What is Under Keel Clearance (UKC) in navigation?

Under Keel Clearance (UKC) is the vertical distance between the lowest point of the ship and the seabed. A planning balance can combine charted depth, predicted water level, actual draft, estimated squat and other required allowances. The governing UKC limit and allowances must come from the approved passage plan, company SMS, port rules and vessel-specific guidance; there is no universal percentage or fixed minimum for every passage.

What is squat effect and how does it affect UKC?

Squat is the hydrodynamic sinkage and trim change that can occur when a ship moves through shallow or confined water. Empirical methods such as a Barras-style estimate use speed through the water, not speed over ground, and remain sensitive to hull form, depth, channel geometry and blockage. Treat the result as an estimate and verify it against approved vessel-specific data and local requirements.

What is a safe UKC for harbour approaches?

Required UKC is passage-specific. Use the value and allowances required by the approved passage plan, company SMS, port or terminal rules, pilot information and vessel-specific guidance. Do not substitute a generic fixed distance or draft percentage.

How do you calculate net UKC step by step?

Build the UKC assessment from the controlling depth and water-level source, the vessel's actual draft and trim, an approved or appropriately bounded squat estimate using speed through the water, and every allowance required by the passage plan and local rules. If the result does not meet the governing requirement, stop and revise the plan through the vessel's approved procedures and required bridge, pilot, company or port coordination.

Does ship speed affect squat in shallow water?

Yes — squat increases with the square of speed. Doubling speed quadruples squat. This is why speed reduction is the primary tool to manage squat in shallow water. Reducing from 12 knots to 8 knots reduces squat by 56%. The critical speed (where squat accelerates dramatically) is typically reached when the depth-to-draft ratio falls below 1.5.

Mastering Under Keel Clearance & Squat Effect

Sailing into shallow waters, ports, or canals changes how a vessel behaves. Squat and the available Under Keel Clearance (UKC) must therefore be assessed as part of the approved passage plan.

The Danger of the Squat Effect

Squat is the bodily sinkage and trim change that can occur as a moving ship alters the flow and pressure field around its hull in shallow or confined water. Its magnitude depends on the hull, loading condition, speed through the water, depth, channel geometry, blockage and environmental conditions.

Using an Empirical Squat Estimate

A common Barras-style empirical estimate uses the vessel's speed through the water (STW) and block coefficient (C_b). SOG is not interchangeable with STW when current is present.

Open-water squat estimate (m) ≈ (Cb × STW²) / 100

This is an empirical estimate, not a universal prediction. Confined-water effects must not be represented by an automatic fixed doubling; use an approved method that accounts for the applicable channel geometry and blockage.

Because the speed term is squared, a change in STW can materially change the estimate.

Building the UKC Assessment

A planning structure may subtract static draft, squat and other required allowances from the available charted depth plus tide:

Estimated dynamic UKC = (Charted depth + tide) - (corrected draft + squat + required allowances)

The allowances and acceptance limit must come from approved ship-specific information, company procedures, port or waterway requirements, and the passage plan. Include the applicable trim/list, density, wave and motion allowances and uncertainty. CaptainCalc is decision support only; verify every operational result with the approved sources for the vessel and transit.

Sources and verification

Use these references as the starting point for verification; always follow current flag-state, company, port, and approved shipboard documents for operational decisions.

Calculate Squat Instantly with CaptainCalc

Don't rely on paper charts and guesswork in confined waters. CaptainCalc features a built-in Squat & UKC calculator that deterministically applies your vessel's exact Block Coefficient and confined-water multipliers instantly, completely offline.