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Under keel clearance (UKC)

Under keel clearance is the vertical margin between the deepest point of the hull and the seabed beneath it. It sounds like a single static number — the difference between charted depth and the ship’s draft — but in practice it is a moving target: several dynamic effects reduce the clearance a ship actually has while under way, and passage planning through shallow or draft-constrained water has to account for all of them, not just the static figure read off a chart.

Static draft vs dynamic draft

Static draft is the draft the ship reads alongside, at rest, in calm water — the figure confirmed by a draft survey. Dynamic draft is the effective draft the ship presents while moving and responding to sea conditions, and it is almost always deeper than the static draft. UKC calculations for a passage through shallow water must use the dynamic draft, not the static draft, or the calculated clearance will be optimistic.

Factors that reduce UKC underway

  1. Squat. As a ship moves through shallow or confined water, the flow of water accelerating past the hull lowers pressure beneath it, causing the vessel to sink bodily and typically trim by the head or stern. Squat increases with speed and worsens in narrow, shallow channels — it is one of the largest and most speed-controllable contributors to dynamic draft.
  2. Heel. A ship heeled by wind, turning, or an athwartships weight distribution presents a deeper effective draft on the low side than its upright static draft.
  3. Wave response (heave). In a seaway the ship heaves up and down with the waves; the trough of that motion adds to the effective draft at the moment the hull is lowest.
  4. Negative surge / wave-induced trim. Passing waves and swell can also induce dynamic trim changes beyond the vessel’s static trim.
  5. Water density (salinity) change. A ship moves lower in less-dense water. Moving from salt water to brackish or fresh water — for example transiting a river mouth or estuary — increases draft for the same displacement, an effect masters routinely allow for using the Fresh Water Allowance.
  6. Tide height above chart datum. Charted depths are referenced to a chart datum (commonly a low-water datum); the actual depth of water available at the time of transit is chart depth plus the height of tide at that moment, which varies through the tidal cycle.

Company UKC policy

Shipping companies set a minimum UKC policy in their Safety Management System, typically expressed as a percentage of the vessel’s draft or as a fixed minimum clearance in metres, with the more conservative of the two often applied in practice. The applicable margin generally varies with berth/channel type — open water passages, confined channel transits, and berthing/unberthing moves are commonly treated differently — and the actual figures a company applies should be read from its own SMS UKC procedure rather than assumed, since values in use across the industry vary by vessel type, trade, and company risk appetite.

Tidal window calculation for a draft-constrained transit

For a passage limited by a shallow bar, channel, or lock sill, the passage plan typically works backward from the vessel’s required UKC to establish the minimum height of tide needed, then uses tide tables/tidal predictions to identify the window of time around high water during which that height of tide is available — the “tidal window” — before deciding whether the ship can sail at any state of tide, only within the window, or must wait for a specific tide.

Dynamic UKC systems

Some draft-critical ports and channels — where large vessels operate close to the available depth — provide or require dynamic UKC systems: shore-based or shipboard tools that combine real-time tide, swell, squat prediction, and vessel motion data to monitor available clearance continuously through a transit, rather than relying solely on a pre-voyage calculation using worst-case assumptions.

Primary sources

See also

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