Draft (draught)

A ship’s draft (British spelling: draught) is the vertical distance from the waterline to the lowest point of the keel — in effect, how deep the ship sits in the water. Draft increases as cargo is loaded and is the key constraint that determines which ports, berths, channels, and canals a vessel can use.

Draft marks are painted on the hull at the bow, stern, and midships, and reading them before and after loading is the basis of the draft survey, a standard method of determining the weight of bulk cargo loaded.

Maximum permitted draft is set by the vessel’s load lines (Plimsoll marks), which vary by season and zone — summer, winter, tropical, and fresh water. Deep-draft chokepoints shape trade: the Suez Canal, the Malacca Strait (“Malaccamax”), and individual port approach channels each impose their own draft limits, and terms like “Suezmax” derive from them.