Figure 13-60.--Developing a curve without locked-in stresses.
and no danger of snapping the curve out later by
accidental overload.
Figure 13-61.--Water spray torch used for reducing cooling
time.
Water Spray Equipment
The water spray torch shown in figure 13-61 may
--Select the correct material for the job. Selecting
be used to reduce cooling time. The torch consists of a
materials for the fabricating plate structures is
nozzle, a "Y" fitting, a control valve, and a l/4-inch
essentially a design problem, requiring a thorough
rubber hose. The air hose is connected to the ship's
knowledge of the properties of materials. The materials
service compressed air and the rubber hose is run to a
for most shipboard fabrication jobs are specified in the
pail of water. When you open the control valve, the rush
job order or on the blueprints. You must ALWAYS
of air past the orifice in the "Y" fitting draws some
consult these sources of information before the material
water into the air stream, creating an atomized spray.
is selected. However, you should know enough about
When the spray strikes the hot plate, it turns into steam
the properties of materials to be able to make an
and absorbs a substantial amount of heat. One pound
intelligent selection when this responsibility is left up
of water will absorb 142 Btu while vaporizing into
to you.
steam. Because of the heat requirements for
--In making a repair or replacement, substitution
vaporization, the cooling is rapid, even with the use of
of one material for another requires a knowledge of the
only a small amount of water. Since all of the sprayed
service conditions that the piece must withstand.
water is vaporized, the work will remain dry.
Low-carbon steel has an ultimate tensile strength of
60,000 psi and cast iron has an ultimate tensile strength
FABRICATION EXAMPLES
of 20,000 psi. When you substitute low-carbon steel for
cast iron, the cross-sectional area of the steel need
The variety of fabrication problems that arises on
only be one-third that of the cast iron if tensile
board ship is almost endless. The examples in this
strength is the ONLY consideration. In many cases,
chapter are not to be considered as a complete listing of
rigidity will be the controlling factor. When it is
jobs you may have to perform.
necessary to maintain the same rigidity, a low-carbon
steel piece would have to be four-fifths the thickness
The following basic considerations apply to most
fabrication jobs:
of the cast-iron piece. When substituting another
13-52
