Thanks for your thoughts! I appreciate everyone's feedback. I contacted the company to confirm that the rheostat would work on 12 V DC motor, and the reply I got back was ambiguous. They said if it didn't work, I could return it. It seems like it was a simple yes/no question, and I'm not going to order it to find out it doesn't work.

Good suggestions on looking for wheelchair speed controllers. I'll see if one of those will float my way.

Thanks for suggestions to just upgrade my trolling motor. I'll keep it in mind for when my current gear gives out.