Squat Effect

Squat is an effect that every mariner must understand because it can turn a safe under-keel clearance into a grounding. When a ship moves through the water, especially in shallow or narrow channels, the water flowing past the hull speeds up and its pressure falls, and the ship responds by sinking bodily lower in the water and altering its trim. This increase in effective draught is called squat, and it means the ship sits deeper (and its clearance above the seabed becomes smaller) precisely in the shallow or confined waters where clearance is already tight, such as port approaches, rivers, canals and over bars.

The amount of squat depends on several factors, but above all on the ship's speed: squat increases sharply with speed, roughly with the square of the speed through the water, so slowing down is the single most effective way to reduce it. It is also greater in shallow water and in narrow channels, where the water is more constrained, and it is influenced by the fullness of the hull, so large, full-bodied ships such as loaded bulk carriers and tankers squat more. The change in trim can be by the bow or the stern depending on the hull form and the depth, which matters for where the least clearance occurs.

Because squat can amount to a significant increase in draught at speed, it must be allowed for in passage planning and ship handling. Navigators calculate the expected squat and keep a safe under-keel clearance margin above it, and when clearance is tight the ship reduces speed to keep squat small. Failing to account for squat has caused groundings of ships that appeared, on paper, to have enough water. Squat is closely related to other shallow-water effects that reduce clearance and affect handling, and managing it is a fundamental part of navigating safely in the confined waters where ships are most at risk.