Draft

Draft ranks among the most important operational figures on a ship because it decides where the vessel can and cannot go. It is measured from the surface of the water to the bottom of the keel, and it increases as the ship loads cargo, fuel and ballast. Draft marks are painted on the hull at the bow, stern and amidships on both sides, numbered so that the crew and a pilot can read the actual draft directly by eye. Reading the marks forward and aft also gives the ship's trim, the difference between the two.

Adequate under-keel clearance, the gap between the keel and the seabed, is a constant concern in shallow water, in port approaches and over bars at the entrance to rivers and harbours. A ship that loads too deep for the depth available cannot sail until the tide rises or cargo is lightered off. Terminals and canals publish maximum permissible drafts, and a vessel's draft on arrival is planned carefully against the tide so it can cross the shallowest point of the passage with a safe margin.

Several related effects change the effective draft while a ship is moving. Squat causes a vessel to sit deeper and trim as it makes way through shallow water, reducing clearance just when it is most critical. Density matters too: a ship floats deeper in fresh water than in denser salt water, so drafts are adjusted between sea and river passages. Air draft, by contrast, is the height of the highest point of the ship above the waterline, and it governs whether the vessel can pass under bridges and power cables. Managing draft, trim and clearance together is central to safe loading and passage planning.