water line

The national Load Line or Plimsoll Line (waterline),


positioned amidships, indicates the legal limit to which a ship may be loaded for specific water types and temperatures. To an observer on the ship the water appears to rise or fall against the hull. Temperature affects the level because warm water provides less buoyancy, being less dense than cold water. The salinity of the water also affects the level, fresh water being 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 length of the waterline can change significantly as the boat heels, and can dynamically affect the speed of the boat.

In aircraft design, the term waterline refer 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 purpose of a ‘load line’ is to ensure that a ship has sufficientfreeboard and thus sufficient reserve buoyancy. 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,[1] 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. Legally, a ship may be loaded only till the Plimsoll line remains visible only above the level of the sea. The exact location of the Load Line is calculated and/or verified by a Classification Society and that society issues the relevant certificates.

This symbol, also called an international load line or Plimsoll line, indicates the maximum safe draft, and therefore the minimum freeboard for the vessel in various operating conditions