There is a vast difference in the performance (efficiencies) between a “straight” high aspect foil with a small flap on the trailing edge and the same high aspect ratio foils raked backwards with the whole foil “turning” (up and down). Then to complicate matters even more if very small “tip winglets” are added then the efficiencies increase dramatically again. (The adage applies – “There are many more than one way to skin a cat”) Consider that a low aspect ratio foil set “straight” to all intent and purpose becomes a much higher aspect ratio in practice when raked, it’s chord length becomes much smaller in ratio to the surface that it is exposing to the direction of the fluid travelling over it. (If these principles hadn’t been exploited they would still be trying to break the sound barrier in aircraft today)

Yes there is all sorts of foils but hydrodynamics has been around a long time now and there is an awful lot known about it. My question is and still hasn't been answered is the following. How much actual drag will there be created from a foil of 0.5 metre sq and how much force can a Spinnaker create in foward motion.

Maths boys, and there are a number of you who regularly post on this forum, get your calculators out and do the sums for us dummies and please don't go all shy on me, show us your credentials.

There is no answer to you question in the form that it is. For both parts IE foil - the drag is a variable calculatable only with additional criteria. the same applies to the "power" generated by the sinnaker

Very dependent on speed of flow around the spinnaker or foil, angle of attack, etc. etc...The question is too vague.

A vague answer about the force on the spinnaker...

As long as the boat isn't accelerating...

The force from the sails is equal to the force of drag on the hull and appendages, just in the opposite direction. (This is on all points of sail, upwind, downwind, reaching...)

Probably a big percentage of that force is developed by the spinnaker.

Glen, Give it a try and make some foils! I would hate to stop progress! If ever you are in the UK give me a call and come and try the only 2 tri foilers in Great Britain! Good Luck!

David