A ship's speed is calculated in knots, which were originally calculated based on the knots in a long rope that was tossed into the water. However, a ship's speed is now calculated using a maritime log that uses Doppler radar or ultrasonic sensors. The knots in the rope used by early sailors were tied 14.4 meters apart, then an hourglass or sand glass was used to time the seconds between each knot as it descended into the water.
Today, a single nautical mile is still called a knot, and measures 1.852 kilometers. It is usually measured with a pitometer, or boat speedometer. It looks similar to the speedometer on a car except that it uses sensors in the water to measure how quickly the water passes through the pitometer's hose.