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On 30 Oct 2003 19:13:45 GMT, (William R.
Watt) wrote:

Is there a formula relating boat sail area, wind speed, and
horsepower?

The best I can find at the public library are some ranges, ie
hp/sq ft of sail area = 0.015 @ 7-10 kt
0.020 @ 11-16 kt
0.040 @ 17-21 kt
0.070 @ 22-27 kt

I'm trying to get a better idea of how much sail I'd have to put
on a light 15 foot plywood cabin cruiser with appropriate
underwater shape, whose total weight with one occupant and a
week's gear (including accumulated human waste) is 630 lb., to get
it to plane in less than a full gale. TF Jones says 1 hp to plane
each 40-50 pounds.

For comparison, the above table applied to an Albacore racing
dingy (250 lb) with two people aboard (300 lb) and 125 sq ft of
sail says the wind has to be off the above scale for the boat to
plane.

A 505 dinghy weighs 286 lbs (min) plus two crew and planes in smooth
water in less than 10 kts of breeze. Its main is 117 ft **2, jib 55,
IIRC. The old spinnaker was 210 but they changed to a larger one
recently.

A Thistle weighs closer to what you are proposing (500 lbs, I think).
If you can find a local fleet you can probably observe when they start
to plane.

These boats are sailed very actively. Just sitting in the cockpit
makes planing more difficult. I don't think equivalent hp is likely to
be a useful concept for your purpose.

I also found general wind power formulae in books at the library for
generating electicity but there are constants in the formulae, eg
efficiency, which I don't know for boat sails.

I've looked at numbers for several racing boats of about the same
displacement in library books to compare sail sizes but I'd like
to do the calculation for this boat if I can manage it.

Rodney Myrvaagnes NYC J36 Gjo/a

"Religious wisdom is to wisdom as military music is to music."

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