Squat is the bodily sinkage and change of trim a ship experiences when moving through shallow or restricted water. Faster water flow under the hull lowers the pressure there, drawing the ship deeper and reducing the clearance beneath the keel.

Which Barrass formula does this use?

For open shallow water it uses maximum squat equals Cb times speed to the power 2.08 divided by 100. For a confined channel it uses Cb times the blockage factor to the power 0.81 times speed to the power 2.08 divided by 20, where the blockage factor is the ship's midship area over the channel cross-section.

Does the ship squat by the bow or the stern?

Barrass found that full-form ships with a block coefficient above 0.700, such as tankers and bulk carriers, squat by the bow, while finer ships below 0.700 squat by the stern. The tool reports which end based on the block coefficient you enter.

How does speed affect squat?

Squat rises roughly with the square of speed, since the speed term is raised to the power 2.08. That means halving your speed cuts the squat to about a quarter, which is why slowing down is the most effective way to recover underkeel clearance in shallow water.

When is shallow-water effect significant?

Squat becomes important once the water depth to draught ratio falls below about 1.4, and grows rapidly as the ratio approaches 1.1 to 1.2. Below those ratios you should reduce speed and watch the net underkeel clearance closely throughout the passage.

Ship Squat Calculator

In shallow or narrow water a moving ship sinks deeper than its static draught because the water rushing under the hull drops in pressure. That extra sinkage, called squat, has grounded ships that looked to have ample charted depth. This calculator uses Dr C. B. Barrass’s empirical formulae to estimate the maximum squat and the underkeel clearance left at your chosen speed.

How it works

Squat depends on hull fullness, speed, and how much the water is restricted. The Barrass formulae are:

open shallow water:  δ_max = Cb · V^2.08 / 100
confined channel:    δ_max = Cb · S^0.81 · V^2.08 / 20
   blockage factor S = (beam × draught) / (channel width × depth)
net underkeel clearance = depth − (draught + squat)

Because the speed exponent is just over 2, squat scales almost with the square of speed. The ship squats by the bow when Cb is above 0.700 and by the stern when it is below — the tool flags which end and warns when squat eats all the clearance.

Example and notes

A loaded bulk carrier with Cb = 0.80 doing 12 knots in 13 m of open water with an 11 m draught squats about 0.97 m by the bow, leaving roughly 1.0 m of net clearance — and the H/T ratio of 1.18 confirms a strong shallow-water effect. Slow to 6 knots and the squat falls to around 0.23 m. Treat every figure as planning guidance: real underkeel clearance also needs allowances for heel, wave response, tidal uncertainty, and survey accuracy, and the pilot’s judgement always governs.