Eliminating Electrical Interface
 

Was out fishing lastnight with my buddy and everytime he would run the trolling motor the fish finder would get all these blue lines on the screen. He said this just started happening since he tied all the batteries (3) on a maintainer. Anything he can do other than putting them on seperate circuits?

I think you meant "interference"

Look up electrical noise suppressor.

Noise generated from the trolling motor is most likely coupling back into the fish finder by way of the tying of the powers together. This is a common thing. Proper filtering adds cost to a product so it is a corner typically cut on most consumer products. :(

The most immediate solution is to separate the batteries so that the fish finder is now isolated from the trolling motor. However I suspect that your buddy doesn't want the hassle of having to deal with moving the battery maintainer around. Another possibility is to put a selector switch in line with the maintainer which would then permit easy selection of the which battery is being maintained. Simply rotate the selector every couple of days.

Your buddy should be able to purchase a power line choke which should reduce the amount of noise the trolling motor throws out onto the power lines.

On another note, batteries in most situations are not meant to be ganged directly in parallel and you mentioned that is what your buddy has done. I would suggest another power topology. For example the simplest would be a single battery which is sized appropriately to handle all the power requirements. Power topology aside however the power choke still needs to be there to remove the noise when everything is all electrically interconnected together.

Put a quick-disconnect on the battery that feeds the accessories like the fish finder. When you go out fishing, disconnect it to isolate that battery. When you get back any put the maintainer on, re-connect it. You could even do it with a racing type electrical cut-off switch.

Just ditch the trolling motor and use paddles! Total analogue will never mess up the fish finder :D

I think you have the answer in your post. Why not run the finder off it's own battery? Like a motorcycle battery.

I'm guessing a bad battery= low voltage in one, and cross-seepage or whatever it's called.

Batteries are liquid/solid chemical and mostly stable.
Isolate and test individually.