Why Are There Marks Painted on a Ship's Side? Somebody Counted the Drownings

Painted on both sides of a ship amidships is a circle crossed by a horizontal line, with a ladder of further lines beside it. The circle's line shows the maximum depth to which the ship may be loaded in summer in salt water. The additional lines allow for the fact that a ship floats at different depths in different conditions, sitting deeper in fresh water because it is less dense, and being permitted to load less in winter and in the north Atlantic where seas are heavier. A loading officer reads the waterline against the marks, and a ship loaded beyond them cannot legally sail.

The incentives pointed in one direction and the protections were absent:

• •Freight was paid by quantity carried, so more cargo meant more revenue
• •Owners insured ships and cargo, so a loss could be profitable
• •Crews had no right to refuse to sail and were imprisoned if they did
• •No survey or limit existed, so nothing was being broken
• •Ageing vessels were loaded hardest, since they were worth least
• •Losses were treated as the ordinary hazard of the sea

Samuel Plimsoll, a coal merchant who became a member of parliament in 1868, took up the issue after learning how many ships were being lost and published a book in 1873 setting out the evidence and naming owners, which cost him a libel action. His most famous moment came in 1875 when a bill was dropped and he shouted at the House, shook his fist at members and refused to withdraw, which was reported everywhere and made the subject impossible to ignore. Legislation followed in 1876 requiring a mark, although it left owners to decide where to paint it, which produced marks painted on funnels and other absurdities. Compulsory official positioning came in 1890.

The ladder of lines beside the circle carries letters that are readable once explained and that record the reasoning behind the whole system. Marks for tropical water sit highest, permitting the deepest loading, since warm calm seas demand less reserve buoyancy. Summer sits below that, then winter, then a lowest mark for the north Atlantic in winter applying to smaller vessels on that crossing. Fresh water marks sit above their salt water equivalents, since a ship floats lower in fresh water and will rise as it reaches the sea. Letters beside the circle identify the organisation that surveyed and assigned the marks, which makes the certification visible on the hull itself.

A national rule protects nobody from foreign ships, and since shipping is international the arrangement had to become so. A convention agreed in 1930 and replaced in 1966 sets the rules now, assigning each ship a permitted depth calculated from its dimensions, its watertight subdivision, the height of its deck above the water and the strength of its structure. Surveyors of a recognised organisation calculate and certify it, and port inspectors worldwide check it as part of a system under which any state may inspect any ship in its ports. The marks therefore work as a universal legible statement, and a ship sitting below its line is visible to anybody on the quay.

A circle with a line through it marks the deepest a ship may legally float, with extra lines for fresh water, for winter and for heavy northern seas. Freight paid by quantity, insurance that made losses profitable and crews imprisoned for refusing to sail made overloading rational. Plimsoll's campaign produced a law in 1876 that let owners position their own marks, and official positioning followed in 1890.