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The unit derives from a very primitive speed measurement system – called nacelle -, which began to be used in the 16th century. The instrument basically consisted of a string with one end tied to a heavy wooden plank and the other to a spool of the same material. This rope was tied with knots at regular intervals of approximately 14.5 meters. When the boatman wanted to know the speed at which he was sailing, the board was thrown overboard. With the boat moving, the water braked the wood, causing the rope to come loose from the spool that remained on the boat. With the help of an hourglass, the boatman watched how many knots unwound in a given period of time. Today, this rudimentary method is no longer used, but the word knot is still in vogue to calculate the speed of vessels around the world. Today, 1 knot equals 1.852 kilometers (or 1 nautical mile) per hour.