The squat effect is the

hydrodynamic In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids— liquids and gases. It has several subdisciplines, including ''aerodynamics'' (the study of air and other gases in motion) ...

phenomenon by which a vessel moving through shallow water creates an area of reduced pressure that causes the ship to increase its draft and thereby be closer to the

seabed The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'. The structure of the seabed of the global ocean is governed by plate tectonics. Most o ...

than would otherwise be expected. This phenomenon is caused by the water flow which accelerates as it passes between the hull and the seabed in confined waters, the increase in water velocity causing a resultant reduction in pressure. Squat effect from a combination of vertical sinkage and a change of trim may cause the vessel to dip towards the stern or towards the bow. This is understood to be a function of the Block coefficient of the vessel concerned, finer lined vessels Cb <0.7 squatting by the stern and vessels with a Cb >0.7 squatting by the head or bow. Squat effect is approximately proportional to the square of the speed of the ship. Thus, by reducing speed by half, the squat effect is reduced by a factor of four. Squat effect is usually felt more when the depth/ draft ratio is less than four or when sailing close to a

bank A bank is a financial institution that accepts Deposit account, deposits from the public and creates a demand deposit while simultaneously making loans. Lending activities can be directly performed by the bank or indirectly through capital m ...

. It can lead to unexpected groundings and handling difficulties. There are indications of squat which mariners and ship pilots should be aware of such as vibration, poor helm response, shearing off course, change of trim and a change in wash. It was a cause of the 7 August 1992 grounding of the ''

Queen Elizabeth 2 ''Queen Elizabeth 2'' (''QE2'') is a retired British ocean liner converted into a floating hotel. Originally built for the Cunard Line, the ship, named as the second ship named ''Queen Elizabeth'', was operated by Cunard as both a transatlant ...

'' (QE2) off

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Island, near

Martha's Vineyard Martha's Vineyard, often simply called the Vineyard, is an island in the Northeastern United States, located south of Cape Cod in Dukes County, Massachusetts, known for being a popular, affluent summer colony. Martha's Vineyard includes the ...

. The liner's speed at the time was and the draft was . The rock upon which the vessel grounded was an uncharted

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later determined to be , which should have given her room to spare, were it not for the "squat effect." U.S.

National Transportation Safety Board The National Transportation Safety Board (NTSB) is an independent U.S. government investigative agency responsible for civil transportation accident investigation. In this role, the NTSB investigates and reports on aviation accidents and inci ...

investigators found that the QE2's officers significantly underestimated the amount the increase in speed would increase the ship's squat. The officers allowed for of squat in their calculations, but the NTSB concluded that squat at that speed and depth would have been between . Squat is also mentioned as a factor in the collision of the bulk carriers ''Tecam Sea'' and ''Federal Fuji'' in the port of Sorel,

Quebec Quebec ( ; )According to the Canadian government, ''Québec'' (with the acute accent) is the official name in Canadian French and ''Quebec'' (without the accent) is the province's official name in Canadian English is one of the thirte ...

, in April 2000. The third largest cruise ship in the world, MS ''Oasis of the Seas'', used this effect to obtain an extra margin of clearance between the vessel and the Great Belt bridge,

Denmark ) , song = ( en, "King Christian stood by the lofty mast") , song_type = National and royal anthem , image_map = EU-Denmark.svg , map_caption = , subdivision_type = Sovereign state , subdivision_name = Kingdom of Denmark , establish ...

, 1 November 2009, on a voyage from the shipyard in

Turku Turku ( ; ; sv, Åbo, ) is a city and former capital on the southwest coast of Finland at the mouth of the Aura River, in the region of Finland Proper (''Varsinais-Suomi'') and the former Turku and Pori Province (''Turun ja Porin lääni''; ...

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, USA.Wright, William, "Clearing a Landmark", ''Captain's Log, Day Three'', Royal Caribbean a
Oasis of the Seas

, KN-online (31 October 2009) (German) The new cruise liner passed under the bridge at 20

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(37 km/h) in the shallow channel, giving the ship extra clearance due to a 30 cm squat.

References

Further reading