The physical location of the transducer depends on the vessel's design and construction, how the hull is shaped, and how the water runs along the hull. There are however a number of important
guidelines, and some of these are even conflicting.
The information here must be considered as general advice. Each transducer installation must be handled separately depending on the hull design.
Bow thruster propellers are extremely noisy. When in operation, the noise and cavitation bubbles created by the thruster make the echo sounder or sonar useless, almost no matter where the transducer is installed. And when not in operation, the tunnel creates turbulence. If your vessel is pitching, the tunnel may be filled with air or aerated water in the upper position and release this in the lower position.
In general, all transducers must therefore be placed well away from the bow thruster. In most cases, a location forward of the bow thruster is advantageous.
However, this is not an invariable rule. Certain thruster designs combined with its physical location on the hull may still offer suitable locations near the thruster. If you are in doubt, consult a naval architect.
(Photo: Brosen, Wikipedia)
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