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.
Worked example — speed reduction saves the clearance
A loaded bulk carrier (Cb = 0.80, draught 11 m, beam 32 m) transiting a 200 m wide channel at 14 m depth:
| Speed | Blockage factor S | Max squat (bow) | Net UKC |
|---|---|---|---|
| 12 knots | (32×11)/(200×14) = 0.126 | ~0.93 m | 3.07 m |
| 8 knots | 0.126 | ~0.43 m | 3.57 m |
| 6 knots | 0.126 | ~0.25 m | 3.75 m |
Halving speed from 12 to 6 knots cuts squat by about 75%, which illustrates why pilotage speed restrictions in confined channels and ports target shallow-water squat directly.
Why H/T ratio matters
The depth-to-draught ratio (H/T) signals when shallow-water effects become significant:
- H/T above 1.5 — squat is modest; open-sea Barrass formula applies comfortably
- H/T between 1.2 and 1.5 — shallow-water effects are noticeable; start monitoring squat
- H/T between 1.1 and 1.2 — significant squat; reduce speed and use confined-channel formula
- H/T below 1.1 — extreme shallow water; squat is large and wave wash effects also grow; navigate very slowly with pilot
Full-form vessels (tankers, bulk carriers, Cb typically 0.78–0.86) are more susceptible to squat than fine-form vessels (container ships, ferries, Cb typically 0.55–0.70) at the same speed, because a fuller hull displaces a greater fraction of the cross-sectional area of the waterway.
What the Barrass method does not capture
The Barrass formula gives a maximum squat at mid-ship based on block coefficient and speed. Real squat in operations also involves:
- Trim change — squat may manifest as a bodily sinkage, a bow-up trim, or a bow-down trim depending on the hull’s longitudinal centre of buoyancy. The Barrass rule for bow versus stern squat (Cb above or below 0.700) is a simplification.
- Heel in a channel — a vessel heeled to one side has a reduced effective UKC on the low side that adds to the squat-reduced clearance.
- Wave return from channel walls — in a confined canal, wall-return effects can add sinkage beyond the blockage-factor formula.
- Tidal height uncertainty — charted depths are from survey datum; actual depth depends on the tidal state and may carry a prediction error of 10–20 cm in complex tidal waters.
Always treat this tool’s output as planning guidance. Port and pilotage authorities specify minimum UKC requirements (often expressed as a percentage of draught), and the pilot’s on-the-day judgement takes precedence over any formula.