Submarine hull A 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 ; this 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 . 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 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 slow 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 . 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 . 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 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 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 . Dive depth . 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 usually had test depths of no more than 100 metres ( 328 feet ) . During World War Two , high - strength alloyed steel was introduced , allowing for depths up to 200 metres ( 656 feet ) , post - war calculations have suggested crush - depths exceeding 300m for late - war German Type VII U - boats . High - strength alloyed steel is still the main material for submarines today , with 250 – 350 metres ( 820 to 1 , 148 feet ) 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 ( 3 , 281 feet ) for the Soviet submarine " Komsomolets " , the deepest - diving military submarine . An Alfa - class submarine may have successfully operated at 1300 metres ( 4 , 265 feet ) , 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 . Other types . There are examples of more than two hulls inside a submarine . The light hull of Typhoon - class submarine s 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 .