Wiring help: grounding 24v trolling motor.

[Rugger]

I get very bad interference on my graph whenever I turn on my trolling motor. Circuits are totally separate, brought out my portable with it's own battery and it had the same problem as the one mounted to the TM. Looked into it and seems most likely is a grounding issue. The fix is to ground the TM housing to the starting battery neg and to ground the deep cycle TM battery(s) to the cranking battery neg.
Questions: 1: do I need to wire both (24v) deep cycle negatives to the crank neg or just the one at the end of the series of batteries?
2: where does the cranking battery ground? Doesn't it just ground into the water?

Thanks

[TroutSlayer]

Could be bad sheilding on your transducer cable. Try disconnecting the transducer while watching the interference. If it goes away, try a new transducer.

[Rugger]

Should have been more specific; the portable FF i used had it's own transducer which I hung off of the side of the boat about 5' from the TM. Same interference.

[Legend Boats]

Which graphs and TM are you running? Here is a little blurb I found on Minn Kota for you: Add Ground Wire - Adding an external ground wire from the Minn Kota lower unit to the trolling motor battery negative/ground and depth finder battery negative/ground will help prevent interference. This can be done externally by scraping a small amount of paint from the lower unit just behind the external transducer clamp. Attach an 18 gauge wire to the bare spot under the clamp then run the wire up the trolling motor shaft with the transducer wire. Connect this end of the wire to trolling motor battery negative/ground and to the depth finder battery negative battery ground. Experiment with disconnecting trolling motor ground and depth finder ground one connection at a time to determine the connections that result in the least amount of interference.

Cheers Henry