Hi Chris,
Thanks for providing your specifics, these conversations get a lot easier when people provide boat info. It's been my experience that an electric drive system that is sized at 1kW per ton of displacement will drive a auxilary sailboat at 90%+ of hull speed at full throttle.
Looking at your boat's specs, that works out to 7kts at 13.5kW. If you're sticking with a 48V system, your batteries and wiring will get hit with 270A at full throttle. Going to a higher voltage will reduce that amperage to more reasonable levels. As an example, Steve's 41' catamaran (which is considerably lighter than your boat) is running a 144V system. ABYC does have additional recommendations for drive systems that exceed 50V, but these addditional safety features are a good idea anyway. You can check out ABYC TE-30 for specific information about these specifications.
If you look at the specs for the ME0913, the max continuous amp rating is 125. The max voltage is 96V. Doing the math, at 48V the motor rates to 6kW, at 96V it's continuous rating is 12kW, exactly matching Motenergy's specs . So running two ME0913 motors at 48V probably won't provide your target 13.5kW, close but 7kts would require exceeding the manufacturer's continuous rating. Moving to 72V (9kW continuous) each, two ME0913s should have the power that you're looking for.
If you check out the Electric Yacht 360ibl, you can see that they are running two ME0913 motors to a common belt reduction. The 360 designation apparently comes from the each motor's 180A DC input rating, which only makes sense if they're running the Motenergy controllers, which they're not. They appear to be using the Sevcon Gen4 controllers, which I agree are much better controllers, but that doesn't change the motor's 125A continuous rating. Perhaps 250ibl would be a more accurate model designation, but that's really their business. Anyway, I don't beleive that an EY 360ibl should be run at more than 250A continuous which at 48V is 12kW. Running that same drive at 72V will push the drive rating to 18kW, if they can provide the proper Gen4 controller (model G8035) for each motor.
If you're the fabricating type, you can put together the same type of setup. My experience with the ME0913 is that 1900RPM is a good balance for power and speed, right next to your prediction of 1860RPM, so I would agree that for your current prop, a reduction of 2.1:1 or 2.2:1 is right on the mark. Going to 2:1 will load the motor a little more, but at 72V, that shouldn't be a problem. Going to 2.5:1, and you will lose your 7kt top speed.
So you're right there. Let us know if you've got more questions.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Chris Hudson <clh5_98@...> wrote:
>
> Andrew &Eric,
> I found a more informative datasheet for the ME-0913 that shows more than just the graph. I've attached it here in case you guys have not seen it. For me it was pretty enlightening. I think I've solved my puzzle but please let me know if I've gone astray.
>
> The particulars on my boat: 27,000 lb displacement, 34' LWL. Engine 62 HP Perkins with a 2.57:1 reduction. A reputable electric drive mfg predicted it would take 17 SHP to push my boat at 7 knots. This puts me deep into the 2 motor realm. In fact this pushes each motor to nearly 180 amps into each controller which is max. Before pulling the engine out I ran the boat at 7 knots and measured 856 shaft RPM.
>
> Motenergy states that the torque constant for the ME-0913 is 1.6 lb-in/amp into the motor controller. Taking the torque constant (1.6 lb-in X 360 amps) / 12 gives 48 lb-ft. Plugging 48 lb-ft and 17 HP into the torque formula and solving for RPM yields 1860 motor RPM. 1860/856 gives a gear ratio of 2.17:1.
>
> Your Thoughts?
>
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