Our bottom finder is only as good as the signal sent and received by the unit’s transducer. While some manufacturers prefer a transducer mounted on the transom for optimum signal, such mounts can be vulnerable to striking debris. In-hull mounting can reduce the signal strength, but most experienced boaters prefer that to the less-protected exterior transom mount. Here are three ways to get a great signal from a reliable transducer mount.
These transducers, mostly made by Airmar, give an extremely reliable and sensitive signal even at high boat speeds. But putting them in takes a little finesse and a big belt of courage. You’ll need to choose one to match your sonar unit and the deadrise angle of your hull, so when installed, it directs its beam at 90 degrees to the bottom. If you don’t, your transducer will give inaccurate readings — the deeper the water, the larger the error, as the signal cone will have to travel farther at an angle than when transmitting vertically.
Measure your hull’s deadrise — if you can’t find written specs on it. Deadrise is the angle in degrees of the hull from keel to chine. Use a level on the keel and a contractor’s protractor to measure the angle. Through-Hull Mounting You can fi nd a variety of transducers to fit any deadrise angle at airmartechnology.com
Examine your boat’s hull and, if you have a trailer or hoist, note where the hull and bunks meet. Choose an area in the bilge near the transom where there are no chines, throughhull fittings or support bunks on the same line. Mounting closer to the keel gives better readings at high speeds, when the hull is likely to ventilate. Choose a spot that gives adequate clearance to service any fittings after installation too.
Using a drill and circle cutter adjusted to the diameter of your transducer, drill your hole. Start a pilot hole through the hull at the center of the mounting position from the inside of the bilge. Use the pilot hole to position the circle cutter from the outside of the hull, and carefully drill through the hull and fiberglass liner (if your hull’s bilge has liner).
Dry-fit the transducer, using a file or rasp to broaden the hole if necessary. Your transducer will have an arrow on the top where the wire comes out, indicating how it should be oriented toward the keel. Point the orientation arrow or index toward the keel to align the transducer element inside the housing to a vertical position for accurate readings.
Spread sealant on the inside of the hole to seal the core between hull and liner. Put a bead of sealant around the flange of the transducer and press it in place. Have a friend hold it while you apply sealant around the base of the transducer inside the hull. Screw down the flange, stopping just short of finger-tight. Finish tightening after the sealant has cured 24 hours for a secure gasketlike seal.
In shallow-water applications, many boaters simply glue their transducer to the inside bottom and it works pretty well. But keep in mind that your signal is reading the water slightly abeam of the boat. You can also purchase an in-hull transducer angled to match your hull’s deadrise.
Follow steps 1 and 2 on the previous page. Don’t worry about trailer bunks on this installation — it’s one of its advantages. If you have a bilge liner, you’ll need to remove a section of the liner large enough to fit your in-hull transducer. You’ll need a router and cutting bit to do that. Seal the gap between the liner and hull with 3M 101 sealant. Be generous with the sealant — you don’t want this joint to fail.
Using a silicone sealant, secure the face of your transducer to the hull’s surface and allow it to cure 24 hours before going boating.