Avoid ANOTHER hole in the bottom of the boat
Considerations
Mounting the depth sounder 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 (2 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.
On
the left is the area I chose for installation. It's a locker well forward
in the boat and has about a 25-degree slope up to the right and a vertical
side on the left. The mushroom flange on the bottom kept the transducer
from fitting nicely into the space. I carefully cut the flange sides (shown
on the right) to permit mounting it as low as possible in the V-shaped
area.
In order to get a vertical orientation
of the transducer; it was necessary to fill the void between the bottom
of the transducer and the hull with something that would permit direct
contact between the transducer and the hull. As with a cored hull, I knew
that wood would NOT work. I chose to use an underwater epoxy compound,
Splash Zone, as the filling material. I wasn't 100% sure this would work
so I did not want to make it a permanent job from the beginning and, perhaps,
waste the investment in the transducer. My approach was to do the job in
stages.
Prior to putting the transducer into the
epoxy filler I covered it with plastic film, taped securely in place to
make a smooth covering of the surface.
I did not want any voids between the surface
of the transducer and the filler material. This, I knew, could cause a
degradation of performance.
After filling the area with Splash Zone,
I pushed the transducer firmly into place.
On
the left is the transducer in place during curing and, on the right, the
result after curing was completed and the transducer removed. The bottom
of the mold is a perfect image of the bottom of the transducer.
To give myself confidence this would work
before taking the fatal step of gluing the transducer in place, I filled
the mold with water and inserted the transducer.
With a bit of trepidation, I turned on the unit and was relieved to see that it worked!
To complete the job it was only necessary
to use a thin coat of epoxy glue to affix the transducer to the molded
shape in the filler material.