100 sq. ft of sail area is about 2 HP?

What you've got is a balance between the driving force of the sails and the resistance of the hull. Now knowing either of these with any certainty is difficult. However,

The Delft resistance series will give a reasonable approximation for resistance (think power over speed).
Hazen's model for sail forces will give you an approximate driving force given sail type, area, wind speed etc. You could derive an equivalent power (ie Driving force * speed), if you wish.

Both these series of equations are well published, so you should not struggle too much.

Tim B.

 

Skenes has some tables that estimate sail pressure with respect to windspeed. Sail pressure is the beginning of any estimate of horse power. Horse power is a calculated figure. We can not measure horsepower directly. We measure torque instead. (Ft.lbs x RPM)/5250 = HP O.K. we could actually measure torque even though it might be sideways, but we do not have an RPM figure to use in the simple equation. We'd need to approach the question from a different direction. One HP is capable of lifting 33,000 pounds one foot in one minute. Perhaps we could contrive some machinery to do such a measurement. Or maybe not. It is a moot question anyway. I'll accept Pars figure and lose no sleep over the positive or negative validity of the ball park estimate.

 

I think downwind efficiency is nearly identical from boat to boat if you consider the sail alone. At right angles to the wind, the wind acting on the sail imparts a given (watts per square ft, etc.) potential at a given wind speed. The boat's efficiency in converting that thrust to speed through the water will vary wildly, of course, and different points of sail have different levels of efficiency.
Windmills, regardless of design, have the same potential. I didn't look it up, but every wind speed has an exact amount of energy per square meter at sea level.
What matters is the boat design and the average wind speed in one's geographical area during the sailing season. Matching the boat and rig perfectly to expected wind conditions is important in achieving a high overall efficiency.