Belt armor
Belt armor is a layer of heavy metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

 armor plated on to or within outer hulls of warship
A warship is a ship that is built and primarily intended for combat. Warships are usually built in a completely different way from merchant ships. As well as being armed, warships are designed to withstand damage and are usually faster and more maneuvrable than merchant ships...

s, typically on battleships, battlecruisers and cruiser
A cruiser is a type of warship. The term has been in use for several hundreds of years, and has had different meanings throughout this period...

s, and on aircraft carrier
Aircraft carrier
An aircraft carrier is a warship designed with a primary mission of deploying and recovering aircraft, acting as a seagoing airbase. Aircraft carriers thus allow a naval force to project air power worldwide without having to depend on local bases for staging aircraft operations...

s converted from those types of ships.
Typically, the main armor belt covered the warship from its main deck down to some distance below the warship's waterline
The term "waterline" generally refers to the line where the hull of a ship meets the water surface. It is also the name of a special marking, also known as the national Load Line or Plimsoll Line, to be positioned amidships, that indicates the draft of the ship and the legal limit to which a ship...

. If the armor belt was 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
Shell (projectile)
A shell is a payload-carrying projectile, which, as opposed to shot, contains an explosive or other filling, though modern usage sometimes includes large solid projectiles properly termed shot . Solid shot may contain a pyrotechnic compound if a tracer or spotting charge is used...

 (from ship or from shore) or an underwater torpedo
The modern torpedo is a self-propelled missile weapon with an explosive warhead, launched above or below the water surface, propelled underwater towards a target, and designed to detonate either on contact with it or in proximity to it.The term torpedo was originally employed for...

, the belt armor is designed to prevent penetration into the heart of the ship, either by being too thick of an obstacle 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 detonates. Frequently, the main belt's armor plates were supplemented with a torpedo bulkhead
Torpedo bulkhead
A torpedo bulkhead is a type of armor common on the more heavily armored warships, especially battleships and battlecruisers of the early 20th century. It is designed to keep the ship afloat even if the hull was struck underneath the belt armor by a shell or by a torpedo...

 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 can contain either 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 short distance "short" of the targeted ship. Those shells, especially armor-piercing shells, can pass through a short stetch of water and strike the warship some distance below her waterline, and then 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
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...

 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 makes 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 of more: for example, the sister ships USS Lexington (CV-2)
USS Lexington (CV-2)
USS Lexington , nicknamed the "Gray Lady" or "Lady Lex," was an early aircraft carrier of the United States Navy. She was the lead ship of the , though her sister ship was commissioned a month earlier...

and USS Saratoga (CV-3)
USS Saratoga (CV-3)
USS Saratoga was the second aircraft carrier of the United States Navy and the fifth ship to bear her name. She was commissioned one month earlier than her sister and class leader, , which is the third actually commissioned after and Saratoga...

, 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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