There are two types of hull installations for a marine transducer: through-hull and in hull. Through-hull installations are critical for some applications, such as the operation of a recording fathometer on an oceanographic research vessel or for certain commercial fishing activities. However, where a through-hull installation requires the boat owner to put a hole in the bottom of the boat, in-hull transducers are entirely inside the hull and respond to sound conducted through the hull. They work well and give a good reading at any speed when properly installed.
Decide where you would like to mount the transducer. As a guideline, the best location is at the lowest point of the centre line of the boat, namely, on the bottom of the bilge at or near the stern. Finding the best mounting location, though, might require several repetitions of a trial and error process.
Test specific locations on the hull before permanently installing the transducer by putting just enough water in the bilge to cover the transducer, then have a friend hold the transducer in place while the boat is under way. At slow speeds, the transducer should provide clear, unscrambled depth readings. If it does not, then move the transducer around until good depth readings are obtained.
Increase the boat speed until the boat is up "on plane." If the depth reading is not scrambled, mark the location with a waterproof pen or by some other means. However, if the depth reading becomes garbled, this means the transducer is picking up feedback from the wake generated by the boat and the transducer must again be moved around until good depth readings are obtained. Once good depth readings at speed are found, reduce the boat speed and check the location at slow speed again. Repeat this process until a good location for the transducer is found.
Dry out the bilge completely and clean the hull surface where the transducer will be mounted. The transducer will be attached to the hull with a two-part epoxy glue and water or dirt will degrade the inherent sound transmission characteristics of the glue.
To prevent air bubbles in the glue (another factor that distorts sound transmission), mix the glue and spread a thick layer on the hull at the chosen location. Then push the transducer down into the glue and move it around slightly to work out any air bubbles and allow the glue to dry completely before connecting the 3-pin push-and-screw connector (attached to the transducer) to the depth finder to complete the installation.
Determining the on-board location for the transducer is best done on a calm day. Gluing the transducer in place is best done on a warm, dry day. The installation procedure is the same for all kinds of hulls; however, aluminium hulls require a specialised transducer.