What the Buoyancy Tanks Are For.—We said previously that a submarine never loses its buoyancy completely. If she were built like an ordinary ship and simply fitted with ballast tanks, she would sink when these are filled with water and never come up again, for her natural buoyancy would be destroyed. As it is, a submarine has tanks filled with air which keeps her buoyant, and these will bring her to the surface the moment the ballast tanks are empty.
Make this experiment and you will quickly understand how these buoyancy air tanks work: Take an empty bottle and cork it up tight. It looks empty, but as a matter of fact it is filled with air. Push the bottle to the bottom of a bucket of water; let go of it and the instant you do so it will rise to the surface.
A large number of steel air tanks, or buoyancy tanks, or high pressure air-flasks, as they are called, are placed both fore and aft in a submarine, and these are filled with compressed air at a pressure of 2,000 pounds to the square inch. These air-flasks have a tremendous supporting power—which is only another way of saying that they are extremely buoyant.
It must be clear now that even though the submarine is resting on the floor of the ocean it can always rise to the surface by the reserve buoyancy provided by these air-flasks.
Compressed Air and Air-Compressor Pumps.—The air-flasks are filled with compressed air by air-compressor pumps which are driven by the engines when the submarine is running light or awash.
The air compressor is formed of several air pumps coupled together; each air pump is made very much like a water pump, but it sucks the air in from the outside and then forces it into the air-flasks until it is under a pressure of 2,000 pounds to the square inch. To overcome this back pressure, as it is called, the pumps must be extra powerful.
These pumps also supply compressed air for the torpedo tubes, the trimming tanks, and to help blow out the water from the ballast tanks.
Inside the Torpedo Compartment.—The torpedo compartment contains the extra torpedoes in their cradles. Near the tubes from which the torpedoes are shot is the compressed air which furnishes the propulsive power needed to make the torpedoes leave their tubes.
Why Trimming Tanks Are Used.—As a torpedo weighs nearly 1,000 pounds, it is plain that whenever one is shot from the craft it will very greatly disturb the balance of the boat unless some means is used to add weight to it which is exactly equal to the weight of the torpedo.
This is done by what is called the trimming tanks. These are usually placed fore and aft and in or near the torpedo and mine compartments. As soon as a torpedo is shot, or a mine is laid, the trimming tanks are filled with water which makes up for the weight lost and keeps the craft on an even keel.