Overloading once sank ships without warning. The Plimsoll line is a set of marks on a ship’s hull indicating the maximum depth to which the vessel may be safely loaded in different seasonal zones and water densities.

Named after Samuel Plimsoll, the nineteenth-century reformer who campaigned against coffin ships, the marks include the deck line and a circle with a horizontal bar, plus additional lines for tropical, summer, winter, and freshwater conditions. The ship may not be loaded so that the appropriate mark is submerged.

Load line certificates and periodic surveys enforce the rules. Officers check freeboard and draft against the marks during cargo operations. The Plimsoll line remains one of the most visible and effective safety regulations in shipping.

• Hull marks showing maximum safe loading depth
• Varies by seasonal zone and water density
• Enforced by load line certificates and surveys
• Prevents dangerous overloading