Squat effect
Encyclopedia
The squat effect is the hydrodynamic
Fluid dynamics
In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...

 phenomenon by which a vessel
Hull (watercraft)
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...

 moving quickly through shallow water
Shallow water
Shallow water may refer to:* Shallow water blackout* Waves and shallow water** Shallow water equations** Boussinesq equations * Shallow Water, Kansas...

 creates an area of lowered pressure that causes the ship to be closer to the seabed
Seabed
The seabed is the bottom of the ocean.- Ocean structure :Most of the oceans have a common structure, created by common physical phenomena, mainly from tectonic movement, and sediment from various sources...

 than would otherwise be expected. This phenomenon is caused when water that should normally flow under the hull encounters resistance due to the close proximity of the hull to the seabed. This causes the water to move faster, creating a low-pressure area with lowered water level surface (See Bernoulli's principle
Bernoulli's principle
In fluid dynamics, Bernoulli's principle states that for an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy...

).
This squat effect results from a combination of (vertical) sinkage and a change of trim that may cause the vessel to dip towards the stern or towards the 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
Bank effect
Bank effect refers to the tendency of the stern of a ship to swing toward the near bank when operating in a river or constricted waterway.The asymmetric flow around a ship induced by the vicinity of banks causes pressure differences between port and starboard sides...

. It can lead to unexpected groundings and handling difficulties.

It is believed to have been one of the causes of the 7 August 1992 grounding of the Queen Elizabeth 2
RMS Queen Elizabeth 2
Queen Elizabeth 2, often referred to simply as the QE2, is an ocean liner that was operated by Cunard from 1969 to 2008. Following her retirement from cruising, she is now owned by Istithmar...

 (QE2) off Cuttyhunk
Cuttyhunk
Cuttyhunk Island is the outermost of the Elizabeth Islands in Massachusetts. It was the first site of English settlement in New England. It is located between Buzzards Bay to the north and Vineyard Sound to the south...

 Island, near Martha's Vineyard
Martha's Vineyard
Martha's Vineyard is an island located south of Cape Cod in Massachusetts, known for being an affluent summer colony....

. It is also known to have been a factor in the collision of the bulk carriers Tecam Sea
MV Tecam Sea
The Motor Vessel Tecam Sea is a bulk carrier built in 1984. The ship was built in Hitachi Innoshima, Japan, and purchased by Sea Quality, a shipping company based in Athens, Greece. The ship was involved in a collision with the M/V Federal Fuji in the port of Sorel, Quebec on 27 April 2000...

 and Federal Fuji
MV Federal Fuji
The Motor Vessel Federal Fuji is a bulk carrier built in 1986, by Nippon Kokan in Shimizu, Japan. The ship was built for Viken Lakers, based in Bergen, Norway. The Federal Fuji was involved in a collision with the M/V Tecam Sea in the port of Sorel, Quebec on 27 April 2000...

 in the port of Sorel
Sorel-Tracy, Quebec
Sorel-Tracy is a city in southwestern Quebec, Canada and the geographical end point of the Lake Champlain Valley at the confluence of the Richelieu River and the St. Lawrence River, on the western edge of Lac Saint-Pierre downstream and east of nearby Montreal. The population as of the Canada 2006...

, Quebec
Quebec
Quebec or is a province in east-central Canada. It is the only Canadian province with a predominantly French-speaking population and the only one whose sole official language is French at the provincial level....

, in April 2000.

At the time of the QE2's grounding she was reportedly traveling at 24 knots (13.1 m/s) and her draft was 32 feet (9.8 m). The rock upon which she grounded was an uncharted shoal later determined to be 34.5 feet (10.5 m), which should have given her room to spare, if not for the "squat effect." U.S. National Transportation Safety Board
National Transportation Safety Board
The National Transportation Safety Board 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 incidents, certain types of highway crashes, ship and marine...

 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 2 foot (0.6096 m) of squat in their calculations, but the NTSB concluded that her squat at that speed and depth would have been between 4.5 and 8 ft (1.4 and 2.4 m).

The world's largest cruise ship, Oasis of the Seas
MS Oasis of the Seas
MS Oasis of the Seas is an Oasis-class cruise ship in the fleet of Royal Caribbean International. The first of her class, she was joined by her sister ship Allure of the Seas in December 2010. Both vessels cruise the Caribbean from Port Everglades in Fort Lauderdale, Florida...

, used this effect as a contributing factor to pass under the Great Belt bridge
Great Belt Fixed Link
The Great Belt Fixed Link is the fixed link between the Danish islands of Zealand and Funen across the Great Belt. It consists of a road suspension bridge and railway tunnel between Zealand and the island Sprogø, as well as a box girder bridge between Sprogø and Funen...

, Denmark
Denmark
Denmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...

, 1 November 2009, on her voyage from the shipyard
Shipyard
Shipyards and dockyards are places which repair and build ships. These can be yachts, military vessels, cruise liners or other cargo or passenger ships. Dockyards are sometimes more associated with maintenance and basing activities than shipyards, which are sometimes associated more with initial...

 in Turku
Turku
Turku is a city situated on the southwest coast of Finland at the mouth of the Aura River. It is located in the region of Finland Proper. It is believed that Turku came into existence during the end of the 13th century which makes it the oldest city in Finland...

, Finland
Finland
Finland , officially the Republic of Finland, is a Nordic country situated in the Fennoscandian region of Northern Europe. It is bordered by Sweden in the west, Norway in the north and Russia in the east, while Estonia lies to its south across the Gulf of Finland.Around 5.4 million people reside...

 to Florida
Florida
Florida is a state in the southeastern United States, located on the nation's Atlantic and Gulf coasts. It is bordered to the west by the Gulf of Mexico, to the north by Alabama and Georgia and to the east by the Atlantic Ocean. With a population of 18,801,310 as measured by the 2010 census, it...

, USA
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

. Without the presence of the squat effect, the ship would not have been able to clear the bridge safely - the margin would have been very slight. However, travelling at 20 knots in the shallow channel, Oasis experienced a 30 cm squat, allowing sufficient room to clear the bridge safely.

Further reading

  • Society of Naval Architects and Marine Engineers
    Society of Naval Architects and Marine Engineers
    The Society of Naval Architects and Marine Engineers is a professional society that provides a forum for the advancement of the engineering profession as applied to the marine field...

    (SNAME), "Principles of Naval Architecture", 1989, Vol. II "Resistance and Propulsion"
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