Belt may also refer to the installation of belt armor on a warship. Warships were traditionally most vulnerable to incoming fire at and just below the waterline - damage taken there can cause massive flooding of the interior, sinking the ship. In addition, the machinery spaces of modern vessels (and the magazines of both modern and sailing warships) are below the waterline. Consequently this area of the ship required additional protection. It's not practical to armor the entirety of a warship evenly - not only can this cause stability issues due to the changing center of gravity, but you don't need as much armor on the superstructure. Therefore, there tended to be a region of heavy armor reaching from below the waterline to just above it. In this location, it offered some protection against shallow-running torpedoes as well as protection against direct or plunging projectile fire for this critical area. Since it ended up forming a band around the middle of the ship, it was referred to as the belt, and the armor there as belt armor.

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