Just have to jump in here with my experience.
I'm developing an electric boat with twin trolling motors and differential steering. Both MinnKota and MotorGuide are using 4" pitch props as a standard no matter which size or voltage motor you buy.
In free air the motors spin at about 1080 rpm. In water, under load, you can expect somewhere between 400 to 600 rpm. With slippage, one can expect no more than 4 mph from trolling motors, no matter how many, regardless of voltage or thrust.
My boat, passenger, and equipment weigh less than 350 lbs. With two MinnKota motors/props (of any size), it does 4 mph with a very efficient hull.
I have now designed augers with 8" pitch and my speed has now almost doubled, with no significant increase in amperage. The augers are 24" long and require a bearing at the other end, so they only work with my application.
The way to gain speed and battery longevity to first lessen hull resistance by design and balance, then increase prop pitch to satisfactory acceleration vs speed. Lastly, increase horsepower. It takes 4 times the power to double the speed.
Torqeedos have a more efficient motor and prop, but they are 8 times the cost of a MinnKota. Unfortunately, No off-the-shelf props are available for MinnKota in varying pitches. What a shame, we can't tweek a Minnkota for a specific task and then measure the difference with a Torqeedo.
John Zimmerlee
www.streamdancer.
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