Waterline

The waterline is the line where the hull of a ship meets the surface of the water, in concept or reality. Specifically, it is also the name of a special marking, also known as the national Load Line or Plimsoll Line (positioned amidships), that indicates the draft of the ship and the legal limit to which a ship may be loaded for specific water types and temperatures in order to safely maintain buoyancy, particularly with regard to the hazard of waves that may arise. Temperature affects the level because warm water provides less buoyancy, being less dense than cold water, as does salinity because fresh water is less dense than salty seawater. For vessels with displacement hulls, the hull speed is determined by, amongst other things, the waterline length. In a sailing boat, the waterline length can change significantly as the boat heels, and can dynamically affect the speed of the boat.