Seamanship
Squat and under-keel clearance
How much a ship sinks and trims when moving in shallow water (squat), what clearance is left under the keel, and the speed that keeps your minimum.
Tankers 0.80–0.85, containers 0.60–0.70
From company or port rules
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How it works
In shallow water the flow under the hull speeds up and the ship sinks bodily and trims. The simplified Barrass formula gives the maximum squat: Cb × V² / 100 metres in open water (depth/draught 1.1–1.4), Cb × V² / 50 in a confined channel, with V in knots through the water. A full ship (high Cb) squats by the bow, a fine one by the stern.
Under-keel clearance = charted depth + height of tide − draught − squat. Halving the speed cuts squat to a quarter: the quickest way to gain clearance. Allow also for swell, heel and the accuracy of the chart.
For training and cross-checking. Not a substitute for approved navigation equipment, nautical publications and the ship’s own procedures. Calculations run in your browser and nothing you type is stored; a PDF you download is made on our server and not kept.
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