Why is the speed of a ship measured in knots? What is a knot? A knot is the time it takes to travel one nautical mile, a distance of 1.852 km or 1.5078 miles. Before the late 1500s, the only way to calculate the speed of a ship was by throwing a piece of wood or a log into the sea and, with the aid of an hourglass, see how long it took to pass between two points of the ship, typically from bow to stern. From this simple procedure, the speed of the vessel could be approximated. This method was known as a Dutchman’s log, despite the fact that Portuguese mariners were the first to use it from about the late 1400s. The next development called a ‘chip log’, weighted so that it would sink down under the water a little way, when it was dropped over the stern attached to a line. The line would be fed out and the time would be measured by means of a 30-second hourglass. When the line and the chip log were then hauled back in after 30 seconds, the length of line that was played could be used to calculate the ship’s speed. The first description of such a device in print is in William Bourne’s A Regiment for the Sea published in 1574. Bourne designed a 30-second hourglass especially for this device, but it is not known if he invented the chip log. At first, the line was not knotted, but knots in the line were added by the mid-17th century, when the concept of the nautical mile, as a distance of one minute of arc, was generally accepted as approximately 6,000 feet (based on assuming the Earth’s circumference at the equator is a circle of 360 degrees, with each degree made up of 60 minutes). Knots were then put into log-lines at equidistant intervals, proportional to a nautical mile. To avoid miles and miles of log-lines, it was calculated that knots every 50 feet, would give a fairly accurate reading after 30 seconds. Therefore, if ten such knots disappeared overboard in 30 seconds, ten knots was the speed at which the ship was travelling. This method delivered about 98% accuracy and continued in use even when engines replaced sails. In the early 20th century, engine revolution counters, which became known as speedometers, finally superseded the old method of knotted lines. Sailboats, however, had no choice but to continue with the old method. Nowadays, all ships calculate their speed and position instantly by means of GPS co-ordinates, but nautical speed is still recorded in knots to this day. See also Rate of knots.