<P> Submarines in a neutral buoyancy condition are not intrinsically trim - stable . To maintain desired trim, submarines use forward and aft trim tanks . Pumps can move water between the tanks, changing weight distribution and pointing the sub up or down . A similar system is sometimes used to maintain stability . </P> <P> The hydrostatic effect of variable ballast tanks is not the only way to control the submarine underwater . Hydrodynamic maneuvering is done by several surfaces, which can be moved to create hydrodynamic forces when a submarine moves at sufficient speed . The stern planes, located near the propeller and normally horizontal, serve the same purpose as the trim tanks, controlling the trim, and are commonly used, while other control surfaces may not be present on all submarines . The fairwater planes on the sail and / or bow planes on the main body, both also horizontal, are closer to the center of gravity, and are used to control depth with less effect on the trim . </P> <P> When a submarine performs an emergency surfacing, all depth and trim methods are used simultaneously, together with propelling the boat upwards . Such surfacing is very quick, so the sub may even partially jump out of the water, potentially damaging submarine systems . </P> <P> Modern submarines are cigar - shaped . This design, visible in early submarines, is sometimes called a "teardrop hull". It reduces the hydrodynamic drag when submerged, but decreases the sea - keeping capabilities and increases drag while surfaced . Since the limitations of the propulsion systems of early submarines forced them to operate surfaced most of the time, their hull designs were a compromise . Because of the slow submerged speeds of those subs, usually well below 10 kt (18 km / h), the increased drag for underwater travel was acceptable . Late in World War II, when technology allowed faster and longer submerged operation and increased aircraft surveillance forced submarines to stay submerged, hull designs became teardrop shaped again to reduce drag and noise . USS Albacore (AGSS - 569) was a unique research submarine that pioneered the American version of the teardrop hull form (sometimes referred to as an "Albacore hull") of modern submarines . On modern military submarines the outer hull is covered with a layer of sound - absorbing rubber, or anechoic plating, to reduce detection . </P>

What is the benefit of the shape of the submarine