The Plimsoll line is a reference mark located on a ship’s hull that indicates the maximum depth to which the vessel may be safely immersed when loaded with cargo. This depth varies with a ship’s dimensions, type of cargo, time of year, and the water densities encountered in port and at sea.
"Nineteenth century merchant shipping was far more dangerous than it is today. The enormous growth in world trade had reached all four corners of the globe. As trade increased and shipping became more competitive, unscrupulous owners overloaded their ships to maximise profits, resulting in the loss of thousands of lives and ships.
In 1835, Lloyd’s Register introduced the Lloyd’s rule in an attempt to curb this practice of overloading. They recommended that the distance from the waterline to the weather deck should be 3 inches of freeboard per each foot of depth in the hold.
This rule stayed in place until 1880, though many ship owners ignored it and losses continued to rise. Samuel Plimsoll, a British coal merchant and Member of Parliament at the time was shocked by the scale of life lost at sea.
With fierce opposition from other merchants, he published Our Seaman in 1872 which detailed evidence of reckless overloading, the rotten condition of hulls and equipment, undermanning, filthy crew accommodation, the prevalence of over-insurance and the deliberate sinking of unsound and unprofitable Coffin ships.
In 1874 Lloyd’s Register made it a condition of their classification that a load line should be painted on newly built awning deck steamers. This original load line was a diamond with a centre line and the letters L R next to it. This of course only applied to ships inspected by Lloyd’s Register.
In the meantime, Plimsoll’s book had started a campaign which led to a Royal Commission.
In 1876 the Merchant Shipping Act made load lines compulsory on all British vessels and on foreign vessels using British ports. However, the act was flawed as it left ship owners to decide where a load line was to be painted. Some apparently painted them on the funnel! Unscrupulous owners could carry on being unscrupulous!
The Lloyd’s Register surveyors continued to gather data including information on a vessel’s strength and construction. This information was used to draw up the UK’s Board of Trade Load Line Tables in 1886 and ensured the fixing of the position of the Load Line on all ships by law in 1890. In Britain this line became known as the Plimsoll Line.
By the early 1900s many countries had adopted their own regulations. An international solution came with the first International Convention on Load Lines in 1930.
In 1966, the International Maritime Organization (IMO), a United Nations agency responsible for ship safety, adopted a new Convention ensuring the ship has enough reserve buoyancy and covering, which allowed freeboard for a ship in different climate zones and seasons via a load line zone map.
The Plimsoll Line and the Summer Line are the same. All the other lines take their positions from there.
When a ship is commissioned, the exact location of the load line is calculated by a classification society, its position on the hull is verified and a load line certificate is issued.
The Chief Officer is responsible for the safe loading of the cargo. On a winter route from Greenock, Glasgow to Ho Chi Minh City in Vietnam, the calculations would need to consider adequate stability for all stages of the voyage and the load line for the route and season. The ship would pass through the Atlantic winter zone and Mediterranean summer zone, on to the Suez Canal, Red sea, the Straits of Malacca, finally arriving in Ho Chi Minh City, which are all in the tropical zone at this time of year.
Thanks to the campaigning of Samuel Plimsoll over 150 years ago, countless lives and vessels have been saved. Today the International Maritime Organization and classification societies continue working to improve maritime safety across the globe."
To learn more, read this blog on the Plimsoll Line.