A double hull is a ship
Since the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing,...
A hull is the watertight body of a ship or boat. Above the hull is the superstructure and/or deckhouse, where present. The line where the hull meets the water surface is called the waterline.The structure of the hull varies depending on the vessel type...
design and construction method invented by Leonardo da Vinci 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 bottom
A double bottom is a ship hull design and construction method where the bottom of the ship has 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 higher in the ship, perhaps a few feet, which forms a...
s, 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 the Safety Of Life At Sea
International Convention for the Safety of Life at Sea
The International Convention for the Safety of Life at Sea is an international maritime safety treaty. The SOLAS Convention in its successive forms is generally regarded as the most important of all international treaties concerning the safety of merchant ships.- History :The first version of the...
or SOLAS Convention.
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
The metacentric height is a measurement of the static stability of a floating body. It is calculated as the distance between the centre of gravity of a ship and its metacentre . A larger metacentric height implies greater stability against overturning...
Oil tankersDouble 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
Exxon Valdez oil spill
The Exxon Valdez oil spill occurred in Prince William Sound, Alaska, on March 24, 1989, when the Exxon Valdez, an oil tanker bound for Long Beach, California, struck Prince William Sound's Bligh Reef and spilled of crude oil. It is considered to be one of the most devastating human-caused...
disaster, when that ship grounded on Bligh Reef
Bligh Reef, sometimes known as Bligh Island Reef, is a reef off the coast of Bligh Island in Prince William Sound, Alaska. This was the location of the 1989 Exxon Valdez oil spill. After the incident, US Code 33 § 2733 mandated the operation of an automated navigation light to prevent future...
outside the port of Valdez
Valdés is a surname of Asturian origin.Both in North and South America the spelling Valdez is very common.The exact meaning of "Valdés" is not clear...
Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...
, 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
Oriental Nicety, formerly Exxon Valdez, Exxon Mediterranean, SeaRiver Mediterranean, S/R Mediterranean, Mediterranean, and Dong Fang Ocean is an oil tanker that gained notoriety after running aground in Prince William Sound spilling hundreds of thousands of barrels of crude oil in Alaska...
penetrated sections of the hull (the slops oil tanks) that 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 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
Free Surface Effect
The free surface effect is one of several mechanisms which can cause a craft to become unstable and roll over . It refers to the tendency of liquids — and of aggregates of small solid objects, like seeds, gravel, or crushed ore which can act as liquids — to slosh about: to move in response to...
. However, this problem is easily corrected with the addition of anti-slosh baffles and partial bulkheads.
SubmarinesIn submarine hull
The term light hull is used to describe the outer hull of a submarine, which houses the pressure hull, providing hydrodynamically efficient shape, but not holding pressure difference...
s, the double hull structure is significantly different, consisting of an outer light hull and inner pressure hull.
- Naval ArchitectureNaval architectureNaval architecture is an engineering discipline dealing with the design, construction, maintenance and operation of marine vessels and structures. Naval architecture involves basic and applied research, design, development, design evaluation and calculations during all stages of the life of a...
- BulkheadBulkhead (partition)A bulkhead is an upright wall within the hull of a ship or within the fuselage of an airplane. Other kinds of partition elements within a ship are decks and deckheads.-Etymology:...
- SubmarineSubmarineA submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...
- MultihullMultihullA multihull is a ship, vessel, craft or boat with more than one hull.-Description:Multihulls include: Proas, which have two differently shaped or sized hulls with lateral symmetry; catamarans, which have two hulls with longitudinal symmetry; and trimarans, which have a main hull in the center and...