Depth Sounder Installation

[Steve Kidson]

I am in the process of installing transducers for depth and speed/log instruments in my boat. I have bought Navman 3100 series instruments.

My question is that in the installation guide a maximum of 15 degrees offset is recommended for the depth transducer, but the only position forward of the keel that I can install transducer is greater than that.

Should I try to machine a wedge to bring into line with recommended offset, or can I compensate in other ways; ie with the setup menu manipulating waterline and keel depth data??

Any suggestions/advice will be appreciated.

Fair winds

Steve

[GordMay]

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.

HTH,
Gord