How far away from the trolling motor to not get RF interference would be dictated by just how bad the RF interference is. The stronger the interference the farther away the transducer and all of its cables would have to be (this would also include the unit and any cables that run from the trolling motor near the unit, its transducer or even the power cables for the unit). I would temporarily install the transducer on the trolling motor, put the boat in the water and test it as various trolling motor speeds to see if you will even have any RF interference from the trolling motor.