The squat effect is the hydrodynamic phenomenon by which a vessel moving quickly through shallow water creates an area of lowered pressure under its bottom that causes the ship to "squat" lower in the water than would otherwise be expected. This is due to a reduction in buoyancy caused by a downward hydrodynamic force created by flow-induced pressures. It is caused by similar forces as lift in aircraft, except that the low pressure area is beneath the hull. It can lead to unexpected groundings and handling difficulties.
This phenomenon is caused by hydrodynamic effects between the hull of the ship and the sea floor. 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.[1] Squat effect is usually felt more when the depth/draft ratio is less than four[1] or when sailing close to a bank.
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 under the bow of the ship, creating a low-pressure area. This counteracts the force of buoyancy, causing the vessel to dip towards the bow.[2] 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 (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.[2]
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."[3] 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).[4]
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.[5]
See also
Society of Naval Architects and Marine Engineers (SNAME), "Principles of Naval Architecture", 1989, Vol. II "Resistance and Propulsion"
References
- ^ a b "Navigation and Vessel Inspection Circular" (PDF). http://www.uscg.mil/hq/g-m/nvic/2_97/n2-97ch1.pdf. Retrieved February 11, 2008.
- ^ a b "Transportation Safety Board of Canada". http://www.tsb.gc.ca/en/reports/marine/2000/M00L0039/M00L0039.asp. Retrieved February 11, 2008.
- ^ [1] Marine Surveyors Find Uncharted Rock That May Have Damaged Hull of the QE2, New York Times, 15 August 1992
- ^ [2] NTSB Letter to Cunard
- ^ Wright, William, "Clearing a Landmark", Captain's Log, Day Three, Royal Caribbean at Oasis of the Seas; „Oasis of the Seas“ hat Kurs auf Fehmarn, KN-online (31 October 2009) (German).
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
