![DoubleBottomDoubleHull](https://upload.wikimedia.org/wikipedia/commons/a/aa/DoubleBottomDoubleHull.png)
A double hull is a
ship
A ship is a large watercraft that travels the world's oceans and other sufficiently deep waterways, carrying cargo or passengers, or in support of specialized missions, such as defense, research, and fishing. Ships are generally distinguished ...
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 sometimes used for storage of 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 collisions, double hulls can prevent flooding beyond the penetrated compartment. In high-energy collisions, 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
The International Convention for the Safety of Life at Sea (SOLAS) is an international maritime treaty that sets minimum safety standards in the construction, equipment and operation of merchant ships. The International Maritime Organization c ...
or SOLAS Convention.
Uses
Double hulls are significantly safer than double bottoms, which in turn are safer than single bottoms. 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 low-energy collisions to the sides of the vessel, double hulls also prevent flooding beyond the penetrated compartment. In high-energy collisions, however, the distance to the inner hull is not sufficient and the inner compartment is penetrated as well.
A double bottom or hull also conveniently forms a stiff and strong
girder
A girder () is a support beam used in construction. It is the main horizontal support of a structure which supports smaller beams. Girders often have an I-beam cross section composed of two load-bearing ''flanges'' separated by a stabilizing ...
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.
Double hulls can also:
* be used as inboard tanks to carry oil, ballast water or fresh water (ventilated by a
gooseneck)
* help prevent pollution in case of liquid cargo (like oil in tankers)
* help to maintain stability of ship; and
* act as a platform for machinery and cargo.
Oil tankers
Double hulls' ability to prevent or reduce oil spills led to double hulls being standardized for other types of ships including
oil tankers by the
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.
(2006): ''The Tankship Tromedy, The Impending Disasters in Tankers'', CTX Press, Tavernier, Florida
After the ''Exxon Valdez'' oil spill disaster, when that ship grounded on Bligh Reef outside the port of Valdez, Alaska
Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alaskaq''; tli, Anáaski) is a state located in the Western United States on the northwest extremity of North America. A semi-exclave of the U.S. ...
, the US Government required all new oil tankers built for use between US ports to be equipped with a full double hull.
Submarines
In submarine hulls, the double hull structure is significantly different, consisting of an outer light hull and inner pressure hull, with the outer hull intended more to provide a hydrodynamic shape for the submarine than the cylindrical inner pressure hull. In addition to tailoring the flow of water around the submarine (also known as ''hydrodynamic bypass''), this outer skin serves as a mounting point for anechoic tiles
__NOTOC__
Anechoic tiles are rubber or synthetic polymer tiles containing thousands of tiny voids, applied to the outer hulls of military ships and submarines, as well as anechoic chambers. Their function is twofold:
*To absorb the sound waves ...
, which are designed specifically to absorb sound rather than reflect it, helping to hide the vessel from sonar detection.
See also
* Coulombi Egg Tanker
*Naval architecture
Naval architecture, or naval engineering, is an engineering discipline incorporating elements of mechanical, electrical, electronic, software and safety engineering as applied to the engineering design process, shipbuilding, maintenance, and ...
* Bulkhead
* Submarine
* Multihull
*Whipple shield
The Whipple shield or Whipple bumper, invented by Fred Whipple, is a type of hypervelocity impact shield used to protect crewed and uncrewed spacecraft from collisions with micrometeoroids and orbital debris whose velocities generally range be ...
References
{{Reflist
Shipbuilding
Watercraft components