Holding bottom at high speed is a common problem with transom mounted fishfinder transducers, and location is usually the reason. Just yesterday, a friend who bought a new boat was asking me about this issue. The one problem he has with his new boat is a flaky bottom reading; at speeds over seven or eight mph, the fishfinder totally loses its grip.
The first question I asked him, and the first you should ask yourself if you encounter the same problem: Is the bottom face of the transducer even or ever so slightly lower then the bottom of the boat? He answered “no,” and went on to tell me that it’s mounted about an inch higher then the boat’s bottom. Well, problem solved – if the face of the transducer is higher then the bottom of the boat, turbulence will roll off the edge of the boat bottom and interfere with the unit. The solution is simple: lower the transducer. Most installation manuals will tell you to make it dead-even or slightly below the bottom, and I’ve found it best to mount the leading edge of the transducer even with the bottom edge of the boat, and cock it back ever so slightly, so the back of the transducer is about 1/16th of an inch lower then the bottom of the boat.
If this isn’t the root of your problem, there are a few other potential issues to look for. Is the transducer mounted as low on the hull as possible? If not, it could be riding clear out of the water when the boat comes onto plane. Is there a strake, high-speed water pick-up, or any other irregularity forward of the transducer? If so, it could be creating turbulance at planing speeds.
The bottom line? Make sure your transducer is as deep as possible, not one iota higher then the boat’s bottom, and in a turbulence-free area. If you do, you should get decent performance up into the 20 to 30 mph range. You want your unit to work at speeds faster than this? You’ll probably have to go to a through hull transducer – even when mounted as well as possible, transom mount transducers have their limitations.