The Navigators toolbox-marine log
[1] Background The compass, sextant, chronometer, and radar are tools that fix the ships position on a chart, meaning the position is a known place “on the ground”. The speed of a ship is another parameter important to navigation and historically has been achieved by measuring the passage of an object alongside the ship. The principle is that an object thrown overboard stays stationery as the ship moves past it. If we can measure the time it takes for the ship to pass the object, we can measure the speed of the ship. There is an important provision here. It measures distance travelled through the water not “over the ground”. Therefore the effects of water mass movement, tide and wind on the ship are not taken account of. The structure of the log is simple. A quadrant-shaped piece of wood is weighted with lead to hold the quadrant vertical in the water. A bridle is attached to each corner of the quadrant and then to a log line that is wound on a spool. A release line is ...