Speed

Max squat

Dynamic UKC

4 kn

0.26 m

2.24 m

5 kn

0.40 m

2.10 m

6 kn

0.58 m

1.92 m

7 kn

0.78 m

1.72 m

8 kn

1.02 m

1.48 m

9 kn

1.30 m

1.20 m

10 kn

1.60 m

0.90 m

11 kn

1.94 m

0.56 m

12 kn

2.30 m

0.20 m

13 kn

2.70 m

-0.20 m

14 kn

3.14 m

-0.64 m

How to use the squat calculator

Enter the block coefficient (Cb) from the ship's stability booklet or hydrostatic tables for the current draught. Full-bodied ships such as tankers and bulk carriers squat more than fine container ships.
Enter the speed through the water, not over the ground, together with the static draught and the water depth (charted depth plus the height of tide).
Choose open shallow water or a confined channel, such as a canal, river or dredged channel, and set your minimum under-keel clearance.

What is squat?

As a ship moves through shallow water, the water flowing past and under the hull speeds up and its pressure drops, so the ship sinks bodily and changes trim. That increase in draught is squat. It grows with the square of speed, with the fullness of the hull and with how much of the waterway the ship blocks, so it is greatest in narrow, shallow channels.

Barrass's formulas

Open shallow water: max squat (m) = Cb × V² ÷ 100 Confined channel: max squat (m) = Cb × V² ÷ 50 Detailed (channels): max squat (m) = Cb × S^0.81 × V^2.08 ÷ 20 V = speed through the water (knots), S = blockage = (B × T) ÷ (channel width × depth)

The simple formulas give the maximum squat at the bow or stern and are the ones taught for deck officer examinations. The open-water version is intended for depth/draught ratios of about 1.1 to 1.4, and the confined version for typical channel blockage factors of about 0.10 to 0.265.

Bow or stern?

Cb above 0.7 (tankers, bulk carriers): maximum squat at the bow, so the ship trims by the head.
Cb below 0.7 (container ships, passenger ships): maximum squat at the stern.
Cb about 0.7: the ship sinks almost evenly.

Worked example: bulk carrier in a canal

A bulk carrier with Cb 0.80, drawing 12.0 m, proceeds at 8 knots through a dredged channel with 14.5 m of water:

Maximum squat = 0.80 × 8² ÷ 50 = 1.02 m, at the bow
Static UKC = 14.5 − 12.0 = 2.50 m; dynamic UKC = 2.50 − 1.02 = 1.48 m
To keep at least 0.60 m under the keel, squat must stay below 1.90 m: V = √(50 × 1.90 ÷ 0.80) = 10.9 knots

In open shallow water the same ship would squat only about 0.51 m, which is why many groundings happen when a ship carries her open-sea speed into a channel. Canal and port authorities set speed limits for this reason.

Signs of squat and good practice

Reduce speed early. Halving the speed cuts squat to a quarter.
Expect sluggish steering, a larger turning circle and more vibration in shallow water.
Squat increases further when passing or overtaking another ship in a channel (interaction).
Plan the under-keel clearance before arrival and confirm it with the pilot.

Going through the Suez Canal? Estimate the dues with the Suez Canal toll calculator, and check load line allowances with the FWA & DWA calculator.

Frequently asked questions

What is ship squat?

The increase in a ship’s draught, and the change in trim, caused by the drop in water pressure around and under the hull when the ship moves through shallow water. It reduces the under-keel clearance.

How do you calculate squat?

With Barrass's formulas: maximum squat in metres equals the block coefficient times the speed in knots squared, divided by 100 in open shallow water or by 50 in a confined channel.

Why does reducing speed reduce squat so much?

Squat is proportional to the square of speed. Going from 10 to 5 knots reduces squat to a quarter, which is why slowing down is the most effective way to gain under-keel clearance.

Does a ship squat by the bow or the stern?

Ships with a block coefficient above about 0.7, such as tankers and bulk carriers, squat most at the bow. Finer ships with a Cb below 0.7, such as container ships, squat most at the stern.

What is a safe under-keel clearance?

There is no single figure. Companies, ports and pilotage authorities set minimum under-keel clearances, often as a percentage of draught or a fixed margin, and they also account for squat, swell, tide and the accuracy of the depth data.

What is the blockage factor?

The ratio of the ship’s immersed midship cross-section (breadth × draught) to the channel’s cross-section (width × depth). The higher the blockage, the faster the water must flow past the hull and the larger the squat.

Training and planning estimate using Barrass's empirical formulas. Actual squat depends on the ship, channel geometry, trim and passing traffic; always follow pilot advice and your company's under-keel clearance policy.

Last reviewed 8 Nov 2026.

Ship Squat Calculator — Barrass Formula & UKC

How to use the squat calculator

What is squat?

Barrass's formulas

Bow or stern?

Worked example: bulk carrier in a canal

Signs of squat and good practice

Frequently asked questions

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