Torpedo belt The torpedo belt was part of the armouring scheme in some warship s of between the 1920s and 1940s . It consisted of a series of lightly armoured compartments , extending laterally along a narrow belt that intersected the ship ' s waterline . In theory , this belt would absorb the explosions from torpedo es and thus minimize internal damage to the ship itself . Torpedo belts are also known as Side Protection Systems or SPS , or Torpedo Defense System or TDS . The problem . Armoured warships ( dreadnought battleship s , armoured cruiser s and later light and heavy cruisers ) of the early 20th century carried their main protective armour above the waterline – the " main belt " – which was intended to stop flat - trajectory gunfire from piercing the hull . Below the belt , the armour generally tapered away , to reduce overall weight . This , however , makes a ship vulnerable to hits below the waterline from torpedoes ; and the occasional large - calibre shell . Extending the belt downwards is impractical , since this increases displacement , which increases draught and can result in reduced speed , and decreasing seaworthiness . Solutions . Another solution was needed . Originally , torpedo net s were tried . These were heavy metal mesh nets hung out on booms at some distance from the sides of ships to catch or detonate torpedoes short of the ship . These proved largely ineffective against torpedoes since they could only be deployed when ships were stationary , were useless against shell hits below the waterline , and were ineffective against mines . Some ships were fitted with underwater armour in vulnerable areas below the waterline . This stopped torpedoes from penetrating the ship but the shock from an underwater explosion s could cause damage to weapon mountings and sensitive machinery , along with violently stressing the ships ' structure . Anti - torpedo bulges . The outbreak of World War I increased the urgency to devise an effective torpedo defense system ( TDS ) , thus the British Director of Naval Construction introduced the anti - torpedo bulge . Originally retrofitted to older ships , this was soon added to ships already under construction . In 1915 , the British s and in 1917 American s introduced anti - torpedo bulges . Most later capital ships would have at least one torpedo bulkhead inboard of the external hull plating along the area of the ship protected by the armour belt . Most Battleships built after World War I had sophisticated and complex side - protection systems , as illustrated by the cross sectional drawing of and . Torpedo belts . It was not until 1922 , in the wake of the Washington Naval Treaty that curtailed ship weights and with the introduction of the British s , that a true layered torpedo belt was introduced . The two " Nelson " s used a water - filled belt , which was written off in the tonnage limits , as water was not part of the calculations for allowed displacement . Over the next 20 years many innovative designs of TDS were tried by various nations . After the end of World War II torpedo belts , much like many of the warships they protected , were rendered obsolete by the widespread use of aircraft , and eventually - especially in the late 60s and into the 70s - by the use of guided missile s which attacked above the belt and a new generation of " smart " , deep - diving torpedoes ( such as the Mark 48 ) which are designed to detonate under a ship ' s keel and break its back .