480 NICHOLLS'S SEAMANSHIP AND NAUTICAL KNOWLEDGE
Figure 79 is an example of an Isherwood ship for general cargo
purposes, in which longitudinal framing is adopted for the double
bottoms, and decks, with deep frames at the sides and transverse beams
at every third frame Note the vertical stifieners on the bulkhead
bracketed at top and bottom to the longitudinal frames, and the
tubular piUars at the hatch corners. Tubular pillars have solid heads
and heels.
Corrosion.—When steel is exposed to the action of air, or of moisture,
particularly if it be salt-laden, rust soon appears on its surface which
penetrates into the plating and corrodes it away. Oxygen is necessary
to the formation of rust, hence the reason why it is necessary to
exclude air from direct contact with iron and steel by keeping it well
coated with paint. New ships should be well coated, especially round
rivet heads, butts and laps.
When corrosion sets in all rust must be scaled or hammered ofi,
the bare metal thoroughly scraped, cleaned and dried before applying
a priming coat of paint. Several coats should be given in succession,
allowing sufficient time between each to allow the previous coat to
dry hard.
Sometimes butt straps and plate landings show signs of moisture
and when this occurs the " weeping " joints are made tight by cleaning
out the crevices with a wire brush and caulking the edges, which is
just burring up the edges of the plating with a chisel and hammer to
close up the joint.
Paint is composed of two ingredients, a pigment and a vehicle. The
pigment is the solid particles, the vehicle is the oil or liquid portion.
The principal pigments are white lead, zinc oxide and red lead; the
principal vehicles used on board ship are linseed oil, turpentine and
varnish. Linseed oil is a drying oil and turpentine is used for thinning
out. Driers are used to expedite the drying of the paint. The drier
acts on the oil but not on the pigment, and too much drier causes the
paint to crack and peel off. Copal varnish is mostly used for woodwork.
Red lead is supplied as a dry powder. It is a very good first coating,
is strongly adhesive, sets hard and dries quickly. The following quan-
tities will make up 1 gallon of mixed paint ready for application, its
covering power being about 50 square yards:—20 Ibs. red lead; 5 pints
linseed oil; 2 gills turpentine; 2 gills drier.
White lead produces a hard layer and is used as an undercoating.
It has more body than white sane, which is a thinner and lighter pigment
and retains its pure colour better than white lead. White zinc is used, for