"Robert Haston" wrote in message
link.net...
I am looking for data on the hull speed one can expect on high aspect
ratio
(narrow) hull.

Just as a rough rule of thumb, at L/B ratios 8, the hull wave making
resistance is very low and you're limited to skin friction resistance.
However a 17-21' boat with a 2' or so beam is basically a kayak shape and
doesn't have a ton of stability. An outrigger (powered proa) design
suggests itself.

from
http://www.maths.adelaide.edu.au/App...on/taxdrag.htm

The viscous resistance Rv can be written as Rv=1/2 r V^2 S Cv where r is the
water density (herein 1025.9 kg/m3) and S the wetted surface area of the
hull. V is velocity in m/sec. Cv is the drag coeff.

When skin friction dominates, the drag coefficient Cv approximately equals
Cf, where Cf is a skin friction coefficient which can be estimated using the
ITTC 1957 ship correlation line (Proc. 8th ITTC).
Cf = 0.075/(log10(Rn)-2)2 where Rn = UL/n is the Reynolds number. Here L is
the overall length of the hull, and n is the kinematic viscosity (herein,
1.1883x10-6m2s-1).

Probably a lot more math than you'd like!

However if I was to guess (and it's a reall wild assed guess 'cause I don't
know how bit a prop you're planning), I'd say you'd get about 7 knots
with your 1.3 HP motor


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Evan Gatehouse

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