Setting aside efficiency differences it still takes the same amount of power to move the boat at a given speed. AC, DC, gasoline, diesel. Doesn't matter. The boat wants what the boat wants. Higher voltage will be lower amps. Lower voltage will be higher amps. The watts remain the same. The sizing of the wires may change but the power needs remain the same.
An AC motor can indeed be used with a controller that allows for forward and reverse and all the rest. Your battery bank challenge will remain. It may even get larger from more losses through added inverter stages.
The generator can also work. For a 6 ton boat you'll need something in the neighborhood of 2000 watts to go 4 kts? Doesn't matter where you get it or what conversions in may go through making it's way to the propeller. If you're going to loose half of it on the way you need to start with 3200. An AC motor will work but it won't pick up the check at lunch.
Dan Pfeiffer
This is probably a really silly question, but....Is it possible to find a 120V AC motor that can be speed controlled (and maybe even reversed??) for at least a proof of concept on a boat?For those of us that prefer electric motors to gas or diesel engines, but are challenged by the investment of a huge battery bank, wouldn't it be cool if we could run an AC motor off an inverter from whatever battery bank we have handy, and use a generator to supplement - or just run off the generator entirely at first?We've discussed the efficiency gain of running off a higher voltage / lower current.I'm guessing that there just isn't a suitable way to control and/or reverse an AC motor??John
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