Most Doppler logs have Janus Configuration where
- Transducers pointing ahead measures speed.
- Transducer pointing Astern is used for accuracy check.
- Transducers point abeam to measure athwart ship speed (while berthing).
Use of Doppler logs for measuring athwartship speed has become mandatory on large tankers. Human judgment of speed and the distance is not accurate enough to handle huge tankers while mooring or unmooring (alongside or single point) or low speed cruising. For example, when a 200,000 Tonnes tanker with a residual speed of 0.2 knots is tying up, the pier, dolphin or the ships side has to absorb energy which is = 0.5 mv2 – 10,00,000 joules. In this situation, an accurate Doppler Log can be of great help here. Currently, a combination of Doppler and EM log provides a robust, reliable system for the measurement of ship’s speed over the ground and water, in both forward and athwart ship direction.