If you’re experiencing difficulty with your sonar, it’s more than likely caused by an improperly mounted transducer.
Transom-mounted transducers come out of the box with most sonar units because they are the easiest to install. This easy installation appeals to many do-it-yourselfers, and that results in loss of performance from shoddy installation. The key to clear readings at any speed is smooth water flow across the face of the transducer. Bubbles off the prop or turbulence caused by elements on the hull can skew readings, and a transducer that comes out of the water on a turn won’t read at all.
To achieve this smooth water flow, transducers should be mounted far enough starboard of the prop to clear bubbles and prop turbulence. Usually this is 18 to 24 inches. Also, choose a flat surface of the hull between lifting strakes, and do not mount the transducer behind any hull fittings or rivets that could cause turbulence.
The next important aspect is angle, both vertical and side-to-side. The transducer should skim through the water rather than plowing through the water nose-down. When mounted parallel to the hull, water flow at high speeds will sometimes bend the transducer nose-down. A slight, maybe 3-degree, nose-up angle will keep the nose of the transducer from dipping when you’re running. The side-to-side angle should be parallel with a straight edge laid across the gunwales at the transom.
Take care and choose the location wisely before drilling holes. In most cases, the angle of the transducer can be tinkered with once you see its performance on the water.