Under-Keel Clearance
Under-Keel Clearance (UKC) is the vertical distance between the bottom of the keel and the seabed — the water the ship has to spare beneath it. It is worked out from the charted depth, the height of tide, the ship draft, and allowances for effects that reduce clearance such as squat, the swell, and changes in water density.
Ports and companies set a minimum UKC that must be maintained, and in shallow, tidal or deep-draught port approaches the passage is planned around a tidal window that guarantees it. UKC is monitored underway with the echo sounder. Too little clearance means grounding, one of the most common and costly casualties.
Grounding a laden ship is catastrophic and expensive, and under-keel clearance is the margin that prevents it. Because tide, squat and swell can eat that margin fast in shallow water, managing UKC — and planning the tidal window around it — is a fundamental piece of passage-planning safety.
What reduces under-keel clearance underway?
Squat (the ship sinking bodily and trimming as it moves through shallow water), the effect of swell and waves, heel in a turn, and a reduction in water density. These are allowed for on top of the static draft and tide.