A submarine hull has two major components, the light hull and the pressure hull. The light hull (casing in British usage) of a submarine is the outer non-watertight hull which provides a hydrodynamically efficient shape. The pressure hull is the inner hull of a submarine that maintains structural integrity with the difference between outside and inside pressure at depth.
Shapes
Modern submarines are usually cigar-shaped. This design, already visible on very early submarines, is called a "teardrop hull". It is structurally efficient for withstanding external pressure, and significantly reduces the hydrodynamic drag on the sub when submerged, but decreases the sea-keeping capabilities and increases drag while surfaced.
History
The concept of an outer hydrodynamically streamlined light hull separated from the inner pressure hull was first introduced in the early pioneering submarine Ictineo I designed by the Spanish inventor Narcís Monturiol in 1859. However, when military submarines entered service in the early 1900s, the limitations of their propulsion systems forced them to operate on the surface most of the time; their hull designs were a compromise, with the outer hulls resembling a ship, allowing for good surface navigation, and a relatively streamlined superstructure to minimize drag under water. Because of the low submerged speeds of these submarines, usually well below , the increased drag for underwater travel by the conventional ship-like 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. 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 (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.
Types
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.
Light hull
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. It defines the hydrodynamic performance of submarine, which affects the amount of power required to drive the vessel through the water. This term is especially appropriate for Russian submarine construction, where the light hull is usually made of thin steel plate, 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 vessel's integrity, as long as the pressure hull is intact.
Pressure hull
Inside the outer hull there is a strong hull, or pressure hull, which withstands the outside pressure and has normal atmospheric pressure inside. The pressure hull is generally constructed of thick high-strength steel with a complex stiffening structure and high strength reserve, and is divided by watertight bulkheads into several compartments. The pressure and light hulls are separated by a gap in which numerous steel structural elements connect the light hull and pressure hull and form a three-dimensional structure which provides increased strength and buckling stability. The interhull space is used for some of the equipment which can tolerate the high external pressure at maximum depth and exposure to the water. This equipment significantly differs between submarines, and generally includes various water and air tanks. In a single-hull submarine, the light hull is discontinuous and exists mainly at the bow and stern.