Bob: I recently purchased an in-hull P79 depth transducer from you and it works quite well with one exception. It seems that quite often when I am in shallow water, for example a channel into a marina, the transducer indicates a dept of 1.2 feet. Sometimes this reading is for a very short, momentary, time and it goes back to the actual reading right away. Other times it reads 1.2 or 1 until I get out of the channel and increase my speed. Since there are no under-water appendages that could pick up debris and block the signal, I was wondering what would cause this mis-reading of the depth?
JackRabbit: Does it do this when you are stationary in a channel, or only underway?
If it is only when underway then what is probably happening is that the transducer is reading a return signal from turbulence under the hull. It may be that the location that you have chosen is one that is subject to turbulence from some hull projection or shape. When you accelerate, that cleans up the water flow.
You could try relocating the transducer. You can try out new locations by sealing the transducer to the hull using KY Jelly and having someone hold it there while you move down the channel. The best location is going to be, in general, somewhere near the centerline but just a short distance either side.
Your transducer is a smart NMEA 0183 model and they have good noise filters built into the software. In deep water it's easy to tell the difference between the main signal and reflections from turbulence as they come back at very different times. But in shallow water, the return signal from turbulence can come at almost the same time as the 'real' return from the bottom, and it's not so easy to filter that out.