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A marking indicating the extent to which the weight of a load may safely submerge a ship; also called Plimsoll line.
The load line, or Plimsoll mark, is positioned amidships on both sides of a vessel. Its purpose is to indicate the legal limit to which a ship may be loaded for specific ocean areas and seasons of the year. The basic Load Line Certificate is issued after a complex calculation is made to determine exactly where the Plimsoll mark should be positioned. These certificates take several forms, such as international voyage, coastwise traffic, and Great Lakes operations.
By calculating the load line, the agency issuing a certificate has determined, among other aspects of seaworthiness, that a vessel has enough volume of ship (reserve buoyancy) above the waterline so that it will not be in danger of foundering or plunging when under way in heavy seas. In the United States the U.S. Coast Guard issues load line regulations; routine assignment of load lines is handled by the American Bureau of Shipping.
A series of multilateral treaties has been executed to impose on signatories the responsibility of seeing that ships flying under their flag have safe load lines designated and that they are observed. The principal treaty is the International Convention on Load Lines 1966. The use of load lines on vessels sailing under the flag of the United States is mandated by federal law (46 U.S.C.A. 86 ). The treaties typically do not apply to ships of war, small ships, pleasure boats, and fishing vessels.