A couple of things to remember. The voltage/rpm constant is generally quoted as a no load constant, so the speed reduction changes the more amps you draw. Also remember that folk quote nominal ratings at usually what they call a 48V system, If your controller, like mine, accepts voltages anywhere between 24 to 48 there is no requirement to use all the volts, so you could for example set the software throttle percentage to give you a peak of e.g. 37 or whatever volts and tune accordingly. Also your voltage and amps multiplied coming out of the controller to the motor compared to that in kW at the motor or the battery will all be different in terms of kW subject to the efficiency of the components, so you need to calculate for that in the overall serial efficiency equation.
Additionally as each boat is different in terms of drag and water flow, you may not get the continuous running current figure matching your continuous voltage choice (to save you pulling too many amps and not enough volts), the first time around.
I started on purpose with the prop I had with the diesel so I could compare what worked with that in comparison to how it did with electric. With known data I could then calculate the intended prop, which becomes a little more difficult with direct drive as you don't have the choice of gearing and you may then discover that your motor voltage and torque constants won't work.
I deliberately specified the motor that I knew would work with my intended later prop but fitted initially (as I had a motor exchange agreement) a higher RPM per volt and lower torque per amp motor, than I now use. Out of interest my initial prop/motor matched the old diesel prop shaft RPM of around 1250 but pulled way too many amps.
With my exchanged motor and new prop my max prop RPM is now 950 where controller max volts is 22.7 and max current once accelerated is 104 Amps which is within 5% of what I would expect for a nominal 24V/110Amp continuous rated motor. Which all proves my calcs worked. The bottom line is doing it yourself you really need to understand what is going on from experience. Luckily 2 years research for me prevented mistakes.
John
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