Yes, machine a wedge (fairing block).
Locate the transducer amidships and away from keel at the point of minimum deadrise angle. The transducer must be fully immersed in water
and not subjected to air bubbles*, or turbulent water
flowing across the radiating face. A fairing is strongly recommended if your deadrise angle exceeds 10 degrees.
Nearly all vessels have some deadrise angle at the mounting location. If the transducer is mounted directly to your hull
, the sound beam will be tilted sideways at the same angle as the deadrise. A fairing aims the sound of the beam straight down by mounting the transducer parallel to the water, and it minimizes aerated water flowing over the transducer's face by mounting it deeper in the water.
When cutting a fairing, carefully orient it on the saw so the wider end will be facing forward (toward the bow) after the fairing is installed. A wider leading edge creates less turbulence* as it moves through the water. The goal is to create a smooth surface, so the water will flow cleanly over the fairing and transducer. Boat drag will be minimized. And more importantly, water will be free of bubbles and turbulence for the best sounder performance. Also, If the transducer is recessed more than 1/64 inch (0.5mm) inside the fairing, shim the transducer or carefully file/sand the fairing until they are flush.
A backing block is mounted inside the hull
to provide a level surface for tightening the threads that hold the transducer in place. It is fabricated with the interior
deadrise angle. After cutting the fairing block the remaining section can be used as the backing block.
* Turbulent water contains air bubbles, and a transducer cannot transmit through air.
There’s a little more to it, if you have a “cored” hull.