Double hull

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 often 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 casualties, double hulls can prevent flooding beyond the penetrated compartment. In high-energy casualties, however, the distance to the inner hull is not sufficient and the inner compartment is penetrated as well.

Double hulls or double bottoms have been required in all passenger ships for decades as part of theSafety Of Life At Sea or SOLAS Convention.^{[citation needed]}

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.