Double hull

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Single hull, Double bottom, and Double hull ship cross sections. Green lines are watertight; black structure is not watertight

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 somewhat further into the ship, perhaps a few feet, which forms a redundant barrier to seawater in case the outer hull is damaged and leaks.

The space in between the two hull layers is often used as storage tanks for 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 and not the sides.

Double hulls are significantly safer than single hulls. In case of grounding or other underwater damage, most of the time the damage is limited to flooding the bottom compartment, and the main occupied areas of the ship remain intact. In case of collision with another ship, most of the time the damage is limited to flooding the side compartment, and the main occupied compartments also remain intact.

For these reasons, double hulls or double bottoms have been required in all passenger ships for decades as part of the International Convention for the Prevention of Pollution from Ships or MARPOL Convention.

After the Exxon Valdez oil spill disaster, when that ship grounded on a rock outside the port of Valdez in Alaska, the US Government required all new oil tankers built for use between US ports to be equipped with a full double hull. This was somewhat ironic, as the damage to the Exxon Valdez was so extensive that it would have penetrated the now-required double hull structure, and was limited to the ship's existing and properly designed double bottom structure. The new regulations, while enhancing general safety, would not have prevented that spill. ^{[[Citing sources citation needed]]}

A double hull also conveniently forms a stiff and strong girder or beam structure with the two hull plating layers as upper and lower plates for a composite beam. This greatly strengthens the hull in secondary hull bending and strength, and to some degree in primary hull bending and strength.

In case of submarines, the double hull structure is significantly different, consisting of an outer light hull and inner pressure hull.

Double hull

See also