Under-Keel Clearance Calculator

Running aground is one of the most frequent and expensive incidents in commercial shipping, and it almost always comes down to a margin that looked fine on paper but ignored squat, water density, or a heel angle. This calculator combines charted depth, tide, water density, and three subtractive allowances into a single net under-keel clearance (UKC) figure — the number that determines whether a vessel safely clears the bottom.

Why UKC calculations go wrong

Four factors catch masters and port captains out:

1. Forgetting squat. A vessel moving through shallow water creates a pressure drop under its hull, causing bodily sinkage and a trim change. Squat increases with speed and decreases with depth of water. In a confined channel, squat can exceed 0.5 m for a large vessel at modest speed — a margin that matters when clearances are tight.

2. Density change on approach. Nautical charts are referenced to a standard sea-water density (typically 1.025 t/m³). River estuaries and tidal inlets often hold fresher or brackish water. Archimedes’ principle means a hull floats deeper in less dense water: a vessel with 10 m draft in sea water may sit 15–20 cm deeper in river water at 1.000 t/m³. That change is silent — it requires a calculation, not an instrument reading.

3. Tidal prediction error. Published tide tables give predicted heights. Meteorological surge (wind-driven or barometric) can push actual water levels well below or above prediction, especially in enclosed bays and estuaries. The UKC figure this tool produces is only as good as the tide height you supply.

4. Heel from loading or wind. A vessel with a list or experiencing a heeling moment in a turn presents a deeper point at the bilge than the static draft indicates. Even a 2° heel on a wide-beamed vessel can add 20–30 cm to the effective under-keel depth required.

The calculation

The formula builds available depth, adjusts draft for density, then subtracts each clearance consumer:

available depth  = charted depth + tidal height
effective draft  = static draft × (chart reference density / actual water density)
net UKC          = available depth − effective draft − squat allowance − heel allowance

The density correction scales draft proportionally. In sea water (1.025) at chart datum, no correction is needed. In brackish water at 1.010, the effective draft increases by a factor of 1.025 ÷ 1.010 ≈ 1.015 — so an 11.0 m draft becomes approximately 11.16 m.

Worked example

A bulk carrier with 11.0 m static draft approaches a berth where the charted depth is 10.5 m. At transit time, the predicted tide is +2.0 m. The channel holds brackish water at a density of 1.015 t/m³ (chart reference is 1.025). Predicted squat at transit speed is 0.5 m; the vessel is expected to develop a 0.2 m heel on the turn.

Whether 0.69 m is acceptable depends on the port’s minimum UKC requirement. Many ports require 10–15% of draft as minimum UKC, or a fixed absolute minimum. At 11.0 m draft, 10% = 1.1 m — so this example would likely require a later departure time when the tide is higher, a reduction in draft, or a reduced transit speed to lower squat.

Notes on use

This calculator is a planning tool. It does not predict actual tidal height, measure squat, or account for sounding uncertainty, chart errors, or dynamic loading changes. Always follow the port authority’s pilotage rules and cross-check against mandatory minimum UKC requirements before committing to a transit.