I'm not sure affordable fits in with this subject at all. But, I've been doing a hybrid electric refit of our 57' sailboat and it's starting to come together. I have a few preconceived notions at this point. Anyone who would like to dispel them, is encouraged to speak up.
Seems to me that the real advantage of PM DC motors (or generators) is at low RPM. Induction motors like to run fast. If your propulsion motor wants to run at 5000RPM, you'll need a gearbox which contributes heavily to losses, especially for power generation. You are also limited by the water flow on regen, theoretical best case for regen is about half of the shaft hp that it takes to drive the boat at hull speed. In reality, you'll get about half of that.
For my boat that comes to 43kw to drive the boat. I'm figuring that best case of power generation will be about 8-10kW. If I can achieve that, I'll be very happy. I'm using a UQM 50kW motor with a 326vdc battery bank (Odyssey).
I'm not a believer in DC generators. Seems to me that you want to optimize the generator to run at the sweet spot of the diesel. Typically about 1500-1800 RPM for a nice heavy and quiet diesel like a Kubota. So I'm using a traditional 3-phase alternator at 240vac. Then that gets fed to a six-pack full bridge rectifier to generate DC at 326 volts. I can goose the voltage regulator to deliver 350 vdc for bulk charging. Very simple and cost effective solution.
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