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
International Load Line,
Plimsoll line or
water 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
seawater. For vessels with
displacement hulls, the
hull speed is determined by, among 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.