Hey there... I did a little research through Humminbird and here's what I gather...
Before moving the transducer, have him check to see if it is "noise" caused by electrical interference from the motor. To do this he just needs to increase rpms while in neutral and not moving. If this has the same effect as travelling at higher speeds its electrical interference. Good luck figuring that out...LOL
If the above test has no affect, time to move the transducer. The face should be slightly ABOVE the plane of the hull. Slightly, like 1/8-1/4". Also, like was stated before, moving more toward the center will put it in "quieter" water less likely to have air pockets and such that distort the readings going to the transducer.
Thanks for the info guys. I'll pass the tips on to Paul he can try it and see if it works. I told Paul that if anybody could help it would be the great guys on this site. Thanks again.
