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.

Length LBP (m)Beam (m)Draft (m)Cb (block coeff)0.55 fast container · 0.85 VLCCSpeed (knots)Water depth (m)

Barrass III

3.42 m

Open / confined waters

Hooft

0.41 m

Open water

ICORELS / PIANC

0.68 m

Confined channels

Depth Froude number Fnh: 0.411

h / T ratio: 1.33

Typical UKC (under-keel clearance) requirements

DWA (dock water)

0.60 m

Open port

1.20 m

Open sea (SMS)

2.40 m

Rules of thumb only — the ship's SMS, port-state minimum UKC, and the pilot's advice control.

Estimate, not for piloting. Bank effect, channel width, passing vessels, and cargo distribution all change the actual squat. Always cross-check against the ship's squat tables, the pilot card, and the PIANC squat guidance.

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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.

When to use this tool

Passage planning through shallow-water approaches — Suez Canal, Panama Canal (both channel + Culebra Cut), Baltic tanker approaches (Kokkola, Luleå), West African crude terminal approaches (Bonny, Escravos), Amazon (Manaus, Belém, Vitória). Any leg where minimum UKC is under 20% of draft. Pilot generally does the operational check but the Master must own the numbers.

Worked example

Scenario: Panamax bulker Cb 0.83, LOA 225 m, B 32.3 m, draft 12.5 m, transiting a 14.5-m depth channel at 8 kn.

Barrass III result: ~1.0 m maximum bodily sinkage. Trim-by-head ~0.3 m.

Interpretation: Effective draft at 8 kn = 12.5 + 1.0 = 13.5 m in a 14.5-m channel = 1.0 m nominal UKC but only 0 m of margin. Reduce speed to 5 kn (squat drops to ~0.4 m) — UKC becomes safer 1.6 m.

Regulatory basis

Not directly regulated but every voyage plan under SOLAS V/34 + IMO Res. A.893(21) must include UKC calculation with speed-adjusted squat. Some ports mandate UKC minimums (Suez Canal, Panama Canal, Australian ports under AMSA UKC Management System, MPA Singapore).