I’ve always been loath to cut holes in the hull below the waterline, so I prefer to install depth finders inside the hull on my boats.
With inside-the-hull installed transducers, I’ve never encountered the slightest variance from the known depth. This holds true in shallow water, where accurate readings are critical, and in great depths.
The key to a successful installation is placing the transducer where you’ll get accurate readings. Avoid installing the transducer too close to the keel, over a void in the hull or over a thick spot in the hull.
Finding the sweet spot
First find the exact water depth under your boat where you plan to install the depth finder. In shallow water, this can be done with a boat pole or a lead line.
Next, following the depth finder’s instructions, hook up its electrical source and connect the transducer to its cable, but don’t turn on the unit.
Fill a large plastic freezer bag with seawater. Find an area on the hull as far forward as practicable. Place the transducer in the bag of water, and turn on the depth finder. With the transducer at the bottom of the bag, move the bag over the hull, checking the readings.
When you find a location where the reading agrees with the known depth, mark the place on the hull with a pencil and turn off the depth finder.
Installing the transducer
Sand off the paint where you plan to install the transducer.
Then, cut the bottom out of a plastic container with a diameter just large enough to accommodate the transducer. Invert the container, placing its smooth top flush against the hull.
Fill the container with an inch or so of marine-grade silicone sealant, and insert the transducer in the sealant until you feel it bottom out. Turn on the depth finder, and recheck the reading against known water depth.
When the sealant cures, the installation is complete. Should your transducer ever fail, simply pull it off the hull and install a new transducer the same way.
I’ve used this installation method on several boats. It works, it’s quick, and it eliminates another hole in the boat. –Don Baker
Individual boats and equipment vary, so consult your owner’s manual or the equipment manufacturer before beginning any installation.