Ship squat is a phenomenon observed when a vessel is travelling at speed. Ship squat is measured as a decrease in the under keel clearance (UKC). Ship squat is a hydrodynamic effect which occurs when the water that typically flows over the hull creates an area of low pressure. The low pressure forces the ship down closer to the seabed, until the the squat can be counteracted by the inherent bouyancy of the ship. When this effect interacts with the seabed or shore, such as in narrow and/or shallow water, the squat effect increases. This effect has been attributed as the cause of many shipping disasters, most notably the Queen Elizabeth 2 (QE2).
Depending on the shape of the hull, a vessel will naturally assume a trim by the bow or stern. Large, full vessels, such as tankers, have a tendency to trim by the bow; while more shapely vessels tend to trim by the stern.
The United States Coast Guard states that ship squat is equal to the square of the ships speed. This means that a deduction in speed say halfing the speed, would yield a net draught saving of 4 times. Shallower and narrower seaways also increase squat, requiring a further reduction.
Authorities of rivers, channels, canals, ports, etc. often impose speed restrictions (though more often aimed at navigational safety and environmental protection) and under keel clearance UKC requirements.