Knot

The knot takes its name from a centuries-old method of measuring speed. Sailors streamed a length of rope astern with a wooden log tied to the end and knots spaced evenly along it. As the log dragged in the water the rope ran out, and the crew counted how many knots passed through their hands in a fixed interval timed by a sand glass. The number of knots counted was the ship's speed, and the name stuck even though the method has long been replaced by instruments.

Because the knot is defined against the nautical mile, it works seamlessly with chart distance: a vessel making 10 knots for three hours has run 30 nautical miles, a calculation a navigator can lay off directly on the chart. This tight relationship between speed, distance and position is why the knot survived where other traditional units were dropped in favour of metric measures.

Modern ships measure speed in two ways that a watchkeeper must not confuse. Speed through the water is read from a log (usually an electromagnetic or Doppler speed log) and reflects the ship's motion relative to the surrounding water. Speed over the ground comes from GPS and reflects true progress across the seabed. The difference between them reveals the effect of current and tide, which matters for passage planning, fuel calculations and estimated time of arrival. Wind speed at sea is also quoted in knots, as is aircraft speed, making the knot one of the few units shared across the maritime and aviation worlds and standard on every bridge and in every weather forecast for mariners.