You open the throttle, plane the boat, tickle the trim until you can’t go any faster, and then your sonar loses the bottom. Chances are, the problem lies with your transducer installation.
The transducer is your sonar’s direct connection with the water and its few basic needs are non-negotiable. It fires sound pulses into the water, receives echoes from them, and sends the echoes to your sonar unit where they are processed into your screen picture. The transducer’s working face must remain in unobstructed contact with the water during the whole process.
The transducer’s mounting position and angle are both critical to good performance. Transducers must not be mounted far enough from the keel to leave the water during turns but they should also not be mounted directly in front of the circle cut by an outboard’s propeller. They also can’t be installed behind turbulence-producing things like through-hull fittings or grooved hull features like strakes or sponsons that form air bubble streams. Transducers need a smooth flow of water across their working surface.
A transom-mounted transducer can generate its own harmful turbulence when mounted at a nose-down angle. Tilting one between three and five degrees nose-up from parallel with the hull bottom is usually about perfect and the front of its working surface should run just far enough below the hull to stay submerged. Streamlined transducers like the Lowrance Skimmer can usually be mounted deep enough to run completely submerged if necessary to get below turbulence from a problem hull that doesn’t offer an ideal mounting location. The side-to-side mounting angle of the transducer’s face should be parallel with the top of the transom or the cockpit floor.
Gluing down a transducer inside the sump area where a hull is thinnest usually works well for fast boats. Choose a spot right over a flat, smooth section of the hull’s outer surface. Since you can’t see through the transom and hull, find an ideal location on the outside and then measure the distance to it from the transom drain hole. Then you can go inside the sump area, locate the drain hole, and use the same measurement to reach the spot. Transducers have an easier time shooting through fiberglass than through aluminum and they rarely deliver acceptable performance through wood.
You can test-run a potential sump area mounting location by placing the transducer on it, pouring enough water in the sump to cover the transducer, and then anchoring it in place with a sandbag. Test it at speed and move it around until you find the best performing spot and then epoxy the transducer in place. Shoot-through-hull mounting turns the hull area under the transducer into part of the transducer’s face. When the transducer vibrates the hull must also vibrate so use an epoxy that dries rock-hard for best sound transfer.
Mount it right, check it often and it will support your need for speed!