NAVY DEPARTMENT

THE DAVID W. TAYLOR MODEL BASIN:
Washington 7, D.
<a

RESISTANCE AND STABILITY TESTS OF THE
WOODS HOLE OCEANOGRAPHIG INSTITUTION
CURRENT-INDIGATOR BUOYS

BY R.A. EBNER

NOVEMBER 1948 REPORT 667

DISTRIBUTION
Initial distribution of copies of this report:
3 copies to Bureau of Ships, Project Records, Code 362.

4 copies to Woods Hole Oceanographic Institution.

RESISTANCE AND STABILITY TESTS OF THE WOODS HOLE OCEANOGRAPHIC
INSTITUTION CURRENT-INDICATOR BUOYS

by
R.A. Ebner

ABSTRACT

The Woods Hole Oceanographic Institution has developed an instru-
ment which in principle uses the drag of a buoy to measure current velocities.
At their request the David Taylor Model Basin measured the drag of 1/3-scale
models of three proposed forms for the buoy and a full-scale model of one of
these forms to determine which of the forms was best suited to their purpose.
The full-scale model was also tested for stability. From the tests conducted
Buoy 1 was found to have the most satisfactory performance.

INTRODUCTION

At a conference held at the David Taylor Model Basin (1),* repre-
sentatives from the Woods Hole Oceanographic Institution explained a proposed
device for obtaining measurements of the vertical distribution of velocity in
ocean currents in water up to 200 feet deep. A system for determining this
distribution was proposed. A buoy which was attached to an anchor by means
of a mooring line was to be released from the ocean bottom. As the buoy slow-
ly ascended to the surface the angle of the mooring line at the buoy was de-
termined by the resultant direction of the weight, buoyancy, and drag forces
acting on the buoy. A recording pendulum for indicating the angles which the
cable assumed at various depths was to be attached directly under the buoy on
the mooring cable; see Figure 1. From the angles obtained, the drag of the
buoy could be computed. Hence, if the drag of the buoy was known as a func-
tion of the current velocity, the recorded angles would be made a direct
measure of the velocity of the current.

It was requested that the Taylor Model Basin make resistance tests
of three 1/3-scale model buoys and one full-scale model. The models were
supplied by the Woods Hole Oceanographic Institution. It was also requested
that tests be made to determine whether the presence of various towing bails
attached to the buoys had an appreciable effect on the drag, and that a test
be made to observe the stability of the full-scale model when it was towed
from an underwater towpoint, simulating actual moored conditions.

* Numbers in parentheses indicate references at the end of this report.

Water Surface

\
Electrical Recording
Pendulum for Indicating Direction of
: oS
Mooring-Line Angle \ Current
\
(F9) -i~
eth ot aN
(c eS ier a)
cit Tk TN
1 ' \
0 Kee
Bev) LaaN
N
\
\
\
anal Ni
ae WON
a | DY
Payee OD
Roth ade SRE
L-4

Buoy Release and
Mooring-Line Reel

Channel Bottom

Figure 1 - Path of Current-Indicator Buoy As the Buoy Rises to the
Surface After Being Released from the Bottom of the Channel

DESCRIPTION OF THE MODELS

Three different 1/3-scale models and one full-scale model were
tested for resistance in the deep-water basin of the Model Basin. The con-
struction of the models is as follows:

The 1/3-scale model of Buoy 1 is a sphere 5 1/3 inches in diameter,
with a projecting ring 6 inches in outside diameter around the vertical trans-
verse equator. When the buoy was towed the ring was normal to the direction
of motion. Protruding 1 1/2 inches from the front of the sphere, forward of
the ring, was a pin 1/32 inch in diameter, simulating the mooring line; see
Figure 2. Provision was made for the pin to be placed at 10, 20, or 30 de-
grees from a vertical plane through the center of the sphere.

The 1/3-scale model of Buoy 2 is a sphere 5 1/3 inches in diameter
with a projecting ring 6 inches in outside diameter. This buoy was fitted
with a bail for mooring, which necessitated placing the ring aft of the
vertical transverse equator; see Figure 3.

\N

Direction of Motion

1
a
6" When Towed

Supporting Shaft

Pin

Figure 2 - Sketch of 1/3-Scale Model of Buoy 1

Direction of Motion
When Towed

Supporting Shaft

Direction of Motion
When Towed

Shy
UG

Towing Bail

Figure 4 - Sketch of 1/3-Scale Model of Buoy 3

The 1/3-scale model of Buoy 3 is a short cylinder 6 inches in diam-
eter with a disk 7 3/8 inches in diameter fastened on the forward end. The
direction of motion on towing was along the axis of the cylinder. This
model is shown in Figure 4.

The full-scale model of Buoy 1 is a 16-inch laminated wooden sphere
with a projecting ring 18 inches in outside diameter around the vertical
transverse equator. This sphere was mounted on one end of a 1 1/2-inch rod
and four plastic fins were mounted at the other. These fins were 24 inches

Supporting Shaft

Mooring Line
Direction of Motion 4

je 6" When Towed

Figure 5 - Sketch of Full-Scale Model of Buoy 1

aft of the sphere and were 6 inches wide, 1/16 inch thick, and had a span of
24 inches. The lower forward quadrant of the vertical longitudinal plane of
the sphere was slotted to permit the passage of the mooring line to the cen-
ter of the sphere. Figure 5 shows the general arrangement of this buoy.
After the sphere had been immersed during several tests, separation occurred
between laminations and the layers became displaced relative to each other,
which produced a corrugated effect with irregularities as great as 1/2 inch.
This necessitated the construction of another full-scale model at the Model
Basin to complete the required tests.

Photographs of the models are shown in Figures 6, 7, 8, 9, and 10.

} wy Direction of Motion
; When Towed

TMB 12222

Figure 6 - 1/3-Scale Model of Buoy 1

The buoy is shown fastened to the towing strut in position for drag tests. Arrows on the buoy point
to holes which receive steel pins used to simulate the mooring line of the full-scale model.

Direction of Moficen
When Towed

TMB 12223

Figure 7 - 1/3-Scale Model of Buoy 2

The buoy is anchored to the end of the towing strut in the towing position. The figure shows
clearly the towing bail arrangement used.

Direction of Motion
When Towed

TMB 12225

Figure 8 - 1/3-Scale Model of Buoy 3

The model with the towing bail attached is fastened to the towing strut. Excessive vibration
was encountered while the buoy was being towed in this position.

Direction of Motion
When Towed

TMB 12224
Figure 9 - Full-Scale Model of Buoy |
The buoy is shown fastened to the end of the towing strut. Separation of the laminations is clearly

shown. The wire cable extends to a pivot shaft at the center of the sphere and serves as the towing
bridle for the buoy.

Direction of. Motion E
When Towed ~

Figure 10 - Full-Scale Model of Buoy 1

This buoy was built after separation of the laminations of the Woods Hole buoy made further tests
on that buoy impossible.

TEST PROCEDURE

To measure the resistance, the buoy to be tested was fastened to
the end of a towing strut which was submerged to a depth of 34 5/8 inches.
The other end of the strut was clamped to the towing beam of the drag dyna-
mometer of the towing carriage, as is shown in Figure 11. The strut was a
standard towing strut for submerged bodies (2) of ogival section with a chord
length of 4 inches and a thickness of 1 inch.

The 1/3-scale model of Buoy 1 was tested at speeds between 0.20 and
13 knots for various positions of the pin simulating the mooring line.

The 1/3-scale models of Buoys 2 and 3 were tested with and without
the towing bails at speeds from 1 to 9 knots. The towing bails were mounted
on the models 45 degrees below the horizontal axis of the buoy.

The full-scale model of Buoy 1 was tested for drag at speeds from
0.50 to 3.50 knots. The model was also tested for towing stability by towing
a 200-pound weight from the towing carriage 18 feet below the water surface,
with the mooring line from the buoy fastened to the weight. The mooring line
was short enough so that the buoy would tow submerged. A sketch of this ar-
rangement is shown in Figure 12.

To obtain the net drag of the buoys it was necessary to determine
the tare drag of the supporting strut. This was done in two ways. The drag
of the strut was first measured alone and then in the wake of the buoy in
question. On both tests the strut was clamped to the towing beam of the drag
dynamometer of the towing carriage, but in the latter test the buoy, support-
ed independently of the towing strut by means of a U-frame, was maintained as
close as possible to its original position relative to the strut without ac-
tually touching it; see Figure 13.

The tests of the 1/3-scale models of Buoys 1 and 2 were limited to
a maximum speed of 13 knots. The towing speed of Buoy 3 was limited to 9
knots because of excessive vibrations above that speed.

Direction of Motion

a
When Towed

TMB 12227

Figure 11 - General Towing Arrangement for Determining
Drag of the Buoys

The 1/3-scale model of Buoy 2 is shown fastened to the end of the towing strut. The strut is
clamped in a bracket fastened to the drag dynamometer beam of the towing carriage.

10

Waterline

Direction of Motion
Se ara EEEEEEEEEEEEEEEEEEeeee el

Towline to Carriage

Full-Scale Model

of Buoy | 200-Pound Faired Weight

Figure 12 - Towing Arrangement for Stability Tests
of Full-Scale Model of Buoy 1

Waterline

Direction of Motion
When Towed

TMB 12230

Figure 13 - Arrangement for Measuring the Tare vrag of the Towing Strut

The U-frame is used to support the buoy independently of the strut. while the tare drag of the
strut in the wake of the buoy is measured.

11

TEST RESULTS

The pins inserted in Buoy 1 tend to increase the drag slightly, as
can be seen in Figure 14.

The bail when fastened to Buoy 2 definitely increases the drag, as
shown in Figure 15.

An interesting effect may be seen in regard to Buoy 3. At low
speeds up to 5 1/2 knots the bail causes an increase in drag, but from 5 1/2
to 9 knots it leads to a decrease; see Figure 16.

The full-scale model of Buoy 1 with fins was towed for drag meas-
urements; the results are presented in Figure 1/7.

The drag data obtained from tests on the full-scale model of Buoy
1 and the computed full-scale data as obtained from test data of the 1/3-
scale model are compared in Figure 18. Drag data of the full-scale model of
Buoy 1 include the drag of the fins.

During the stability tests of the full-scale model of Buoy 1, the
buoy oscillated from side to side at the end of the towline in a plane nor-
mal to the direction of motion. As the speed increased, the period of oscilla-
tion decreased. There apparently was no pitching oscillation. It is interest-
ing to note that the period of oscillation of the buoy is very nearly the same
as that of a cylinder. The observed periods of lateral oscillation of the buoy
are compared in Table 1 with the computed period of oscillation of a cylinder.

TABLE 1
Oscillation of Buoy 1

Period of Buoy Period of Cylinder
Speed Knots Oscillation Oscillation
seconds seconds

The period of lateral oscillation of the full-scale model of Buoy
1 when towed at the end of a short section of towline from an underwater
towpoint is compared with computed values for a cylinder towed with its axis
normal to the stream. If the same value for the Strouhals number is assumed
for the buoy as for a cylinder of the same diameter, the period of oscilla-
tion of the cylinder can readily be computed. A free cylinder in a stream

(Text continued on page 16.)

12

70

Mean Drag Curve for 1/3-Scale Model of
Buoy | without Peg and with Peg at 10°
20°, and 30° to Axis of Buoy Parallel
Be to Direction of Motion

50

TS
°

Drag in pounds

30

Drag of Buoy with the Tare
Drag of the Strut in the
Wake of the Buoy Deducted

20

Mean Tare Drag of Strut in Wake of
1/3-Scale Model of Buoy |

34 2 Immersed

Speed in knots

Figure 14 - Drag of 1/3-Scale Model of Buoy 1

V3

70 >|
1/3-Scale Model of
Buoy 2 with Bail

60 -——_|_ |

173-Scale Model of Buoy 2 /
with Bail /
Tare Drag of Strut in Wake /

of Buoy Deducted
50 —_ ae I
i

/
/
hi
i
40
= /
: iff
a /|
3 y]
. ee eee lic cane /
30 ij
/
/
[
{/ nll ee
// 1/3-Scale Model of
20 Buoy 2 without Bail
/
/

Tare Drag of Strut in Wake of
1/3-Scale Model of Buoy 2

34 2 Immersed

Y 1/3-Scale Model of Buoy 2
without Bail
6 VA, Tare Drag of Strut in Wake
of Buoy Deducted
10 Y

Speed in knots

Figure 15 - Drag of 1/3-Scale Model of Buoy 2

100

90

80

70

a (2)
te} to}

Drag in pounds

py
te}

30

20

14

1/3-Scale Model of Buoy 3
on Strut,without Bail

\/3-Scale Model of Buoy 3
Tare Drag of Strut in Wake
of Buoy Deducted

1/3-Scale Model of Buoy 3
on Strut,with Bail

Vj 1/3-Scale Model of Buoy 3
Tare Drag of Strut in Wake
yA of Buoy Deducted

Tare Drag of Strut in Wake of
\73-Scale Model of Buoy 3

34 3 Immersed

SS SED STEER ES (S aIT] PSE ee are |
| 2 ©) 4 5 6 7 8 9 10
Speed in knots

Figure 16 - Drag of 1/3-Scale Model of ‘Buoy 3

Full-Scale Model of
Buoy | on Strut

Drag in pounds

Full-Scale Model of
Buoy |
[rere Drag of Strut in
Wake of Buoy Deducted!
T mine

Tare Drag of Strut in Wake of
Full-Scale Model of Buoy |

/
(0) | 2 3 4
Speed in knots

Figure 1/ - Drag of Full-Scale Model of Buoy 1 with Fins

V,

16

: Pe
90 |
Computed Drag
of Buoy 3 :

80

Computed Drag
of Buoy |

70

Test Drag
of Buoy |

60
wo
Uv
c
=]
a
< 50
= Computed Drag
8 of Buoy 2
a
40

30

20

Speed in knots

Figure 18 - Computed Full-Scale Drags of Buoys 1, 2, and 3 Compared
with Experimental Full-Scale Drag of Buoy 1

flowing normal to the axis of the cylinder tends to oscillate with frequency
nm given by Strouhals number
d

Wes 0.185

where d is the diameter of the cylinder in feet and
V is the velocity in feet per second (3)

TARE DRAG RESULTS

The tare drag of the strut, which was deducted from the total drag
of the buoys and the strut combined, is the mean of the tare drag data ob-
tained from tests on the strut when it was towed in the wake of the buoys.
Figures 14 to 18 are curves in which the mean tare drag for the respective
buoys is plotted against speed.

Figure 19 presents test data of the buoys in the form of a plot of
drag coefficient versus Reynolds number. The drag coefficient is defined as

ig See ee SS SS Se Se
\ rr rs [a | meee || [ree
: ae eral [ena | (nr [a fee a
r SS ee
ae Raa a
iia Re ea Aa ee ee eee See “is caro | [eee
aa 1/3-Scale Model Buoy 3 without Bail >| a
1.2 — eee
ae 73Scale Model Buoy 3withBal | | (|__|
1o| = Svante JL
Full-Scale Model Buoy | with Fins +>
aA poo
08 Se + 1/3-Scale Model Buoy |
ss i SSS
Eitaumeneers= Ee
06 — ied tes oe

1/3-Scale Model Buoy 2 without Bail
1/3-Scale Model Buoy 2 with Bail

04

Drag Coefficient

02

fu pee
ee 2 3 4
Reynolds Number x 10°

Figure 19 - Drag Coefficient Versus Reynolds Number for
Various Buoy Models

Gyo 2
tpvrA
where Dis the drag of the buoy in pounds,
p is the density of the water in slugs per cubic foot,
A is the maximum projected area of the buoy into a plane normal to the
flow, in square feet, and
V is the towing speed in feet per second.

The Reynolds number is defined as
Rie Ve
v

where V is the towing speed in feet per second,
d is the maximum diameter of the buoy, and
vy is the kinematic viscosity in feet® per second.

18

CONCLUSIONS

From the analysis of the test data on Buoys 1, 2, and 3 it was pos-
sible to select that buoy which would meet the requirements established by
the Woods Hole Oceanographic Institution (1 Ne

The requirement that the drag coefficient remain fairly constant
for a range of Reynolds numbers from 1.0 x 10° to 7.5 x 10° was satisfied by
both Buoys 1 and 3; see Figure 19. The other requirement, that the buoy se-
lected be reasonably stable in currents up to 5 knots, full-scale speed, was
satisfied only by Buoy 1. The full-scale model of Buoy 1 was selected as the
one most nearly fulfilling these requirements.

Buoy 1 will be further developed by the Woods Hole Oceanographic
Institution. .

REFERENCES

(1) Conference at the David Taylor Model Basin 15 March 1944, between
Dr. J.L. Hough and Mr. P. Osborn of the Woods Hole Oceanographic Institution
and Messrs. L. Landweber, G. Grimminger, and R. A. Ebner of the Taylor Model
Basin staff.

(2) Equipment Information Booklet, David Taylor Model Basin, July 1942.

(3) Goldstein, S., "Modern Developments in Fluid Dynamics," Vol. 2, p.
571, 1938.

PRN C-7614-10-22-48-76

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