This is probably nothing new, but I'll mention it anyway. I was running with sidescan and downscan on split screen, and lowered the 55 Riptide into the water. When I turned the handle, the HDS screen got dramatically lighter, and I seemed to lose contrast. I could still make out things on the screen, but it was difficult. When I turned the trolling motor off, the normal screen started scrolling into view. I have separate batteries for the engine, trolling motor and electronics. The only connection between the electronics battery and the trolling motor battery is a new on-board charger. When I detached the on-board charger leads to the trolling motor battery, the interference stopped. Might have got acceptable results by adjusting the contrast setting or something, but it didn't occur to me to try that.

The only unit setting that may have some effect is the Noise Filter.
Basically your motor is broadcasting on the SI frequency and the wiring is the antenna. There are chokes available that may or may not take care of it and there are also grounding measures that may or may not work.
Read Doug's post at the bottom of this thread for suggestions:
http://www.bbcboards.net/zerothread?id=655131

Thanks. It wasn't bands, streaks, or anything like surface clutter. I could still see the shad near the bottom that I'd been seeing. The screen just got a lot lighter and brighter, so much so that it was hard to make out the shad. Maybe the "back light" was triggered or something. For now, I'll just figure out a way to quickly disconnect the charger leads to the trolling motor battery before I put in. It has occurred to me that the charger may be missing some sort of filter.

I think I know what you were seeing, I delt with that issue for several months last year. If your screen is clear when you are not running the trolling motor and the screen gets lighter when you turn it on, that is the RF interference.
If you run the motor for 5 seconds, turn it off for that long and turn it on for that long, repeat, you will get the bright bands across the imaging view.
Been-there-done-that-have-the-t-shirt.
This is how it affected the Humminbird Side Imaging. RF interference can affect any frequency and is entirely random with trolling motor brands. Your task is to "change the broadcasting channel frequency" or prevent the broadcast from reaching the unit.

Yep - that is interference from the TM. I had that on my HDS-7. It was visible when a Structure Scan screen was displayed on the HDS-7

My problem was fixed when I grounded the TM shaft (MG TM)to the same point that the HDS-7 power (negative was connected.

C.

Reading about these grounding procedures is giving me a headache, and the trolling motor and electronics batteries are a good 7 ft. apart. The trolling motor battery has holes in the top, with little 1/2 (maybe 3/4) inch bolts to connect the leads. I'm going to get some twist off bolts, with good thumb holds, so I can quickly disconnect and reconnect the charger leads (won't need it when I'm on the water.) There's probably an easier way to go about it, and I'll appreciate any suggestions.