Per Yanmar, they say that engine should be run between 2400 and 3200 rpm to prevent glazing of the cylinders. I don't know much about diesel engines, but it is my understanding that the need to be worked in their designed range and not underworked. So that's where the concern about 1800 rpm comes from. I insulated the engine with 2" thick, 2lb per sq ft SoundDown material, and at 1800 the noise is tolerable. At 2200+ ear plugs become required.
As to my current internal debate between using the existing prop/shaft and v-drive versus the external pods, I share your concern with the small props on the pods. The default prop is 12.6" x 8.7" 3 bladed aluminum, turning at a max rpm of 1500. I don't know if 1/4 power input (1500 watts per pod) translated to 1/4 of max rpm, but if it does that would be 325 rpm. I don't imagine the prop speed would be faster than 1/4 of max at 1/4 power, so that probably a safe upper limit on the prop rotation speed.
I've seen similar results to the one you mention, of around 3500-4000 watts for 5 knots of speed. That would be my planned cruising speed. All of the electronics will be Victron, with the current plan to use the Multiplus 2 48/5000 which provides a max battery charge current of 70 amps. I don't tend to like to run electronics near their max ratings, so I'd probably limit that to 50 amps in the software settings and know that if it's cloudy and my batteries are completely flat, I'll be limping home at a slower rate or just staying on anchor for a bit while my batteries charge.
Thanks for the input.
Lars
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