Hi Eric,
Wow, that has to be the best response on a forum I've had in years. Thank you very much. It was so complete, I don't even have any follow on questions about the sizing. The 30HP engines were specified by the designer for this boat, as it weighs only 5.5 tons. 9 tons is severely loaded down to the waterline, which will not happen (I keep my cats on a strict diet). :)
I think I made a mistake in my post... a terminology error.
When I said, "pods", I did actually mean "retractable electric outboards." I guess I had that terminology wrong. I'm thinking something like those well known Torqueedos, but something that can also move my boat. It has to be custom, since they don't make any off the shelf units in this power range.
The catamaran has more windage, but also has a very easily driven hull form. The numbers are pretty much the same (or better) than the Gunboat 48. This is a cruiser with a big slant toward performance. I suppose that's why the designer suggests 30HP per side.
It is also currently a blank canvas, so this will be a complete system design. It is a brand new boat.
Again, thank you for the answer to that question.
Anyone know of anywhere to look at submersible electric motors appropriate for use on retractable outboards in the KW/HP range specified by Eric?
--- In electricboats@yahoogroups.com, "Eric" <ewdysar@...> wrote:
>
> Hi Sean,
>
> There are two different rules of thumb for converting ICE to electric drive. The first is based on the HP of the existing engines. Typically, you can convert HP at 2.5-3HP ICE to 1HP electric. Using this conversion, each of your 30HP engines can be replaced with 10-12HP motors or 7.5 to 9kW drives. (1HP = 750W). The problem is that this conversion is dependent on how well the IC engines were sized to your boat.
>
> Alternatively, most displacement boats typically require 1kW for each ton of displacement. So using that conversion your 9 ton boat would need 9kW of total drive to power at 95% of regular hull speed. However, your catamaran desn't act just like a regular displacement monohull, and cats typically have more windage than a monohull of similar displacement, so your power requirements will probably be higher.
>
> So while I believe that 9kW is too little for your boat, you should get safe propulsion to reasonable speeds with 15-18kW of electric drive as listed in the first paragraph.
>
> Hopefully Steve, a member here with an all electric 41' cat, will weigh in with specifics about his boat, I think that you would see similar results with similar drive units.
>
> I believe that pods look good onthe design board, but some pod owners have reported problems with pods that are submerged 24/7. Many successful electric conversion have been done with inboard style drives, retractable outboards or drive legs have also worked well. I'm not saying that pods are inherently bad, but they haven't built a solid reputation to date. Properly designed and executed pods might be able to change that perception, but the cost might be higher than some of the regular electric drive conversions.
>
> Welcome to the group!
>
> Fair winds,
> Eric
> Marina del Rey, CA
>
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