Speed and distance measuring log
Speed logs, depending upon their type, will provide either speed through the water or speed over the ground measurements.
Types of speed measurement
In general terms, speed through the water is used for radar collision avoidance, and speed over the ground is used for navigation. Speed made good can also be measured on ships, and represents the speed that the ship has achieved over a period of time. Speed made good can be measured from the chart between position fixes, and is also calculated and transmitted by electronic position-fixing systems.
Direction of speed measurement
Doppler-type logs can both be single-axis and measure speed in the fore and aft direction or dual-axis and measure fore and aft and arthwartship movement. Coupled with rate of turn measurement, dual-axis logs are also able to calculate the speed and direction of movement of the bow and stern. Electro-magnetic logs provide single-axis measurement only.
Recording of distance travelled
As well as indicating ship's speed, logs record and display distance travelled. It is good navigation practice to initialise the log distance trip at the start of each new track, and record log distances in the logbook at the end of each watch.
