Editing Submarine (section)

====Single and double hulls====
[[File:U995 2004 1.jpg|thumb|{{GS|U-995||2}}, Type VIIC/41 U-boat of World War II, showing the ship-like lines of the outer hull for surface travel, blended into the cylindrical pressure hull structure.]]

Modern submarines and submersibles usually have, as did the earliest models, a single hull. Large submarines generally have an additional hull or hull sections outside. This external hull, which actually forms the shape of submarine, is called the outer hull (''[[Casing (submarine)|casing]]'' in the Royal Navy) or [[light hull]], as it does not have to withstand a pressure difference. Inside the outer hull there is a strong hull, or [[pressure hull]], which withstands sea pressure and has normal atmospheric pressure inside.

As early as World War I, it was realized that the optimal shape for withstanding pressure conflicted with the optimal shape for seakeeping and minimal drag at the surface, and construction difficulties further complicated the problem. This was solved either by a compromise shape, or by using two layered hulls: the internal strength hull for withstanding pressure, and an external fairing for hydrodynamic shape. Until the end of World War II, most submarines had an additional partial casing on the top, bow and stern, built of thinner metal, which was flooded when submerged. Germany went further with the [[Type XXI]], a general predecessor of modern submarines, in which the pressure hull was fully enclosed inside the light hull, but optimized for submerged navigation, unlike earlier designs that were optimized for surface operation.

[[File:SRH025-p40.jpg|thumb|left|[[Type XXI]] U-boat, late World War II, with pressure hull almost fully enclosed inside the light hull]]
After World War II, approaches split. The Soviet Union changed its designs, basing them on German developments. All post-World War II heavy Soviet and Russian submarines are built with a [[double hull]] structure. American and most other Western submarines switched to a primarily single-hull approach. They still have light hull sections in the bow and stern, which house main ballast tanks and provide a hydrodynamically optimized shape, but the main cylindrical hull section has only a single plating layer. Double hulls are being considered for future submarines in the United States to improve payload capacity, stealth and range.<ref>[http://www.nationaldefensemagazine.org/issues/2000/May/Virginia-Class.htm]. National Defense magazine. {{webarchive|url=https://web.archive.org/web/20080405194626/http://www.nationaldefensemagazine.org/issues/2000/May/Virginia-Class.htm|date=5 April 2008}}</ref>