Plimsoll mark

The Plimsoll mark is the load line painted on a ship’s side, a circle crossed by a horizontal line marking the maximum permitted loaded draft, named after Samuel Plimsoll, whose campaigning led to the UK Merchant Shipping Act 1876. Alongside it sit the seasonal and zonal lines (tropical, summer, winter, winter North Atlantic, and fresh water), since safe freeboard varies with water density and weather. The mark is governed by the International Convention on Load Lines 1966, which fixes minimum freeboard so the ship keeps adequate reserve buoyancy. Loading past the applicable line is an offense and voids the vessel’s load line certificate.