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 force.

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.

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 desired rate.

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 submarine.

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 trim operations.).

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.

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.

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