William R. Watt wrote:

yes, but that's true of most things in nature and yet applied mathematics
works. for example, formulae are used to design aeroplane wings. the two
approaches used are to make the most appropriate assumptions and verify by
testing. the results only have to be accurate enough to be effective
design tools.

I don't know that that is a valid comparison. Aerodynamic formulas for
aircraft calculate the amount of horsepower required to overcome drag
and provide lift but that is not what you are after. Also aircraft
foils operate in a very limited angle of attack and with predictable
wind speeds.

If a single horsepower factor for a particular sail area/wind speed
existed the polars developed from VPP programs would be circular. You
can't take a single horsepower and compare it to the horsepower
delivered to a prop. The propeller delivers all its power in one
benificial direction. Wind force on sails has a lift and drag component
and the net benificial power varies depending on the direction of the
lift to the relative heading you want to steer. That is constantly
varying. You might determine an equivelent horsepower for one wind
speed and one aparent wind angle but it would be valid for only that
condition.

--
Glenn Ashmore

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