I have installed transducers in fiberglass hulls in two different ways.

One way is to drill (cut) a hole through the hull, in a location that will be accessible from both sides (inside and outside).
This usually requires one to build a fairing block to go on the outside of the hull to insure that the transducer is looking,
reasonably, downward.  Bed everything with a good caulking compound, including a backing ring inside of the hull.

The alternative is to install the transducer inside of the hull.  It still must look down.  One can build a small reservoir
(sealed to the hull), fill it with a fluid like mineral oil, and insert the transducer into the fluid.  The alternative is to
bed the transducer directly to the hull, using a silicon bedding compound.  Seat the transducer in such a way that there are no
air bubbles trapped in the bedding.  In either case, one is trying to obtain a coupling between the transducer and the water
outside of the hull.

There will be some attenuation of signal when using the inside installation.  Depending on how you the depth sounder, one may
or may not notice a difference.

RBourke@aol.com wrote:

--
Nick Meloy
mulatfl@pcola.gulf.net
(850) 994-6165

RBourke@aol.com writes:

Installed mine last summer near midship and at a place inside  the
hull that was close to horizontal.
Bedded it in plenty of G.E. Silicone compound to avoid airbubbles
between the sensor and the hull. Even though the sensor is not
"looking" perfectly straight down, it seems to work quite well in
depth up to 60 ft (I don't care about deeper readings). My hull is
fiberglass and about 3/8 thick where I installed the sensor.
Ciao - Scaramouche

P.s: The sensor warned me in plenty of time last summer before I hit
the rocks and took a chunk out off my keel - only I ignored its
"Beep" because I was being distracted elsewhere....

How about using a bracket at the bottom of the transom. EZ to install, align, and service plus no thru hull???

Michael Wilkie
Jammin'
MDR, Cal

RBourke@aol.com wrote: