That's going to call for something in the neighborhood of 40:1 reduction for most common propulsion motors of appropriate size. Perhaps a gearhead motor of the sort often used for conveyor belts? This would be an AC motor unfortunately. Or use a standard 4kw to 5kw Brushless DC motor or equivelant sized brushed motor and two stages of reduction, such as an enclosed gearbox of 8:1 ratio and then belt or chain drive at 5:1 to drive the paddlewheel. Sorry to just be throwing ideas out there with no first hand experience with such slow final output, but maybe that would work. If you could have a large driven gear cut by CAM and a worm gear on the motor output shaft, (this would require a thrust bearing to protect the motor from axial load) you could achieve the required reduction. Custom machining would add considerable cost to the project, though.
Typical propulsion motors look for a cruise motor speed of 2200 to 3000 rpm with 48v. If you can find a slower wound motor that would be a big help. And I hesitate to recommend this but it may be more efficient to run at around 24v which would lower the motor speed but double the current and exponentially increase copper losses. Turning the motor too slowly with a heavy load will impair the cooling efficiency of the internal fan and so external cooling may be required, but even this won't perform miracles or repeal the laws of physics.
You should try to include a stage that allows slippage in the event of debris stopping the wheel from turning. A belt drive might accomplish this with careful tension adjustment.
I rather like the small cast iron gearbox made by Baldor, and currently use a 2:1 unit on my boat, and will be fitting a 3:1 when I get home from my current job. They make I believe up to a 10:1 ratio, in whole number ratios only.
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