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 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.[1]
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 second largest cruise ship in the world, MS 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] 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, traveling at 20 knots (37 km/h) in the shallow channel, Oasis experienced a 30 cm squat, allowing sufficient room to clear the bridge safely. However, the shipping-industry magazine Shipgaz states that this claim was made about her sister ship, MS Allure of the Seas, passing under the same bridge, and claim that this statement was mere media exaggeration. The media claim was that the Allure (and therefore the Oasis) had a mere 30 cm of clearance under the bridge, and that was due to her squat in the water, while in fact the vessel had several meters of clearance between her funnel and the bridge, meaning that 30 cm of squat would have made little difference in her safe clearance, other than to increase the margin of error. They also claim that the water of the channel under the bridge is too deep to create a squat effect.[6]
References
- ↑ 1.0 1.1 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 2.0 2.1 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ [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)
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
Further reading
- Society of Naval Architects and Marine Engineers (SNAME), "Principles of Naval Architecture", 1989, Vol. II "Resistance and Propulsion"
