Plimsoll mark: lines that save lives

In the 19^th century, seafaring was a thriving industry, but for sailors themselves, the work was life-threatening. Ships were sometimes deliberately loaded as heavily as possible in order to maximise profit on each voyage. Many of these ships were also outdated and poorly maintained. Some ships were even over-insured, making them worth more if they sank.

The figures speak for themselves. In 1871 alone, 856 British merchant ships were lost off Britain’s own coast. Between 1870 and 1872, as many as 1,628 sailors ended up in prison for refusing to sail on so-called ‘coffin ships’, which they rightly considered unsafe.

Samuel Plimsoll was a Member of the British Parliament in the late 19^th century. He cared deeply about the plight of sailors and, in 1873, wrote the book 'Our Seamen', an indictment of coffin ships. He fought the establishment in Parliament for years. Many parliamentarians were shipowners themselves and had an interest in keeping the rules flexible.

In the end, public pressure became too great to ignore. In 1876, the Merchant Shipping Act was passed, which established a mandatory load-line on all ships. That line henceforth bore his name: the Plimsoll mark.

The Plimsoll mark consists of a circle with a horizontal line through it. That line indicates the maximum load-line: if the water rises above it, the ship is overloaded. Next to it are two letters indicating the classification society, such as LR for Lloyd’s Register or NV for Det Norske Veritas.

A ship is safely loaded when the water-line remains below the appropriate line for that particular voyage. If the water rises above it, the ship is overloaded and the crew’s lives are in danger.

But water is not the same everywhere. Fresh water is less dense than salt water, causing a ship with the same cargo to sit deeper in it. Warm tropical water is less dense than cold winter water. Therefore, additional letters to the right of the circle indicate the maximum draught according to the type of water and the season:

Today, the Plimsoll mark is a global standard, mandatory on all ships over 150 gross tonnes. Along the Thames in London stands an impressive monument honouring Samuel Plimsoll. But his greatest monument still sails on every cargo ship in port.

A few simple lines on a ship’s hull that still save lives.