Load line (watercraft)

The load line, also known as Plimsoll line, indicates the legal limit to which a ship may be loaded for specific water types and temperatures in order to safely maintain buoyancy,^[1] particularly with regard to the hazard of waves that may arise. The load line is a waterline that corresponds to the maximum draft of the ship, thus yet another name, load waterline.^[2] Varying water temperatures will affect a ship's draft, because warm water is less dense than cold water, providing less buoyancy. In the same way, fresh water is less dense than salinated or seawater, with a similar lessening effect upon buoyancy. The rules for international load lines are defined by the International Convention on Load Lines from 1966. For inland water transport regional, national or local rules apply.

Load lines are indicated by special markings on the hull. The marking for the main load line, the summer load line, is called load line mark or Plimsoll mark (positioned amidships), the marks for other conditions are named after the condition suffixed with "load line" (e.g. winter load line).