In the Age of Sail, sailors measured the speed of their vessel by tying knots at intervals (every 47’3”) in a rope, called a log-line, which was attached to a wooden board called a chip log, also known as a common log, ship’s log, or just log. The board was dropped over the stern of the ship and the number of knots that passed in thirty seconds while the rope unwound were counted. The wooden board and line were attached to a reel and weighted on one edge to float perpendicularly to the water surface, offering substantial resistance to the water moving around it. The knots were tied on the line at a distance of 47 feet 3 inches (14.4018 m). They passed through a sailor’s fingers, while another sailor used a 30-second sand-glass to time the operation. The knot count would be reported and used for navigation. 1 knot equals 1 nautical mile or 1.151 land miles per hour. The term ‘knots’ for speed resulted from this process.
Use of a chip log did not give an exact speed measurement and several other factors had to be taken into consideration, such as the currents, temperature, humidity, state of the sea and stretch of the line. This method was used until the development of mechanical speed logs, also know as taffrail logs, which were in widespread use by the mid 19th century.