Freeboard

Freeboard is the vertical distance from the waterline to the freeboard deck (broadly, the uppermost continuous watertight deck). It is the inverse of draft: the more deeply a ship is loaded, the smaller the freeboard, and the smaller the reserve of buoyancy protecting the vessel from being overwhelmed by the sea.

Minimum freeboard is fixed by the International Convention on Load Lines (1966) and marked on the hull with the Plimsoll mark — a circle bisected by a horizontal line, flanked by a “comb” of seasonal marks (Summer, Winter, Winter North Atlantic, Tropical, Fresh water). Loading a ship beyond its applicable load line is a serious offence in virtually every jurisdiction.

The Plimsoll mark is named after Samuel Plimsoll, the British MP whose 1870s campaign against dangerously overloaded “coffin ships” led to compulsory load lines — one of the earliest and most influential pieces of international maritime safety regulation.