Sunday, February 8, 2009
The speed of a ship at sea is measure in knots, each knot being equal to a nautical mile (6076 feet), which is slightly longer than a mile measured on land (5280 feet). In the old days the speed of a ship was ascertained by means of the log line. A long cord with knots at equal distances was released, freely from the stern. By counting the amount of knots in a certain time frame, the sailors could calculate the speed. Now we use a Doppler log. Doppler’s method can calculate the ships speed by the change in frequency of a signal that is send out under water and reflected by the ground or a water layer, back to the ship.