The term light hull (casing in British usage) is used to describe the outer hull of a submarine, which houses the pressure hull, providing hydrodynamically efficient shape, but not holding pressure difference. The term pressure hull is used to describe the inner hull of a submarine, which holds the difference between outside and inside pressure.
Modern submarines are usually cigar-shaped. This design, already visible on very early submarines is called a "teardrop hull", and was patterned after the bodies of whales. It significantly reduces the hydrodynamic drag on the sub when submerged, but decreases the sea-keeping capabilities and increases the drag while surfaced.
Since the limitations of the propulsion systems of early military submarines forced them to operate most their time on the surface, their hull designs were a compromise with outer hulls allowing for good surfaced abilities, and a relatively streamlined superstructure. Because of the slow submerged speeds of those subs, usually well below 10 knots (19 km/h), the increased drag for underwater travel by the conventional ship type outer hull was considered acceptable. Only late in World War II, when technology enhancements allowed faster and longer submerged operations and increased surveillance by enemy aircraft forced submarines to spend most of their times below the surface, did hull designs become teardrop shaped again, to reduce drag and noise. On modern military submarines the outer hull (and sometimes also the propeller) is covered with a thick layer of special sound-absorbing rubber, or anechoic plating, to make the submarine more difficult to detect by active and passive SONAR.
All small modern submarines and submersibles, as well as the oldest ones, have a single hull. However, for large submarines, the approaches have separated. All Soviet heavy submarines are built with a double hull structure, but American submarines usually are single-hulled. They still have light hull sections in bow and stern, which house main ballast tanks and provide hydrodynamically optimized shape, but the main, usually cylindrical, hull section has only a single plating layer.
The double hull of a submarine is different from a ship's double hull. The external hull, which actually forms the shape of submarine, is called the outer hull, casing or light hull. This term is especially appropriate for Russian submarine construction, where the light hull is usually made of steel that is only 2 to 4 millimetres thick, as it has the same pressure on both sides. The light hull can be used to mount equipment, which if attached directly to the pressure hull could cause unnecessary stress. The double hull approach also saves space inside the pressure hull, as the ring stiffeners and longitudinals can be located between the hulls. These measures help minimise the size of the pressure hull, which is much heavier than the light hull. Also, in case the submarine is damaged, the light hull takes some of the damage and does not compromise the boat's integrity, as long as the pressure hull is intact.
Inside the outer hull there is a strong hull, or pressure hull, which actually withstands the outside pressure and has normal atmospheric pressure inside. The pressure hull is generally constructed of thick high-strength steel with a complex structure and high strength reserve, and is separated with watertight bulkheads into several compartments. The pressure and light hulls aren't separated, and form a three-dimensional structure with increased strength. The interhull space is used for some of the equipment which doesn't require constant pressure to operate. The list significantly differs between submarines, and generally includes different water/air tanks. In case of a single-hull submarine, the light hull and the pressure hull are the same except for the bow and stern.
The task of building a pressure hull is very difficult. No matter how large the submarine is, its hull must be constructed with high precision. Inevitable minor deviations are resisted by the stiffener rings, but even a one inch (25 mm) deviation from roundness results in over 30 percent decrease of hydrostatic load. The total pressure force of several million tons must be distributed evenly over the hull and be oriented longitudinally, as no material could resist such force by bending. A submarine hull has to use expensive transversal construction, with the stiffeners rings located more frequently than the longitudinals. All hull parts must be welded without defects, and all joints are checked several times with different methods. This contributes to very high cost of modern submarines (for instance, a Virginia-class attack submarine costs 2.6 billion dollars, over $200,000 per ton of displacement).
The dive depth cannot be increased easily. Simply making the hull thicker increases the weight and requires reduction of the weight of onboard equipment, ultimately resulting in a bathyscaphe. This is affordable for civilian research submersibles, but not military submarines, so their dive depth was always bounded by current technology.
The World War One submarines had their hulls built of carbon steel, and could not submerge below 100 metres. During World War Two, high-strength alloyed steel was introduced, allowing for depths up to 200 metres. High-strength alloyed steel is still the main material for submarines today, with 250-350 metres depth limit, which cannot be exceeded on a military submarine without sacrificing other characteristics. To exceed that limit, a few submarines were built with titanium hulls. Titanium is stronger and lighter than steel, and is non-magnetic. Titanium submarines were especially favoured by the Soviets, as they had developed specialized high-strength alloys, built an industry for producing titanium with affordable costs, and have several types of titanium submarines. Titanium alloys allow a major increase in depth, but other systems need to be redesigned as well, so test depth was limited to 1000 metres for the Soviet submarine Komsomolets, the deepest-diving military submarine. An Alfa-class submarine may have successfully operated at 1300 metres, though continuous operation at such depths would be an excessive stress for many submarine systems. Despite its benefits, high costs of titanium construction led to abandonment of titanium submarines idea as the Cold War ended.
There are examples of more than two hulls inside a submarine. The light hull of Typhoon-class submarines houses two main pressure hulls, a smaller third pressure hull constituting most of the sail, two other for torpedoes and steering gear, and between the main hulls 20 MIRV SLBMs along with ballast tanks and some other systems. The Royal Netherlands Navy Dolfijn- and Potvis-class submarines housed three main pressure hulls.
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