Torpedo belt
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The torpedo belt was part of the armor of warships between the 1920s and 1940s. It consisted of a series of lightly armored compartments, extending along a narrow belt across and just below a ship's waterline. The idea was this belt would absorb the explosions from torpedoes, thus minimizing damage to the ship itself.
Warships of the early 20th century had their heaviest armor above their waterlines - the "main belt" which was intended to stop flat trajectory gunfire. Below this line, their armor was generally thinner to reduce overall weight but making them extremely vulnerable to torpedo hits and the occasional large calibre shell. Increasing the lower armor was impractical, since this increased tonange which led to a slower ship but greatly decreasing their seaworthiness and increased costs as well. Another solution had to be found. Originally torpedo nets were tried. These were simply metal mesh nets hung out at some distance from the ships' sides to detonate or catch torpedoes before they struck. These proved largely ineffective against torpedoes since they were only deployed when stationary useless against shell hits below the waterline and not really effective against naval mines either. Some ships were fitted with armor patches, increasing their armor in vulnerable areas below the waterline. These stopped torpedoes from penetrating the ship, but the shock from the blast of underwater explosions often wreaked havoc with the ships' weapons mounts and machinery in addition to violently rocking the entire ship. With the outbreak of World War I the urgency to devise an effective torpedo defense system (TDS) was greatly increased. In 1915 the British introduced the Renown class battlecruisers and in 1917 the Americans began building the Tennessee class battleships , both of which incorporated innovative TDS designs featuring the forerunners of the first true torpedo belts. It was not until 1922, in the wake of the Washington Naval Treaty that curtailed ship weights that the introduction of the British Nelson class battleships that a true layered torpedo belt was introduced. The two Nelsons used a waterfilled belt, which was written off in the tonnage limits under water and not armor. Over the next 20 years many innovative designs of TDS were tried by various nations.
By 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 guided missiles which attacked above the belt and a new generation of "smart", deep-diving torpedoes (such as the Mark 48) which are capable of detonating below a ships' keel effectively breaking its "back".
The ARA General Belgrano, a World War II cruiser purchased by the Argentine Navy, was sunk when hit by two conventional Mk 8 mod 4 torpedoes of 1920's design, each with an 800 lb (363 kg) Torpex warhead, by a British submarine during the Falkland War, despite having a torpedo belt.
