For correcting any alteration in the longitudinal distribution of weight, a "trimming tank" is fitted at either end of the vessel. They are partially filled in the normal diving condition and connected by a pipe through which water is forced from one to the other as required.
"Water around torpedo" tanks are fitted below the centre of length of the torpedo tubes, and contain sufficient water to fill the spaces around the torpedoes when in the tubes. By their use the torpedoes can be left dry in the tubes and withdrawn for examination. If it is desired to fire a torpedo the tube is closed, the water from the "water around torpedo" tanks is blown into the tube, and when the cap of the tube is opened, no water enters and the weight of the submarine is unaltered. An experienced submarine officer keeps a log of the variations of weight of the stores and of the amount of water in all his tanks, and checks the records on every convenient occasion by submerging his vessel.
The compressed air for blowing tanks and for the torpedo service is stored in reservoirs or air bottles at 2,500 lb. per square inch. The capacity of a reservoir varies in dif ferent navies, but in British submarines is about 3 cubic feet and the number sufficient to blow the ballast tanks three times. They are recharged by compressors when the vessel is on the sur face with hatches open. A shore connection is fitted for use when charging from the shore or parent ship. It was formerly the practice to clear the tanks entirely by air from the reservoirs, but the vessel is now brought to the surface by partially clearing one or more ballast tanks with stored air, and fully clearing the tanks by motor driven air or blowers which, with the conning tower hatch open, supply air at 15 or 20 lb. per square inch. Divers' connections are fitted in the superstructure for supplying air to the compartments in case of accident.
At least two motor driven pumps are provided for pumping out a submarine. These are capable of pumping against any pressure up to that at the greatest depth the vessel is designed to navigate. Both deliver overboard, and their suctions are connected with at least two of the main ballast tanks for use in the event of the compressed air failing.
The rudder is like that of a surface ship. Hydroplanes, or horizontal rudders, are used for the depth and horizontal control of submarines when navigating submerged. In earlier submarines, a pair of hydroplanes were fitted at the stern, but now a pair is fitted forward and a pair aft, the latter generally in the propeller races. The hydroplanes are rectangular or elliptical in shape, and are of the balanced type. They can be turned through 30 or 35 degrees either way by a motor and mechanism. The after pair are of greater area than the forward, and when in the neutral position act as fins, giving a certain amount of stability to the motion of the vessel. The for ward hydroplanes are generally fitted below the surface water plane, but in fast submarines they are often placed above water, and are of the housing type, stowing in or against the superstruc ture. The resistance to surface propulsion is thereby decreased, and the hydroplanes are not subjected to the forces of the sea. When a vessel is diving the hydroplanes are used to overcome the small reserve of buoyancy usual when submerged.
The operating motor and mechanism of each pair of hydroplanes are controlled by a handwheel mounted on a pedestal in the con trol room, close to the commander. The two pedestals are placed side by side, with a diving coxswain at each wheel. In front of each is a depth gauge showing the depth below water, and a clinometer giving the longitudinal inclination of the vessel. The coxswain at the forward pedestal moves his wheel, and by it the forward hydroplanes, to maintain the depth ordered by the com mander, whilst his colleague moves the after wheel and the after hydroplanes to maintain the submarine horizontal. In the earlier types with after hydroplanes only, change of depth was attended with large longitudinal inclinations of the submarine. In the event of a breakdown of the motors operating the hydroplanes, hand gear can be used. Hydroplane guards are fitted to prevent wires catching in the hydroplanes when passing through nets fitted as a defence to entrances to harbours, etc.