The Plimsoll line (also known as a Load Line or the International Load line) is a reference mark located on a ship’s hull that indicates the maximum depth to which the vessel may be safely immersed when loaded with cargo.

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Why it is useful

It is evident that there is not a standard maximum height into which a ship is allowed to immerse, but the maximum allowed depth varies depending on the conditions, for example:

• the ship’s dimensions,

• the type of cargo carried,

• the time of year, and

• the water densities encountered in port and at sea.

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Why ‘Plimsoll’?

The name comes from Samuel Plimsoll (1824–1898), a member of the British Parliament, who expressed concerns in regard to the loss of ships and crews from vessel overloading.

In 1876, he persuaded Parliament to pass the Unseaworthy Ships Bill. This mandated marking a ship's sides with a line that would disappear below the waterline if the ship was overloaded.

The line is found midship on both the port and starboard hulls of cargo vessels and is still used widely in shipping.

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The original Plimsoll Mark was a circle with a horizontal line through it to indicate the maximum draft of a particular ship. Additional marks have been added over the years, taking into consideration different water densities.