Submarine

Subdivision.

Except for the tanks essential for their opera tion, earlier submarines had little or no watertight subdivision. In later types the hull is subdivided to the maximum extent, experience having proved the value of this feature in case of accident. Transverse bulkheads are the main feature of subdivi sion, a submarine of medium displacement having at least six main compartments, whilst at the extremities trimming and other tanks are added (fig. 6). Starting from forward the main compartments in order are : the fore torpedo room, into which the bow torpedo tubes project and from which they are loaded and operated; the battery space, the lower portion of which contains the battery cells whilst accommodation for the crew is provided in the upper portion ; the control room, in which all controlling mechanisms for the submerged operations of the sub marine are fitted, whilst below is a battery space and over the conning tower giving access to the bridge ; the engine ruom with the two Diesel engines, the spaces below them being utilised for the lubricating oil tanks; the motor room with the main motors and switchboards and some accommodation for the crew; and finally, the stern torpedo room containing the breech ends of the torpedo tubes, spare torpedoes, and the motors and gear for rudder and hydroplanes. A deck further subdivides the two torpedo rooms, the spaces below it being divided by water tight bulkheads, boundaries to the compensating and fresh water or fuel oil tanks and provision and other stores. In a single hull submarine the main ballast tanks are below and at the sides of the battery, and at the bow and stern, whilst in a double or partial double hull submarine the space between the two hulls is divided by transverse bulkheads and flats to form main ballast and oil fuel tanks.

The main bulkheads at the after end of the fore torpedo room and at the fore end of the stern room are generally tested to 35 or 5o lb. per sq.in., the two rooms being sometimes termed refuge compartments, where in case of accident the survivors may take refuge. The other main bulkheads are tested to 15 to 20 lb. per sq.in., being primarily intended to limit the entry of water in surface collisions. The bulkheads at the extremities are tested to 5o to 75 lb. whilst those below the decks are subject to 5o lb. test and those between the hulls to 15 or 20 pounds. In many submarines the transverse bulkheads are of curved form to better withstand the pressure. Watertight doors of a special quick closing type are provided, with manholes, and other means of access.

Tanks in Submarines.

The most important tanks are the "main ballast tanks" by the filling of which almost all the positive buoyancy is destroyed. Each tank has a valve at the bottom for the entry or exit of water; a vent valve at the top for the escape of air; and a pipe through which compressed air enters the tank and forces out the water. Some tanks are also fitted with drain pipes for emptying them by pumping. When cruising on the sur face the bottom valve may be open and the vent valve closed. Water then enters the tank compressing the air until the pressure prevents further entry, or a slight air pressure is put into the tank to limit the water to the least amount. When submerging

all main ballast vent valves are opened, water floods the tanks, the air escaping through the vent valves, which, when the tanks are full, are closed. The submarine "Narval" on her first trials submerged in 25 minutes, reduced to 12 when the crew were trained. War experience has shown that the time for submersion is a vital factor in escaping gun attack or ramming, and the valves have been increased until i to 2 minutes is now required. The flooding valves are open and vent valves closed when sub merged, and the submarine is brought to the surface by forcing air into the tanks, thus clearing out the water. The vent and blow ing valves are all operated from the control room. When enemy ships may be met, a submarine navigating on the surface has her main ballast tanks partially filled to secure more rapid submergence. If the tanks are half full she is at "half buoyancy," and if three-quarter, at "quarter buoyancy," if empty she is at "full buoyancy." Some of the main ballast tanks are arranged for the stowage of fuel, in case extra cruising endurance is required.

A submarine is at "main ballast trim," or in "diving trim," when the flooding of the main ballast tanks alone will bring her to the desired submerged condition, with practically no reserve of buoyancy. During war or practice operations, a submarine is always in "main ballast trim," or in "diving trim," and can at once submerge. To maintain this trim the officers have to exer cise a ceaseless vigilance, any loss or gain of weight being met by letting in or blowing or pumping out water. For this purpose "compensating tanks," or "auxiliary ballast tanks," are pro vided, generally one on either side amidships, and one at either end. These have flooding, venting, pumping and blowing arrange ments, and gauges to indicate the amount of water in them under all conditions.

The oil fuel is the main item for which compensation is needed. The fuel is stored in the "oil fuel tanks," sometimes placed in the spaces exterior to the pressure hull in the "double hull" or partial "double hull" type, and necessarily inside in the "single hull" type. The fuel tanks are completely filled with oil, and are positioned symmetrically transversely and longitudinally. When fuel from a tank is required, it is forced to the engine room by pumping sea water into the bottom of the tank, the oil passing out through a pipe at the top. The difference in densities between sea water and oil fuel prevents mixing, but involves an increase in weight as fuel is used. If, for example, the oil fuel is .9 specific gravity, and a tank of Soo cubic feet has been emptied of fuel and filled with sea water, the added weight is 3,900 lb., and this weight of water must be discharged from the compensating tanks. Other items, i.e., lubricating oil, ammunition, shell, food, fresh water, etc., when consumed, must be compensated for by adding an equal weight of water, the compensating tank used for the purpose being as close as possible, longitudinally, to the item consumed. In minelaying submarines special tanks are fitted for compensating for the release of mines.