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Hi Chris,
Interesting numbers for a bigger boat. I note you invite the group to comment. Jame's knows his ME0913 motors better than anyone I think, so I won't attempt to steal his thunder.
What I would like to comment on is: " In your opinion, is my ratio correct? 12KW from a "20 KW" system,"
I won't comment on the gearing/prop as such even though of course there is a relationship to power. It is more the "20kW" system. I think this is where a lot of folk get confused. i.e. continuous/rated/peak ratings. I know I did, but now after 3 years of research/build and my first 600 miles under my belt with electric, I feel qualified to comment.
You system is a nominal 48v @ 50 RPM/Volt (no load speed constant I assume?) so if a low voltage is at the motor and the motor is on tickover the load is light but then at 48 x 50=2,400 RPM it does not account for load. If at that voltage you pull your rated current of 125 Amps, then 48 x 125 = 6kW/shaft nominal and as the voltage/speed constant rating can only be held within a certain range subject to the combination of speed and load, rated power may be less. V=IR and V x I= Power shows us this.
Out of interest how did you arrive at your RPM figures? Were they derived or measured with a shaft tacho and if so did you correlate/chart that with measured controller voltages and current, battery and motor readings with temperatures too?
The 20 kW is an intermittent rating as I'm sure you know based on peak controller/motor ratings for a given voltage and current. We can make power with loads of volts and little current or the other way round as I'm sure you know. For me a good way to see where you are in terms of peak with your particular system/gearing is to do a kind of bollard pull test. I tied my boat up securely and slowly raised the throttle and at close to full throttle (my prop wasn't optimised then and it was starting to cavitate) I noted the current draw which naturally was above my continuous rating. It was about 30% greater, so for me this simulates the kind of real world current I could experience if I chose to accelerate my boat hard(ish) or push up the face of a wave if you like. A bit like going up a hill in an electric car.
For me gearing a motor and matching the prop is all about where I want the peak efficiency of my motor to run. In my case I'm looking to run at 90% efficiency at normal cruise speed (that is where the most time is spent) and sacrifice max continous top speed and efficiency. As long as I have a sufficient reserve I'm happy. So my approach is to maximise range at cruise rather than worry about peak/continuous powers. In that sense only you can decide what prop and what gearing you want, to achieve your desired/stated goals.
A bit like you can't have your cake and eat it, I guess. I welcome comments.
John
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