Finger trouble in the post below! The first phrase should read:
"I agree, Eric, the ratio is NOT linear,"
Jeremy
--- In electricboats@yahoogroups.com, "Jeremy" <jeremy_harris_uk@...> wrote:
>
> I agree, Eric, the ratio is linear, but as I mentioned, if we knew the LWL, beam and draft we could have a stab at getting a better estimate of WSA, using something like Taylors method, or one of the other methods better suited to yacht hull forms if it has a deep keel.
>
> Earlier we did debate the oft-used rule of thumb that does relate displacement directly to power requirement, with several posters confirming that the "1000W per ton" rule gave reasonably good results. The good correlation between the BSFC derived power requirement, the 1000W per tone rule of thumb and the comment that modern boat diesels are often assumed to us 1 US gallon per hour per 18hp all tends to support the figure of 15 to 16kW as being a reasonable estimate.
>
> Because of the approximate cube law relationship between speed and power (half the speed needs 1/8th of the power) it is difficult to make comparisons with boat power requirements at other speeds.
>
> Jeremy
>
> --- In electricboats@yahoogroups.com, "Eric" <ewdysar@> wrote:
> >
> > Hi Jeremy,
> >
> > It is true that surface area is proportional to volume (displacement) but the ratio is not 1:1. Typically, three times the volume comes to about 2 times the surface area. If that is the case then three times the displacement should take twice the power for the same speed. Since we're dealing with typical sailboats, the hull shapes should be similar enough in LWL, beam and draft ratios that the WSA to displacement ratios should be fairly predictable. Of course, barges, powerboats or multihulls would throw more wrinkles into the calcs, but I believe we can skip that for now.
> >
> > So I'll go out on a limb and state that I believe that a 16 ton sailboat like what Michael is considering will require between 8 and 10kW to hit 6kts. Your prediction sounds like 15-16kW for 6kts. It will be interesting if anybody can provide real world obeservations for a boat in the 10 to 16 ton range. Then we could validate which prediction is closer to what people see on the water.
> >
> > It's funny that the only performance numbers that I could find in previous posts were for boats in the 4-5 ton range. Even Mike with his 8 ton Nonsuch regularly states that he can run at 2.9kts with his Honda 2000 generator and Zivan charger, but I couldn't find any watts to knots data for auxiliary sailboats outside the 4-5ton range at common speeds like 4, 5, or 6 kts.
> >
> > Eric
> >
>
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