Shallow-water squat estimator

When a vessel moves through water, the local pressure drop under the hull lowers the waterline — the vessel squats. In shallow water this effect grows non-linearly with speed and reaches a maximum at the depth Froude number Fnh = 1. Squat is one of the leading causes of grounding incidents and groundlines on bunker barges.

The calculator computes maximum bodily sinkage using three published methods. Results vary because each formula was derived from different model-test programmes — Barrass tends to be most conservative, Hooft and ICORELS apply best in deep-channel conditions.

Practical rules

• Squat increases with speed roughly as V². Halving speed reduces squat by 75%.
• The trim direction depends on Cb: full-form ships (Cb > 0.7, e.g. tankers, bulkers) squat by the bow; fine-form ships (Cb < 0.6, e.g. container, fast ferries) squat by the stern.
• In a confined channel (narrow channel, lock approach), squat can be 2× the open-water value.
• Bank effect adds further bow-out yaw and squat asymmetry; pilots compensate with rudder bias.
• Always reduce speed when entering shallow water — the canonical 12 → 6 kt reduction quarters the squat.

See also

• · Load lines — fresh water allowance, dock water allowance.
• · Vessel stability — GM, GZ curve, free-surface effect.
• · Passage planning — UKC budget per leg.
• · Draft survey — measured draft corrections.

Last updated 2026-04-27