I recently purchased a Hummiin Bird Helix 7 to mount on the bow of my boat. Get an a lot of electrical interference when trolling motor is on. They are connected to different batteries and I have turned off all other devices when in use, and that does not solve the problem. Unit is just sonar and Down Imaging with no mapping or GPS. Have used Lowrance and Garmin in the same situation and did not have the same problem. If anything, the problem seems to be getting worse as I try to use the unit. Self installed the unit but have done so many  times over the years with no difficulty.

Any suggestions as to how to solve the problem?

Contact Humminbird for help with your RFI issue.  

Your trolling motor is the problem, there are many remedies that may fix it. 

X2 what Wayne said. HB will ask you a bunch of questions and have you check certain things. They may more may not send you ferrite rings. I've added the HB ring with the addition of a Radio Shack ferrite sleeve which has removed about 80% of the interference from the bow unit.