Speedometer

A speedometer is an instrument used to measure the speed of the vessel and this information is fed into the navigational equipment. Instruments vary from a propeller log, (which calculates the speed relative to the number of rotations of a small propeller set under the hull and which functions like an anemometer for wind speed), a Propeller RPM (which measures the rotation rate of the propeller on a motor boat), Doppler Shift (which measures the vessel's speed by bouncing pulses off the ocean bottom) and GPS (which uses satellites to measure a ship's speed and its position. Speed is measured in knots.