Mounting the depthsounder transducer in a fiberglass boat in a conventional, through-the-hull fashion may give optimum performance of the instrument but has a few shortcomings. Another hole in the bottom of the boat doesn’t thrill me, though failure should be rare. The angle at which the transducer faces the bottom, and the resulting readings, will be influenced by the angle of the bottom of the boat where it is installed. Although the effects of heeling produce greater than actual readings, if the transducer is aligned with the vertical it will read correctly when the boat is on an even keel. Finding a horizontal area on the boat’s bottom isn’t easy on a sailing vessel, especially one with a long keel. Some means of mounting the transducer in a vertical orientation will be required.
Another consideration is that optimum performance happens only when the face of the transducer is kept clean of marine growth, a continuous chore.
Placing the transducer inside the boat can, according to some manufacturers, cause slightly degraded performance. But I have done this on three occasions, using different techniques, with perfectly reliable performance.Oil bath versus adhesive mounting
The method I used on two installations was oil bath. In the first case the transducer was mounted inside a pipe, shaped at its junction with the bottom of the boat to be vertical, and the pipe filled with vegetable oil. I used a short section of PVC pipe and an end cap to seal the entire unit. In the second case the transducer and through-hull fitting were separate units. I cut off the bottom of the plastic through-hull fitting to fit the desired shape since there was enough height within the fitting to allow the transducer to seal. It was also filled with vegetable oil. Performance was fine in both cases.
On my most recent installation the transducer and through-hull fitting came as an integral unit. The transducer is quite large in diameter (two inches) and, with the bottom flange, would take an overly large PVC pipe to use the oil-bath method. I decided to try adhesive mounting.