Belt armor Belt armor is a layer of heavy metal armor plated onto or within the outer hulls of warship s , typically on battleships , battlecruisers and cruiser s , and aircraft carrier s . Typically , the main armor belt covered the warship from its main deck down to some distance below the waterline . If the armor belt were built within the hull , rather than forming the outer hull , it would be installed at an inclined angle to improve the warship ' s protection . When the warship is struck by an artillery shell ( from ship or from shore ) or an underwater torpedo , the belt armor is designed to prevent penetration into the heart of the ship , either by being too thick for the explosive warhead to penetrate or else by being sloped to a degree that would deflect downwards the artillery shell or the torpedo , and also its explosive force if it detonated . Frequently , the main belt ' s armor plates were supplemented with a torpedo bulkhead spaced several meters behind the main belt , designed to maintain the ship ' s watertight integrity even if the main belt was penetrated . Furthermore , the outer spaces around the main belt in some designs were filled with storage tanks that could contain fuel oil , seawater , or fresh water . There , the liquids in the tanks absorb or scatter much of the explosive force of warheads and shells . In other designs , as illustrated by the cross sectional drawing of " Tirpitz " and " King George V " , the outer spaces were left empty which allowed some the initial blast wave to dissipate , while the inner liquid layers then absorbed any splinters and spread out the shock wave over a larger area , while the armored , " holding bulkhead " , prevented leakage into the ship from the liquid layers . In combat , a warship can be seriously damaged underwater not only by torpedoes , but also by heavy naval artillery shells that plunge into the ocean a little distance " short " of the targeted ship . Those shells , especially armor - piercing shell s , can pass through a short stretch of water and strike the warship some distance below her waterline and detonate there . Hits by such shells present a severe risk of sinking the warship , just as torpedoes do . An air space between the armor belt and the hull would also add to the buoyancy of the warship , and this was often done to increase protection against either torpedoes or the shells mentioned above . Some kinds of naval warships also had belt armor that was thinner than was really necessary for their protection . This was done with some warships , especially battlecruisers and aircraft carriers , to make them significantly lighter and faster in steaming through the seas , in order to take heavy striking power to the enemy rapidly , or in the case of aircraft carriers , so that their speed made them much more capable of launching their warplanes and recovering the warplanes from flight . This is always done by steaming the aircraft carrier rapidly into any wind that is present , and nearly all large aircraft carriers have had speeds of 30 knots or more : for example , the sister ships " USS Lexington ( CV - 2 ) " and " USS Saratoga ( CV - 3 ) " , the second and third aircraft carriers to enter the U . S . Navy , in 1927 . These were completed on the hulls of under - construction battlecruisers with powerful engines , giving them very high speed .