Okay brando, I don't think that option will work and here is why.
I understand what you are wanting to do, but to get an electrical part (variable resistor) capable of handling the huge current capabilities is near impossible. I don't know of any variable control part that can handle such current and fit into the trolling motor housing. Yes I know they have controller circuits that do, but the variable knob part is not actually controlling the current. Other parts are and the knob only controls the biasing voltages and currents to those parts.
Let me see If I can try to explain this without getting into technical terms and theory. In order to vary the motor's speed you either have to have different motor windings (and a lot of trolling motors used to do it that way) or electronic parts that can. Meaning the variable knob controls other parts that control the speed. So unless you understand electronic pretty well, a simple variable potentiometer (variable resistor) is not going to work...at least for more then a few seconds.
If you buy a PWM motor speed controller, then you can do that. But you also have to know the absolute max current your trolling motor can possibly draw and go a little above that amount when buying the controller speed control.
Not sure if that explains it properly, but let just say, one simple variable resistor is NOT going to work. :sorry:
