One method I've heard to test transducer placement and function is to take a
small coffee can and temporarily attach it to the inside of the hull using
silicone. Fill it with water and drop the transducer in. This should allow
the sonar signal to pass through the hull and into the water. If you like
the results, remove the coffee can and silicone, dry the area, and epoxy
the transducer to the hull. I used Marine-Tex. Don't apply the silicone to
the coffee can until you're ready to try this on the water; it could be
difficult to remove if you let it cure.
-- Eric L. Pederson P-19 #970, Necessity Bloomington, MN