What is under-keel clearance?

Under-keel clearance is the vertical distance between the lowest point of a vessel's hull and the seabed. A positive value means the ship floats clear; a negative value means it would touch bottom.

Why does water density matter for draft?

A ship floats deeper in less dense water. If you transit from salt water (1.025) into brackish or fresh water, the vessel sinks lower, reducing clearance. The tool applies a density correction to the effective draft.

What is squat and why subtract it?

Squat is the bodily sinkage and trim a moving vessel experiences in shallow or confined water due to reduced pressure under the hull. It increases with speed and can be over a metre, so it must be reserved as clearance.

Is a fixed safety margin enough?

No. Many ports require a minimum UKC of 10 to 15 percent of draft, or a fixed figure plus allowances for swell and uncertainty. Treat this tool as a planning aid and always follow your port's pilotage rules.

Does the calculator predict tide for me?

No. You supply the predicted tidal height from an official tide table or prediction service. The tool then combines that height with charted depth, density, draft, squat, and heel to give net clearance.

Under-Keel Clearance Calculator

Running aground is one of the most common and expensive groundings in commercial shipping, and almost always comes down to a margin that looked fine on paper but ignored squat, density, or a heel angle. This calculator combines charted depth, tide, water density, and three subtractive allowances into a single net under-keel clearance figure.

How it works

The available water depth is built up from the chart, then every consumer of that depth is subtracted:

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

The density term reflects Archimedes’ principle: in water less dense than the chart’s reference (typically 1.025 for sea water), the hull sits deeper, so the effective draft is scaled up. Squat and a heel allowance are reserved separately because both reduce the lowest point of the hull during the transit.

Example and notes

A vessel with 11.0 m draft entering a channel charted at 10.5 m, with a 2.0 m tide, 0.6 m predicted squat, and 0.2 m heel, in sea water at chart density, has an available depth of 12.5 m and a net UKC of 12.5 − 11.0 − 0.6 − 0.2 = 0.7 m. If that same berth held brackish water at 1.010, the effective draft would rise to about 11.16 m and clearance would fall to roughly 0.54 m. Always cross-check against the port’s mandated minimum and add margin for swell and chart accuracy.