The squat effect is the hydrodynamic phenomenon by which a vessel moving quickly through shallow water creates an area of lowered pressure under its keel that reduces the ship's buoyancy, particularly at the bow. The reduced buoyancy causes the ship to "squat" lower in the water than would otherwise be expected. It can lead to unexpected groundings and handling difficulties. It is caused by similar forces as lift in aircraft, except that the low pressure area is beneath the hull.
This phenomenon is caused by hydrodynamic effects between the hull of the ship and the sea floor. According to the United States Coast Guard, the squat effect is approximately proportional to the speed of the ship squared. 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.
The 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, especially towards the bow of the ship, creating a low-pressure area. This counteracts the force of buoyancy, causing the vessel to dip towards the bow. The reduced pressure on the bottom of the boat sucks the boat slightly downward until the increased displacement counteracts the force generated by the reduced pressure. (See Bernoulli's principle)
It is believed to have been one of the causes of the 7 August 1992 grounding of the Queen Elizabeth 2 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 and her draft was 32 feet. The rock upon which she grounded was an uncharted shoal later determined to be 34.5 feet deep, which should have given her room to spare, if not for the "squat effect." American NTSB 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 two feet of squat in their calculations. Whereas the NTSB concluded that her squat at that speed and depth would have been between 4.5 and 8 feet.
Society of Naval Architects and Marine Engineers (SNAME), "Principles of Naval Architecture", 1989, Vol. II "Resistance and Propulsion"