 
oxygene, des bathyscaphes et tout un tquipement sous-marin ont 
et6 utilises normalement, mais en plus, le sous-marin special 
Severyanka a permis de faire des observations de longue duree et 
des photographies. 

Evoluckta de las tecnicas sovitticas de pesca al arrastre 


Las actividades sovieticas se encaminan hacia la pesca de gran 
altura empleando principalmente el arte de arrastre de fondo. 
1 autor da ilustraciones de cinco fases de su evoluci6n desde 
1878. Actualmente la pesca al arrastre se practica en fondos de 
150 a 250 m, a velocidades hasta de 4-5 nudos por periodos de 
1*5 a 3 hrs, con artes de las siguientes caracteristicas generates: 
longitud, 60 m; abertura horizontal de 25 a 40 m y vertical hasta de 
3 m. 1 Novator en 1948 y posteriormente los barcos de la clase 
Pushkin han culminado en el modernisimo arrastrcro con rampa 
a popa con motor de 1,900 hp y rnanipulaci6n y elaboracion 
completamente mecanizadas. La experiencia aconseja el empleo de 
redes de abertura en altura pequefta para la pesca de peces pianos; 
de mayor abertura en altura y velocidad para el arenque; y varia- 
ciones en las aberturas horizontal y en altura para bacalao y 

Continued from page 205 

If the design objectives include the avoidance of damage 
to submarine cables, there is good reason to expect that 
the international communications interests will be willing 
to assist. 

Only a single, specific development programme has 
been discussed here. If it is carried through successfully 


there should be strong incentive to retain the organisa- 
tional framework and to take up other problems. In 
many industries the most healthy and prosperous 
companies are those which allocate a significant part 
of their budgets to research and development. Can 
trawler owners afford not to follow this example. 

eglcfino. Los arrastreros sovteticos emplean pucrtas ovaladas, 
con ranuras, que duran mas, son mas ligeras y tienen un 1 9 por ciento 
mas de fuerza de abertura que las normales de dimensiones ana- 
logas. La pesca entre dos aguas se practic6 por primera vez en la 
tercera d6cada de este siglo, pero esta todavia en la fase experimental. 
Esta pesca, practicada en el Atl&ntico septentrional, ha producido 
en ocasiones mas de 20 tons de pescado en lances de 30 min de 
duraci6n. Se da un esquema del proyecto para regular la velocidad 
a que se llena la red. Un transmisor metido en una esfera ligada 
a la relinga de corchos csta conectado con un transductor instalado 
en el saco; el transmisor envia senales a un receptor instalado en la 
caseta del timbn. Con los m6todos e instrumentos actuales s61o se 
pueden estudiar algunos parametros de los artes. El autor da 
esquemas de lo que es a su juicio la instalacion mas avanzada para 
ensayar artes. En esta, dos cables de acero sinfin se sujetan a un 
pontdn montado en dos flotadores; el arte que se va a ensayar se 
ata a los cables; el pont6n sirve cpmo plataforma para camaras, 
luces y otros instrumentos que registrar) los cambios en los para- 
metros cuando el arte se remolca a diyersas velocidades por un 
estanque natural en el que no hay corrientes y est protegido del 
viento. Este mdtodo permite ensayar modelos a la mitad y un 
cuarto de la escala total. El autor describe otro instrumento 
especial, un dinam6grafo submarino que registra continuamente 
durante 90 min los esfuerzos a que estan sometidos Jos cables de 
fibra y de acero a profundidades de 350 m. Da un esquema de 
otro instrumento que registra la abertura en altura de la red por 
medio de un dispositivio de expansidn que, a traves de una man- 
guera llena de liquido, transmite las variaciones de la presi6n a un 
registrador. El comportamiento de los peces y la capacidad de 
pesca de los artes se estudian por buzos aut6nomos, batiscafos y 
equipos submarines, asi como con el submarino especial Severyanka 
que permite hacer observaciones y tomar fotografias durante 
mucho tiempo. 

THE Soviet fishing industry is directed mainly toward 
active high sea fisheries practised from extremely 
sea-worthy vessels. 

The main fishing gear is the bottom trawl (Fig. 1). 
The most widely used type is about 60 m long with a 
horizontal opening from 25 to 40 m; mouth to 3 m, 
towing at up to 3*5-5*0 knots. 

Trawling is mostly at 1 50 to 250 m but sometimes, in 
the redfish (Sebastes) fishery the trawl is down to 500 m 
and deeper. Usually trawlers up to 800 to 1,000 hp 
bring in salted fish. There are now new freezer trawlers 
of 1,900 hp. The first Soviet sterntrawler, Novator, was 
built in Murmansk in 1948. 

At present trawlers catch demersal species (such as 
cod, haddock and redfish) and pelagic species (such as 
herring and sardine). Trawling practice indicates that 
operations and trawl design for various species should 
be changed. For plaice, for instance, the large vertical 

Ma inn 

Fig. 1. Principal development stage of bottom trawl. 

opening of a trawlnet and high trawling speed are not 
required, while for herring the vertical opening and 
trawling speed are of primary importance. To a certain 
extent, a simultaneous increase in vertical and horizontal 
openings can be obtained by higher trawling speeds. 
In addition to adjustments by rigging the headline and 
footrope, the trawl parameters can be considerably 
changed by simply employing a more rational design of 
otter boards. The Soviet trawling fleet uses oval-shaped 
slotted otter boards (Fig. 2). These do not cut so deeply 
into the bottom, are more durable, relatively light in 
weight and have 19 per cent greater spreading force than 
comparable rectangular boards. 

Midwater trawls 

First attempts to use trawls to catch fish in midwater 
were made by Murmansk skippers as early as the 1920's, 
but results were poor. After many trials a new method 
of midwater .trawling has been worked out which is 

Fig. 2. Oval-shaped slotted otter board. 


used by big trawlers on dense concentrations of Atlantic 
herring. In the North Atlantic catches after 30 minutes 9 
trawling sometimes exceed 20 tons. 

Apart from fish location aids, successful "aimed" 
midwater trawling requires special apparatus. A catch 
indicator which works on a hydro-acoustic principle is 
presently used in research work but it is not yet perfected 
for commercial use. 

The transmitter is enclosed in a ball about the same 
size as the ordinary metallic trawl floats. This instru- 
ment is attached to the headline and connected by cable 
with the transducer installed in the codend. The receiver 
is on the pilot bridge. A more detailed description of the 
instrument may be found in the journal "Rybnoye 
Khozyaistvo" No. 7, 1959. Scientific research in trawl 
design development must be closely connected with 
studies of fish behaviour near trawl gear. 

Gear testing 

The determination of technical parameters the openings 
of fishing gears, stresses and strains imposed on warps 
and rigs, shapes assumed by gears and so on is a rather 
complicated task and requires special measuring devices, 
especially when operating at great depth. 

Present methods and instruments so far allow study 
of only some of the factors involved, such as resistance 
to movement, depth of submersion and vertical and 
horizontal openings. The shape assumed by a trawlnet 
under different conditions and corresponding internal 
stresses in the gear are difficult to study during commer- 
cial fishing. 

Experimental methods for laboratory investigations 
of trawl gear have been mainly oriented toward experi- 
ments in natural waters because model experiments in 
hydrocanals and wind tunnels with relatively small-sized 
models characterise only the qualitative aspect of pheno- 

mena, whereas the quantitative results obtained in this 
way are not sufficiently reliable. 

There are several methods of conducting laboratory 
investigations of fishing gear in natural waters. At 
present the most advanced method, in the author's 
opinion, is that based on the use of a hydrodynamical 
installation on steel ropes. In this case a ropeway formed 
by two endless steel ropes is used (Fig. 3) running through 
snatch blocks in a natural basin with motionless water 
sheltered from wind. The endless ropes (1) are driven at 
a desired speed by an onshore electric winch (2). To 
these ropes is attached a pontoon (3) mounted on two 
floats. The tested gear or gear component (4) (trawl, etc.) 
is secured to the endless ropes under the pontoon by 
special hangings. During the process of towing the 
pontoon may shift its position in relation to the gear 
being tested. On the pontoon are measuring and con- 
trolling instruments (5) which register changes in para- 
meters of the gear tested ; movie cameras, searchlights 
and other aids. 

The return movement of the pontoon is assured by a 
reverse operation of the winch. Speed of towing is 
regulated by a gradual change of winch r.p.m. through 
a frictional speed variator. This method permits testing 
both full-sized trawling gear and models scaled down to 
2 and . The tests are conducted under conditions as 
near as possible to commercial fishing. 

A special underwater dynamograph has been devised 
for measuring loading fishing gear operating at great 
depths. The working principle is based on the measure- 
ment of tractive effort reduced by a lever mechanism 
by means of a calibrated spring, and registered by a 
self-recorder supplied with multiplying device. The 
instrument consists of three essential parts : body, receiv- 
ing mechanism and multiplying and recording mechan- 
ism. Its purpose is to measure stresses in ropes and 

Fig. 3. A hydrodynamical installation on steel ropes. 


Double-Rig Shrimp Beam Trawling 


Since 1959 the Belgian shrimp trawlers have steadily been convert- 
ing from otter trawling to the double-rig beam trawling. All new 
shrimp trawlers have been especially adapted for this fishing method 
since 1962. The trawls used have, on average, a beam length of 
8 m using a net of 550 meshes width of 26 mm mesh size in the 
upper part and 18 mm. in the codend. The whole net is made of 
nylon 420 R Tex. The paper describes the operational method and 
compares the catch efficiency of otter trawls versus double-rig 
beam trawls. The calculation of the resistance and effective mouth- 
opening for the two fishing methods shows that the theoretical 
advantage of double-rig beam trawls over otter trawls for shrimp 
fishing is of the order of 30 per cent. In double-rig operations in 
areas having strong tides, a hook-up with one gear is dangerous 
and the levering force acting at the boomtip can cause the vessel 
to capsize. Full details are given of a safety-rig incorporating a 
special safety hook. A survey of the catch results obtained with 
both methods in 1962 shows that, for the same horse-power, the 
double-rig method resulted in, on average, a 30 per cent higher 
catch, which compares well with the calculated 40 per cent greater 
horizontal opening. 

Chalutage de crevettes avec double-chalut a perche 

Depuis 1959, les chalutiers a crevettes beiges ont converti progressive 
ment la peche au chalut a panneau en peche avec des chaluts & 
perches a double agreement. Depuis 1962, toutes les nouvelles 
unites de pfcche aux crevettes sont adapters specialement pour 
cette methode de peche. Les chaluts ont une perche d'environ 8 
mitres de long et utilisent un filet de 550 mailles d'ouverture dont 
les mailles ont 26 mm. dans la partie superieure et 18 mm dans le 
sac. Le filet entier est fabriquS en nylon 420 R Tex. L'etude decrit 
la methode d'operation et compare Pefficacit de capture des chaluts 
a panneaux avec celle des chaluts & double greement. Le calcul de la 
resistance et de Pouverture effective du filet pour les deux m6thodes 
d6montre qu'en theorie les chaluts a perches a double greement 
sont surerieurs auc chaluts a panneaux de 30 pour cent. Cependant, 
au cours d'une operation a double greement, dans les eaux a fort 
courant, un accroshement avec un des engins pounait etre dangereux 


J. Verhoest 


A. Maton 

University of Agriculture, Ostend 

car la force s'exeroant alors sur le bout de la bigue risque de faire 
chavirer le bateau. Des details concernant un greement dont la 
securitd est basee sur un crochet de stirctd sont tournis dans cette 
communication. Une 6tude des resultats de capture obtenus par les 
deux methodes en 1962 montre que pour une mme puissance, le 
systeme & double greement produit des captures d'environ 30 pour 
cent plus importantes ce qui confirme assez bicn Taugmentation 
thgorique calculee pour une ouverture horizontal 40 pour cent 
plus grande. 

Arte de vara doble para la pesca del camaron 

Desde 1959 aumenta el ntimero de arrastreros belgas que pescan 
camar6n que sustituyen el arte de puertas por otro de varas dobles 
Desde 1962 todos los arrastreros para la pesca del camarbn se 
adaptan especialrncnte para este metodo de pesca. Por tdrmino 
medio, los artes tienen varas de 8 m y emplean paftos de 555 mallas 
de 26 mm en la parte superior y de 18 mm en el saco, todos de 
nylon de 420 R Tex. 1 autor describe la maniobra y compara su 
rendimiento de pesca con el de los artes de puerta. El calculo de la 
resistencia y abertura efectiva de la boca en ambos metodos 
demuestra que la ventaja te6rica del de vara doble sobre el de 
puertas, en la pesca del camar6n, es del orden del 30 por cicnto. 

Continued from page 208 

cables at up to 350 m depth. Depending on the elasticity 
of the spring, the measured effort may vary between 
500 and 5,000 kg. The measurements may be recorded 
continuously during 90 minutes. The instrument has a 
positive buoyancy so that its use does not greatly inter- 
fere with gear operation. 

The vertical opening can be determined with the help 
of an instrument consisting of an expanding tank fas- 
tened to the upper point and a recording mechanism at 
the lowest point; the tank and the recorder are connected 
by an incompressible hose. The principle of this device 
being to measure the pressure created by a column of 
liquid contained in a closed system. 

The manufacture of these devices present certain 
difficulties, since they have to be hermetically sealed. 
But they are much simpler and easier to handle than the 
equipment used in studies of fish behaviour and catching 
capacity of fishing nets, which require far more complex, 
long-term observations. Recently, such observations have 
been conducted with the help of aqualungs, bathyscapes 
and underwater television. In the U.S.S.R., a special 
submarine, Severyanka, is also used. The employment 
of the submarine permits long-term direct observations 

of fish near the gear and gives the opportunity to photo- 
graph the most interesting phenomena. Fig. 4 shows a 
photo of a trawl taken from the submarine. 

Fig. 4. Photo of a trawl taken from a Russian submarine. 


Cuando sc cmptean artcs de vara doble en fondos de corricntcs 
fuertea, los enganches y embanes ion pcligrosos porquc la fuerza 
que actiia sobre el extremo de la botavara pucde hacer que vuclquc 
el barco. Se dan detalles completes de un aparejo con un gancho de 

i en 1962 demuestra que, a igualidad de potencia, el arte 
de vara doble dio, por termino mcdio, capturas superiores en un 
30 por ciento, lo que resulta muy favorable en comparaci6n con la 
abertura horizontal calculada en un 40 por ciento mayor. 

IN 1959 two traditionally equipped Belgian shrimp 
otter trawlers were converted to a double rig beam- 
trawling system. The operations were completely 
successful from the start, resulting in other vessels follow- 
ing suit, so that, by 1962, 28 of the 66 shrimp trawlers 
operating on the Belgian coast had changed over to 
double-rig operation. This conversion was the result 
of a scientific research, undertaken in charge of the 
Commission for Applied Scientific Research in Marine 
Fishery, presided over by gen. dir. F. Lievens. 

Since 1962 new vessels are being built especially 
equipped for this type of operation. The fishery has a 
definite future and bigger vessels are under construction, 
such as those already operating from the Netherlands. 


The vessels converted up to the present have been mainly 
wooden with an overall length of 12 to 17 m, powered 
by 40 to 120 hp diesel engines (Fig. 1). 

The conversion work consists mainly in erecting a 
steel mast based down to the keel which carries the two 
booms and the necessary rigging as shown in Fig. 2. 
Practically no changes are required to deck arrangements. 

The winch used is the traditional double-drum otter 
trawl winch; the warps are now led over the booms to 
carry one complete fishing gear on each side. The booms 
are set to the required stance by tackles, which can be 
done by hand as long as the fishing gear is inboard; 
once fishing operations have started, the load is too 
high. For fishing, the booms are set to an angle of about 
30 to the horizontal so that the footrope hangs just 
clear of the water (Fig. 3). During bad weather the boom 
is lowered to near the horizontal to improve stability. 

Fig. L Common Belgian shrimp trawler. 

Fig. 3, Diagram showing the length of the derrick (L) as a function of 
the beamlength (/) and the vessel's beam (b). 



topper "block 


hoisting taoklt 

Fig. 2. Details of boom rigging on converted side trawler. 


Adjustment of the position of the booms between hauls, 
according to fishing conditions, is done by heaving on 
the warping heads. 

Several new double-rig vessels have lately been built 
for this work and the main difference between these 
new vessels and the older converted type seems to lie in 
the position and type of winch. The new winches are of 
the friction clutch type with four drums, two for the 
warps and two for topping the booms. The winch, 
furthermore, is now housed under the bridge within 
reach of the skipper so that the manipulation of the gear, 
regulation of the booms' position and steering of the 
vessel can now all be done from one position by one 
man. Only three men crew these vessels (Fig. 4). 

By doing away with the skylight over the engine room 
and by leading the warps high above deck, these vessels 
now have a much larger working space on deck. 

if- > 

1M X 

Fig. 5. Construction details of a beamnet. 

a. warp 

to , stopper 

c. stopper block 

d boon 

e. hoisting tackle 

Fig. 4. Details of boom rigging on newly constructed double-rig trawler. 


The net is the usual type consisting of an upper and lower 
piece seamed at the sides. Two wedges are inserted 
between the square and the lower wings to allow full 
lift (Fig. 5). The size depends on the length of beam. 
The more generally used type at present has a beam 
length of 8 m, a net width on top of 550 meshes of 
26 mm mesh size in the upper part and 18 mm in the 
codend. The whole net is made of nylon 420 R Tex, 
except for the codend where 480 R Tex is used. The 
footrcpa, 9-80 m. long, is made of mixed wire rope of 

14 mm diam. Full indications for assembling and 
mounting the net are given in Figs. 6 to 8. The above 
particulars cover both big and small nets as only the 
main dimensions differ, the general assembly of the nets 
being the same. 

The length of beams differs according to the size of 
vessel and there is some slight constructional difference 
between ports (Fig. 9). The width of the beam shoe at 
the sole depends on the type of bottom fished, the softer 
the bottom, the broader the sole. Each beam shoe 
carries several eyes; one each for the headline, the foot- 

Fig. 6. Side panel and square with headline. Fig. 7. Side panel and belly with bolchllne. 

Fix, B. Junction of wing and bosom. 


Cftntrtl fete* ttotl** 

bridlt tytt 

feotropt and bolehllitt 

. 9. De/fli/s of beam and beam shoe. 

Fig. 10. Details of rig-up of beam and foot rope. Inset A: normal 
bolchline length. Inset B: shortened bolchline. 


rope and the bridle. The beam itself is normally construc- 
ted in three sections; the outer parts, which are secured 
by a chain, slide over the middle. The centre part is 
often of a lighter diameter than the outer parts so that 
when a hook-up occurs this part will bend and is then 
easily replaced without interference with the beam shoes. 
The footrope is made up of 13 cm diam wooden rollers 
or rubber discs, which are all joined by links at a distance 
of 30 cm. centre to centre. At each link there is a connec- 
ting chain to the bolchline. For the type of net under 
description 32 such rollers are used which together make 
up a length of 9*60 m. Under normal conditions the 
bolchline operates behind the rollers but in areas where 
much bottom trash, starfish, crabs, etc. are picked up, 
the bolchline is shortened so that it operates approxi- 
mately parallel and above the rollers, allowing the bottom 
trash to pass between the rollers and under the net (Fig. 
10). The relative tension on headline and footrope can 
be adjusted by attaching the footrope ends more or less 
forward at the beam shoes. 

Operational method 

At the fishing grounds both beam nets are hoisted ready 
for operation on the booms, which are set at an angle of 
approximately 30 and pointing a little more aft than 
abeam. The vessel steams ahead on a straight course, 
while wearing out both gears, until sufficient warp has 
been paid out in relation to the depth. During poor 
weather the beams are normally lowered to an almost 
horizontal position to improve stability. After hauling, 
the process described above is reversed and, when both 
nets are heaved to the boomtips, the vessel steams ahead 
for a short while to rinse the catch. Only the bat is then 
hauled on board with the gilson, the codend emptied 
and the gear prepared for the next haul. 

Advantages of double-rig fishing 

It can be shown that for the same engine power a larger 
area is swept with double-rig beam trawls than with one 
otter trawl. 

The relation between the resistance of nets of identical 
construction can be expressed by: 


R 2 

Where: R= resistance; M= stretched mesh size: 
N~ number of meshes in rounding; 
e= density of twines ; R=runnage of twines ; 
v=:towing speed. 

Indices 1 and 2 refer to each net (otter trawl; double- 
rig beam). 

When both nets are constructed of the same material 
and mesh size, and are towed at the same speed, the 
relation can be simplified to: 

In the above equation N* can be substituted by / x H ; 

li /M^y /MA / Cl R^ 
L 2 \M 2 N 2 / \M 2 / \e 2 R 2/ 

when the relation of the length (/) of the headlines and 
the vertical height (H) of the nets is : 

H 2 


in which case 


/iXH 1 


/, x H B 

The combined resistance of two small congruent 
nets whose total resistance is that of one big net can be 
given by: 2R a = R L 

, 4* "I .fclVo M _ 

therefore -i = - = 2 = s* 
K z K 2 

and s - 

Ml; '/ = 


= 1-41 

The length and height of the opening of one small net 
would then be 0-7 of that of the larger net having the 
resistance of two such small nets. The shrimp otter trawls 
used on the Flemish coast have very short wings and are 
therefore of practically the same construction as the 
beam trawls. 

The length of the headline is on average about 10 m. 
and the net is attached to the boards generally without 
legs. The trawl boards used are 1-8 m xO*9 m. The open- 
ing of such nets differs very little from the height of the 
trawl board and, as only few floats are used, the headline 
will hardly rise much above the board height. Using the 
opening of such a net as a basis, one could calculate the 
opening of two small beam trawls having the same resis- 

l a = 0-7 x 10 = 7m. 

H 2 - 0-7 x 0-9 = 0-63 m. 

The height of the beam shoes, which represent the 
opening height of the beam net, is normally found to be 
60 cm, which is very near to the height found for H 2 . 
The horizontal spread of two beam nets would then be 
2x7 = 14 m, and is therefore 40 per cent higher than 
that of one otter trawl having the same resistance. 

Resistance of otter boards and trawl beams 

Otter boards Taking for granted that the otter boards 
have an angle of attack giving them an optimum sheering 
force for a certain speed, the resistance of the boards of 
0-9 x 1-8 m having a towing speed of two knots can 
be approximated as below: 

p C g- A - v2 

R = 2 

C specific resistance coefficient -0*65 

sec 2 
Q = density of seawater - 104 kg - 

A = area of boards in m 2 . 
v = speed in m/sec.; v = 2 knots = 1*028 m/sec 
R = 58kg. 
Fhe resistance for two boards works out to 116 kg. 

Trawl beams The resistance R of the cylindric part 
of the beam can be read: 

P _C. Q. A. v* 

K _ 2 . 

C = 1. 

A. =0-09 x 8 = 0-72 m 2 

v = 1-028 m/sec. 

R =39-5 kg. 

In the same manner we can define resistance of the 
beam shoes with C = 1-3 as: R = 8-5 kg. 

Total resistance of two beams with beamshoes = 
2(39-5 8-5) = 96 kg. 

Bottom friction resistance 

The weight of one trawl beam with beam shoes is more 
or less the same as a pair of otter boards and the frictional 
resistance of an otter trawl should therefore be the same 
as that of the beam trawl. However, trawl boards plough 
through the bottom at an angle whereas, owing to the 
broad soles on the beam shoes, which act as sleds, the 
trawl beam glides over the bottom. It would therefore 
seem fair to assess that frictional resistance of two otter 
boards amount to about the same as that of two trawl 
beams. Summing up, it may be said that for a same 
condition of resistance, towing power and speed, the two 
beam trawls sweep 40 per cent more bottom, so that the 
increase in catch of shrimp and flatfish should also be 
about that amount; this is in fact corroborated by sta- 
tistical analysis of the catches during one year (see later). 

Advantages of the beam trawl 

(a) The length of warp has much less influence on 
the beam trawl than on the otter trawl ; 

(b) The opening does not change during course altera- 

(c) The influence of tides is much less on a beam trawl 
than on an otter trawl as the opening is fixed 
whereas with otter trawls the resistance and shear 
of the otter boards, is highly affected when fishing 
in tidal areas; 

(d) Shrimp trawling is usually carried out in muddy 
and soft bottoms. This does not affect the opening 
of the beam trawl as the opening is constant and 
the footrope catanary adjusted to an appropriate 
and constant "raking" light in the bosom. Such 
bottoms do affect the shearing effect of the boards 
so that the opening of the trawl varies, with the 
result that the footrope tends to dig in soft bottoms 
when the boards close; 

(e) Double-rig operations allow easier adjustment of 
the gear to the fishing conditions as it is possible to 
conduct comparative fishing at all times because 
two nets are constantly in operation at the same 

The direction of wind and tide has no influence on 
the shooting of the gear. As only the codend is hauled 
aboard, the whole can be mechanized, leading to faster 
operation with less crew. 



One disadvantage of the method is that the stability of the 
vessel can be largely impaired when one of the trawls 
becomes hung-up by a bottom obstruction. The forces 
acting on the boomtop become so great that the vessel 
may capsize. This has in fact occurred on two occasions. 
The forces arising from a hang-up of one net may be 
taken to culminate at a point at the top of the beam 
which is high above and far outside of the centre of 
gravity and tends to heel the vessel over. Investigations 
were carried out to develop a security release system 
which would guarantee stability under all conditions. 
Such a release must: 

(a) Allow the forces acting at the top of the boom to 
be brought to a point lower down on the vessel, 
even when there arc winch or other defects which 
prevent veering out of the warps; 

(b) Transfer the forces to a point which allows the 
vessel to manoeuvre the gear clear of the obstacle. 

(c) Allow the net to be hauled quickly and in a safe 

Fig. 1 1 shows such a security system which can be used 
on any type of double-rig vessel. In this system the warp 
coming from the winch is led through a block "a" 
attached far forward to the stern and runs over a boom- 
block "b" to the gear (Figs. 12 to 16). The boomblock 
is itself attached to a second block "c" by a runner and 
this runner is secured by a sliphook "d". 

When the gear meets an obstruction the sliphook 

is released, which in turn releases boomblock "b", so that 
the towing forces now act on block "a", which ensures 
full stability of the vessel The free net is then hauled over 
its own boom, after which the vessel is manoeuvred in 
the usual manner to free the hung-up gear from this 
obstruction. The gear is hauled hard on to block "a" 
where it is stoppered. Boom-block "b" is then hauled 
back to its position at the boomtip and the gear is further 
hauled in the usual manner. 


The construction of the sliphook is given in Fig. 17. As 
can be seen, the gear consists of a holder B in which the 
hook H can turn freely round its axis E, but blocked by 
the lock F. Lock F itself has an axle which has two flat 
areas (as shown in detail, cross-section AB) and the 
axis carries handle h. The construction of hook H is 
such that the forces acting on it tend to turn it round its 
axis E, which is however prevented by F when the cross- 
section AB locks the hole G at the back end of hook H. 
By turning handle h, axis F releases the hook. 

The axes E and F are constructed of stainless steel 
while the whole mechanization can be released by remov- 
ing two slip pins so that maintenance and greasing are 

Economic results 

During 1962 the Belgian Fisheries Services made a survey 
of the catches of shrimp trawlers. Data were collected 
on the engine power of the vessels, the number of hours 

Fig. 11. Safety release rig-up, (a) forward block, (b) fishing block, (c) safety block,(d) safety hook, (e) safety runner,(f) warp. 

Fig. 12. Warps(f) coming from the winch, running over the forward blocks (a) to the fishing blocks. 
Fig. 13. Arrangement at the top of the boom, (b) fishing block, (c) safety block, (e) safety runner, (/) warp. 

Fig. 14. The release arrangement with old type of sliphook (d) and runner (e). One can easily see the bowblocks (a) and the warps ( /). 
Fig. 15. The safety arrangement. The sliphook (d) was opened and the fishing block (b) came down. The warp (f)is pulling directly on the ship s 

bow (e) safety runner. 

Fig. 16. The safety arrangement. When the net is hauled and stopped, the warps are loosening and the boomblock is hung in the top of the boom 

(h) bridle of the beamnet, ( n wan. 


Fig. 17. Construction details of sliphook. 

at sea for each trip, the number of hours spent in actual 
fishing operations and the total catch per trip. These 
data were analysed and statistically investigated for the 
purpose of ascertaining whether the double-rig fishing 
method was superior to the traditional otter trawl fishing 
method. The data pertaining to the fishing vessels belong- 
ing to the harbours of Ostend and Zeebrugge respectively 
were processed separately, partly because the fishing 
grounds for these vessels differed slightly but mainly 
because the relation of the number of beam and otter 
trawlers used was different. 

Shrimp catch of Ostend vessels 

The shrimp catches were analysed biometrically by two 
approach methods. In the first case, the average catch 
per hour for every week was calculated from the catch 
data for all vessels. In this calculation the time required 
to go to and from the fishing grounds was subtracted 
so that the catch per hour is in relation to actual fishing 
time. The data collected is given in Tables la and Jb. 

The results of the biometrical analysis of these figures 
in respect of both double-rig and otter trawlers is given 
in Table II. As can be seen from this table, the double- 
rig trawlers during 1962 caught, on average, 6-21 kg 
of shrimp per hr, whereas the otter trawlers caught, on 
average, 4-49 kg per hr. The biometrical analysis shows 
that there is a significant difference ( 99 per cent) between 
the average catches calculated, in favour of the double- 
rig vessels, amounting to 1-72 kg per hr which works out 
to 38 per cent more catch. 

Table II. Reimlt of tht variance analyaoe of the 

weekly avera/rea in re] 
both fiehin." methods 1 

ation to oat oh per hour for 



f. Theoretical 


* g 







s ** 





























Total 95 324,2300 
1. Correction term C 


2. 9*tel 3080,5566 - C - 324,2300 

3. Method* . (298,42) 2 + (215,9&) 2 - . 70,7954 




Average ihriap patch/hour for 1962 

1. Average *g/)r (double-riff )i 

2. Average k$/hr(otter trewl)i 

. 4,49 

Difference! 6,21 - 4,49- - 1,72 

This difference ie iifnifioanti r. oal. (59*28 )> ?.th.(719> 


Details of shrimp catches by 
Oat end otter trawlers for 19 

'Table Ib - Details of shrimp cutchoe by OB tend 







Total trawling 
tine per week. 






per week 

































































































































71 1 









3 '*2 











































































































































The second method consisted of using the data con- 
cerning the daily catch of all vessels in the biometrical 
analysis involving 2,868 figures. The reason is that the 
first biometrical analysis depends on the average of 
weekly catches, which reduces the spread of the data 
considerably, because of the nature of these averages, and 
gives rise to the theoretical possibility that a significant 
difference could be found, where in fact there is none. 

This second analysis, carried out over individual catch 




Average hp 
of vessels 

Total trawling 
tine per week 


per week 



















' 4 

























2 - 




































18 . 
































































































































131 ,,92 

' 906 
- 893 





149, 75' 1*066 






























































data, confirmed that there is a significant difference 
(99 per cent) between the catches of the double-rig traw- 
lers and the otter trawlers (Table III). 

In assessing the much larger catch efficiency of the 
double-rig system, trawlers with an average propulsion 
power of 76*2 hp were equipped with about 10 per cent 
more horsepower than the otter trawlers (69-1). On the 
other hand, the catching capacity of a vessel does not 
increase linear with the potential engine power. This 

fJ22TLl^g^^igJ? ttWttggjSi 











2867 18051,9 
1. Oomotlon trm C (14270,14) 2 . 203. 

.6196 . 71-003 

2. Total . 89054,9 - C . 89054,9 - 71. 003 - 18051,9 

3. SubolMMd . 76301,34 - C - 5298,3 

4. With th aubolamd! 18051, 9 - 5298,3 - 12753,6 

5. ** 72516,5-C . 1513,5 

6. Mrthodt 

3,223 for the double-rig system. The opposite occurred 
in Ostend where 603 double-rig catches could be compared 
with 2,265 otter trawl catches. 

Notwithstanding, the collected data was analysed and 
it was found that the double-rig trawlers caught, on 
average, 8-88 kg of shrimp, compared with 5*63 kg per 
hr for the otter trawlers, resulting in a difference of 3*25 
kg per hr, or 57 per cent in favour of the double-rig 
trawlers. These figures are, however, not directly com- 
parable as there exists an appreciable difference in aver- 
age horsepower between the vessels in Zeebrugge. The 
average horsepower of the double-rig vessels was found 
to be 87-7 whereas that of the otter trawlers was 76-2, 
so that the average horsepower of the latter is about 
25 per cent lower than the double-rig trawlers. This 
must, of course, be taken into account and when the 
catch per hour of the double-rig trawlers is reduced for 
this difference in horsepower the comparable catch 

8-88 x 67-2 


108.755.280.8 - C 

From the above it is clear that the catch of the double- 
trawlers is still superior to the otter trawlers by 1*12 kg 
per hr. It must be noted that the comparison is based, 
however, on a linear increase of catch with the horse- 
power of the vessel, which is by no means sure. 

24.473,6 + 48207,1 - C . 1677,7 
7. Interaction 5298,3 - 1677,7 * 1513,5 2107,1 
Avrag> hriaip oatohAour for 1962 

1. Avr*c kg/hr (double rig)i 6,21 kg/hr 

2. Average kg/hr (ottr-trnrl)t 4,49 kg/ 17 
Difference* 6,21 - 4,49 1,72 kg/hr 

ThU difference ! significants F oal. (6,69)>,52) 

was clearly shown in the study in the matter by De 
Poorter (1962). Yet even if we were to accept that the 
catch capacity of a vessel is in direct relation to its horse- 
power, the catch per hour of the double-rig trawlers 
would only be reduced to: 

6-21 x 69-1 
" 76-2 

= 5-63 kg/hr. 

which would still mean a difference in catch per hour 
of 5-63 - 4-49 =1-14 kg/hr, or 25 per cent, in favour of 
the double-rig shrimp beam trawling. 

Vessels working from Zeebrugge 

Here again a survey was conducted for all vessels; 
however, the number of vessels which had converted 
to the double-rig system was much greater and did not 
provide enough figures for otter trawlers to lead to a 
. valied mathematical comparison between the two 
methods. During the whole of 1962, only 291 otter 
trawl catch figures could be collected, as compared with 


The survey conducted has shown that the double-rig 
shrimp trawlers operating from Ostend have a catch 
efficiency that is about 38 per cent higher than that of the 
traditional otter trawlers. Even when reducing the actual 
catches in respect of the difference in horsepower, the 
net higher catches of the double-rig trawlers can be 
expected to be 30 per cent. This 30 per cent increase in 
catch shows a good relation to the theoretical 40 per 
cent increase in horizontal opening of two beam trawls 
as compared with one otter trawl. 

The double-rig method has recently been introduced 
on fish trawlers and is being applied more and more 
to the catch of flatfish. There is not enough data available 
at present to assess whether the increase in catch of 
fish is of the same order as for shrimps. 


Bom, J. on de Visser, T.: Rapport betreffende de visserij met 
boomkorren in Nederland. -Directic van de Visserjen van het 
Ministerie van Landbouw en Visserij. 

de Wit, Ing. J. : Het Kromtrekken van de bomen bij de boom- 
It or re visserij. Vicuws No. 4. 1962. 

de Wit, Ing. J.: Opmerkingen over de veiligheid bij de boom- 
korren visserij. 

De Poorter, H. : Verder beschouwingen over de invlocd van het 
motorvermogen van een vissersvaartuig op zgn rendement. Land- 
bouwtydschrift No. 67 juni-juli 1962. 

Dickson, W.: Trawl Performance A study relating models to 
commercial trawls. Department of Agriculture and Fisheries for 
Scotland. Marine Research Service, No. 1. 1961. 

Verhoest, J.: Een veilghcidsinrichting vopr de bokkenvisaerij. 
Commissie voor toegepast wetenschappeUjk onderzoek in de 
zecvisscrij Publicatic No. 9. 1962. 


Discussion on 
Bottom Trawling 

A.R. Margetts 

Mr. A. R. Margetts (UK) Rapporteur : The bottom trawl 
itself has changed remarkably little over the years and those 
changes that have been made are certainly not commensurate 
with the changes which, in the same period, have occurred in 
trawling vessels. The big development in recent years had 
been in midwater trawling, while more attention has been 
paid to developing from something like a standard bottom 
trawl special trawls for particular types of fish and fisheries. 
Both experience and experiments have shown that two rather 
similar trawlers with not very different trawls could take 
catches of quite different composition as regards both quan- 
tities and species of fish. Since the last Congress there has 
been progress in fitting instruments to gear to discover the 
mechanics of how it works. A common assumption in 
efforts to improve bottom trawls has been that to increase the 
mouth area of the trawl would increase the catch. The 
validity of that assumption is perhaps questionable. 

The approaches are broadly divisible into two groups. 
The first start from the bottom or bottom trawls and work up 
by increasing the headline heights, as instanced by Crewe, 
Dale and Hamuro. The second group start from the truly- 
midwater trawls and work down by adapting them for fishing 
just off the bottom, as described by Sch&rfe and Okonski. 

Dickson outlined the steps taken to find out by instruments 
the dimensions and operating forces in respect of a standard 
bottom trawl. The measurements of the standard Granton 
trawl given, and generally accepted as typical, are that the 
spread of the net (with a 24 m headline) is of the order of 16 m 
with headline height or vertical opening of about 2 m, the 
whole towed at a load of about 7 tons. Hamuro also meas- 
ured existing trawls in his investigations, and to achieve 
similar knowledge the Russians, as described by Treschev, 
had photographed a trawl from a submarine. It would be 
interesting to know more of their technique. 

Hamuro was dissatisfied with the behaviour of otter 
boards and designed improvements. Crewe and Dale and 
Moller also detailed their investigations into the hydro- 
dynamics of otter boards. Hamuro measured the flow of 
water in the trawl and showed how much water undesirably 
spilled out of the front of the net. He then redesigned the 
net to successfully reduce this and improve flow characteristics. 
Crewe outlined how otter boards and warps behaved and 
showed the effect of camber in improving efficiency of the 
boards while requiring a different fishing technique. Instru- 
ments showed even that warps could appear crossed at the 
trawler yet the otter boards be still holding the net somewhat 
open. Crewe studied the netting component of the trawl and 
tackled the problem of netting resistance at the low angles of 
incidence occurring in the trawl; this was relatively very 
different from that of sheets of plane netting at big angles of 
incidence such as had been investigated in most of the tank 


experiments hitherto. There was a striking similarity in the 
development of trawls by all authors in this section; each had 
improved otter boards and two or three used a long leg or 
bridle system allowing floats to give a vertical opening to a big 
net which had side panels of netting to allow it to be opened* 
In the making of these bigger nets, new synthetic materials 
had helped by maintaining the necessary strength with finer 
twines which gave less drag. The opening of Hamuro's new 
net measured 26 m wide by 1 1 m high, an enormous increase 
on the 16 m x 2 m mouth of the Granton trawl. 

On the mechanical side, big advances had been made and 
the engineers now knew enough to develop any particular 
trawl required. On catching, Hamuro, with his 26 m X 1 1 m 
trawl compared with the Granton, claims that his trawl 
(which had been adopted for commercial fishing) has given a 
catch increase of 150-200 per cent but he does not mention 
what fish are caught. If they are mainly bottom species 
it may be that it is only the spread which is mostly responsible. 
Sch&rfe, in his midwater trawling, at first got poor catches, 
but he persisted and was rewarded by success in catching 
herring in a couple of hauls at Iceland. It would be wrong 
to expect any newly developed gear to start catching at 
maximum efficiency straight away. Time must be allowed 
for a new gear to be properly tried and worked with the 
co-operation of skilled fishermen before final judgment is 
passed on it. 

It is abundantly clear from many papers that the link 
between gear development and behaviour of fish in relation to 
that gear is a very tenuous one and one which must be 
strengthened. We still do not properly know how or why a 
trawl catches fish. We know that herding of fish in front of 
the net does occur but we are not sure how it occurs and this 
we must know in order to make more efficient gear. Bottom 
trawling has the advantage over midwater in that there is the 
bottom against which to trap the fish. But there is always the 
problem of how to trawl rough ground. Overfishing and 
experience of fish distribution, drives or draws fishermen 
to rough ground, and there is need for both rough and smooth 
ground trawls. 

Contributions by Kreutzer and Wathne might well be 
important for future development. Earlier it was thought that 
electric fishing in the sea would not be practical. Now, 
however, Kreutzer has shown that pulsed DC can overcome 
a lot of difficulties. He has already, perhaps, eliminated fish 
behaviour as a factor in trawling by electrocuting the fish in 
front of the trawl which then swept them up. His first 
electrified trawling results indicated improvements in catch 
but more thorough testing is needed. Wathne used pulsed 
DC (a different technical method of pulsing) to disturb 
shrimps from the sea bed and make them jump into the path 
of the trawl. 

In his paper Nicholls urged more co-operation in gear 
development by the inclusion of biologists as well as techno- 
logists, engineers and fishermen. A start had been made in 
that direction in Britain by a trawl development programme 
involving the Saunders Roe Division of Westland Aircraft 
Ltd., and in Europe there is an unofficial international group 
working for the development of pelagic fishing methods, 
which consisted of actively interested workers exchanging 
ideas and experiences. 

Mr. Robert Lenier (France): It is obvious that the more 
bottom trawls work the fishing grounds the more will they 
debilitate the bottom of the sea by destroying the natural feed 
and larvae. To avoid that damage by heavy trawls, they 
should lighten the trawling gear and preserve within the 

territorial waters a three-mile limit within which trawling 
should not be permitted and so protect the spawning grounds. 

Mr, Margetts: I must flatly contradict Mr. Lenier that 
trawling badly damages the sea bed and its food value. 
Experiments in the United States, Britain and Denmark were 
convincing that even heavy trawling did not appreciably 
damage the grounds or most of the animals contained in the 
surface of the sea bed. The sea bed was not denuded of food. 
Grounds had been worked for centuries and were still pro- 
ducing fish. Further, most spawning grounds were not inside 
the three-mile limit, but there were nursery grounds in inshore 
waters and some fish could be protected by keeping trawlers 
away from those nursery grounds. 

Mr. J. G. de Wit (Netherlands): Our fishermen are using 
the double-rig beam trawl the same as the Belgians for shrimps 
(described by Verhoest) but they also use it for flat fish. 
This was a very old rig which had come into use in an extensive 
way. Our fishermen used a number of tickler chains in 
front of the footrope up to five and this exerted a tremen- 
dous force first on the shoes and afterwards on the beam, and 
bending of the beam was causing a lot of trouble. The beams 
were up to 1 1 metres in length and it was therefore essential to 
rig the beams so as to minimise the bending movements as 
much as possible. There was a strong tendency to use the 
double rig beam trawl for stern trawling. The small trawlers 
using double-rig beam trawls had only one mast. If this was 
placed aft, the vessel was easily handled, but when working 
in strong currents the mast should be put forward as much as 
possible in order to give greater manoeuvrability, so that in 
the event of the gear becoming fast and the boat swinging 
in cross-currents it would be less in danger of capsizing. 

Mr. J. A. Tvedt (UK): Many skippers use different types 
of gear on the same grounds for the same purpose and each 
swears by his particular choice. Obviously they cannot all 
be right. It was therefore important to try to find an im- 
proved standard gear. As regards the desirable headline 
height and net size, his company had worked out a design for 
a trawl to work a little way off the bottom. They had reduced 
the heavy ground gear so that it will partly roll and partly skim 
the surface without scooping up the bottom. To further 
reduce the churning up of the bottom their trawl boards were 
made more buoyant and operated lightly over the ground 
and were given increased lift. They did not subscribe to the 
theory that sheer net size was the way to get a better catch. 
They had gone the other way and were trying to produce a 
reasonable standard mouth area but they had changed the 
proportions of the mouth and had settled on something like 
twice the Granton headline height and had also reduced the 
twine in the trawl to the minimum size they thought necessary 
to withstand rough usage. This design had been tried in 
laboratory tests and they expected to have practical results 
shortly. They expected to get greater spread with it. 

Mr. D. L. AKerson (USA): Fishermen have to choose 
whether they are going to catch fish on the bottom or above it 
and fishermen in some areas carry two nets: one for flounders 
on the bottom and one for redfish higher above the ground. 
As to raising the trawl a little off the bottom to preclude 
damage, his department had carried out some experiments. 
For their specific purpose they towed three ways: (1) on the 
bottom, (2) half a metre above the bottom, and (3) with a 
box form of trawl just above the bottom, for towing half a 

metre above the bottom they caught only one-eighth of the 
quantity of the fish they caught on the bottom. 

When fishing alongside some Russian vessels in the northern 
area they found that the Russians rigged their gear so that they 
had no weight on the groundrope itself but used a number of 
droppers like sash weights each fastened on a fathom of rope 
so as to just touch the bottom. They then waited till their 
netzsonde showed the redfish had moved off the bottom and 
then began towing and got good catches. When the Ameri- 
cans in that same area put their gear on the bottom they 
got very little fish. By timing their operation the Russians 
did very well. 

Dr. Miyazaki (Japan): A company in our area which 
specialises in shrimp trawling uses two methods. In the first 
method a bottom trawl is raised to midwater and in the second 
the trawl is in midwater from the beginning. In the first 
case, if the presence of shrimp is known, the trawl is raised 
from the bottom while carefully watching the net conditions 
through the netzsonde. The ratio between the depth and the 
warp length should be between 1 to 2-5 and 1 to 2-3 and 
the difference between the upper and lower otter board 
back-strops about 2-50 m. In the second method, with the 
trawl in midwater from the beginning it is generally about 
10 metres above the seabed the ship's speed must be main- 
tained after shooting the net. The netzsonde is again carefully 
used. If the school of shrimp happens to be above the net 
an adjustment should be made by the same method just 
described. When the school is at a lower level, the adjust- 
ment has to be made by slowing down the engines. The 
maximum speed of lifting the net is 12 metres per minute 
and the average about 9 metres per minute. Midwater 
trawling can be carried out within a range of about 40 metres 
by continuous adjustment in the vertical distance. No precise 
formula exists for these operations but the greatest efficiency 
depends on the captain's experience and skill. 

Mr. Eldon Nichols (USA): Breaks in international cables 
due to trawling are 40 to 50 a year in the North Atlantic alone. 
Any one break might interrupt 100 telephone conversations 
or cablegrams and it would take a boat sometimes a week to 
get to the cable to mend it. A new cable had been laid across 
the Atlantic between Britain, Iceland, Greenland and Canada 
for International Air Control. Since last October up till 
May there had been six breaks in that cable because of trawling 
and it was still out of action. (Nevertheless he hoped dele- 
gates at the congress who were flying the Atlantic would have 
happy landings!) In his paper he stressed the desirability 
of improving trawl board design if at all possible so that they 
would not hook and catch the cables. If the otter boards 
could be kept off the bottom it would help, or alternatively 
the points of attachment might be made so that they would 
fend off rather than hook on the cables. 

Mr. Harper Gow: Your company and mine have been in 
contact over the years on this problem. These cables spread 
completely haphazardly over the best fishing grounds in the 
world and the surprising thing really is that so few get caught 
by trawlers as they carry out their operations. 

Mr. Nichols: The number is being reduced by improved 
methods and some cables are being lifted. 

Mr. P. R. Crcwc (UK): The present method of attaching 
the warps to the otter boards is designed to give them some 


sort of stability. There was probably more hope of avoiding 
damage to the cables by taking the oner boards completely 
off the bottom but this might have disadvantages in regard to 
fish behaviour, but it might also have advantages. He had 
never done any work on fish until required by the research 
work for the White Fish Authority, the British Trawlers 
Federation and the British Government to take part in the 
recent investigation. They were not charged with investigat- 
ing fish behaviour but only the engineering aspects of trawl 
boards and nets. The nature of their project nevertheless 
made it necessary to do some general investigation regarding 
the trawls and their mouth area and so on, and he had 
attempted to develop general mathematical theories as to how 
they behaved. These were set out in his paper which gave a 
range of values for mouth areas and the warp lengths. 

Dr. Schiife (Germany): In the Mediterranean the sweep: 
lines attached to the otter boards are 120 fathoms in length 
and are thus so long that they make their trawl something 
between a trawl and a Danish seine. Without the long 
sweeplines their catch would decrease very much. 

Mr. A. Gronningsaeter (Norway): The cable routes should 
be marked by electronic devices in the form of a grid so that 
they can be avoided. 

Mr. Nichols: The United States authorities mark the cables 
on all the official charts that are, now issued. These charts 
will also show the Loran grid and he understood that the 
British Admiralty's charts in preparation would show the 
cables as well. The cable companies had distributed tons of 
Shifting the point of attachment might not provide the right 

charts throughout the world to help trawlers know where 
cables were and avoid them if possible. Some countries 
had given excellent co-operation but in others progress was 
very slow. They could not expect that it would ever be 
100 per cent and that was why they looked, if possible, for 
the fishing industry to do something about fishing gear. 

Mr. Dennis Roberts (UK) agreed with Dr. Scharfe that 
otter boards and sweeplines were essential. It was their 
experience on the Humber that skippers who used longer 
bridles caught more fish. Certainly expenses went up. For 
bottom trawling to be successful they would have to be on the 
bottom and use maximum length of bridles. 

Mr. Margetts summing up said that many of the points 
made related to fish behaviour and they were questions to 
which biologists and technologists believed the answers to be 
important for ensuring future development. On existing 
engineering information, it was possible to design nets to 
definite specifications, but what they did not know were the 
particular values for any parameter as being most suitable for 
catching fish. They required to know much more about fish 
behaviour and he hoped that that theme would be developed 
in other sessions of the Congress. Trawl designers should 
heed the comments about submarine cables, but the most 
helpful immediate measure will probably be the accurate 
charting of cable positions, particularly with fixes by such 
navigational aids as Loran, so that fishermen might keep 
away from them. Mr. Alversorv s point showed that, in some 
fisheries, if the net was raised off the bottom, the fishing would 
become non-paying, and that problem related to fish behaviour 
as well as to fish distribution. 


Part 2 Bulk Fish Catching 

Section 7 Midwater Trawling 

One-Boat Midwater Trawling from Germany 


From 1958/59 the German efforts towards the development of 
midwater trawling for herring were intensified. The present paper 
deals only with the one-boat technique for which special nets, 
otter boards and a net-depth telemeter ("netzsonde") were improved 
step by step. The trials were started with small (24-m length and 
150 to 180 hp) trawlers and gradually extended to the big long- 
distance sterntrawlers with a stern ramp (70 m length, 2,100 hp). 
The objective was to develop a trawl gear suitable for on-the-bottom 
trawling, near-the-bottom trawling as well as real midwater trawling. 
The gear was designed for operation with normal side- or stern- 
trawlers without requiring additional auxiliary gear or special 
training of crews. The trawlnets were made of nylon continuous 
multifilament twine. The first model was a two-seam, high-opening 
design requiring three bridles at each side and with 500 to 800 
meshes (200 mm stretched) circumference. Recently a new four- 
seam model with 1,200 to 1,400 meshes circumference and rec- 
tangular cross-section (side panels narrower and shorter than top 
and bottom panels) gave very satisfactory results. Curved all-steel 
trawl boards (Suberkrlib) proved to be better than combined steel- 
wood construction. Sizes of up to 6 m 2 each have been used, 
depending on the size of the vessel and the gear. The "netzsonde" 
telemeter is an echo sounder connected by cable with a transducer 
mounted on the headline, and gives information regarding the net- 
depth, the opening-height and fish distribution in and below the 
netmouth. Such information is of value for gear studies as well as 
for commercial fishing. The catching efficiency of the one-boat 
midwater trawl was satisfactory for spawning herring, but at first 
rather poor for non-spawning herring in comparison with the two- 
boat trawlers. Improvements to the design and size of the net and 
boards increased the distance between the boards from about 30 
to 50 m up to 65 to 90 m, and resulted in remarkable increase of 
catching efficiency for non-spawning herring. Comparative fishing 
trials indicate that this improved one-boat trawl will catch practi- 
cally the same as two-boat trawls of comparable size. Herring has 
been fished experimentally in the North Sea, the English Channel 
and the Irish Sea, with main emphasis on the Norwegian coast 
(Egersund and north up to about 60"N). In recent trials (February 
1963) promising catches of non-spawning herring were also obtained 
off South Iceland. It is expected that the new one-boat trawling 
technique will become valuable when the conditions for bottom 
trawling are less favourable. Apart from herring, also cod, saithe, 
mackerel, sardines and other clupeids have been caught in quantities 
and it seems likely that other fish stocks could also be fished with 
this gear. 

Chalutage ptlagique a un bateau, en allemagne 


Depuis 1958 les Allemands ont intensifie leurs efforts en vue de 
developper le chalutage pelagique pour les harengs. La presente 
etude ne traite que du chalutage pelagique a 1 bateau, pour lequel 
des filets speciaux, des panneaux et des tglemetres de profondeur 
des filets (Netsonde) ont 6te amdliores progressivement. Les 
essais ont commence avec des petits chalutiers (24 m de long et 
150 a 180 cv) puis ont etc etendus aux grands chalutiers hauturiers 
pechant par Farricre (70 m de long et 2,100 cv) ayant une rampe a 
rarriere. L'objectif etait de deyelopper un engin pouvant opercr 
sur le fond, pres du fond, aussi bien qu'entre deux eaux. Les engins 
ont etc construits pour des chalutiers normaux operant par le cdte 
ou par Tarriere, sans necessity d'installation supplementaire d'acces- 
soires auxiliaires ni entrainement special de l'6quipage. Les filets 
ont etd fabriqucs en fil de nylon continu, multifilaments. Le premier 
modele etait un chalut a grande ouverture, a deux coutures avec 
trois entremises de chaque cote et avait de 500 a 800 mailles de 
circonference (200 mm mailie etirte). Recemment un modele a 
quatre coutures, de 1200 a 1400 mailles de circonfference, de coupe 
rectangulaire (panneaux latc"raux plus etroits que les panneaux 
superieur et inf6rieur) a donne des resultats tres satisfaisants. Des 
panneaux de chalut, courbes (SUberkrUb) construits tout en acier 
se sont reveles superieurs & ceux d'acier et de bois combines. 
Suivant la longueur du bateau et la tailte du filet, des surfaces 


J. Scharfe 

Institut ftir Netz-und Materiai- 
forschung, Hamburg. 

ayant jusqu'a 6 m2 chacune, ont etc utilisees. Le telemetrc 
Netsonde est un echo-sondeur reli par cable electrique & un 
dmetteur mont6 sur la ralingue superieure. 11 donne des informations 
concernant la profondeur du filet, son ouverture verticale et la 
repartition du poisson a 1'interieur et au-dessus de P ouverture du 
filet. Ces informations sont de grande importance pour Petude des 
engins de peche et pour la peche commerciale. En comparaison avec 
celle des chaluts a deux bateaux, 1'efficacite de capture de ce chalut 
pelagique a un bateau 6tait satisfaisante pour le hareng en frai 
mais beaucoup moins pour le hareng ne frayant pas. Des modifica- 
tions apportees au dessin et aux dimensions du filet et des panneaux 
ont augmentd de 30-50 m jusqu'a 65-90 m la distance entre les 
panneaux et ont permis un accroissement remarquable de Peffica- 
cit6 de capture des harengs ne frayant pas. Des essais de peche 
comparative indiqnent que ce chalut a un bateau amelior peut, 
pratiquement, pecher autant que les chaluts a deux bateaux de 
dimensions comparables. La peche au hareng a M conduitc 
experimentalement dans la Mer du Nord, la Manche et la Mer 
d'lrlande mais surtout sur la cdte Norvegicnne (Epersund) et au 
Nord, jusqu'a 60. Au cours d 'essais rccents (fevner 1963) on a 
obtenu, au Sud de PIslande, de bonnes captures de harengs ne 
frayant pas. On suppose que la nouvelle technique de chalutage 
a un bateau deviendra tres int6ressante surtout lorque les conditions 
de chalutage sur le fond seront mouns favorables. Outre les harengs, 
des cabillauds, des monies charbonnieres, des maquereaux, des 
sardines et autres clupeides ont 6t6 pris en quantit6s et il semble que 
d'autres especes de poissons pourraient aussi &tre pechtes par cet 

La pesca al arrastre entre dos aguas con una embarcac!6n en aienunia 


Desde 1958/59 se han intensificado las experiencias alemanas para 
perfeccionar la pesca del arenque con artes de arrastre flotantes. 
Esta ponencia se ocupa solamente de la tecnica en la que se emplea 
una sola embarcaci6n, y para la cual se mejoraron poco a poco 
redes y puertas especiales y un tetemetro que indica la profundidad 
a que esta el arte (netzsonde). Los ensayos se iniciaron con arras- 
treros pequsftos de 24 m de eslora con motores de 150 a 180 hp y 
gradualmcnte se emplearon grandes arrastreros de altura con rampa 
a popa (70 m de eslora y motores de 2,100 hp). La finalidad era 
encontrar un arte que sirviera para pescar en el fondo, cerca de 
6ste y entre dos aguas, y que lo pudieran emplear los arrastreros 
normales que pescan por el costado o por la popa sin tener que 
instalar material auxihar adicional o capacitar especialmente a las 
tripulaciones. Las redes eran de nylon de ftlamentos continues. 
El primer modelo construido consistia en un arte de deux reiingas 
laterales, gran abertura en altura, tres bridas en cada lado y de 500 
a 800 mallas de 200 mm de circunferencia. Recientemente un nuevo 
modelo de cuatro reiingas laterales de 1,200 a 1,400 mallas de 
circunferencia y secci6n rectangular (los paftos laterales son mas 
estrechos y cortos que los superiores e inferiores) dio muy buenos 
resultados. Las puertas de acero cuvado (SUberkrttb) rcsultaron 
ser mejores que las mixtas de acero y madera. Se han empteado 
hasta de 6 m 1 , segun las dimensiones del barco y del arte. El 
tel^metro "netzsonde" es una ecosonda conectada mediante un 


cable a un transductor roontado en la relinga de corchos que da 
information respecto a la profundidad a que estd la red, su abertura 
en altura y distribucidn de los peces ante la boca y debajo de ella. 
Estos datos son de importancia para los estudios de los artes y en 
la pesca industrial. El rendimiento de pesca de un artc flotantc 
arraitrado por una embarcaci6n fue satisfactorio en el caso del 
arenquc en desove, pero al principio, y con respecto a las parejas, 
no lo fue para el arenque que no desova. La mejora de la forma y de 
las dimensiones del artc y de las puertas incremento la distancia 
cntre estas de entre 30 y 50 m hasta 65 y 90 m y produjo un notable 
incremento del rendimiento de pesca de arenque que no desova. 
Los ensayos de pesca comparativa indican que este arte mejorado 
para una sola embarcaci6n pesca practicamente lo mismo que una 
pareja de dimensiones analogas. El arenque se ha pescado experi- 
mentalmente en el mar del None, Canal de la Mancha y mar de 
Irlanda y, sobre todo, en la costa noruega (Egcrsund y hacia el 
none hasta cerca de los 60N). En ensayos recientes (febrero de 
1963) al sur de Islandia tamb&n se obtuvieron captures alentadoras 
de arenque que no desova. Se anticipa que la nueva tecnica de 
arrastre con una cmbarcaci6n adquirirf gran importancia cuando 
las condiciones para la pesca en el fondo scan menos favorables. 
Adcmas de arenque, tambicn se nan pescado importantes cantidadcs 
de bacalao, abadejp, caballa, sardinas y otros clupeidos y es probable 
que tambi6n pudieran capturarse en estos artes otras poblaciones 

A schematic view of the prototype trawl gear developed 
/\ during 1958/59 and thoroughly tested later is 
given in Fig. 1. Details of the otter board and of the 
one-boat midwater trawl net (two panels) are given in 
Figs. 2 and 3. 

This gear, tested in different sizes and with different 
types of trawlers, performed apparently satisfactorily 
from a technical point of view. On several occasions 
during trials, and also on commercial fishing trips it gave 
satisfactory catches of spawning herring in the North 
Sea, the Irish Sea and the English Channel, and of cod 
off the west coast of Greenland. However, extensive 

Meh- Twine 
nix* ntmbcr 

too no /M 

Cutting Mh 

io tie/ 4i 

110 HO/ IB 



40 IIO/BB 

> IIO/BB doublo 

Fig. 3. One-boat midwater trawlnet, two-sideseam type, consisting 
of two equal panels. 

otter boards warps 

Fig. 1. Schematic view of a midwater trawlgear with two-seam net and Netzsonde. 


fishing trials until mid-1962 showed that the catching 
efficiency of this gear for non-spawning herring was not 
adequate for commercial operation and very poor indeed 
in comparison with that of the two-boat midwater trawls 
which at this time were already to some extent in com- 
mercial operation. 

Fig. 2. Dimensional drawing of a Suberkrub type otter board of 
3-2 m z as used in the trials with larger trawlers of up to 1,200 hp. 

Observations and experiences collected during these 
fishing trials indicated that two shortcomings were 
probably mainly responsible for this inferiority of the 
one-boat gear, namely (a) insufficient width of the area 
swept and (b) less suitable design, particularly mesh 
shape, of the two-seam net. To overcome these short- 
comings, the distance between the otter boards had to 
be increased considerably and the trawlnet design had to 
be modified or even a new design had to be developed. 
Interesting details covering these preliminary trials and 
earlier efforts are available to all interested in the original 
paper submitted to the Congress and in the reference 
papers listed at the end of this article. 

Effective modifications 

After a last comparative fishing test in October 1962 
(Table I), drastic modifications were made. 

Table /.- Comparison of herring catches (in baskets of 50 kg each) 
by experimental one-boat midwater trawler Sigofiich with pair- 
trawlers Everting and Dahrendorf : Skate Hole, in October, 1962. 
S&gefisch Everting, Dahrendorf Catch 


baskets per hour baskets per hour fishing day 
Oct. per day towing per day towing Slgefisch 





















Total: 796 







Modification of the trawl gear 

To increase the distance between the otter boards their 
size was almost doubled to 6 m 2 each and the bridles 
were lengthened to 100 to 150 m. For these larger otter 
boards the proportions of the Suberkrub design were 
retained but the construction was strengthened (Fig. 4). 

Fig. 4. Suberkriib otter board of six m 2 size constructed for the 
modified one-boat trawlgear, during shooting from a side trawler. 
The bridles are operated with the trawl winch by means of one mutual 

Since this larger size corresponds closely to that of the 
ordinary flat otter boards of large trawlers, while the 
weight was even less, no operation difficulties were 
encountered. The mode of operation of the boards and 
bridles remained unchanged. For the bridles steel wire 
of 16 to 18 mm diameter was chosen. 

Depending on the type and size of net, the length of 
the bridles, the warplength and the towing speed, these 
modifications almost doubled the distance between the 
otter boards to about 65 to 90 m without unduly stretch- 
ing the net itself sideways. 

In order to improve the catching efficiency of the 
trawlnets not only a modified two-seam net was designed 
but also a new four-seam type, adapted for one-boat 
trawling. (Most of the development work regarding 
nets was done in close co-operation with the net-making 
firm H. Engel, Kiel.) For the practical fishing tests a 


sidctrawler of 1,250 hp was used, which represents in 
type and power a good number of trawlers of the German 
deep-sea trawler fleet (Fig. 5). 

Fig, 5. The German deep-sea trawler Johannes Kriiss on which the 
modified trawlgear was tested. This vessel represents, in regard to type 
and power, a good number of sidetrawlers in the German trawler fleet. 

A good deal of the midwater trawling for herring in 
the North Sea and the North-East Atlantic is conducted 
very near to or even on the bottom, with extremely high- 
opening nets. In order to reduce the risk of tearing the 
net when occasionally fishing on the bottom, the front 
edge of the netting along the footrope was reinforced, 
the footrope itself served with rope and the front part 
of the lower net in some of the nets was made of stronger 

So far the new two-seam type with 1,000 meshes 
circumference, shortened wings and modified hanging 
for more closed mesh shape has not been thoroughly 
tested. It is therefore still in an experimental stage and a 
detailed description here appears premature. 

Of the four-seam type two sizes were made, one with 
1,200 and one with 1,400 meshes circumference (Fig. 6). 
The two main differences of this type, as compared with 
the ordinary four-seam nets of the pairtrawlers, are the 
rectangular (or rather oval) shape of its opening and the 

slightly protruding lower panel. The rectangular shape 
with the side panels narrower and shorter than the upper 
and lower panels was chosen because earlier experience 
had shown that it is difficult with the one-boat rig to open 
sufficiently in vertical direction a net with equal panels. 
This general trawl form, which is well known, e.g., in 
Japan and the U.S.A., is so far rarely found in Northern 
Europe. The forward extension of the lower panel is 
meant to give the net a downward sheering tendency 
to secure good bottom contact if desired. This feature, 
together with the in-built additional extension of the 
lower net by means of special hanging, also makes up 
for the sag of the lower bridles caused by the heavy ground 
weights (370 to 430 kg each). It furthermore allows the 
otter boards to travel above the centre line of the trawl- 
gear and this removes the warps, with their supposedly 
disadvantageous scaring effect, even more out of the 
path of the trawl. 

The modified trawlgears technically performed very 
satisfactorily. The main technical characteristics, as 
were found during the limited number of fishing trials, 
are compiled in Table II. 

Propeller revolutions and speed were taken from the 
instruments installed on board. Net-depth and opening- 
height were measured with the Netzsonde and the 
trawler's echo sounder. The distance between the otter 
boards was deduced from the spread of the warps in the 
sliphook. The distance between the wingtips was 
calculated from the distance between the otter boards 
considering the length of the bridles and of the net bag up 
to the front edge of the tunnel. 

Since so far only a limited number of measurements 
could be taken during two trial trips in December 1962 
and February/March 1963 to the Norwegian coast 
(Egersund-Stavanger) and the Icelandic south coast 
(Skeidardr Deep), the values reported here should not 
be taken as final. They give, however, a clear enough 

Table II 

Gear type 

1 ,400 meshes, 4-seam, bridles 1 50 m, ground- 
weights 470 kg each 

Dittc groundwcights 370 kg each 

1,200 meshes, 4-seam, bridles 1 50 m, ground- 
weights 370 kg each 

Ditto bridles 100m.. 

1,000 meshes, 2-seam, bridles 90 m, ground- 
weights 289 kg each 

Ditto bridles 100m, groundweighto 370 
kg each 




























Net depth 




length fm 



approx. m 

approx. m 





















































































s s ? 






Table III 

Bail/ midwater trawling oatohee in baakete (50 kg eaoh) of 
J Kiflee (1,200 hp) rereue the oatohea of two-boat trawler* of 

the one-boat trawler 
different eiie and power 




Dan eke r 



Bet mold 

and *a 




6 D*o. 

390/30 1 -/ 







7 H 






Stf 2 






a 7-9 

9 " 





WN 5 hUh 


10 " 





mr 7 







12 " 






HW 7-10 

13 H 



o o o^/ 




mr 4-5 


LJ&) . 







S-V 5-7 

15 " 







( 30/-) 

(o o o) 

I 6-7 

16 " 

l*ft out b*oau** J.Kra** was in harbour to tak* salt 

17 " 






ao 7-8 



* X X X 




80 8 

19 " 






SO 2-6 








p*r day 
on th* 







Brarling/Dahrtndorf/ - modazn sidatravlaxv of aiailar ! and povar aa 
Thiala/tf ! anall combination drifter/trawlers of 300 hp aaoh 

Bmakar/Sohul ta , Mindan/Bialafald and Dtmold/3tadthagan - larga oombination drif tar/ 

trawl rn of 600 hp aoh 

tha oatoh 

1'ht figUMa in front of tha daah gir* tha herring oat oh. th f iguraa bahind 
of other ark table 0peoie* 9 mainly aaitbe 
* - - - not proaent on the grounde 

xxx- not fishing beoauee of bad weather 

000* net damage 
5/ oatoh of one tow with partly damaged net not included 


impression of the improvements achieved. As regards 
the size of the opening, the experimental modified one- 
boat trawls are now almost equal even to large two-boat 
trawls (up to 1,600 meshes circumference). The variations 
of shape and size of the net opening due to speed varia- 
tions are within acceptable limits. Also in regard to 
sturdiness the modified gears seem to be satisfactory for 
commercial operation under reasonably favourable 

Catching efficiency 

After some technical trials, which were required for 
determining the proper rig of the modified one-boat gear, 
comparative fishing trials were made versus experimental 
and commercial two-boat trawlers fishing on the same 
grounds. A first series of such tests was conducted in 
December 1962 off the Norwegian coast near Egersund 
where good schools of non-spawning herring were being 
fished by midwater pairtrawlers of several nationalities. 
The results are compiled in Table III. 

When comparing this table with Table T, a very distinct 
change in the relation of the one-boat catches to the two- 
boat catches is quite obvious. While in comparison with 
the former one-boat trawlgear the pairtrawlers caught in 
average about five times more, with the new trawl gear 
the one-boat trawler now caught as much and even more 
than any of the German pairtrawlers fishing at the same 
time on the same grounds. These satisfactory results 
show that the one-boat midwater trawl must not neces- 
sarily be inferior to the two-boat gear even for non- 
spawning herring, and that the modifications made did 
actually aim in the right direction. 

The table shows further that during the trial period 
fishing suffered a good deal from rough weather and 
that, according to expectations, the pairtrawlers were 
more affected by this than the single trawler. This fact 
is of significance for winter fishing in the North Atlantic. 
Pairtrawlers usually stop fishing at wind force 5 to 6 
while a single trawler can carry on until about wind force 
7 to 8. The superiority of single trawlers in this respect 
is shown in the last line of the table, i.e., "Catch per day 
on the grounds" where all days were considered during 
which the vessels fished or could have fished except for 
the weather. 

The results of this first trial period of only 13 days can, 
naturally, not be taken as final. The total catches also 
of the pairtrawlers were not too impressive which must 
be attributed to the poor weather. There are, however, 
no obvious reasons so far why also under more favourable 
conditions the one-boat trawler with the new trawl gear 
should not be able to keep pace with the pairtrawlers as 
well. The largest catch per tow of the one-boat trawler 
was 21 tons, and the best daily catch was 31 tons of 
herring. The saithe caught was of large size and prime 

These promising results encouraged the testing of the 
modified one-boat gear on another non-spawning herring 
stock, i.e., the winter herring off the Icelandic south coast. 
The programme of the next trial trip, again with the 
sidetrawler J. KrUss, included therefore a short trial 

period in this area. Thanks to the generous assistance 
of the Icelandic colleagues and authorities, good herring 
concentrations could be spotted without delay outside 
the national fisheries limits at Skeidarr Deep-Sidugrunn. 
At this time of the year (February) this herring is fished by 
Icelandic purse seiners during night when it is near the 
surface. During the day it migrates downwards and 
stays even near the bottom. 

Because of the very poor weather (the purse seiners 
could not fish) and some technical trouble, /. Kriiss 
could actually fish only for one day, on 24th February, 
1963. On this day the modified one-boat gear, with the 
new large (1,400 mesh circumference), rectangular four- 
seam trawlnet, was used. The first tow in the morning 
yielded nine tons and the second tow at noon and early 
afternoon 23 tons of herring. Due to a misinterpretation 
of the Netzsonde echogram, the third tow was extended 
too long in order to clear first the deck from the earlier 
catch (Fig. 7). Consequently the bulk of the fish, which 

Fig. 7. Large catches are taken on board by splitting into bags of about 
two tons each as usual on sidetrawlers. The photo shows a catch of 
about 23 tons of non-spawning herring taken with the 1,400 mesh, 
four-seam net off the Icelandic coast on 24th February, 1963, in a tow 
of three hours. 

was caught in the very beginning of the tow, had died 
and become very heavy. When hauling this catch, the 
auxiliary lines (heavy nylon rope) parted and it was only 
with the greatest difficulty that the net could be hauled 
until the beginning of the completely filled-up tunnel 


came near the surface. Thus at least a good guess could 
be made, according to which the catch amounted to 
somewhat more than 20 tons of herring. Unfortunately 
the net could not withstand the stress of this heavy catch 
which was held from the mast of the vessel rolling in 
rather high seas. It parted before splitting could be 
started and tunnel and codend with the catch were lost. 

A catch of about 30 tons in two tows, or even per day, 
is considered good herring trawling anywhere. Regarding 
the total catch of this day, together with the 20 tons lost, 
the catching efficiency of the modified one-boat trawl for 
this Icelandic herring must be considered as very satis- 
factory indeed. Although of a preliminary nature, this 
positive result could become of significance for the 
German sea fishery and it is hoped that the fishing indus- 
try will soon start to take advantage of the new possibi- 
lities offered. 

Together with herring some cod and redfish were also 
caught, indicating that the gear may prove to be efficient 
also for these species. 


The preliminary results of the comparative fishing 
trials off the Norwegian coast and of the fishing trial off 
the Icelandic south coast appear to indicate that, with 
the modifications applied, a significant step forward has 
been achieved towards one-boat midwater trawling as 
well as for active non-spawning herring in the North 
Atlantic. Naturally a great deal of further experience will 
be needed for the adaptation of this fishing technique 
to other fish stocks and fishing conditions even in this 
general area. 

For spawning herring, which in the North Sea often 
occurs in very dense schools yielding large and extremely 
heavy catches, a smaller and much stronger trawlnet will 
be required. For cod, saithe and eventually redfish, on 
the other hand, the same trawlnet as for non-spawning 
herring, only with larger meshed aft belly, tunnel and 
codend, will probably be suitable. The operation from 
sterntrawlcrs with sternramp will require the usual 
strengthening of the aft net, tunnel and codend. Parti- 
cularly for large trawlnets, of 1,000 meshes circumference 
and more, the four-seam type appears to be better 
suited than the two-seam type. 

Not only can four-seam nets be made shorter, but they 
are also easier to handle because they need only two 
instead of three bridles at each side. In order to fully 
utilise the large towing power (2,000 hp and more) 
of the big, modern sterntrawlcrs, even bigger nets will 
be required. In view of the limitation of the length and 
also the hauling of heavy catches over the sternramp, 
rather new and unorthodox designs will probably 
have to be developed for the construction of such large 
nets of more than 1,600 meshes circumference. 

Further modifications of the rigging will for instance 
be concerned with trawling on, or rather close over, 
rough bottom. For trawling near the surface, operational 
means like towing in circles will probably have to be 
developed in order to tow the net outside the propeller 
wake which may scatter or disturb the fish schools. 

For rational midwater trawling, navigational aids 
for spot plotting and optimal means for fish detection 
are of particularly great importance. Echo sounding 
and echo ranging are actually indispensable for detecting 
fish schools in advance and also for observing their 
movements until the net arrives. 

With the introduction of other modern techniques 
new means will, of course, be found for further increasing 
the catching efficiency of trawling. This includes the 
possibilities of applying electric fields, optimum relation 
of towing speed to size and design of the net, improved 
release of water through the net to decrease towing 
resistance, attraction and guiding of fish, which are 
being considered already. Completely new ones will 
certainly develop in the future. 

This paper deals only with some phases of a continuing 
development during which, however, results of some 
practical importance have already been achieved. The 
one-boat midwater trawl design reached so far is already 
considered suitable for careful application in commercial 
fishing. Work will be continued to develop this gear 
into a still better combined midwater and bottom 
trawlgear suitable for a wide range of fishing conditions, 
including part of the present conventional bottom trawl- 
ing. This gear will, however, not attempt to substitute 
for the conventional bottom trawl, in particular the 
models for trawling on rough bottom, but rather to make 
trawling more versatile and thus help to improve the 
economy by extending the scope of operations. 


v. Brandt, A. and Steinberg, R.: Schwimmschleppnetzversuche 
im Gebiet von Egersund (1961). Protokolle zur Fischereitechnik, 
Bd. VII, pp. 256-328. 1962. 

Sch&rfe, J. : Fernmeldender Tiefenmesser fUr Schwimmschlepp- 
netze. Fischereiwelt, Bd. VIII, pp. 24-26. 1956. 

Sch&rfe, J.: Das Messen der Tiefenlage von Schwimmschlepp- 
nctzen. Rcferat auf der Diskussionstagung des Fachausschusses 
Akustik im Fachausshuss flir Funk-und Schallortung, Hamburg. 

Sch&rfe, J. : Report on one-boat midwater trawling experiments 
in the North Sea in December 1958 and February/March 1959 
FAO Report 59/11/9452, FAO Fishing Gear Section, Rome. 1959. 

Scharfe, J.: A New Method for "Aimed" One-Boat Trawling in 
Midwater and on the Bottom. Studies and Reviews No. 13, General 
Fisheries Council for the Mediterranean. 1960. 

Scharfe, J. and Steinberg, R.: Neue Erfahrungen mit Schwimm- 
schleppnetzen. Protokolle zur Fischereitechnik, Bd. VIII, Heft 37. 

Steinberg, R.: Einschiff-Schwimmschleppnetz Versuche mit 
eincm Logger vom 17.11. 3.12.1959 in der Irischen See und in 
Kanal. Protokolle zur Fischereitechnik, Bd. VI, pp. 235-253. 1960. 

SUberkrUb, F.: Otter Boards for Pelagic Trawling. Int. Fishing 
Gear Congress, 1957 Paper No. 71 and "Modern Fishing Gear of 
the WorlcT, 1959, pp. 359-360. 1957. 


Universal One-Boat Midwater and Bottom Trawl 


During recent years experiments were carried out in Poland to 
develop a universal one-boat midwatcr and bottom trawl for the 
sprat and herring fisheries. The experiments were carried out with 
the 1,350 hp deep-sea trawler Miedwie, using a four-seam trawl 
of 680 mesh circumference, 220 mm mesh size in the rounding, 
with 3,050x1,340 mm flat trawlboards. The paper gives full 
details of the gear and the instruments used, as well as the specific 
results of the experiments which cover the fishing depth of the 
trawl in relation to warplength and towing speed, its resistance and 
horizontal and vertical openings. The graphs allow clear inter- 
pretation of the results. During the experiments it was found that 
by increasing the engine revolutions, the trawl could be lifted safely 
to pass over bottom obstructions seen on the depth recorder. 

Chalut a un bateau pour la pfehe p^lagique et sur le fond] 


Depuis quelques annees des expeiimences ont 6t conduites en 
Pologne, dans le but de d6vclopper un chalut & un bateau pour la 
ptehe pdlagiquc et sur le fond, destin6 a la pfcche du hareng et de 
resprot. Ces experiences ont ite effectuees avec le chalutier hautu- 
rier Miedwie de 1,350 cv, utilisant un chalut & quatre coutures, 
de 680 mailles de circonference, en mailles de 220 mm, avec des 
panneaux ordinaires mesurant 3, 050 x 1,340 mm. La communica- 
tion donne les details de la construction des engins et instruments 
de mesure employes aussi bien que les resultats obtenus et traite de 
la relation entre la longueur des funes et la vitesse de touage d'une 
part, la profondeur d'op&ation, la resistance et Touverture hori- 
zontale et yerticale d'autre part. Les graphiques permettent 
rinterpr&ation claire des rtsultats obtenus. Au cours de ces expSri- 
ences on a remarque qu'en accelerant la vitesse des machines, on 
pouvait facilement lever le chalut au-dessus des obstacles observ6s 
sur Fecho sondeur. 

Embarcacifa universal para la pesca con artes pelagicos y de fondo 


En los ultimos afios se ban realizado experimentos en Polonia para 
encontrar una embarcaci6n que pueda emplearse indistintamente 
para pescar espadin y arenque en el fondo y entre dos aguas. Los 
experimentos se realizaron con el arrastrero de altura Miedwie, de 
1,350 hp, empleando un arte de cuatro costuras, de 680 mallas de 
circunferencia, mallas de 220 mm, y pucrtas planas de 3,050 x 
3,340 mm. El autor da detalles completos del material y de los 
instrumentos empleados, asi como de los resultados especfficos 
de los experimentos, que comprenden la profundidad de pesca del 
arte con relaci6n a la longitud del cable y velocidad de remolque, 
su resistencia y las aberturas horizontal y vertical. Las graficas 
permiten interpretar claramente los resultados. Se observb durante 
los experimentos que aumentando las revoluciones del motor el 
arte se levantaba y pasaba por encima de obst&culos del fondo 
indkados en el registrador de la profundidad. 

POLAND'S first experiments of midwatcr trawls were 
conducted on the sprat schools in the Baltic Sea in 
1959, during which two-boat mid water trawls were used. 
It was then observed that the swimming speed of sprat 
allowed them to be caught by one-boat midwater trawls 
operated by cutters. So trials were made with one-boat 
trawls of synthetic fibres equipped with the ordinary flat 
otter boards to operate in midwater. 

These proving successful the experiments were extended 
to the bigger vessels so that by 1960 the research vessel 
Birkut (33 m in length) caught 250 barrels (bbl) of 
herring during midwater trawling on Faren Deep grounds 
in the North Sea. Subsequently, plans were made for 
conducting such a fishery with a supertrawler. The traw- 
ler used was the Miedwie, 1,350 hp, and all measuring 
equipment was prepared by the technologists of the 


S. Okonski 

Sea Fisheries Institute, Gdynia. 

Sea Fisheries Institute and commercial fisheries com- 
panies. The objectives were to investigate the behaviour 
of the nets and the possibilities of applying ordinary 
flat trawlboards to midwater operations, as well .as 
to bottom fishing, with this type of vessel. 

Although other types of boards have better hydro- 
dynamical characteristics, the flat rectangular trawlboards 
were used on purpose to avoid difficulties in introducing 
the new method in commercial fisheries. It is well known 
that fishermen are often unwilling to accept radical 
changes, whereas a not too different rig can be readily 
accepted. The gear was fully designed, constructed and 
assembled ashore to avoid any major work or changes 
on board ship. However, some small problems were 
encountered during the experimental trip, but they 
were solved by the technologists. 

The gear 

Full specifications of the net are provided in Fig. 1. 
All mesh sizes are given for stretched mesh and lengths 
in metres. The wings and front part of the body are made 
of polyamide (nylon type fibre) double twine of 1*8 mm 
diam, while the rest of the body is constructed of single 
twine, same diameter. The whole net is made up of four 
identical panels, one of which is seen in Fig. 1. Bosom 
corners of each panel have a wedge piece to even out 

The headline has 55 spherical aluminium floats of 
200 mm diam attached at every 40 cm in the bosom, 
every 50 cm at the quarters and every 60 cm in the 
wings. The footrope construction was very light, with 
a total of 30 kg chain. The chain was divided into short 
lengths; six pieces were attached to the bosom of the 
footrope, two of them at the extremities of the quarter 
wedges and one piece in the middle of the wings. At 
the tip of each bottom wing, a weight of 50 to 60 kg 
was attached by 60 cm long strops (Fig. 2). 

For pulling up the codend, the net was equipped with 
two porklincs, one attached to the codend bccket while 
the second was attached to the middle of the body. 

Both lines were of synthetic fibre of 14 mm diam. 
The net had two quarter ropes of 55 m length attached 
at the belly quarters and led through the upper quarters. 

The net was operated with upper and lower legs which 



12 44 12 







22 01 doubl. | 

r i.eoi 


- Wl 






14 01 



* 10 mm I 



8 01 / 


... / 




9 i 

4 01 

'4 i 

F/. 7. De/a// of trawl constructiorTjipnc panel). 

differed in length by 2-5 m, the upper legs being 82-5 m 
each and the lower legs 80 m. All legs were made of 
12 mm diam steel wire, although in some cases the 
lower legs were replaced by thicker combination rope 
of the type used for the bridles. 

The otter boards 

The boards were of the normal flat oblong type of 3,050 
mm X 1,340 mm, weighing approximately 900 kg each, and 
were specially fitted for midwater trawling by means 
of two sets of bridles instead of the usual single bridles 

at the back of the board (Fig. 3). The angle of attack of 
the boards had been calculated for bottom trawling and 
the length of the strops differed to ensure that the boards 
took up a stable position during operations in midwater, 
so that the upper strops were 2-1 m and 3*6 m, while the 
lower strops were 2*25 m and 3*75 m, attached as shown 
in Fig. 3. 

The experiments 

The measuring instruments used during the experiments 

71*. 4* Baadlim d*pth at rarlou* 
- " i (190 r.p..) 

fi. 4b Bmdlin* tepth t rariou* projllr p**d 
(at 1^0 Unffta of vacpa) 

Fig. 2. General arrangement of" Universal" trawl. Symbolic haulinfjrawler indicated on top. 



lower leg 

t* legs 




Fig. 3. Details of trawlboard rig. 

comprised : 

(a) A sal log. 

(b) Two dynamometers with a range of 0-15 tons for 
measuring warp tension. 

(c) A bathygraph with a range of 0-120 m for measur- 
ing operational depth of the trawl. 

(d) Differential bathygraph with a range of 0-20 m 
for measuring the vertical opening of the net. 

(e) Wind gauge. 

(f) A tachometer for measuring the propeller speed. 

(g) An anglemeter for measuring the angle of the 
warps at stern. 

During the experimental hauls, measurements were 
made to ascertain the shape of the trawl in action whilst 
towing at different speeds and with different lengths of 
warp. The data obtained during these trials were worked 
out to averages, which are presented in the graphs and 
illustrations given in Figs. 4 to 11. 

Fig. 4a shows the graph of the headline depth obtained 
during the trials in which different warp lengths were used 
at constant engine revolutions. In Fig. 5a this information 
is plotted as a curve and, as can be seen, the relation 
between the warplcngth and the trawl depth is not linear 
at constant speed. 

Fig. 4b shows the graph of the headline depth obtained 
when changing the towing speed while keeping the warp- 
length constant. The values are plotted in Fig. 5b and 
the curve shows that the change in headline depth, due 
to change of speed, is not constant and is greater at 
low towing speed than at high towing speed. 

Figs. 5a and b (on tow) (a) Relation of headline depth to warplength at con- 
stant speed; (b) Relation of headline depth to towing speed (150 m warplength). 

Figs. 6a and b (a) Relation of warp tension to towing speed for 150 m 
length; (b) Relation of warp tension to warptength at constant speed. 

Drawl resistance ID relation to towing speed and warp 

The warp tensions obtained during several hauls are 
shown in Fig. 6. The total tension at five knots was 
about 10 tons. Whereas the load to speed ratio increases 
rapidly at higher towing speeds, the increase in load as 
more warp is veered out at constant speed is almost 
linear. The amount of warp to be veered out to allow 
the gear to operate one metre deeper differs according to 
the length of warp already veered or, respectively, 
the depth at which the gear is operating. It was found 
that, at constant engine r.p.m., to allow the net to sink 
one metre deeper it required: 

7*8 m more warp when fishing with 100 m warp out 

6-5 m 150m 

5-9 m 200m 

5-0 m 250m 

4-3 m 300 m 

The above values are, of course, only valid for the 
gear used, so that even a change in the rig-up of the 
trawl will result in a different ratio. 

As shown in Figs. 7 and 8, the distance between the 
otter boards depends on the speed of towing and the 
length of warps. Legs of 52 m, 80 m, and 95 m were 
tested and, within this range, the length of legs did not 
affect much the distance between the boards. On the 

Ftgi. 7a and b (top) (a) Relation of horizontal opening to towing speed (150 m 

warplength); (b) Relation of horizontal opening to warplength (constant towing 


tf ! 

other hand, the leg length has great influence on the 
horizontal opening of the net. Longer legs give smaller 
horizontal net opening. From the purely technological 
point of view (leaving out biological considerations), 
60 m was found to be the optimum length of legs for this 
type of midwater trawl. 

Headline height 

The headline height of the trawls was measured by differ- 
ential bathygraphs constructed by S. Okonski and J. 
Zaucha. A typical graph from this instrument is shown 
in Fig. 9, one of which gives the headline height variation 

Ftgi. it and b Co) Effect of leg length on horizontal opening In relation to warp- 
length; (b) Average horizontal opening in relation to warplength. 


Fit . ?b BMdliM toifht in rUtioa to 
varplofth at ooMtut r.p.B. 

in relation to towing speed while the other shows the 
headline variation in relation to warplength. The infor- 
mation contained in the graphs has been plotted as a 
curve in Fig. 10, which clearly shows that the towing 
speed has much greater effect on the headline height 
than the warplength. At 3 knots the opening-height 
was 14-4 m but at 5 knots it was 1 1 -3 m. 

Taking into account the very light rigging and the 
overall dimensions of the trawl used, the average head- 
line height of 12 m at a speed of four knots would appear 
to be very satisfactory. This headline height could, 
however, be increased by a heavier rigging and the use of 
hydrodynamical lifting power. Furthermore, a bigger 


Figs. lOa and b (top) (a) Headline height in relation to warplength at constant 
r.p.m.; (b) Headline height in relation to towing speed at 150 m warplength. 

Fig. 11 The shape of the trawl during towing. 

net could be used as the gear was small for the engine 
power of the vessel. 

Fig. 12a. Shooting. Arrangement of leg strops on otter board. 

Area of the trawlmouth 

Using the above data on headline height and horizontal 
opening, the catanary of the headline and the footrope 
can be calculated and the area of the netmouth is then 
obtained for different conditions of towing speed and 

Keeping the engine at constant revolutions, the area 
of the netmouth was found to change as follows for 
the different warplengths: 

Warplength Area at wingtips Area at bosom 

m m 2 m 2 

100 140 117 

150 150 132 

200 169 141 

250 176 145 

300 178 147 

When the warplength is kept constant with an increas- 
ing towing speed, the area of the trawlmouth was found 
to be maximum at 185 r.p.m. 



Area at wingtips Area at bosom 

m 38 

m 58 

Fig. 1 1 shows the general configuration of the trawl 
in action. The experiments generally conform that the 
pelagic trawl is a delicately balanced fishing gear which 
required close co-ordination of the many factors in- 
fluencing its shape and operation. 

Fig. lib. Releasing the quarter rope during hauling (note sinkers on 
lower leg). 


Operetta techniques 

The operation of the "Universal" trawl is similar to 
that of the ordinary bottom trawls. In hauling and shoot- 
ing the main difference lies in the absence of bridles 
and danlenos and, on the other hand, the use of long 
legs which, being connected through Kelly eyes and 
pennants, are led straight onto the winch drums. 

The gear can be shot while the vessel is stopped or 
steaming round, in the same manner as the ordinary 
bottom gear. The wire of the upper and lower legs should 
preferably be of right and left twist respectively, to avoid 
their twisting together during the hauling. In the rig 
used, the lower leg was stoppered through the Kelly 
eye, whereas the upper leg was connected to the same 
stopper outside the Kelly eye (Figs. 3 and 12 a). The 
addition of two leg-strops at the back of the board 
improved the stability of their performance, especially 
during poor weather. For operation on the shallower 
Sandettie grounds in the English Channel, the legs were 
shortened to 52 m. 

Fishing results 

The 1962 herring season on English Channel grounds was 
very poor and this gave an opportunity to investigate 
the comportment of the gear to other than midwater 
towing conditions. The purpose was to assess the possi- 

bility of operating on the bottom as well as near the 
surface. Three days were devoted to this work and during 
some of the tows, though there had been no trace on the 
echo sounder, up to 10 bbl of herring were caught. 

The distance of the footrope from the bottom could be 
regulated very accurately. With the footrope chain 
arranged as previously described, the footrope functioned 
at 80 cm above the bottom; when 10 kg of chain was 
added to the wings this distance was reduced to 30 cm; 
with 10 kg more chain in the bosom the net touched 

During these experiments the possibilities of lifting 
the gear over obstacles was also investigated and it was 
found that this could be done easily by appropriate 
increase of the engine revolutions. Fig. 13 gives the echo- 
recording of one occasion when the echo sounder showed 
10 to 12 m rocks straight ahead. The dotted lines show 
the track followed by the gear when the engine speed was 
inciyased. About 1*8 min after increasing the speed, the 
net had risen four m; 3-2 min later it had risen llm 
and after a further 3-7 min it was 18 m above the bottom, 
passing clear of the rocks. 

The experiments have shown that this gear can be 
operated easily at any depth. There remain, however, 
some small problems, one of which is the provision of 
cheap and efficient net telemeters for commercial fishing 

Fig. 13. Echogram of the gear being lifted over obstacles. 


Two-Boat Midwater Trawling for Herring 
with Bigger Boats 


In 1961 and 1962, the German trials with two-boat midwater trawl- 
ing for herring in the North Sea were extended to larger vessels 
of the lugger type (300 tons, 40 m long, 450 to 600 hp) and big 
trawlers (400 to 650 tons, 43 m to 53 m long, 600 to 1,250 hp). 
The experiments have shown that the German luggers can adopt 
two-boat midwater trawling for herring on a commercial scale 
without serious technical difficulties. Since good manoeuvrability 
is essential when the boats have to come closer together during 
shooting and hauling, controllable pitch propeller, diesel-elcctric 
drive and/or active rudder are the more desirable the larger the 
vessels are. Even big long-distance side-trawlers can operate a 
two-boat midwater trawl if good manoeuverability is secured by 
one of the above means. The operation costs of such large vessels 
are, however, so high that two-boat trawling would only be profit- 
able under exceedingly good fishing conditions. The German 
combined luggers can operate at moderate costs. Consequently, 
they are adopting two-boat midwater trawling for herring at an 
increasing rate and the economic results have so far been highly 
satisfactory. In the beginning the larger pair of trawlers used four 
seam trawlnets of a similar type as are common for smaller vessels 
in northern Europe, but of appropriate size, i.e., 1,400 to 1,600 
meshes (mesh size 200 mm stretched) circumference in the mouth 
giving an opening height of 14 to 20 m. The trend now is, however, 
to adopt a trawl-type with "rectangular" opening and modified 

Chalutage pelagique a deux bateaux pour grands chalutiers 


En 1961 et 1962 les essais allemands pour developper 1e chalutage 
pelagique a deux bateaux pour le hareng, dans la Mer du Nord, ont 
et& etendus a des bateaux plus grands du type lougre, de 300 tonnes, 
40 metres de long et de 450 a 600 cv et aussi a des chalutiers 
hauturiers (de 400 & 650 tonnes, 43 a 53 metres de long, 600 a 
1,250 cv). Les experiences ont montre que les lougres allemands 
s'adaptaient au chalutage pelagique a deux bateaux, a une &chelle 
commerciale, sans difficultes techniques serieuses. Une helice a 
pas variable, un entralnement electrique Diesel, un gouvernail actif 
sont des facteurs desirables pour les grands chalutiers car une 
grande manoeuvrability est essentielle surtout quand les bateaux 
se rapprochent pour echanger les lignes. M6me les bateaux de 
peche lointaine operant par le cote peuvent utiliser les chaluts 
pelagiques a deux bateaux s'ils obtiennent une bonne manoeuvrabi- 
lite par les moyens decrits ci-dessus. Cependant le cout d'operation 
de ces grands bateaux est si &leve que le chalutage a deux bateaux ne 
peut dtre profitable que sous des conditions extremement favorables, 
tandis que les frais des operations des lougres allemands sont plus 
moderes. C'est pourquoi les lougres allemands ont adopte rapide- 
ment le chalutage a deux bateaux et les resultats ont 6t tres 
satisfaisants. Au premier, les grands chalutiers "pareja" utilisaient 
des chaluts a quatre coutures du meme type que ceux utilises par 
les petits chalutiers dans le Nord de PEurope, mais d'une grandeur 
appropriee, c'est & dire de 1,400 a 1,600 mailles (de 200 mm, ma i lie 
ttiree) de circonference & 1'ouverture ce qui donne une puverture 
verticale de 14 a 20m. Neanmoins la tendance aujourd'hui est pour 
un chalut avec une ouverture rectangulaire et de montage modified 

Empieo de parejas mayores para la pesca al arrastre pdagica del 

En 1961 y 1962 los ensayos practicados por los alemanes con artes 
de arrastre pelagicos remolcados por las embarcaciones que pescan 
arenque en el Mar del Norte, se ampliaron de manera que compren- 
diesen otras mayores del tipo llamado "lugger" (300 ton., 40 m de 
eslora, motores de 450 a 600 hp) y arrastreros grandes (de 400 
a 650 ton, 43 a 53 m de eslora y motores de 600 a 1,250 hp). Los 
experimentos ban demostrado que, sin graves dificultades tecnicas, 
las parejas de "lugger" alemanes pueden pescar arenque con artes 
pelagicos en escala industrial. Como la maniobrabilidad es esencial 
cuando los barcos tienen que acercarse para largar y virar la red, 
cuanto mas grandes son mas conviene el empleo de propulsores de 
paso variable, transmisiones diesel elcctricas y timones activos, o 


Rolf Steinberg 

Institut fur Netz- und Materialforschung, 

uno de los dos ultimos. Incluso los arrastreros de altura que pcscan 
por el costado pueden emplear el arte pelagico remolcadp en pareja 
si se les da buena maniobrabilidad por uno de los medios citados. 
Sin embargo, la explotacidn de barcos tan grandes es tan costosa 
que la pesca en pareja s61o es provechosa en condiciones excepcional- 
mente buenas. La explotaci6n de los "lugger" combinados atemanes 
es rclativamentc modesta, por lo que cada vez hay mas parejas de 
ellos dedicados a la pesca del arenque con artes pelagicos, habiendo 
obtenido hasta ahora resultados muy satisfactorios. Originalmente 
las parejas mayores empleaban artes de cuatro relingas de un tipo 
parecido a acquellos utilizados para las mas pequenas buques 
del norte de Europa que pescan con artes, pero de una dimension 
conveniente, as decir de 1,400 a 1,600 mallas (de 200 mm cuando 
estan estiradas) de circunferencia en la abertura, lo que de una 
abertura vertical de 14 a 20 metros. Hoy dia, pero, la tendencia es 
de emplear las artes con una abertura rectangular y aparejo modi- 

AN extensive and very successful herring fishery with 
two-boat midwater trawls has developed in the 
North Sea. Until a few years ago only smaller boats 
cutters of 60 to 100 tons were engaged in this fishery. 
These cutters are easily manageable and have relatively 
powerful engines (up to 600 hp). 

The good commercial results of the cutters suggested 
that bigger vessels would also be suitable, so two-boat 
midwater trawling trials were carried out by the Institut 
fur Netz- und Materialforschung of the Federal Republic 
of Germany. The main problem in two-boat trawling 
is the passing of lines from one boat to the other during 
shooting and hauling. Tt was found that the suitability 
of vessels for two-boat trawling depends more on manoeu- 
vrability than on size. Accordingly boats fitted with 
controllable pitch propeller or diesel-electric drive 
proved best suited because propeller performance, 
including reversing, can be controlled quickly and 
efficiently from the wheelhouse. Boats wijh an active 
rudder are also relatively easy to manoeuvre and are 
consequently suitable too. Big trawlers without such 
equipment are less suitable for two-boat trawling. 

Four-seam trawlnets are mostly used in two-boat mid- 
water trawling. The German luggers (600 hp) now use nets 
of 1,400 meshes circumference at the mouth (mesh size 
200 mm stretched Figs. 1 and 3). For the first trials with 
large German trawlers (of about 1,000 hp) a trawlnet 
of 1,600 meshes in circumference (mesh size also 200 mm 
stretched) was designed and gave very good results 
(Fig. 2). The opening height of the lugger nets ranges 
between 14 and 18 m, and that of the trawler net between 
16 and 20 m. 

Because of the very big catches taken occasionally, 


mx 99 m 

Jff XM M 





1 f 4* 

1 P O 

1 P 4 * " 

1 P 2 b 00 

1 P 2 b *0 

1 p 1 b W 

2 P 1 b 1 

Jf0 JT 41 IK 

atO 4 4* 

Fig. L Structure of one panel of a four-seam two-boat midwater 

trawlnet for luggers (about 600 hp each). The net consists of four 

such panels. 

11 4* 


** / 4M 

Fig. 2. Structure of one panel of a four-seam two-boat midwater 
trawlnet for larger side trawlers (about 1,000 hp each). The net 
consists of four such panels. 

50-90(00-90) float* L) 

O ($5) m combined ropt 15(30) kg 

4 The rising of tho German two-ton t nidwater tr^iwl gear Tor 
luggero and trawler (numbers in brockets). 

f-Cf m 

Schematic view of the Oermnn two-boat midwater trawl 
gear for lugger a, net with "rectangular" opening. 


In , 

H- i 



A fi 

; i H 




8 S 



i if 

s as 



S 3 





t ft 



* 9 * 




net material of high breaking strength must be used, 
particularly since the netting twine should be as thin as 
possible in order to provide optimum water flow through 
the netting and minimum frightening wake in front of 
and in the mouth of the trawl Nylon or 'Perlon* 
(polyamide 6) continuous multifilament twines were 
mainly used. During the extensive herring fishing trials 
in the North Sea the twine strengths given in Figs. 1, 2 
and 3 were found adequate. Finer netting is not recom- 
mended because it does not withstand the stress of bad 
weather and/or large catches. Sufficient elasticity of the 
net material is of great importance for absorbing shock 
loads and unavoidable distortions of the net. 

Arrangement of the legs and weights used in the first 
trials is shown in Fig. 4. Of the figures given, the first 
one refers to the lugger gear and the second one, in 
brackets, to the trawler gear. According to their size 
the trawlers use longer legs than luggers do. In this way 
the handing over of the legs from one boat to the other 
during shooting and hauling is simplified. The 25 mm 
wire between the upper wingtips is very important be- 
cause it prevents damage to the trawlnet in case the dis- 
tance between the two boats becomes too wide during 
shooting and hauling. For modified rigging of the two- 
boat midwater trawl with rectangular opening see Fig. 5. 
Here the big weights are fastened near the lower wingtips 
rather than at the juncture of the lower wings and legs. 

The warps normally used by these vessels in bottom 
trawling are considered too heavy for midwater trawling, 
causing a small opening-height of the net mouth. There- 
fore the German luggers and trawlers are using warps 
of 16 mm diameter for midwater trawling instead of the 
normal 19 to 24 mm warps. 

As a net depth telemeter, all German luggers and traw- 

Fig. 6. Method of shooting the net from the "netzsonde" boat. 

Fig. 7. Method of hauling the net on the "netzsonde" boar. 

lers use the so-called "netzsonde" so that the depth of 
the net is recorded continuously during fishing, as well 
as the opening-height of the net and the fish passing 
through the net mouth, etc. Moreover, it indicates the 
behaviour and distribution of those fish so that valuable 
conclusions can be drawn for the most efficient tactics 
of operation. (Mohr this Congress.) With the "netzsonde" 
much bigger catches are obtained on average. 

In the two-boat trawling normally only one boat is 
equipped with a complete "netzsonde" unit. The other 
has only a separate headline transducer with about 
120 m of cable attached to her trawlnet. If the net of 
this boat is shot, this cable is connected with the main 
cable and the electronic unit on board of the "netz- 
sonde" boat during shooting (Fig. 6). In this way the 
one "netzsonde" echo recorder unit can be used with 
the nets of either partner at will. 

In two*boat midwater trawling two methods are used 
for shooting and hauling (Figs. 6-9). When the 
"netzsonde" boat is shooting (and hauling), the method 
1 shown in Figs. 6-7 is applied. When the partner boat 
is shooting her net, method 2 (Figs. 8-9) is used. In 

Fig. 8. Method of shooting the net from the partner boat. The cable 

of the headline transducer is connected with the main cable on the 

"netzsonde" boat by means of a special plug. 

this way the "netzsonde" boat is always on the same 
(port) side of the partner boat when towing. This arrange- 
ment is necessary because roller and winch equipment 
for handling the cable is directed to the starboard side 
of the "netzsonde" boat. 

The depth of the trawl is regulated by variations of 
towing speed and/or the length of warps. This is the 
same for one-boat and two-boat trawling. To change 
the net depth within about 10 to 15 m, it is normally 
sufficient to alter only the speed. If greater changes of 
net depth are needed then the warp length has to be 

In two-boat trawling it is difficult to change course 
quickly. When fishing deep with long warps, about 45 
min would be needed to turn to the opposite course. 
Such turns can be achieved much faster by hauling in 
the warps up to 50 fm, turning the boats round in a 

small circle and veering the warps again. In this way only 
about 15 min are needed. 

German luggers are taking up commercial two-boat 
midwater trawling for herring in the North Sea at an 
increasing rate. The economic results have so far been 
highly satisfactory. However, it is still an open question 
whether a similar development will materialise in the 
case of the larger side trawlers. For economic reasons, 
and because only a few of these vessels have the required 
manoeuvrability it seems likely that if they should take 
up commercial midwater trawling at all, they may use 
the one-boat technique. 


Fig. ?. Method of hauling the net on the partner boat. 


Development of the Cobb Pelagic Trawl 
A Progress Report 


A large single boat pelagic trawl utilizing hydrofoil otter boards is 
under development by the Bureau of Commercial Fisheries 
Exploratory Fishing and Gear Research Base in Seattle, Washington. 
The general configuration of the huge gear is based on the theory 
that a large net travelling at relatively low speed through midwater 
or on the surface (Fig. 1) would be more effective in capturing large, 
active fish than a small net towed at high speed. Construction 
details have primarily resulted from direct observation of experi- 
mental nets in action by SCUBA-equipped staff members. These 
observations have allowed a series of modifications to be performed, 
resulting in attainment of a favourable ratio of net size to horse- 
power requirements. A mouth opening of approximately 7,200 
sq. ft. and a towing speed of 2.5 kn. using 350 hp. has been accom- 

Actuellcmcnt, au Centre de la Ptehe Explorative et de la Recherche 
des Engins de Ptehe est dvellopp6, par le Bureau de P&che Com- 
mcrciale des Etats-Unis, & Seattle (Washington), un grand chalut 
ptlagique & un seul navire, 6quip6 de plateaux hydrodynamiques. 
La configuration gn6rale de cct engin immense est fonctee sur 
Thypothese qu'un grand chalut remorqud, soil entre deux eaux, 
soit pres de la surface (Fig. 1), & une vitesse relativement faible, 
serait plus apte & capturer les grands poissons tres actifs qu*un petit 
chalut remorqu6 & une grande vitesse. Les details de sa construction 
r&ultent principalement d'observations sous-marines de 1'engin en 
fonction effectives par des employed du Centre, 6quip6s de scaphan- 
dres autonomes. 

Ces observations ont pcrmis d'apporter toute une s6rie de 
modifications qui ont abouti & une relation plus favourable entre 
les dimensions du chalut et la puissance du moteur. C'est ainsi 
que Ton a augment^ I'ouverture du chalut jusqu'& pres de 700 
metres carres pour une vitesse de touage de 2.5 noeuds et une 
force motricc de 350 c.v. 


Un arte de grandes dimensiones para la pesca al arrastre pel&gica, 
dotado de puertas hidrodinimicas y para ser rcmolcado por una 
sola embarcaci6n se construye en la Base de Investigations de 
Material de Pesca y de Pesca Exploratoria de la Oficina de Pesca 
Industrial, Seattle, Washington. La forma general del cnormc arte 
se basa en la teoria de que una red grande que se desplace a poca 
velocided relativa entre dos aguas o en la superficie (Fig. 1) pesca 
mas peces grandes, activos, que una pequefta remolcada a mucha 
velocidad. Los detalles de la construcci6n se deben principalmentc 
a observacioncs directas de artes experimentales por invcstigadorcs 
equipados con escafandras aut6nomas. Las observaciones han 
permitido realizar una serie de modificaciones que han dado por 
resultado la obteni6n de una relacidn favorable de dimensiones 
de la red a necesidades de fuerza motriz. Se ha logrado una 
abertura de red de cerca de 7.200 pies cuadrados a la velocidad de 
remolque de 2.5 nudos con un motor de 350 hp. 


Richard L. McNeely 

Research Bureau of Commercial 
Fisheries, Seattle 

/COMMERCIAL fishermen and marine scientists have 
\*/ in the past often considered improvements in the 
techniques of harvesting stocks of fish known to inhabit 
midwater. Their interest in mid-depth fishing was 
accelerated with the introduction of echo sounding 
machines which, in addition to registering the depth of 
water under the vessel, indicated marine life at inter- 
mediate depths (Alverson and Powell, 1955). In recent 
years a variety of mid-depth fishing trawls have been 
used experimentally in attempts to efficiently harvest 
pelagic fishes (Parrish, 1959). 

During early 1961 a programme of one-boat midwater 
trawl development was undertaken by the Bureau of 
Commercial Fisheries, Exploratory Fishing and Gear 
Research Base, in co-operation with the Bureau's 
Biological Laboratory at Seattle, Washington. A 
multi-purpose gear was envisioned which would be both 
commercially acceptable by the fishing industry and 
useful as a pelagic fish sampling device. 

Design concepts 

The initial concept of a gear capable of this dual operation 
was that it should be of sufficient size to utilize the maxi- 
mum horsepower available (350 hp) in the Service's 
research vessel John N. Cobb. Prior experiments aboard 
the John N. Cobb with a British Columbia herring trawl 
(Barraclough and Johnson, 1955) the Larsson Phantom 

Fig. L Float line of giant Cobb Pelagic Trawl skims surface 185 fm. aft of the research vessel John N. Cobb. 






" No. 15 NYLON (oil body mtihtt) 




Fig. 2. Otter board and bridle arrangement used to open the Cobb Pelagic Trawl 

trawl (Larsson, 1952), and other conventional midwater 
trawls designed and constructed by the Bureau of 
Commercial Fisheries indicated that fish could be 
captured at towing speeds less than 2 kn. Moreover, 
directing observations (using SCUBA) and televised 
observations (Sand, 1959) of the reaction of fish to the 
approach of trawls show that most large pelagic fishes 
are capable of swimming speeds in excess of that necessary 
to avoid capture. Thus the capturing gear should be of 
sufficient size to entrap the fish before it becomes aware 

of the net. It was reasoned that a four-door otter board 
and bridle system (Fig 2) would be more effective in 
opening the mouth of the net than a conventional dual 
otter board system requiring unusually long bridles and 
unusually large doors. 

Gear design experiments 

In 1960 a partially constructed experimental two-boat 
surface trawl was purchased from a California tuna 
vessel owner-operator who had abandoned pelagic 

Fig. 3. SCUBA-equipped divers mounted on manoeuvrable sea sled prepare to examine experimental net In action. 





WINGS- 4^" No. 15 Contlnous flkjmtnt nylon 
BODY 4^" Na IS Contlnoui fllomtnt nylon 
INTERMEDI ATE- 4" No. 15 Contlnout fllomtnt nylon 
COD END-5i"No.9Centlnout fllomtnt nylon 
HEAD LINE-168'xf Brold.d nylon 
LEAD LINE- 1*6* *il"Glvonlitd choln 
BREAST LINES-168'xf*Broldtf nylon 

RIBLINES- Duol f nylon rop.(249* from wing tip 
through cod tnd-oll four corntrt) 

DANDYLINES-Two 10 fathom Mctlont|"wlrt ropo 
Two 20 fathom toot tons fwirt ropt 
Two 30 fathom actlont^'wiro ropt 
Two 40 fathom toctlont^wiro ropt 

OTTER BOARDS-T*o5'X8' "COBS" hydrofoil 

Two LarMnVhofiW trawl doon 

FLO ATS- 41 Vwillpt" aluminum trawl floats 
2 NaTS^orly^ntumttte floats 


Fig. 4. CM Pelagic Trawl 


trawling experiments. In early 1961 assembly of the huge 
net to its original specifications was completed. During 
this same period a four-door otter board system was 
designed and constructed to provide maximum opening 
and single-boat operation of the gear. A desired perform- 
ance characteristic of high lift at low speeds of midwater- 
otter boards appeared to be similar to aircraft wing 
sections used in the 1930's. Therefore, two experimental 
hydrofoil otter boards constructed of plywood and tim- 
bers were fitted with an appropriate bridle system to 
operate in conjunction with two patented "Phantom" 
trawl otter boards. The gear was then observed in action 
by SCUBA-equipped divers using a sea sled (Fig 3). 

The net was found to have several defects which 
required extensive modifications to correct. The princi- 
pal observed defect was excessive slack or "bagging" of 
webbing around the mouth of the net, apparently caused 
by the box-shaped wingless design of the net. Breastlines 
and footrope were found describing near semi-circular 
parabolic curves when viewed perpendicular to the 
longitudinal axis of the net. No provision for displace- 
ment of these sections from a straight line had been made. 
Subsequent to these findings, the net was disassembled, 
redesigned and rebuilt with long tapering wings, hung in 
along the corner riblines 13.4 per cent. Underwater 
observation of the redesigned net, named the Cobb 
Pelagic Trawl (Fig 4), revealed a considerable improve- 
ment in performance. Most of the slack webbing had 
been eliminated, resulting in a more satisfactory ratio 
of net opening to number of meshes across the mouth. 
The immediate visible effect was a greater mouth opening 
and improvement of individual mesh openings which 
ranged in configuration from 60 diamonds to 90 
squares. A more equalized distribution of load through- 
out the net was observed. Correction of an insufficient 
vertical opening was achieved by installation of ft in. 
chain for the lead line and increasing the number of 
floats on the headrope. 

Otter boards 

The attempt to provide horizontal spread of the net 
through use of large hydrofoil otter doors in conjunction 
with "Phantom" travel otter boards has been successful. 
A measured opening of 80 ft has been achieved during 
trials. The plywood doors which measured 8 ft 2 in in 
height and 5 ft in length, have a blunt nose and pro- 
portionately thick chord section (Fig 5). It was necessary 
to provide sufficient holes on the back (curved) side of 
the doors to allow rapid flooding and spilling of water 
from each of the hollow compartments during setting 
and retrieving operations (Fig 6). A metal shoe was 
placed on the lower side of each door to provide weight 
for stabilization and to allow use of the doors in future 
bottom trawling or near bottom pelagic trawling 
experiments. Observed performance characteristics of 
the doors showed them to be very stable, even when sets 
were made in cross currents. No tendency for the doors 
to cross was observed. Since both sides of the doors were 
covered with a plywood skin, no deflection vanes were 

Fig. 5. 

HB r< ^13------^i 


1 1 

. r . , *.- 


*F T 

i i 


T'p TV, 

,-.. 4*. A.I Jli*XJJJ-_1.-J5t-.M%nS *!* Jp*j4*J 

1 Jj u O, Ct il .1. 

f |0| .. 

r ^ 

^. .".' w- ' " U l 

*p n* 

1 T " " fX 

L^jdp 'Tfel^J 

""^ ".-/..::.;:.;;;::"_" J "i 


! i 
L .^ '^...L-... ^..^.^..r. 

Prototype Cobb Pelagic Otter Board (1961) 

needed to ensure proper setting. The doors responded 
effectually to changes in bridle chain lengths which 
allowed manipulation of angle of attack for maximum 
spread. Changes in differential lengths of the trace chains 
at the rear of the doors provided control of rising or 
diving in the water. This function was useful during 
tests of the gear rigged to fish on the surface. 

Four-door hookup 

Use of Larsson "Phantom" trawl doors in conjunction 
with the larger plywood doors presented initial problems 
in hookup. The Larsson doors were later found easier 
to set and retrieve when placed ahead of the plywood 

Fig. 6. Numerous holes (drilled on curved side only) allow rapid 
flooding and drainage of inner compartments. 


doors. Positioning of the larger doors on the upper 
bridle section and the Larsson doors on the lower bridle 
section was found desirable since a part of the function 
of the upper doors in a four-door arrangement is to 
counteract their own weight and diving effect of the lower 
doors, thereby stabilizing the depth of the net. 

Use of the gear on the surface after use at depths of 
ISO to 200 fathoms resulted in a change in the attitude of 
the plywood doors due to their becoming water logged. 
Prior to mid-depth tows surface tows could be made 
using 150 fathoms of cable. Following the mid-depth 
tows, attempted surface tows using more than 25 fathoms 
of cable resulted in the net sinking beneath the surface. 
This observation indicated the advisability of using all 
metal doors which would maintain a constant attitude 
at all depths. 

Modifications in 1962 

In early 1962 the Cobb pelagic trawl underwent further 
modifications in design to improve operational character- 
istics. The redesigned net, "Cobb Pelagic Trawl Mark 
II" (Fig 7) incorporated the following changes: 

1 1 





(a) Reduction of mesh size to 3 in (stretched measure) 
in the body of the net to eliminate at least part of a 
serious gilling problem (Fig 8) encountered with the 
original model. 

(b) Reduction of length and horizontal size of the net 
to compensate for increased drag of smaller meshes. 

(c) Elimination of multiple tapers along the corners by 
using a straight line taper from wing tip to bag. 

(d) Installation of "criss-cross" riblines for better 
equalization of strain during instances involving unequal 
lengths of towing warp. 


Fig. 7. Cobb Pelagic TrawlMark U 

Fig. 8. Mackerel and dogfish shark, gllled in intermediate section of 
trawl, suggest reduction in size of meshes. 

(e) Use of 6 in webbing in the wing sections to reduce 
<lrag. '''.: .- 

' * ' - * 

Aluminium hydrofoil otter boards ; 

Redesign of the large plywood hydrofoils to all alumin- 

ium construction was also completed early in 1962. The 
new doors (Fig 9) utilized the same basic characteristics of 
air foil cross section with vertical size (8 ft) larger than 
the fore-aft dimension (5 ft). To develop vertical 
stability, 72 3J in glass ball floats were placed in the 




rt wet covcnco t 







sc 'HILI-AHC' wt LOCO 





Fig. 9. Cobb Pelagic Otter Board (1962) 


Fig. JO New aluminium otter board as seen from afterdeck of the John N. Cobb. 

upper two rib chambers of each door. Each glass ball 
was covered with heavy webbing and packed in fibreglass 
to prevent damage during use of the doors. 

Underwater observations 1962 

Following construction of a new Mark II Cobb Pelagic 
Trawl and modification of existing gear to Mark II 
design in early 1962, the trawl, equipped with aluminium 
hydrofoils and Larsson "Phantom" trawl doors, was 
again observed in action by SCUBA-equipped divers. 
With the exception of moderately excess strain noted in 
the wing tips, the net appeared to be fully expanded and 
exhibited the characteristics of a semi-elastic body being 
inflated by an internal force similar to the manner in 
which air inflates an airport wind sock. This condition 
extended throughout the net from the wings to the 
"bitter end" of the codend. A near circular cross section 
shape was noted. Meshes from the wings through the 
codend were examined by sight and feel to gain know- 
ledge of strain distribution. The amount of strain on 
individual meshes in all but the forward sections of the 
wings was found to be small, even in areas adjacent to 
the riblines. The slight increase in strain in wing meshes 
was attributed to a greater amount of stretch in ribline 
in this area, due to the concentration of total load at the 
four wing tips. A further improvement in net per- 
formance is expected when corner riblines are rehung 
using an incremental hang-in ranging from approxi- 
mately IS per cent at the wing tips to approximately 
10% at the codend junction. (See note 1 - p. 248) 

Observations of the new aluminium hydrofoils from 
shipboard (Fig 10) and below water showed them to be 
very stable and responsive to adjustments in bridle and 
trace chains. Through manipulation of chain linkage 
it was possible to tow the net on the surface 185 fathoms 
behind the vessel. 


Direct measurement of horizontal opening was secured 
through two auxiliary launches between which a tight 
line was suspended in the air over wing-tip mounted buoys 
shown in Fig 1. 

Vertical opening measurements were secured by sea- 
sled mounted SCUBA divers descending alongside the 
net and observing differences in depth gauge readings. 

Average towing speed of 2.5 kn was arrived at by 
use of radar and Loran to establish start and end positions 
of a series of a trial drags, usually of one hour duration. 

Depth assessments of the net during experiments at sea 
trials were secured through an electrical Depth Telemeter 
(McNeely 1959) and conventional wire angle-scope ratio 
calculations. Changes in net depth were normally made 
by regulating the length of towing cable at the winch. 

Preliminary sea trials (1961) 

During July and August 1961 a Cobb Pelagic Trawl, 
made of 4| inch webbing utilizing plywood hydrofoils in 
conjunction with Larsson "Phantom" trawl otter boards, 
was fished 25 times in offshore waters to determine 
operational characteristics. Most of the 25 tows were set 
"blind" (no indication of fish). Ten of the tows were 
made in the deep scattering layers (150 to 200 fathoms) 
110 miles off the Washington coast to test the ability of 
the gear to descend to these depths. These tows produced 
small amounts of jellyfish, squid, lantern fish, and 
fanged viper fish. Other blind tows made in shallower 
depths (less than 50 fathoms) off the mouth of the 
Columbia River produced occasional silver salmon and 
up to 160 Ib each of hake and jack mackerel. Other 
species taken in small numbers include blue shark, 
herring, anchovy. English sole, turbot, and black 



t'^&p 3 "". 1 "V'l.^-"t;A' ; ; '* " ' 

^^.-^af^^rr" -/' 

,#jJ^:-v,^%.,,-,-.,Ty'-';- ',;.-'.-/ " 
2 a&Jt^^tt^'^-X< :.' '*J*^-*' .,' 


Fig. /7. T/ie ma/i stands inside the after-body of the net which is stretched out three-dimsnsionally. 

Testing of the gear as a salmon sampling surface trawl 
yielded generally poor results. These drags, made on 
the Swiftsure Bank, and in Prince William Sound, 
Alaska, produced salmon in small numbers ranging 
from a single individual to 29 per drag. Large catches of 
dogfish shark taken on the Swiftsure Bank repeatedly 
damaged the net. Catches of dogfish to 10,000 Ib were 
made. During one drag the entire codend was lost 
and the intermediate section was severely chaffed. 
Jack mackerel and hake were also taken in amounts up to 
200 Ib in the Swiftsure area. Although salmon did not 
appear to gill easily, dogfish, hake and jack mackerel 
catches usually resulted in severe gilling. A reduction 
in mesh size from 4J in (stretch measure) to 3 in was 
therefore indicated for fish in this size group. 

Fishing trials in 1962 

In August 1962 the Cobb Pelagic Trawl Mark II was 
used during an extensive survey to determine the relative 
abundance of all pelagic species offish at pre-determined 
stations off the coasts of California and Mexico. Follow- 
ing this work, tests of the gear were conducted in nearby 
waters off Mexico, California, Oregon and Washington 
to determine its relative efficiency as a biological sampling 
tool and possible utility as commercial fishing equipment. 

(a) Results of offshore pelagic survey 

Forty-four predetermined stations were occupied during 
this phase of testing. Oblique tows from 220 fathoms 
to the surface were made at each station during daylight 
hours. At least one of each series of night tows was 
made on the surface. With the exception of one night 
surface tow in which 24 mackerel were taken, catch 
rates seldom exceeded five Ib per two hour tow. How- 
ever, echo-soundings taken at all stations along the track 
line, which extended over 600 miles offshore, indicated 
no fish concentrations were available. Scatter recordings 
were typical of those associated with the deep scattering 
layers. During part of the offshore survey simultaneous 
sampling was conducted by the Bureau's research vessel 
Black Douglass using plankton nets and stramin nets. 
A subsequent correlation (by co-operating scientists at 
the Bureau's La Jolla Biological Laboratory) of catch 
rates made during these trials and catch rates made at 
other times by small, high-speed midwater trawls shows 
that small, fine mesh nets are as efficient as the Cobb 
Pelagic Trawl in taking zooplankton and small fishes 
such as stomatoids and myctophids. Catch rates and sizes 
of the larger fishes such as anchovy, hake, rockfish, 
bonito, sardine, mackerel, barracuda, and ribbon fishes 
(up to 6 ft in length) indicate that the large net was 


Underwater Telemeters for Midwater Trawls and Purse Seines 


Since 1957 the authors have worked on the development of a wire- 
less net depth telemeter for commercial midwater trawling. The 
general principle and some technical details are given of the five 
types of instruments built and tried so far in the course of this work. 
Toe net depth is measured by determining the hydro static pressure 
and the measured data are coded into ultrasound signals transmitted 
to a sound receiver towed by the trawler. They are converted into 
electrical signals and led by a 25 m long cable to the depth indica- 
tor unit in the wheelhouse. The depth indication was first only by 
pointer on a scale, as well as acoustical. Later a paper-recording 
system was developed to write the net depth in correct scale on 
the echogram of the trawler's echo sounder. For the first four 
instrument types the depth measurements were converted into 
signals consisting of ultrasound impulses (impulse rate 40 per sec) 
the duration of which was in linear relation to the depth. Instru- 
ments of this kind could not be made light, cheap and rugged 
enough and continuous depth indication could not be achieved. 
Then a system of frequency modulation was adopted, i.e., the 
freouency of a continuous ultrasound signal was changed according 
to the measured depth. The frequency of the transmitted impulses 
or signals for all types is approximately 200 kc. The depth-measur- 
ing range is to about 250 m with an accuracy of about 1-0 m 
at about 100 m depth. The transmission range from net to trawler 
is about 1,000 m for the first four models and about 2,000 m for 
the last model. The last model, which is completely transistorised, 
was found to be satisfactory and is available on the market. Already 
in 1961 about 70 units were in commercial use, mainly in conjunc- 
tion with midwater trawling for shrimp in the East China and Yel- 
low Seas. Such depth telemeters are also suitable for measuring 
tlie behaviour (sinking speed) and fishing depth of purse seines. 


} pelagiques et i 


La presente ttude d6crit le principe gnral et quelques details 
techniques des cinq types d'instruments utilises par les auteurs qui, 
depuis 1957, ont travai!16 au d6veloppement d'un sondeur de filet, 
actionn par radio, pour chaluts p61agiques. La profondeur du 
filet est mesuree par la determination de la pression hydrostatique 


Chikamasa Hamuro 

Kenji Ishii 

Fishing Boat Laboratory, Tokyo 

et ks donnees sont transmiscs en signaux ultra-sonores codes et 
recus a bord du bateau par un recepteur courant. Ces signaux 
sonores sont convertis en signaux dlectriques et transmis a 1'indica- 
teur installe dans la timonerie. Au debut, la profondeur 6tait 
indiquee sur un cadran ou par signaux audibles; plus tard, grace a 
un systeme enregistreur plus perfection^, les profondeurs obtcnues 
s'inscrivaient sur I'fchogramme de rinstrument de bord. Les quatre 
premiers types d'instruments convertissaient les donnees sonores 
en signaux 61ectriques ultra-sonores (frequence de 40/sec) dont la 
duree etait en relation lineaire avec la profondeur. Avec octte 
m&thode il 6tait impossible de construire des instruments tegers et 
assez robustes pour un emploi continu. C'est alors que le systeme 
de modulation de frequence fut adopts, c'est-a-dire que la frequence 
de 1'onde porteuse fut modulee selon la profondeur mesuree. La 
frequence de 1'onde porteuse, pour tous les types, est d'environ 
200 kc et Ton peut mesurer des profondeurs de a 250 metres avec 
une precision de 1 pour cent a environ 100 m. La portee de la 
transmission du filet au bateau est d'a peu prcs 1,000 m pour les 
quatre premiers modeles et de 2,000 m pour le dernier modele. 
Ce dernier modele, a transistors, est tres efficace et est maintenant 
disponsible sur le march6. Deja en 1961, 70 unites etaient utilisees 
par des bateaux de commerce, notamment pour le chalutage des 
crevettes. Ces instruments sont aussi ties utiles pour mesurer It 
vitesse de descente du filet et la profondeur de pdche des sennea 

Continued from page 247 

capable of sampling a wide spectrum of the large pelagic 
vertebrates. In most cases the larger fishes are rarely 
if ever taken in small, high-speed midwater nets. 

(b) Gear efficiency tests 

Following the offshore pelagic survey, 16 surface 
towsand 1 1 mid-depth tows were made off the coasts of 
Mexico, California, Oregon and Washington. Catch 
rates during the near shore tests greatly exceeded catch 
rates offshore. Although a wide variety of species were 
taken, the largest catches consisted of 1,850 Ib of hake, 
1,900 Ib of mixed sablefish and hake, 1,000 Ib of 
anchovy, and 600 Ib of ocean sunfish. Most of the 
larger catches were made at mid-depth. 


Utility of the Cobb Pelagic Trawl for gross biological 
sampling 8 was demonstrated during sea trials of the gear 
by the wide variety of fishes taken seventy, in fact. 
Those sets made on schools of fish located by echo- 
sounding in depths greater than 30 fathoms usually 
produced fair amounts of fish. Attempts to capture 
surface swimming schools usually resulted in poor catches. 


It should be noted that the total number of drags made 
to date is small and has entailed many variables inherent 
in development of new gear. 


1. In other gear experiments conducted during 1961 and 1962 
involving use of a giant, small-mesh fyke net to strain the total 
discharge of water from hydro-electric turbines, the author has 
found incremental hang-in to be a distinct advantage to counter 
distortion of meshes near the mouth of the net. 

2. Towing at predetermined stations without prior echo-sounding 
or other observations to determine the presence of fish. 


Alverson, D. L. and D. E. Powell. The open ocean challenges 
the scientist and dares the fisherman. Pacific Fisherman, Vol. 53, 
No. 11, (Oct.) and Vol. 53, No. 12, Nov. 1955. 

Barraclough, W. E. and W. W. Johnson. Canadian midwater 
herring trawl. World Fishing, Vol. 4, No. 8, 1955. 

Larsson, K. H. The "Phantom*' pelagic trawl. Fishing News* 
No. 2067, 1952, 

Parrish, B. B. Midwater trawls and their operation. Modern 
Fishing Gear of the World, Fishing News (Books, Ltd.), London, 

Sand, R. F. Midwater trawl design by underwater observation . 
Modern Fishing Gear of the World/ Fishing News (Books, Ltd.), 


Ntes de arrastre flotantes y dc cerco de 


Trabajan los autores desde 1957 en la fabricaci6n de un tetemetro 
inalambrico registrador de la profundidad a que esta la red, para 
los artes flotantcs empleados en la pesca industrial. Se dan los 
principles generates y algunos detalles tecnicos de los cinco modelos 
de aparatos construidos y ensayados hasta ahora. La profundidad 
a que se encuentra el arte se mide determinando la presidn hidrosta- 
tica y los datos se transforman en seriates ultrasonoras y se transmitcn 
a un receptor que remolca el arrastrero en el que se convierten en 
seftales electricas y a trav6s de un cable de 25 metres pasan al grupo 
indicator de la profundidad situado en el puente de mando. En 
un principio la profundidad se indicaba solamente mediante un 
puntero en una escala y acusticamente ; mas tarde se ideo un sistema 
de papel registrador en el que aparecia la profundidad del arte en 
la escala del ecograma de la ecosonda del barco. En el caso de los 
cuatro primeros modelos la medida de la profundidad se convertia 
en seftales que consistian en impulsos ultrasonoros (40 por segundo) 
cuya duraci6n estaba en relaci6n aritm6tica con aquella. Los apa- 
ratos de esta clase no lograban hacerse lo bastante ligeros, robustos 
y baratos y por ello no se obtenia una indicacibn continua de 
la profundidad. Posteriormente se adoptd el sistema de modulaci6n 
de la frecuencia, es decir: la frecuencia de una seftal ultrasonora 
continua se transformaba de acuerdo con la profundidad medida. 
La frecuencia de los impulsos o seftales transmitidos por todos los 
aparatos es de cerca de 200 kc. La escala para medir la profundidad 
va de a cerca de 250 m, con una exactitud pr6xima al 1 por 
ciento a cerca 100 m. El alcance de la seftal transmitida desde la 
red es de unos 100 m en los primeros cuatro modelos y de unos 
2,000 m en el ultimo. Este esta completamente transistorizado, da 
resultados muy satisfactorios y se encuentra ya en el comercio. En 
1961 se empleaban industrialmente unos 70 aparatos, principal- 
mente en la pesca del camar6n con artes pelagicas en el mar del 
este de la China y en el Amarillo. Estos telemetros tambien se 
pueden emplear para medir la velocidad a que se hunden y la 
profundidad a que pescan los artes de cerco de jareta. 

Q1 UCCESSFUL mid water trawling requires knowing the 
>3 net depth under various conditions. Since 1957, the 
author and his colleagues have been doing research on 
midwater fishing gear and methods and on underwater 
telemetry for midwater trawling. Five kinds of tele- 
meters were designed and manufactured in four years. 
The latest design, Type V, has been installed in fishing 
boats and is performing satisfactorily. About 70 sets 
are in use in one-boat and two-boat midwater trawling 
in the Yellow Sea and other fishing grounds. This tele- 
meter has also been adapted to purse seining. 

As is known, the depth of the net, even if warp length 
and towing speed are constant, can vary due to the 
influences of currents and wind. 

In order to keep the trawl at a desired depth, it is 
therefore necessary to have continuous net depth 
measurements by means of a suitable depth meter. The 
actual problem for such depth telemeters is not so much 
the measurement itself but the transmission of the meas- 
ured data from the net to the boat. With the telemeters 
described here, this is done by wireless ultrasound 

The first instrument (Type I) was completed in Novem- 
ber 1958. This was gradually improved into Type II. 
In January 1960, Type III and in February 1961 Type IV 
were developed. Type V, which contains further innova- 
tions, was completed in June 1961. It is now used in 
commercial operation in East China and Yellow Seas. 

The complete net depth telemeter consists of the depth 
meter and signal transmitter, the signal receiver and the 

Otter board 
The depth signal transmitter 

Sea bottom 

Fig. 1. Arrangement of wireless depth telemeters with midwater 

depth indicator. As shown in Fig. 1, the depth meter and 
signal transmitter unit, which transmits signals corres- 
ponding to the depth of the meter, is attached to one 
warp close in front of the otter board (or the wing tip). 
In order to evade the interference of the propeller wake 
of the trawler, the receiver is also attached to the warp in 
sufficient depth about 25 m astern of the trawler. The 
ultrasound signals are converted by the receiver into 
electrical signals which are transmitted by cable to the 
depth indicator on board the trawler. 

Fig. 2. General construction of depth meter and signal transmitter 

Fig. 2 shows schematically the structure of the depth 
meter and signal transmitter (Types I to IV). For the 
net depth the hydrostatic pressure is measured by a set 
of bellows. The displacement of the diaphragm in 
proportion to the depth actuates a pointer which has 
contact with a wire spiral on a cylinder which is driven at 
constant speed by a micromotor. Since the pointer moves 
in the direction of the axis of the rotating cylinder in 
accordance with the depth, the period during which it is 
in contact with the spiral is proportional to the depth. 
For this period the signal-transmitting circuit is closed 
and ultrasound impulses of 203 kc are transmitted at a 
frequency of 40 per sec. This impulse frequency is 
effected by a commutator attached to the axis of the 
micromotor. The duration of the signal of 40 impulses 
per sec. thus indicates the depth measured by the bellows. 


Variations of the voltage of the electrical source are 
irrelevant to the accuracy of the measurements. 

There is one main shortcoming of this system, i.e., 
measurements are not continuous but can be transmitted 
only at the rate of one per rotation of the cylinder. In 
the Type I to IV meters the cylinder was designed to 
rotate once every 10 sec so that measurements could be 
transmitted once every 10 sec which was considered 
sufficient for practical fishing. 

A pressure switch was provided to cut off the trans- 
mitter in less than 8 m water depth to avoid waste of the 
battery. The ultrasonic transmitter and receiver were of 
the same type, i.e., barium titanate, 30 mm diameter, 
beam angle approximately 20. The unit is shown in 
Fig. 3, A. The ultrasound receiver (Fig. 3, B) converts 
the received sound signals into electrical signals which, 
by means of a 25 m long cable, are conducted to the 
indicator unit in the wheelhouse of the trawler. 

The indicator unit (Fig. 3, C) consists of the amplifier, 

Fig. 3. The wireless net depth telemeter Type /. A depth meter and 
signal transmitter. B receiver. C - indicator. 

the depth-evaluating part, the depth indicator (with 
loudspeaker) and an electrical power source. The depth 
indicator is located near the centre of the instrument 
panel, and on both sides of this are a voltmeter for the 
electrical source and the loudspeaker. The depth signal 

S*Q surface 

Cweep zone of me md- water trowl net 


."'..' '' V-M956I26 - ' ' 

. * - : ' ?Q- 

. ; . .../.. . ..* ,/. 5if 

:: X;:vA;v : ;> ';<:::: ! ^ .:.?: r .::v.;.*-S. 

*;: ,..:. .. 

.*:.*/''** '** 
*'.*' i* ' 

/' . -'^V: .' 
*. ... . 

. . ' . ;.vi. * t .v,jQ 

<%: ,'t* i*r* 

.':' J*-"*"* ^ y .: . ti i.i- . . 

^v.';^^ ; . s .. :V.v..v'-'-.''Mo. 

;.';.";'.';"''" ".'!.-':. "':"' ., -, . - v 

*'.':' -' .' ..-".''. ^V- ':. ;' '*'' v>'''.\?x''^ V-'- ; v"'.l.- ; i-^- '*"*'. "*v.' 

is amplified and led to the depth evaluation part where 
it is converted into a voltage proportional to its duration. 
Thus the depth is indicated directly by a voltmeter with 
depth calibration. A filtering circuit at the input side of 
the depth indicator eliminates the disturbances caused by 
external noise. An additional output circuit is provided 
to make the depth signals also audible through a loud- 
speaker. When the distance to the net becomes too 
great (and the signal-input too weak) the depth indicator 
may not indicate even at maximum amplification. In 
such cases the depth can be estimated from the duration 
of the audible signals. The audible signals also help to 
determine whether faults of the instrument lie in the 
depth meter and signal transmitter or in the receiving 
parts. Finally the audible display helps to find the best 
position for the receiver. 

The Type I depth meter was designed for: range of 
depth measurement to 250 m, range of transmission 
over 1,000 m, accuracy of depth measurement within 
1 per cent, maximum depth for the depth meter and 
transmitting unit 500 m. 

Since the directional beam angle of the transmitter 
and the receiver is rather narrow, it is best to attach the 
depth meter and transmitter unit at the end of the warp; 
that is, right in front of the otter board in the case of one- 
boat trawling or right in front of the wing tip for two- 
boat trawling. The receiver is also attached to the same 
warp but short astern of the trawler and just below the 
propelling wake. The attachment of transmitter and 
receiver to the warp has to be made so that they face 
each other over the distance as directly as possible. 

From December 1958 to September 1959 various 
theoretical and practical experiments were conducted 
with satisfactory results on Type I. 

The Type II meter developed on the basis of these 
experiences incorporated the following improvements : 

(1) The outside diameter of the depth meter and trans- 
mitting unit was decreased by 20 mm to 120