Re: Optimal propulsion system
I hope for your sake an expert chimes in but I've been poking around looking at this this morning.
First a number to use as a reference is that a hp is 0.75kW. This means that if you want to run continuously with the equivalent of a 15hp diesel you need roughly 11kw available. Punching that into a search for generators seems to reveal some bad news - 11kw generators are huge. Panda for example is 265 lbs for just 6kW (The 6kW is electrical output so the conversion losses are already taken into account. I bet it's a 6-10hp engine). And although there are a lot of complexities I think that's a fair starting point.
However it does seem like you have quite a lot of places to make compromises. First, assuming you're making a long passage and know it, you can charge the batteries ahead of time. Then you could run the generator and draw off the batteries at the same time and get good speed for a long time.
Second, even if you're limited to say 1/2 throttle average for a long passage that should still net 3/4 or 2/3 speed given the typical power curves. And you'll be able to throttle up for quick bursts anytime you want. Actually, you can probably specifiy a fairly oversized electric motor without a whole lot of extra cost/weight and have some pretty high speed burst potential. That's a neat option to have if you get into trouble.
If you're willing to comprise your passage ability by installing a smaller generator it might work well, if not it looks like it's heavy and expensive.