Rocks & mirror
Encyclopedia of Marine and Energy Technology

Speed logs


Ship speed measuring devices.

- Doppler log – An acoustic speed log based on the Doppler effect in which the wave lengths of moving objects appear to shift in relation to the observer. This shift can be converted to speed, thereby giving a very accurate result.

The Dual Axis Doppler Speed Log utilizes the Doppler shifted returns from high frequency acoustic energy transmitted into water to provide precise speed data, distance travelled, and water depth below the transducer. The transmitted signal is scattered back from the sea bottom and/or scatters in the water mass. The system amplifies the received signals and processes them to determine the Doppler shift. The time required for the signal to bounce back from the sea bottom is measured to calculate depth. The standard system consists of a Master Display Unit, an Electronics Unit, a Transducer Assembly, a Sea Chest or Tank, and interconnecting cabling.

- Electromagnetic log – The electromagnetic log works by generating a small alternating current in a transducer producing an electromagnetic field in the adjacent water. As the vessel moves through the water, the voltage proportional to the speed is generated at 90 deg to the direction of travel. This signal voltage is detected by the probes and transmitted to the master electronic unit where it is amplified and processed digitally before being passed to the speed and distance displays.