Speed Log
A Speed Log measures how fast the ship is moving. A water-speed log measures speed through the water using an electromagnetic sensor or a Doppler acoustic beam; a Doppler log can also measure speed over the ground in shallow water by reflecting off the seabed. It also totals the distance run.
The name comes from the old chip log — a knotted line streamed astern — from which the unit knot takes its name. Modern speed and distance data feed the radar, ECDIS, autopilot and the noon report, and speed through the water is important for judging ship performance against the currents.
Speed through the water, as distinct from speed over the ground, is what shows the real performance of the hull and engine independent of current — the basis for performance monitoring, charter-party speed claims and fuel analysis. The speed log is the instrument that provides it.
What is the difference between speed through the water and speed over the ground?
Speed through the water is movement relative to the water and reflects hull and engine performance; speed over the ground is movement relative to the earth (from GPS) and includes the effect of current. Both matter for navigation and performance.