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The hull is the main body of the ship below the main outside deck. The hull consists of an outside covering (or skin) and an inside framework to which the skin is secured. The skin and framework are usually made of steel and secured by welding. To achieve high speeds, i.e. speeds greater than 30 knots, various types of hull have been developed, some making use solely of hydrostatic support like monohulls but by increasing the number of hulls, as with catamarans and trimarans in particular; hydrodynamic support has also been used by means of underwater "wings" which serve to lift the load-carrying hull out of the water above a certain speed; other principles have been developed, in particular those using aerostatic support, such as air cushion vehicles.
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