The load line is a means of quickly ascertaining that a ship has sufficient freeboard and reserve buoyancy to remain afloat in the waters in which it is likely to navigate
While load lines had been used on ships at various times and in various locations throughout history, a systematic program for ship loading regulations originated with Lloyd’s Register of Shipping in 1835.
Even then, it was not mandatory for British vessels and was not utilized by foreign shipping.
In the 1870’s, a movement headed by Samuel Plimsoll, a Member of Parliament, resulted in passage of an amendment to the United Kingdom Merchant Shipping Act and made load lines mandatory on British flag commercial vessels.
In 1906, the Act was amended again to require load lines on foreign ships calling in British ports (an example of unilateral maritime legislation long predating the US Oil Pollution Act of 1990).
The 1930 Load Line Convention made application of the load line universal.
A major overhaul of the concept resulted in the International Load Line Convention, 1966.
Though amended various times, this beneficial regulation, often referred to as the “Plimsoll line” remains in effect today.
Due to differences in the density of water, the modern load line generally has six separate marks: (1) TF, for tropical fresh water; (2) F, for fresh water; (3) T, for tropical seawater; (4) S, for summer temperate seawater; (5) W, for winter temperate seawater; and (6) WNA, for winter North Atlantic.
The mark for the type of water in which a ship is currently operating is required to be visible at all times when the ship is on a relatively even keel.
This provides the crew, the regulators, and port officials with a rapid and effective means of determining whether the ship is safely loaded.