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A double hull is a ship hull design and construction method where the bottom and sides of the ship have two complete layers of watertight hull surface: one outer layer forming the normal hull of the ship, and a second inner hull which is some distance inboard, typically by a few feet, which forms a redundant barrier to seawater in case the outer hull is damaged and leaks.
The space between the two hulls is sometimes used for storage of fuel or ballast water.
Double hulls are a more extensive safety measure than double bottoms, which have two hull layers only in the bottom of the ship but not the sides.
In low-energy collisions, double hulls can prevent flooding beyond the penetrated compartment. In high-energy collisions, however, the distance to the inner hull is not sufficient and the inner compartment is penetrated as well.
One of the disadvantages of a double hull is that the stability of a ship can be less than that of a single hull. Because the double hull raises the centre of gravity, the metacentric height is reduced.
Oil tankers 
Double hulls' ability to prevent or reduce oil spills led to their being standardized for other types of ships including oil tankers by the International Convention for the Prevention of Pollution from Ships or MARPOL Convention.
A double hull does not protect against major, high-energy collisions or groundings which cause the majority of oil pollution, despite this being the reason that the double hull was mandated by United States legislation.
After the Exxon Valdez oil spill disaster, when that ship grounded on Bligh Reef outside the port of Valdez, Alaska, the US Government required all new oil tankers built for use between US ports to be equipped with a full double hull. However, the damage to the Exxon Valdez penetrated sections of the hull (the slops oil tanks, or slop tanks) that were protected by a double bottom, or partial double hull.
Furthermore, a double-hulled tanker does not need longitudinal bulkheads for longitudinal strength, as the inner hull already provides this. Eliminating longitudinal bulkheads would result in much wider tanks, significantly increasing the free surface effect. However, this problem is easily corrected with the addition of anti-slosh baffles and partial bulkheads.
In submarine hulls, the double hull structure is significantly different, consisting of an outer light hull and inner pressure hull, with the outer hull intended more to provide a more hydrodynamic shape for the submarine than the cylindrical inner pressure hull.
See also 
- "Chapter II-1: Construction - Structure, subdivision and stability, machinery and electrical installations, Regulation 12: Double bottoms in passenger ships", International Convention for the Safety of Life at Sea (SOLAS), International Maritime Organization (IMO), 2004 , p. 51, retrieved 17 July 2012
- Jack Devanney (2006): The Tankship Tromedy, The Impending Disasters in Tankers, CTX Press, Tavernier, Florida, ISBN 0-9776479-0-0
- The T/V Exxon Valdez, accessed June 14, 2007
- "The Exxon Valdez Oil Spill", US Federal report, accessed June 14, 2007
- COMPARISON OF SINGLE AND DOUBLE HULL TANKERS, Australian Maritime Safety Authority, April 2001