If you have a tunnel boat or a powercat, you may have a tough time getting your fishfinder’s transom mount transducer to work up to its full potential. In fact, due to the quirky water flow these types of boats produce, transducer placement will have a huge impact on how well your fishfinder works.
In many cases, the fishfinder will lose a good bottom reading at speeds over 20-mph or so. In some cases, there will always be several feet of surface clutter at the top of the screen. And in the worst cases, the fishfinder barely works whenever the boat’s moving at more than a few knots.
Again, water flow is the key. If you’re trying to get a transom-mount to work on your cat or tunnel boat, make sure the transducer is placed as deep as possible, and as far as possible from any strakes. Strakes trump depth; if the “puck” is too close to one your fishfinder won’t work right at just about any speed. If you need to move it up on the hull to get away from the strakes, it may come out of the water and you’ll lose the reading at high speeds, but at least it’ll work properly when you slow down.
Another helpful tip is to run your boat at different speeds before you mount the transducer, and simply watch the waterflow at the transom. Some tunnels will shoot bubbles out to the sides, which will ruin your fishfinder’s performance. Others may have a step in the hull bottom just forward of the transom, which also causes turbulence. And any protrusions from the hull will cause performance-ruining turbulence as well.
When all else fails, take your boat to a pro. A full-time electronics installer may have dealt with your specific hullform in the past, and know a trick or two.