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The squat effect is the hydrodynamic phenomenon by which a vessel moving quickly through shallow water creates an area of lowered pressure that causes the ship to be closer to the seabed 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). 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. 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 (QE2) off Cuttyhunk Island, near Martha's Vineyard. It is also known to have been a factor in the collision of the bulk carriers Tecam Sea and Federal Fuji in the port of Sorel, Quebec, in April 2000.
At the time of the QE2's grounding she was reportedly traveling at 24 knots (12 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 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 feet (0.61 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 feet (1.4 and 2.4 m).
The world's largest cruise ship, Oasis of the Seas, used this effect as a contributing factor to pass under the Great Belt bridge, Denmark, 1 November 2009, on her voyage from the shipyard in Turku, Finland to Florida, USA. Without the presence of the squat effect, the ship wouldn't have been able to clear the bridge safely - the margin would have been very slight. However, travelling at 20 knots (37 km/h) in the shallow channel, Oasis experienced a 30 cm squat, allowing sufficient room to clear the bridge safely.