Load Line (Plimsoll Mark)
The Load Line — also known as the Plimsoll mark — is the marking on a ship's side that shows the maximum draft to which the vessel may legally be loaded, ensuring it retains enough reserve buoyancy and freeboard to stay safe at sea. It is a circle crossed by a horizontal line, with a set of lines alongside for different conditions.
Because water density varies, the mark has separate lines for tropical, summer, winter, and fresh water (TF, F, T, S, W, WNA) — a ship may load deeper in dense cold seawater than in warm or fresh water. Established by the International Convention on Load Lines and named after campaigner Samuel Plimsoll, the load line is a fundamental safety limit: loading past the applicable mark is illegal and dangerous, and its position is verified at survey.
The load line is the visible line between a safe ship and an overloaded one. It caps how deep a vessel can load in a given sea and season, protecting reserve buoyancy — which is why it is enforced at survey and by port state control. For cargo planners it is the hard ceiling that draft and deadweight calculations must respect.
Why does the load line have several lines?
Water density changes with temperature and salinity, so separate marks (tropical, summer, winter, fresh) set how deep the ship may load in each condition.
What is the Plimsoll mark?
Another name for the load line — the circle-and-line marking on the hull showing the maximum permitted loading draft.