In the forward compartment is one torpedo expulsion tube, located at the extreme forward end of the vessel, opening outward two feet below the light water-line. The tube is placed with its muzzle in the nose of the craft and its axis inclined somewhat to the longitudinal axis of the vessel. The muzzle of the tube is closed by a watertight door, and this can be lifted from within for the discharge of the five Whitehead torpedoes, each 11 feet 8 inches long, which are carried in the interior. One torpedo is placed in the tube, and the other four are placed side by side above the storage batteries. When the first torpedo is fired a sufficient amount of water to compensate for the loss of weight is automatically and almost instantaneously admitted into the tube, causing only a slight change of time for a few seconds. Such compensation is necessary, as the submarine boat is lighter than the amount of water it displaces and any alteration in its weight tends to send it up to the surface or down towards the bottom. When the second torpedo is placed in the tube the water is run into a special torpedo compensating tank; of these tanks there are four, and they are filled as each torpedo is fired. When the last torpedo has been ejected the expulsion tube is filled with water and is kept thus until the end of the run.
Besides the torpedo tube there is in the forward compartment a series of air flasks, a gasoline tank of 850 gallons capacity, and a trimming tank.
The central compartment contains in its double bottom the main ballast tanks and a circular compensating tank placed between the two sets of batteries. Above the double bottom and below the axis of the vessel are located the storage batteries, and above these are carried the four torpedoes, which are 45 centimetres in diameter and 11 feet 8 inches in length. In the same compartment are a series of air flasks in which air at 2,000 lbs. to the square inch pressure is stored for the purpose of keeping pure the living spaces of the crew.[[9]]
[9]. Also for firing the torpedoes and for blowing the water out of the ballast tanks when it is desired to navigate as a surface boat.
In the rear compartment is the four-cylinder gasoline engine, which is rated at from 160 to 190 actual horse-power, and at from 320 to 390 revolutions per minute. Its net weight is 1,300 lbs., its length over all is 9 feet 7 inches, and its total height above the crank-shaft centre is 5 feet 6 inches. In these engines, which gave satisfaction in the first Holland boat, the distribution of the cranks and the timing of the valves and igniters are so arranged that the operations in the four-cylinders alternate, so that while one is at the expansion stroke the other three are at the suction, compression, and exhaust strokes respectively. By this arrangement the engine is perfectly balanced and vibration is reduced to a minimum.
Submersion.—The first operation to be performed when it is desired to submerge the vessel is the admission of water into her ballast tanks. When these are full she runs “awash” with her conning tower alone above the waves.
In order to completely submerge the boat she is steered below the surface by means of her pair of horizontal rudders at the stern. When at the required depth (normally from 20 to 30 feet) she is brought up on an even keel by the inclination of the horizontal rudders, which may be controlled automatically (by some such arrangement as is found in the Whitehead torpedo) or by hand. All modern submarine craft go under at an angle, and are steered below the waves by rudders: the principle of submersion on an even keel by down-haul propellers, advocated by Mr. Nordenfelt, has been abandoned by all latter-day designers, as has also the submersion of the drawing in of cylinders adopted in the Campbell-Ash boat.
When running submerged the submarine is lighter than her displacement, and is only kept under by her horizontal rudders, which, of course, only act when she is moving.
The great drawback to all under-water craft has been their lack of longitudinal stability, for they have a tendency to rise or sink at the slightest provocation. In our account of the Nordenfelt boats this disagreeable quality is illustrated. In order to keep the vessel at a regular depth recourse must be had to the horizontal rudders and to the two trimming tanks and the one circular compensating tank carried.
In a general way it will probably be found possible to keep the boat on an even keel by means of her rudders, but if she shows any decided tendency either to dive to the bottom or to rise to the surface or to heel over to one side water may be admitted into the above-mentioned tanks, and the admission may be effected either automatically or by hand. A hydrostatic valve will prevent the vessel passing the safe limit of depth.