What to check on a Speed Log (Doppler).
A Doppler speed log sends short acoustic pulses from a transducer through the water at an angle, usually four beams in a Janus configuration, and measures the frequency shift in the echo. When the beams reflect off the seabed, the log calculates speed over ground directly, without any assumption about the surrounding water mass. This is the fundamental difference from an electromagnetic (EM) log, which only ever measures speed through water by sensing the ship's motion relative to the water flowing past a sensor head. In shallow and coastal waters…
What sets a Doppler log apart
A Doppler speed log sends short acoustic pulses from a transducer through the water at an angle, usually four beams in a Janus configuration, and measures the frequency shift in the echo. When the beams reflect off the seabed, the log calculates speed over ground directly, without any assumption about the surrounding water mass. This is the fundamental difference from an electromagnetic (EM) log, which only ever measures speed through water by sensing the ship's motion relative to the water flowing past a sensor head. In shallow and coastal waters, where bottom lock is available, a Doppler log gives a docking officer or pilot a true ground speed reading that an EM log simply cannot provide.
Main components
Transducer array
Four, occasionally three, piezoelectric transducers mounted in a sea chest or hull-mounted housing, angled roughly 30 degrees from vertical. The Janus arrangement, opposing pairs fore-aft and port-stbd, cancels out the ship's pitch and roll error in the calculated velocity.
Transceiver and signal processor
Generates the transmit pulses, digitises the returned echo, and extracts the Doppler shift for each beam. Modern processors correlate successive pings to reject noise from bubbles, wake turbulence and biological scatterers.
Bottom-track / water-track switch
Below the maximum bottom-lock depth, typically 200 to 400 metres depending on transducer power and seabed reflectivity, the unit automatically drops to water-track mode, tracking scatterers suspended in a reference layer of water instead of the seabed.
Selection and sizing
| Parameter | Typical range |
|---|---|
| Bottom-track depth | 200-400 m (some units to 600 m) |
| Speed accuracy, bottom track | +/-0.1 to +/-0.2 knots |
| Speed accuracy, water track | +/-0.2 to +/-0.5 knots |
| Operating frequency | 150-600 kHz |
Lower frequencies reach deeper bottom lock but need a larger transducer footprint; higher frequencies suit smaller vessels where hull space is limited.
Regulations and class
SOLAS Chapter V, Regulation 19 requires a device to measure and display speed and distance on ships of 300 GT and above. It does not mandate the Doppler principle specifically, an EM log satisfies the same regulation, but where dynamic positioning, docking or river pilotage is routine, owners specify Doppler for the ground-speed capability. Class societies treat the log as part of the navigation equipment survey at each periodical survey, checking calibration records and the interface to the VDR and ECDIS.
Typical faults
| Fault | Consequence |
|---|---|
| Marine growth or air bubbles over the transducer face | Weak or lost echo, log drops to water-track or fails entirely near the berth, when accuracy matters most |
| Sea chest valve left partly shut after painting or dry-docking | No signal at all, often mistaken for an electronic fault |
| Gyro or heading input lost | Log cannot resolve fore-aft and athwartship components correctly, longitudinal and transverse speed become unreliable |
| Transducer misalignment after hull repair | Systematic speed error that calibration alone cannot fix |
What to look for in a supplier
- Documented bottom-lock performance at realistic seabed types, not just calm-water trial figures
- Interface compatibility with the existing ECDIS, VDR and speed log alarm system without a protocol converter
- Availability of a diver-changeable transducer or a sea chest design that avoids dry-docking for replacement
- Clear published accuracy figures for both bottom-track and water-track modes, not a single blended number
Before trusting a Doppler log for pilotage or docking, check on the display whether it is actually in bottom-track mode; a log quietly running on water-track gives a confident-looking number that is still only speed through water.