Squat
Squat is the increase in a ship draft — it sinks bodily lower in the water and usually trims — caused by moving through shallow or confined water. As the ship moves, water has to flow faster past the hull in the restricted space, lowering the pressure around it and letting the ship settle deeper.
Squat grows rapidly with speed and in shallow water, and it directly reduces the under-keel clearance just when it is already small. It is one of the main reasons ships slow down in shallow channels and port approaches, and it is a key allowance in the under-keel-clearance calculation.
Squat has grounded ships that looked to have adequate clearance at rest, because it can lower a large ship by a metre or more at speed in shallow water. Understanding and allowing for it — mainly by reducing speed — is essential to keeping enough water under the keel in channels and approaches.
How is squat reduced?
Chiefly by reducing speed — squat increases sharply with speed and in shallow water — which is why ships slow down in shallow channels and port approaches to preserve under-keel clearance.