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
--- In electricboats@yahoogroups.com, "Jeremy" <jeremy_harris_uk@...> wrote:
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> It is worth looking at the relationship between the two displacements here, as I think that should give a reasonable guide as to the power relationship. My reasoning is based on the principle that the power required is directly proportional to the wetted surface area, at least at displacement speeds. WSA is proportional to displacement, hence the common use of the rough rule of thumb methods for power requirement prediction, as already discussed here a few days ago.
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> I would expect a 16 ton boat to require a little over 3 times the power of a 5 ton boat for any given speed. Using your example of 6kts needing 5kW on a 5 ton boat, it would be reasonable to estimate that the power required to do 6kts in a 16 ton boat would be around 16kW, rather than 7.6 to 8kW.
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> Without knowing the LWL, beam or draft of either boat it is hard to be more precise, but if we had those figures, and made some broad assumptions about hull shape, we could use one of the well-known formulae to derive the wetted surface area for each and refine the above estimate. In practice I don't think it would make much difference, as I suspect that the commonly used "1000W per ton" method gives a close enough figure for this size and type of boat.
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> Jeremy
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