1A1. Balance and stability. A modern submarine is designed to dive or surface rapidly
under complete control. It must be able to
proceed on the surface and to submerge at the
desired rate of speed to the depths required.
To do so quickly and efficiently, the submarine must maintain fore and aft balance,
and athwartship stability. The chief function
of the trim and drain system is to maintain
this fore and aft balance by controlling the
amount and distribution of water in the various tanks used for this purpose.
Before proceeding with the functional
description of the trim and drain system, let
us consider the factors affecting the balance
and stability of the submarine. The balance
and stability of the submarine are maintained
by applications of the principles of buoyancy
and the law of the lever.
1A2. Buoyancy. Buoyancy is the force which
tends to keep an object afloat in water or any
other liquid. When an object is immersed in a
liquid, the liquid exerts pressure from all directions on the external surface of the object.
The deeper the object is immersed in the
liquid, the greater is the pressure exerted
against its surface. Also, the upward pressure
exerted by the liquid against the lower surface of the object is greater than the downward
pressure against its top. If the immersed
object weighs more than the liquid it displaces, it is said to have negative buoyancy.
Such an object sinks. If the object weighs less
than the liquid it displaces, the object is said
to have positive buoyancy. Such an object
floats, or if it is thrust under the surface of
the liquid, it rises. When both the object and
the liquid it displaces weigh the same, the
object is said to have neutral buoyancy. If
such an object is submerged, it remains submerged unless it is acted upon by an outside
1A3. Fore and aft balance. The conditions
of positive, neutral, and negative buoyancy
just described apply to submarine operations.
However, these buoyancy conditions must
always be considered with respect to the law
of the lever, or the balancing of forces, on
each side of the center of gravity of the boat.
This is known as fore and aft balance. When
a submarine is on the surface, or when it
reaches a desired depth, the first objective is
to attain perfect, or nearly perfect, trim, that
is, a balancing of the forces. The trimming of
the boat is accomplished by varying, or adjusting, the amount of water in the variable
ballast tanks. The trim system is the means by
which this adjustment is made.
B. TRIM SYSTEM
1B1. Functions. The assumption is made in
the next few paragraphs that the submarine
is in diving trim on the surface. The submarine is so designed that when the main ballast tanks are empty, it has positive buoyancy
and can cruise on the surface. When the main
ballast tanks are flooded, the positive buoyancy
is destroyed, and a state of neutral buoyancy
exists. This enables the submarine to cruise
underwater at any desired depth. Of course,
diving, surfacing, and cruising submerged are
further controlled by means of the bow and
stern planes and rudder and speed adjustments.
However, the trim has been so
carefully adjusted that by flooding the main
ballast tanks and adding the required amount
of water to the special ballast tanks, the
submarine can be made to submerge at the
In actual operation the condition of fore
and aft balance assumed in the preceding
paragraph is obtained by the use of the trim
system. The trim system consists of a trim
pump, a trim manifold, and the connecting
piping leading to the variable ballast tanks.
The trim system admits additional water
ballast to the variable ballast tanks to compensate
for loss of weight, removes water ballast to
reduce excessive weight, and distributes water
ballast to the proper tanks to compensate for
unequal distribution of weight aboard the
For example, if additional stores are
stowed in the forward end of the boat, water
is pumped out of the forward trim tank and
auxiliary tanks in accordance with the compensating sheet, to compensate for this additional weight. The auxiliary ballast tanks,
No. 1 and No. 2 amidships, generally are used
to compensate for over-all weight changes,
and therefore water ballast must be taken into
these tanks or discharged overboard as required.
Since the auxiliary tanks form a U outside the pressure hull and are separated at
the keel, it follows that if a large amount of
water ballast is added, it must be added equally on both the port and starboard sides in
order to prevent listing. Again, as in maintaining fore and aft balance, the trim system
is used in adjusting the distribution of this
water ballast so that athwartship stability is
maintained. Either the port or the starboard
tank may be used individually to correct listing of the ship.
The safety and negative tanks can be
used as additional variable ballast tanks since
they can be flooded and pumped by means of
the trim manifold.
In addition, the trim system can be used
to flood and pump the water round torpedo
(WRT) tanks, both forward and aft. (See
Chapter 2 for a detailed description of the
trim system, and Chapter 4 for procedures for
1B2. Standby trim pump. Because of the importance of the functions of the trim system,
it is necessary that a standby trim pump be
provided to insure operation of the system in
the event of failure of the trim pump. This
standby equipment is provided by cross connecting the drain pump of the drain system
with the trim manifold of the trim system.
Thus each pump serves as a standby for the
other, assuring operation of both systems.
C. DRAIN SYSTEM
1C1. Functions. The chief function of the
drain system is to pump out the free water
that accumulates in the wells, bilge sumps,
and poppet valve drain tank. This water,
which enters the ship from drains, overflow
piping, gland leakage, and condensation, must
be removed, for it may interfere with the
operation of the vessel.
The drain system consists essentially
of a pump and piping extending to the forward
and after sections of the ship. A number
of individually controlled branch suction lines
extend to the bilge sumps and wells.
The drain system is interconnected with
the trim system through the trim manifold.
This enables the trim system to operate
through the drain pump and the drain system
to operate through the trim pump, when necessary.