Re: i have a question about trolling motors.
This contradicts what I've read and heard about electric trolling motors. It's my understanding that a larger motor at half speed will use much less battery than a smaller motor at full speed to move the same boat.
This is true, but as I said, the RPM of the propeller is a consistent curve through the power band of the motors for each size in *this* application. That is, half speed on the 55 does not equal full speed on the 30. In order to get the same speed out of the 55, you have to apply full throttle, which burns almost twice as many amps. Batteries don't like heavy amp draws, which is one reason I'm suggesting to the OP a 30 lb motor in this case.
If you truly want to conserve energy without sacrificing speed, you need to go to a 24v motor which draws half the amps for the same size thrust rating. But now you're talking big bucks and serious overkill for a small inflatable. And still, the speed will remain the same. Trolling motors are matched to the size and weight of the boat they are to push (or pull). However, they are all designed to push that weight at the same speed.
If you want the motor to push a boat faster, you need to design a propeller with greater pitch (because the RPM is set). Some people have used model airplane propellers to accomplish this and many people have burned up their motors because it puts more load on the motors than they were designed to handle.
