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Diagram of common elements of warship armor. The belt armor is denoted by "A".

Belt armor is a layer of heavy metal armor plated onto or within the outer hulls of warships, typically on battleships, battlecruisers and cruisers, and aircraft carriers.

Armor and underwater protection of King George V and Tirpitz.

Belt armor on damaged USS Oklahoma

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.[1]

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 shells, 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.

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