Hi Matt,
There are two common conversion formulae to size an electric conversion. The first is to divide the existing engine hp by 2 or 3 to determine electric hp. Unfortunately, this has to be vague to accomodate the wide ranges of installed engines. For example, many 30' sailboats have engines with more hp that your 39' ketch which would lead some people to consider your boat underpowered. Alternatively, the other conversion ratio is 1kW per ton of displacement. Since this relates directly to the boat itself, I find it much more reliable. Personally, I would find a 10kW drive marginal for your size boat and would look for something closer to 14kW from your description.
Higher power drives come with additional considerations. A 48V drive would draw 300A to deliver 14kW, 96V would halve that, but systems above 48V should comply with ABYC's requirements for high voltage systems addding cost and complexity.
Lots of people want regen, but I have yet to hear of anyone that has reliably generated more than 100w at 5kts. It is my opinion that it takes at least 7 to 8 knots of boat speed to generate any significant amount of electricity back into a battery bank. Most people are getting better results from PV solar panels, again adding cost and complexity.
Realistically, I would never recommend an electric conversion as a way to save money on a re-power. If you're thinking about building a DIY electric drive, you should be comparing that to a DIY install of a good used or rebuilt diesel. If you're using the cost of a turn-key professionaly installed diesel repower, compare that to a turn-key professionally installed electric system. Of course, many people will bring all the money you'll save over the next 10 years, but you still have to pay up front just to get sailing again.
Lastly, when you think about sizing your batteries, every 10kWh of battery bank roughly equals 1 gallon of gas. Once you have an idea of the size of your prospective bank, consider if you would sail your boat with that size gas tank, most electric conversions have less than 2 gallons of equivalent energy storage. I'm converting my 30' ketch because I can accept that I've got about 3/4 of a gallon (8kWh) when I'm full up. Many other boaters would not make the same choice. It all depends how you use your boat.
Your boat is a little larger than the "sweet spot" for electric conversions, which translates to bigger parts, higher voltages, more complex systems and more money. Don't get me wrong, it can still be a good idea, but you should be able to make an informed choice because its your boat and your money. There's nothing worse than learning some of this after you're already thousands of dollars into the game.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "KetchNomad" <KetchNomad@...> wrote:
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> My NOMAD is a 1950 steel hulled 39' ketch (monohull) with a full keel of a J. Murray Watts design built in Hoerneke Boat Yard (WI), estimated at approximately 13 tons (probably a bit more). She is powered by a Universal Gas (not an Atomic 4 but its predecessor) 25 HP engine at 2500 RPM with a gear reduction of 2.25:1. The prop I believe to be a 19" 3-blade with an 11" pitch.....not 100% sure on this....found various old literature on the boat concerning the prop by the previous owner.
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> I have only recently been researching my various options in electric drive, but generally the consensus for pushing this much bulk is about an 8-10 kw motor, and I would prefer a regenerative system. Commercial applications are appearing rather pricey........
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> Any tips, advice, comments, or suggestions would be greatly appreciated.
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> Thanks,
> Matt
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