I don't live far from Tommy and he got me motivated to work on my trolling motor interference. I have a terrova with US2 that was connected to a 998 and then networked back to a 798. When fishing, I always leave the 798 recording on side imaging. When the trolling motor would run, it would cause interference on the 798. I disconnected the network cable and the interference went away. I assumed it was from the US2, so I installed a spare external transducer on the Terrova and had the same interference with the external transducer as with the US2. The fix was to run an additional ground from the skeg on the trolling motor to the neg side of the Humminbird power as per the FAQs on the humminbird website. This appears to have completely cleaned up all the interference.
http://www.humminbird.com/Category/S...-Interference/