The Submarine Torpedo Boat
Its Characteristics And Modern Development

62 THE SUBMARINE TORPEDO BOAT neutralize the effect of the reserve buoyancy, usually about from twenty to thirty per cent of the total displace- ment. This main ballast system is designed to be kept completely filled when submerged in order that the con- tained large bodies of water may not surge forward and aft and so destroy the trim of the vessel. The main ballast tank is supplemented by fore and aft trim tanks and an auxiliary adjusting and compensating tank. The for- ward and aft trim tanks must have sufficient capacity to overcome any change in moments due to a disarrangement or movement of the weights on board, and to bring the vessel back to an even keel by the transfer of water from one tank to the other. The auxiliary ballast tank must be large enough to completely overcome the reserve buoyancy and to compensate for the variations of con- sumable stores and weights on board, and in addition to compensate for the difference in density of the water of flotation. A small adjusting tank is sometimes provided for the purpose of delicately adjusting the bouyancy of the vessel by taking in or blowing out a few pounds of water. To maintain trim when submerged it is essential that the center of gravity of the auxiliary ballast tanks should coincide longitudinally with the center of gravity of the emerged volume of the vessel and this coincidence must remain throughout the process of submergence. Other- wise serious alterations of trim in a fore and aft direction will take place with probably disastrous results. Apportionment OF WeIGHTS No hard and fast rule can be laid down for guidance in the matter of apportionment of weights. This of course depends, in the first place, upon what particular tactical