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.

The purpose of a load line is to ensure that a ship has sufficient freeboard (the height from the water line to the main deck) and thus sufficient reserve buoyancy (e.g., the enclosed volume created by the area between the waterline and the main deck). The freeboard of commercial vessels is measured between the lowest point of the uppermost continuous deck at side and the waterline and this must not be less than the freeboard marked on the Load Line Certificate issued to that ship. All commercial ships, other than in exceptional circumstances, have a load line symbol painted amidships on each side of the ship. This symbol must also be permanently marked, so that if the paint wears off it remains visible. The load line makes it easy for anyone to determine if a ship has been overloaded. The exact location of the Load Line is calculated and/or verified by a Classification Society and that society issues the relevant certificates. This marking was invented in the 1870s by Samuel Plimsoll.

In aircraft design, the term "waterline" refers to the vertical location of items on the aircraft. This is the (normally) "Z" axis of an XYZ coordinate system, the other two axes being the Fuselage Station (X) and Buttock Line (Y).

The plimsoll mark is also used in chemistry as a symbol to denote a standard state.