Squat effect
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.[1]
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.[2] Squat effect is usually felt more when the depth/draft ratio is less than four[2] or when sailing close to abank. 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, nearMartha'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.[1]
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