Thanks. That makes sense. My 11/1 LB ratio kayak tops out in a sprint at
its conventional formula hull speed of 6 knots. Paddling "uphill" against
its small bow wave is part of the problem, but nothing compared to the
insurmountable speed bump my shorter and wider boats create.

I guess instead of designing the boat first, building the motor and putting
it on similar length but fatter hulls (I have three 17 foot kayaks, from 18
to 28 wide) would be a better way to see what I would get.

For what it is worth, the project is to make a "kids boat" without the cost
and dangers of a real powerboat, and easy enough to paddle a long way home
or to safety.



"Evan Gatehouse" wrote in message
...

"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


--
Evan Gatehouse

you'll have to rewrite my email address to get to me
ceilydh AT 3web dot net
(fools the spammers)