What is the Plimsoll mark? The Plimsoll mark is a circle with a horizontal line drawn through it; carried on both sides of British-owned merchant vessels. It provides a safety line; and a ship may not be loaded so that the line is under water. The mark takes its name from Samuel Plimsoll; who; disgusted with "coffin ships" which went to the bottom with their crews through overloading; managed to get a law made to ensure that a load line was put on every shipss side. Sailors call it "Plimsolls Eye"; and it must have saved hundreds of thousands of lives; and a great deal of insurance money. Illustration for one of a set of 50 "Do You Know?" cigarette cards issued by Carreras in 1939.