Glenn,
I wouldn't go smaller than a 90 or 100Ah Li battery bank, small banks have bigger issues with Peukert's Effect, they lose more power under high load rates. You could also consider 4 group 31 AGM (105Ah) to begin with, they would weigh about 280 pounds, but you should get about an 8Nm range at 4Kts to 80% DoD. Then after you win the lottery, you can upgrade to the latest Li batteries. :)
If you're thinking generator, Mike has been successful with his Honda 2000 and a charger, I think that he can put about 15A into his system when the generator is running, that is about half of your 4kt load, so your batteires should last twice as long at that speed. There are some chargers available that can deliver more than 20A at 48V running on a Honda or Yamaha 2000 generator. The generator should be on deck while running. But the good part is that you don't need to set it up until you think that tyou need the range. If it's truly a day sail, you can leave the generator ashore.
It sounds like your getting your head wrapped around this project. Let us know if you need more info as you do more research.
Fair winds,
Eric
--- In electricboats@yahoogroups.com, "Glenn Dennis" <vega1184@...> wrote:
>
> Eric
>
> Thanks for the input, IMO the battery is key here.
>
> The actual power required to push the boat and the reserve capacity was what I was unsure of. It's good to get advice from some one who has experience in the field rather than just going it alone. Truthfully I had been more interested in the mechanical part of the system and did not put much effort in calculating the battery requirements. I was a little optimistic on range but I could start with smaller pack and go larger later. L/A or AGM will certainly add a lot of weight, maybe a smaller lithium pack, say 40 or 50 Ah and generator (Hybrid) would be the way to go. Several things to consider and of course cost; the boat although in very good shape is 39 years old.
>
> Glenn
>
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