Navigation: dead reckoning

Dead reckoning is a way of calculating your location using the simple equation:

distance travelled = speed x time

This sounds a lot simpler than it is in reality. Consider the potential problems as the procedure is described. The navigator will need a long piece of rope, knotted at measured intervals, a timing device, and a float tied to one end of the rope. To measure the ship’s speed the float is thrown over the rear of the boat and the timing is started. The number of sections of rope that pass in the time allowed will indicate the distance that the boat has travelled.

Have a look at the animation on the following website:

Every time the ship changes direction a calculation is needed and the new direction will need to be noted, a ship’s compass will give the new direction. However, magnetic north is not quite the same as true north – ie. the point at which all the lines of longitude meet.

Compasses were first used by the Chinese but it was not until about the 12th century that they became used by European sailors. As people did not understand how the compass needle worked many sailors considered them to be magical and linked with unearthly powers, some captains of sailing vessels had to hide their compass from view!

During the 19th Century, iron was being used in the construction of ships, which caused major problems with the ship's compass - the iron in the hull of the ship caused the needle to deviate from magnetic north. The Scottish mathematician Sir William Thompson developed a new type of compass-housing, or binnacle, to overcome this problem. By incorporating corrector magnets and iron spheres into the structure of the binnacle, it was possible to counteract the effects of the ship's iron.


What are some possible causes of inaccuracies using the dead reckoning method described above?
Speed at sea is measured in knots. What is a knot? Where do you think the word may have come from?
If a knot is tied in the rope at every two metres and 5 knots pass over the end of the boat in one minute, how fast is the boat travelling in [a] metres per second and [b] kilometres per hour?

Find out more about navigation:

Dead reckoning
Early navigation
Improved navigation
Modern navigation

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February 2007