Squat and Under-Keel Clearance: A Field Guide to Safe Speed in Confined Water

A loaded ship moving through shallow water sinks bodily and trims — it squats — and the faster it goes, the more it sinks. In open water that is academic. In a dredged channel with a couple of feet of water under the keel, squat is the difference between a clean transit and a soft grounding that closes the channel and ends up in a casualty report. This guide covers the mechanism, the estimate pilots carry in their heads, and how squat translates into a maximum safe speed.

Why a ship squats

As a hull moves, water has to flow around and under it. In shallow or confined water there is less room for that flow, so it accelerates, pressure under the hull drops (Bernoulli), and the ship settles deeper into the water. Because the effect is not symmetrical fore and aft, the ship also changes trim — typically squatting by the bow when laden, by the stern when in ballast.

Two things drive the magnitude: speed (squat rises roughly with the square of speed) and block coefficient (fuller, boxier hulls — laden tankers and bulkers — squat more than fine ones).

The Barrass estimate

Pilots rarely run a CFD model on the bridge. They use a fast approximation. The widely taught Barrass form estimates maximum squat as:

Squat (m) ≈ C_b × V² ÷ K

where C_b is the block coefficient, V is speed through the water in knots, and K is a constant reflecting how confined the water is — roughly 100 in open shallow water and about 50 in a confined channel. The squared-velocity term is the headline: halving speed cuts squat to a quarter.

A worked example: a laden tanker, C_b ≈ 0.84, making 12 knots in a confined channel (K ≈ 50):

Squat ≈ 0.84 × 12² ÷ 50 ≈ 0.84 × 144 ÷ 50 ≈ 2.42 m

Nearly two and a half meters of additional draft, purely from speed in confined water. Drop to 8 knots and the estimate falls to about 1.08 m. That is why "slow down in the shallows" is not caution for its own sake — it is buying back draft.

How squat eats under-keel clearance

Under-keel clearance (UKC) is the water between the bottom of the keel and the seabed. The honest version is a stack:

1. Start with charted depth, corrected for the height of tide at the time you will actually be there.
2. Subtract static draft (today's, not the marks) to get static UKC.
3. Subtract squat at your planned speed.
4. Subtract allowances for heel, wave response, and survey/datum uncertainty.
5. What remains must exceed your required UKC margin — often expressed as a percentage of draft, set by the port or your association's policy.

If dynamic UKC after squat falls below the required margin, the plan is not safe at that speed. You have two levers: wait for more tide, or slow down.

Backing out a maximum safe speed

Because squat scales with V², you can invert the estimate to find the fastest speed that still leaves your required clearance. Rearranged:

V_max ≈ √( allowable squat × K ÷ C_b )

Allowable squat is the UKC you can spend after reserving your margin. Continuing the example — say the stack leaves 1.5 m of squat budget, K = 50, C_b = 0.84:

V_max ≈ √(1.5 × 50 ÷ 0.84) ≈ √89.3 ≈ 9.4 knots

So in that channel, at that tide and draft, the laden tanker should be doing no more than about 9–9.5 knots to preserve clearance. That number is a planning input for the Master-Pilot Exchange: it is the speed both bridge teams agree the ship will not exceed in the shallow leg.

Making it part of the transit, not a back-of-envelope

Every pilot can do this arithmetic; the value of doing it systematically is consistency and a record. Binnacle Passage includes a squat and UKC tool that takes vessel block coefficient, draft, charted depth, tide, and required margin, computes squat by the Barrass estimate, and returns dynamic UKC and a maximum safe speed for the leg — tied to the transit so the agreed speed lives with the rest of the passage record.

It does not replace the pilot's judgment or the ship's own UKC policy. It removes the arithmetic as a source of error and makes the agreed safe speed an explicit, recorded number rather than a feel.

The bottom line

Squat is real, it scales with the square of speed, and it is worst exactly where you have the least water — fully laden, in a confined channel, near low water. Build the UKC stack honestly, reserve your margin, and let the allowable squat set your speed rather than the schedule. The cheapest grounding prevention available is a knot or two off the speed.

This article is informational and uses simplified estimates. Always apply your ship's UKC policy, port requirements, and professional judgment.