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.
In low energy casualties, double hulls can prevent flooding beyond the penetrated compartment. In low 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 the Safety Of Life At Sea or SOLAS Convention.
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.
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. 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. In addition, the double hull increases the risk of corrosion on the double hull, causing oil to spill into the ballasttanks where oil vapours can create an explosive mixture; 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.
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, 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. 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.
Furthermore, a double-hulled tanker doesn't 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.