Glossary Term
Doppler Speed Log
A Doppler speed log is an instrument that measures vessel speed by analysing the Doppler shift of acoustic signals reflected from the water column or the seabed. On yachts, it is used to provide speed data for navigation, manoeuvring, and bridge awareness. Depending on the equipment and operating conditions, it may provide speed through the water, speed over the ground, and sometimes distance run as well.
This data is valuable because it gives the crew a speed reference that does not rely on satellite positioning alone. During close manoeuvring, passage monitoring, or current assessment, the difference between speed through the water and speed over the ground can be operationally important. On more complex bridge setups, the log may also feed other onboard functions that use steady speed input for tracking and control.
A Doppler speed log should not be treated as interchangeable with GPS speed. GPS normally shows movement over the ground, while the log may be measuring how the yacht is moving through the water around it. Accuracy can also change with depth, aerated water, hull condition, transducer location, and calibration quality. For that reason, crews read log data in context, especially when current, shallow water, or low-speed handling makes small differences in speed matter.