A log is a device used in navigation to measure the speed of a ship.

The original log literally consisted on a piece of wood, the "log", on a length of rope, which was thrown over the back of a ship. The rope had a number of measured knots tied in it. The speed of the ship was indicated by the number of knots passing over the stern during a certain period of time. The unit, knot, for nautical mile per hour, was derived from the knots tied in the rope of a log.

Later technologies used in logs include small forward facing water turbines whose speed of rotation is proportional to the speed of the ship and the measurement of the doppler effect on ultrasonic transmissions through water passing by the ship's hull.