Resources for teachers: PHYSICS

Why do ships need a plimsoll line?

Discipline: PHYSICS

Key Stage: 3 or 4



Why do ships need a plimsoll line?


You will understand why it is easier to float in different sea waters and link this to the density of the water.


The Industrial Revolution that started in Britain in the 18th century was dependent on trade with foreign countries to supply the raw materials for Britain’s factories to transform into finished products. The wealth generated also created a demand for the importation of exotic goods from abroad. As an island, Britain therefore became more dependent on merchant shipping.
The rewards from a successful voyage were enormous, but the risks were huge too. Many a ship was lost at sea, often due to overloading with cargo.
Samuel Plimsoll, a member of parliament from Derby, responded to concerns and demanded that ships ought to operate to safety limits determined by a ‘load line’ on the ship’s hull. It was not until 1876 that the first regulations came into practice in Britain. Since then, every merchant ship has to have the load line, known as a plimsoll line, marked on its hull.
A single load line would not be adequate because the upthrust from the water depends on its density. The density of the water depends on its temperature and its salinity [saltiness].

  1. Cut five drinking straws to the same length of 10cm to 15cm
  2. Seal one end with a small piece of plasticine, make these as equal as possible.
  3. Fill four boiling tubes with solutions of salt water 0.5%,1%,2%,4% and one with distilled water.
  4. Put all of the straws in the distilled water and then mark the water line.
  5. Predict how the floats will be supported by each of the other solutions.
  6. Test your prediction by placing one straw in each of the four other solutions, in each case, mark the water line and label the straw.



  • What forces are acting on the straw to make it float?
  • Are they balanced or unbalanced? Explain your answer.
  • Draw a force diagram to show the interaction of the forces.
  • Make a simple description of your observations
  • Did they match your predictions?
  • Explain how saltiness effects water density, use a particle model to help your explanation.

In the Plimsoll Line, TF stands for Tropical Fresh Water and WNA for Winter North Atlantic.

  • Which of these waters is denser?
  • Why is the WNA mark at the bottom
  • Which of these waters is less rough? Which is of greater concern, density or storminess?

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February 2007