Hi Mike,
The rule of thumb of 1kW per ton (2000 lbs) is for typical displacement auxiliary sailboats. And it is a broad generalization, basically just a place to start. While your displacement is less than 2 tons, I believe that you said this would be a 29' gaffer that will be significantly lighter than a Catalina 25. Because of the increased size of your boat and additional wetted surface, one might find a 2kW drive a little underpowered. So your plan for a 4kW drive seems fine, especially since there aren't too many options between 2 and 4kW. (I't kind of like if you only need 1 cup of oil, you still need to buy a quart)
My 30' full-keel boat is just over 5 tons and my 5.5kW drive powers the boat faster and more powerfully than the Yanmar SVE12 that it replaced. Still, at over 5kW (full power), my boat is right around 6kts, give or take a little. But that's only 92% of the theoretical 6.5 kt hull speed. Most of my time under power is spent around 2-2.5kW. My best speed/range combination is over 20nm at 4.3kts burning 1.5kW. Going slower yields even more range, but slower than 4kts is really too slow to me under power.
So your motor may reach hull speed more easily, but most of the time, you'll be running at much less than full power. What you'll get is a system that runs cooler and could last longer.
I'm looking forward to hearing about your results after you launch.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Sarah Shepherd <shepsinorbit@...> wrote:
>
> Hi Eric,
>
> Quick one... you say 1kW per ton? I assume you're not referring to metric tonnes, so can you tell me in lbs or kgs what a ton in the US is?
> My boat will displace around 3,300 lbs. Would the 4kW I was anticipating would be overkill?
>
> Thanks,
>
> Mike
>
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