Under-keel clearance

Under-keel clearance is the water depth beneath a ship’s keel, and it converts a charted depth into a usable draught. It is the constraint that turns a hydrographic figure into an operating limit, and in several passages it is set by regulation rather than by local practice.

In the Strait of Malacca and the Singapore Strait the rule is regulatory rather than local. It first appeared in IMO Assembly Resolution A.375(X) of 14 November 1977 and was carried unchanged into the Rules for Vessels Navigating Through the Straits of Malacca and Singapore, issued as IMO SN/Circ.198 on 26 May 1998, which defines a deep draught vessel as one of 15 m draught or more and requires that deep draught vessels and VLCCs allow for an under-keel clearance of at least 3.5 m at all times during the entire passage. The same rules direct those vessels to the designated deep water routes and cap them at 12 knots over the ground in named places, because squat increases with speed and eats into the clearance.

That corridor is also the standing example of a clearance rule published without a controlling depth to apply it against, so the margin is computed from the current electronic navigational chart rather than from a headline figure.

The full article will cover the components of the calculation, squat and its speed dependence, tidal windows, port and canal under-keel clearance policies, and the interaction with loading and draught survey.