Draft (Draught)
Draft (spelled draught in British usage) is the vertical distance from the waterline down to the bottom of a ship's keel — in other words, how deep the vessel sits in the water. It increases as the ship is loaded and is read from draft marks on the hull at the bow, midships and stern.
Draft governs how much water a ship needs beneath it, so it determines which ports, berths and channels a vessel can enter and how much cargo it can safely load — a laden ship must keep enough under-keel clearance for the water depth available, tides and safety margins. The related air draft is the height from the waterline to the highest fixed point, which decides whether a ship can pass under bridges. Draft is a fundamental constraint in loading, port access and voyage planning.
Draft is where cargo intake meets physical reality: load too much and the ship can't enter its berth or clear the channel; load too little and you waste earning capacity. Every loading and port-call decision balances draft against water depth, tides and clearance. It is one of the hard constraints that shapes what a ship can carry and where it can go.
What is the difference between draft and air draft?
Draft is the depth below the waterline (to the keel); air draft is the height above the waterline to the highest point, used for bridge clearance.
Why does draft limit cargo?
Loading increases draft; the ship must keep enough under-keel clearance for the available water depth, so shallow ports cap how much it can load.