Hi Jason,
Given your boat's displacement, you should be considering a drive with at least 3.3kW of power. At 24V, you'll need wiring that stands up to 138A. Your batteries will also be hit with a 138A load at full throttle. By upping your voltage to 48V, your max amperage is cut in half.
With a Bristol 27, a prop in an aperture, and a waterline of less than 20', you can ignore regen. If you're sailing at hull speed around 6 knots (how often does that happen?) you might see 50W, but probably not. Any regen should be considered a gift. That said, with your prop in an aperture, I believe that a 3 or 4 blade prop gives a better chance for regen, the 2 blade prop can hide in the keel's shadow and will probably produce even less power.
Most of the less expensive drives; DC, PMAC, BLDC will need reductions to let the motor spin up enough to keep it cool. There are inductive AC drives that are well suited for direct drive, but they generally cost more.
I don't know how old your batteries are, but if they're not too old, you might get away with adding one or two new ones to your traction bank. I know that the battery manufacturers recommend all batteries being the same age, or even from the same lot, but I believe that this is more important in parallel strings. AGMs seem to be more tolerant of mixed ages than flooded batteries.
If you added a battery now and bought (or put together) a system that includes a Sevcon Gen 4 controller that will run on either 36 or 48V, that could give you the opportunity to upgrade later, when you can add another battery. You could also consider the Advanced Marine electric Propulsion Mark II C drive that is rated for 3kW at 24V. When you can add a battery later, the same drive is rated for 4kW at 36V. There are a few 2kW systems that have been sucessfully installed in boats that are similar to yours, but personally, I think that that drive might be marginal if you find yourself exposed to adverse conditions. But I don't know where you are or what type of sailing that you do.
You've got a great candidate for going electric. I look forward to hearing about your progress.
Fair winds,
Eric
1964 Bermuda 30 ketch, 5.5kW Propulsion marine drive, 8kWh Lithium batteries
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "Jason" <jason.sattler@...> wrote:
>
> Good morning all. I've been lurking here for a while now and trying to get up to hull speed on this topic, but not getting the nuts and bolts into my head as fast as I'd like. I'm repowering a Bristol 27 (6600# displacement). The original Westerbeke 10 still runs, but I think the rings are leaking, and I tore something up in the transmission this fall...
>
> Anyways, I have two big AGM 12v batteries (250Ah each) I'd like to use to keep costs down, so I'd like to go with a 24v approach. I've been told this is not as efficient as a 48v system. Whould anyone know why? I'd be willing to upgrade to 4 new batteries, but it would be nice to be able to stage it over a few years...
>
> Prop. I've got the standard 2 blade factory prop. Should I be loking at a 3 blade if I plan to do regen? 4?
>
> Gearing? The 3000rpm Westerbeke was reduced 2.8:1, which gets it down to around 1100 rpms at the prop. Do I do a belt drive? Gears?
>
> Sorry for all the newbie Q's.
>
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