Under-keel clearance (UKC) is the vertical distance between the lowest point of a ship’s keel and the seabed beneath it, at a specific place and time. It is not a fixed number — it changes constantly with tide, the vessel’s own trim and speed, and the actual depth of water over the ground.
The Basic UKC Calculation
At its simplest:
UKC = Charted depth + Height of tide − Draft
But a genuinely safe UKC figure has to account for more than that simple subtraction:
- Chart datum — charted depths are referenced to a low-water datum (commonly chart datum, close to lowest astronomical tide), not to the current actual depth; the height of tide at the time of passage must be added to it.
- Squat — a moving vessel’s stern (and sometimes bow) sinks lower in the water than its static draft due to the pressure/flow effects of moving through water, especially in shallow or confined channels. Squat increases with speed and can add well over a metre of effective draft on a large vessel in a narrow channel, so it must always be added to draft in any UKC calculation, never assumed away.
- Trim and heel — a vessel trimmed by the stern effectively has a deeper draft aft than the mean draft shown on the draft marks; heel from wind, turning, or an off-centre load shifts the deepest point of the hull to one side.
- Density (fresh vs salt water) — a vessel moves lower in the water in fresh or brackish water than in full seawater for the same displacement, which matters when transiting river mouths or estuarine approaches.
Why the Margin Matters
Grounding is one of the most consistently repeated causes of serious marine casualties, and an underestimated UKC — usually from ignoring squat, trim, or a stale tide calculation — is a recurring root cause. A vessel that appears to have adequate clearance on paper using only charted depth and static draft can still touch bottom once squat and trim are factored in, particularly at speed in a narrow, shallow channel.
Minimum UKC Requirements
Port authorities and pilotage organizations typically publish a minimum required UKC for vessels using their channels — often expressed as a percentage of draft (a common rule of thumb is a minimum UKC of around 10% of draft in open water, though individual ports set their own figures based on seabed type, swell exposure, and channel conditions). Soft mud seabeds are sometimes treated differently from rock or sand, since the consequences of touching a soft bottom are less severe — but this is port-specific and never something to assume without checking local regulations.
Practical Passage Planning
Before entering a shallow port, channel, or berth, the passage plan should show the calculated UKC at each critical point along the route, cross-checked against the tide table for the actual time of transit — not just the deepest point of the day. Officers commonly build in a safety margin beyond the bare minimum specifically to absorb any error in the tide prediction or an unexpectedly higher squat than calculated.
Exam Relevance
UKC calculation — including squat, trim, tide, and chart datum corrections — is one of the most heavily tested practical navigation topics in deck officer competency exams (COC), oral assessments, and IMU-CET preparation, precisely because getting it wrong in practice causes real groundings.