Clearance

The available distance between a vessel and an overhead, underwater or nearby obstruction.

Vertical or overhead clearance is compared with the vessel’s air draught when passing beneath a bridge, cable or other structure. Under-keel clearance is the distance remaining between the vessel’s lowest point and the seabed or a submerged obstruction. The unqualified word “clearance” can therefore be ambiguous and should be accompanied by the type being considered.

Safe clearance calculations allow for changing tide and water level, squat, heel, waves, vessel motion, loading, chart and survey uncertainty, and any safety margin appropriate to the passage. Published bridge clearances must also be interpreted using the datum and notes shown on the relevant chart.