# How the heck do you weigh a ship?

If you caught the first episode of our series, where we talked about buoyancy, you may remember that the weight of the water that a ship displaces is equal to the weight of the ship itself. This is known as the Archimedes principle. But how do you establish the weight of that water?

Ship builders have resolved the water-weighing problem by painting displacement marks on every side of the hull. (You’ve probably seen them along the waterline – all ships have them.). Take a look at the film and see how they work:

As we saw, displacement marks indicate the amount of displacement caused by the ship under various load conditions.

Maximum draught is marked with a ring with a straight line in the middle. That mark, called a plimsoll line, is named after Samuel Plimsoll, an English politician who worked to make life safer for seafarers. The clear mark on the side prevents overloading of the ship.

By measuring the distance from the mark closest to the waterline, you can calculate the exact displacement from the ship’s hydrostatic tables – and thus the weight of the ship and its load.

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