G'Day Littleelse, and welcome to CF,
If the instrument responds normally when you spin the paddlewheel with your finger, then there is nothing wrong with it electrically. Rather, it must be something stopping the paddlewheel from turning with the flow of water
along the hull
One thing to check is that there isn't any growth inside the through hull
fitting that the transducer fits into. It is possible that such growth is squeezing the sides of the
assembly in such a way as to jam the wheel
. Also look for anything that could be getting into the wheel
on the outside of the hull. I've had soft growth forward of the transducer that trailed back and jammed the wheel under way, but wasn't obvious when I inspected it in the water at rest.
for NavMan (ex Standard Horizon) instruments is nearly impossible... I have an intermittent wind
angle transducer that I'd like to replace, but they don't seem to be available now. Grrrr.