use compressed air for the operation of the
instruments. Compressed air is also used in
diving bells and diving suits. Our following
discussion on the use of compressed air as an aid
in the control of submarines will help you under-
stand the theory of pneumatics.
Submarines are designed with a number of
tanks that may be used for the control of the ship.
These tanks are flooded with water to submerge,
or they are filled with compressed air to surface.
The compressed air for the pneumatic system
is maintained in storage tanks (called banks) at
a pressure of 4500 psi. During surfacing, the
pneumatic system delivers compressed air to the
desired control tanks (the tanks filled with water).
Since the pressure of the air is greater than the
pressure of the water, the water is forced out of
the tank. As a result, the weight of the ship
decreases. It becomes more buoyant and rises to
As you look around, you see not only that
your ship is constructed of metal, but also that
the boilers, piping system, machinery, and even
your bunk and locker are constructed of some
type of metal. No one type of metal can serve all
the needs aboard ship. Many types of metals or
metal alloys must be used. A strong metal must
be used for some parts of a ship, while a
lightweight metal is needed for other parts. Some
areas require special metal that can be shaped or
worked very easily.
The physical properties of some metals or
metal alloys make them more suitable for one use
than for another. Various terms are used in
describing the physical properties of metals. By
studying the following explanations of these
terms, you should have a better understanding of
why certain metals are used on one part of the
ships structure and not on another part.
BRITTLENESS is a property of a metal that
will allow it to shatter easily. Metals, such as cast
iron or cast aluminum and some very hard steels,
DUCTILITY refers to the ability of a metal
to stretch or bend without breaking. Soft iron,
soft steel, and copper are ductile metals.
HARDNESS refers to the ability of a metal
to resist penetration, wear, or cutting action.
MALLEABILITY is a property of a metal
that allows it to be rolled, forged, hammered, or
shaped without cracking or breaking. Copper is
a very malleable metal.
STRENGTH refers to the ability of a metal
to maintain heavy loads (or force) without
breaking. Steel, for example, is strong, but lead
TOUGHNESS is the property of a metal that
will not permit it to tear or shear (cut) easily and
will allow it to stretch without breaking.
Metal preservation aboard ship is a continuous
operation, since the metals are constantly exposed
to fumes, water, acids, and moist salt air. All of
these elements will eventually cause corrosion. The
corrosion of iron and steel is called rusting. This
results in the formation of iron oxide (iron and
oxygen) on the surface of the metal. Iron oxide
(or rust) can be identified easily by its reddish
color. (A blackish hue occurs in the first stage of
rusting but is seldom thought of as rust.)
Corrosion can be reduced or prevented by use of
better grades of alloyed metals. Chromium and
nickel are commonly used. Coating the surface
with paint or other metal preservatives also helps
Metals and alloys are divided into two
general classes: ferrous and nonferrous. Ferrous
metals are those composed primarily of iron.
Nonferrous metals are those composed primarily
of some element or elements other than iron.
One way to tell a common ferrous metal from a
nonferrous metal is by using a magnet. Most
ferrous metal is magnetic, and nonferrous metal
Elements must be alloyed (or mixed) together
to obtain the desired physical properties of a
metal. For example, alloying (or mixing)
chromium and nickel with iron produces a metal
known as special treatment steel (STS). An STS
has great resistance to penetrating and shearing
forces. A nonferrous alloy that has many uses
aboard ship is copper-nickel. It is used extensively
in saltwater piping systems. Copper-nickel is a
mixture of copper and nickel. Many other
different metals and alloys are used aboard ship
that will not be discussed here.