What is Towed Speed Log?

Towed Log is a type of Speed Log which was used to measure the speed of a ship. An impeller, attached to a special rope,  was lowered from the stern of a moving ship. Due to the flow of water, the impeller rotated. The impeller was connected to a indicator which showed ships speed. Faster the ship, faster the flow of water, faster the rotation of the impeller, greater the speed shown by the dial. This log is now obsolete.


Currently, it is being used on ships-of-opportunity to measure ocean surface currents. The technique used is one of the oldest known to navigators. The ship’s dead reckoned position is calculated from the heading and as speed measured by the towed log. This is then compared to the ship’s true position as obtained from a reference navigation system like GPS, and the difference is due to currents as  encountered by the ship. While it is not capable of making high precision current measurements as would be obtained from moored current meters, it can distinguish features on the order of 20 to 30 cm/sec.