Editing Submarine (section)

===Hull===
{{main|Submarine hull}}

====Overview====
[[File:USS Greeneville (SSN 772) - dry dock Pearl Harbor (1).jpg|thumb|The [[US Navy]] {{sclass|Los Angeles|submarine|0}} {{USS|Greeneville|SSN-772|6}} in dry dock, showing cigar-shaped hull]]

Modern submarines are cigar-shaped. This design, also used in very early submarines, is sometimes called a "[[teardrop hull]]". It reduces hydrodynamic [[drag (physics)|drag]] when the sub is 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&nbsp;[[knot (unit)|kt]] (18&nbsp;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 tile|anechoic plating]], to reduce detection.

The occupied pressure hulls of deep-diving submarines such as {{ship|DSV|Alvin}} are spherical instead of cylindrical. This allows a more even distribution of stress and efficient use of materials to withstand external pressure as it gives the most internal volume for structural weight and is the most efficient shape to avoid buckling instability in compression. A frame is usually affixed to the outside of the pressure hull, providing attachment for ballast and trim systems, scientific instrumentation, battery packs, [[syntactic foam|syntactic flotation foam]], and lighting.

A raised tower on top of a standard submarine accommodates the [[periscope]] and electronics masts, which can include radio, [[radar]], [[electronic warfare]], and other systems. It might also include a snorkel mast. In many early classes of submarines (see history), the control room, or "conn", was located inside this tower, which was known as the "[[conning tower]]". Since then, the conn has been located within the hull of the submarine, and the tower is now called the [[Sail (submarine)|"sail" or "fin"]]. The conn is distinct from the "bridge", a small open platform in the top of the sail, used for observation during surface operation.

"Bathtubs" are related to conning towers but are used on smaller submarines. The bathtub is a metal cylinder surrounding the hatch that prevents waves from breaking directly into the cabin. It is needed because surfaced submarines have limited [[freeboard (nautical)|freeboard]], that is, they lie low in the water. Bathtubs help prevent swamping the vessel.

====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>

====Pressure hull====
[[File:Bathyscaphe Trieste.jpg|thumb|In 1960, [[Jacques Piccard]] and [[Don Walsh]] were the first people to explore the [[Challenger Deep|deepest part]] of the world's [[ocean]], and the deepest location on the surface of the Earth's crust, in the {{ship|Bathyscaphe|Trieste}} designed by [[Auguste Piccard]].]]
{{See also|Pressure hull}}

The pressure hull is generally constructed of thick high-strength steel with a complex structure and high strength reserve, and is separated by watertight [[bulkhead (partition)|bulkheads]] into several [[Compartmentalization (fire protection)|compartments]]. There are also examples of more than two hulls in a submarine, like the {{sclass2|Typhoon|submarine|4}}, which has two main pressure hulls and three smaller ones for control room, torpedoes and steering gear, with the missile launch system between the main hulls, all surrounded and supported by the outer light hydrodynamic hull. When submerged the pressure hull provides most of the buoyancy for the whole vessel.

The [[Submarine depth ratings|dive depth]] cannot be increased easily. Simply making the hull thicker increases the structural weight and requires reduction of onboard equipment weight, and increasing the diameter requires a proportional increase in thickness for the same material and architecture, ultimately resulting in a pressure hull that does not have sufficient buoyancy to support its own weight, as in a [[bathyscaphe]]. This is acceptable for civilian research submersibles, but not military submarines, which need to carry a large equipment, crew, and weapons load to fulfill their function. Construction materials with greater [[specific strength]] and [[specific modulus]] are needed.

WWI submarines had hulls of [[carbon steel]], with a {{convert|100|m|ft|adj=on}} maximum depth. During WWII, high-strength [[alloy]]ed steel was introduced, allowing {{convert|200|m|ft|adj=on}} depths. High-strength alloy steel remains the primary material for submarines today, with {{convert|250|-|400|m|ft|adj=on}} depths, which cannot be exceeded on a military submarine without design compromises. To exceed that limit, a few submarines were built with [[titanium]] hulls. Titanium alloys can be stronger than steel, lighter, and most importantly, have higher immersed [[specific strength]] and [[specific modulus]]. Titanium is also not [[ferromagnetism|ferromagnetic]], important for stealth. Titanium submarines were built by the Soviet Union, which developed specialized high-strength alloys. It has produced several types of titanium submarines. Titanium alloys allow a major increase in depth, but other systems must be redesigned to cope, so test depth was limited to {{convert|1000|m|ft}} for the {{ship|Soviet submarine|K-278 Komsomolets}}, the deepest-diving combat submarine. An {{sclass2|Alfa|submarine|2}} may have successfully operated at {{convert|1300|m|ft}},<ref>{{cite web|url=https://fas.org/man/dod-101/sys/ship/deep.htm|title=Federation of American Scientists|publisher=Fas.org|access-date=18 April 2010}}</ref> though continuous operation at such depths would produce excessive stress on many submarine systems. Titanium does not flex as readily as steel, and may become brittle after many dive cycles. Despite its benefits, the high cost of titanium construction led to the abandonment of titanium submarine construction as the Cold War ended. Deep-diving civilian submarines have used thick [[Poly(methyl methacrylate)|acrylic]] pressure hulls. Although the specific strength and specific modulus of acrylic are not very high, the density is only 1.18g/cm<sup>3</sup>, so it is only very slightly denser than water, and the buoyancy penalty of increased thickness is correspondingly low.

The deepest [[deep-submergence vehicle]] (DSV) to date is [[Bathyscaphe Trieste|''Trieste'']]. On 5 October 1959, ''Trieste'' departed San Diego for [[Guam]] aboard the freighter ''Santa Maria'' to participate in ''[[Project Nekton]]'', a series of very deep dives in the [[Mariana Trench]]. On 23 January 1960, ''Trieste'' reached the ocean floor in the Challenger Deep (the deepest southern part of the Mariana Trench), carrying [[Jacques Piccard]] (son of Auguste) and Lieutenant [[Don Walsh]], USN.<ref>{{cite web|url=http://www.history.navy.mil/danfs/t8/trieste.htm |title=Trieste |publisher=History.navy.mil |access-date=18 April 2010 |url-status=dead |archive-url=https://web.archive.org/web/20100317120249/http://www.history.navy.mil/danfs/t8/trieste.htm |archive-date=17 March 2010 }}</ref> This was the first time a vessel, crewed or uncrewed, had reached the deepest point in the Earth's oceans. The onboard systems indicated a depth of {{convert|11521|m|ft|0}}, although this was later revised to {{convert|10916|m|ft|0}} and more accurate measurements made in 1995 have found the Challenger Deep slightly shallower, at {{convert|10911|m|ft|0}}.

Building a pressure hull is difficult, as it must withstand pressures at its required diving depth. When the hull is perfectly round in cross-section, the pressure is evenly distributed, and causes only hull compression. If the shape is not perfect, the hull deflects more in some places and [[buckling]] instability is the usual [[failure mode]]. Inevitable minor deviations are resisted by stiffener rings, but even a one-inch (25&nbsp;mm) deviation from roundness results in over 30 percent decrease of maximal hydrostatic load and consequently dive depth.<ref>{{cite web|url=http://www.usna.edu/naoe/courses/en200/ch10.pdf|title=US Naval Academy}}</ref> The hull must therefore be constructed with high precision. All hull parts must be welded without defects, and all joints are checked multiple times with different methods, contributing to the high cost of modern submarines. (For example, each {{sclass|Virginia|submarine|0}} attack submarine costs US$2.6 [[1000000000 (number)|billion]], over US$200,000 per [[long ton|ton]] of displacement.)