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.^{[citation needed]} 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.^{[citation needed]} 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.^{[citation needed]}

For these reasons, double hulls or double bottoms have been required in all passenger ships for decades as part of the Safety Of Life At Sea or SOLAS Convention.

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.

One of the downsides of a double hull is that the stability of a ship can be less than that of a single hull. Because the double hull raise the centre of gravity, the metacentric height will be reduced.

1 Oil Tankers
2 Submarines
3 See also
4 References

[edit] Oil Tankers

Their 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.

Opposing viewpoints have argued that the double hull is actually more dangerous than a single hull.^{[citation needed]} Most of the collisions that the double hull prevents are so minor that they would typically spill little to no oil on a single hull tanker.^{[citation needed]} In addition, the double hull increases the risk of corrosion on the double hull,^{[citation needed]} causing oil to spill into the ballast; not only defeating the purpose of the double hull, but in addition making it increasingly dangerous for the crew on a ship who must inspect those areas.

In addition to this, there is a much larger potential for explosive accidents happening due to the increased element of oil mixing with air during a high-energy grounding, as was the case with the Aegean Sea (oil spill).

Possibly the most disturbing fact about the double hull is that it does not protect against major, high-energy collisions or groundings which is what causes the majority oil pollution^{[citation needed]} , despite the fact this is why the double hull was put into United States Legislation.

After the Exxon Valdez oil spill disaster, when that ship grounded on Bligh Reef 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 penetrated sections of the hull (the slops oil tanks) which were protected by a partial double hull.^[1] ^[2] The double hull required by the new regulations would not have prevented extensive loss of oil from the Exxon Valdez, though it might have somewhat limited the losses. ^[3]

Furthermore, a double-hulled tanker doesn't need longitudinal bulkheads for longitudinal strength, as the inner hull already provides this. This results in much wider tanks, significantly increasing the free surface effect.^[4]