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Submarines Surfacing and Diving

Submarines use a system of tanks that can be filled with and emptied of seawater to control the buoyancy of the warship. These tanks, called ballast tanks, permit diving and surfacing. The modern submarine has an array of wrap-around ballast tanks surrounding the inner (or pressure) hull of the submarine, where the crew lives. The ballast tanks are filled with seawater to reduce buoyancy for the dive. To surface, compressed air is ejected into the tanks, forcing out the ballast water and increasing the ship’s buoyancy. By varying the number of tanks that are filled with water, a submarine can run at different underwater depths.

Maneuvering underwater is accomplished by adjusting the ship's rudder and control planes. These are short, hydraulically powered wing-shaped surfaces. The rudder is located at the rear of the sub near the propeller, and controls left and right movement. Two sets of control planes are mounted near the rudder and at the forward end of the sub, either on the hull or bridge structure. The control planes control upward and downward movement through water, and help the submarine surface and dive. World War II-era and later submarines have a separate set of controls for the stern (rear) planes and fairwater (forward) planes, but one person can also operate the submarine at a single station. By flooding the ballast tanks with water and controlling the planes, the submarine can dive through the water to the desired depth.

Three factors govern how a modern submarine functions in underwater combat: its operating depth, sound silencing ability, and navigational capabilities.

A submarine’s operating depth is limited by the toughness of the pressure hull and penetrations for seawater intake pipes, torpedo tubes, and other openings. World War II-era submarines could only venture safely to a depth of about 120 m (about 400 ft), while modern nuclear submarines are believed to be able to descend safely to depths in excess of 460 m (1500 ft)—the precise figures are highly classified. ©2018.