It has always been a source of confusion for non-sailors, but maybe this will help to clear up the mystery. A knot is actually a measurement of speed equal to one nautical mile per hour and is used for measuring the speed of ships and boats. A nautical mile is equal to 1.15 land miles. The word "knot" is derived from the ancient sailor's practice of gradually trailing a knotted rope over the side of the ship. The number of knots fed out over a specific period of time indicated the ship's speed. To hold the rope on place on the surface of the sea, a large piece of wood was attached to the trailing end to prevent the rope from moving with the ship. Hence, yet another definition is explained, Ship's Log.