The Plimsoll line — the mark that stops ships being overloaded

Amidships on every merchant hull is a circle crossed by a horizontal line, flanked by a ladder of short lines lettered TF, F, T, S, W, WNA. That is the load line — the Plimsoll mark — and it is the legal limit of how deep a ship may be loaded. Water touching the relevant line means the ship is full: no more cargo, by law.

The ladder exists because buoyancy varies. Fresh water is less dense than salt (F sits above S); warm tropical seas less dense than winter North Atlantic storm water (T above W, with WNA lowest of all for small ships in the worst ocean). The mark's letters also carry the classification society that certified it.

The idea was forced through the British Parliament in 1876 by Samuel Plimsoll against ferocious shipowner opposition, aimed at the 'coffin ships' deliberately overloaded and over-insured at sailors' expense. It remains one of the oldest pieces of safety regulation continuously in force at sea — and the direct ancestor of the load-line rules that define every deadweight figure on this site.