Archimedes' principle states: "Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object."

To harness this buoyant force, an object must be able to control its displacement or weight. Some centuries-old unfinished designs for submarines used leather walls which would expand or contract to change the vessels' volume, but all real submarines control weight instead, by use of ballast tanks. To surface, the tanks are filled with air. To submerge, they are flooded with seawater, increasing the submarine's weight to overcome its buoyancy.

Ballast tank diagram

Submarines have two hulls. The inner hull, called the pressure hull, maintains conditions for survival inside, while the outer hull gives the submarine its form. Ballast tanks are held between them, and are used to control buoyancy.

Control surfaces

Submarines have to maintain trim to stay in control, meaning that their weight needs to be balanced along their length. To do so, crew carefully manage weight inside the submarine, and special ballast tanks, called trim tanks, can adjust balance.

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