Calling: engineers, physicists, mathematicians-- sail theory

[pmalter]

In my 35 years before the helm, my desire for speed has caused me to play around with sail theory. I understand airfoil theory; but I have a big question: an airfoil generates lift because the air on the top of the airfoil must go a greater distance, hence goes faster than the air on the bottom. But the air goes the same distance on the top and bottom of a sail. Moreover, with the angles of attack used in sailing, drag forces are usually greater than lift forces; hence, especially off the wind, Newton's second and third laws of motion(force vectors) are dominant.

This particular inquiry is generated by my discussion in "learning to sail" of the fact that when the leech of the sail is to windward, the lift on that portion of the sail is aft. That is true pursuant to airfoil theory, however if you analyze it from simple force vectors pushing from the windward side of the sail, that is not necessarily true.

Look at the shape of your genoa when you are reaching -- for maximum speed it will have about 90° of curvature. That doesn't work as an airfoil, but it does if you take the integral of the force vectors pushing on the sail.
I have been very successful in racing by using my own "theory of tangents:" your sails are optimally shaped when the luff is tangent to the direction of the wind and the leech is tangent to the direction of the boat; but I would really appreciate it if somebody could answer the questions I have generated above.

p

[jjablonowski]

You might be interested in this article, The Physics of Sailing, from a recent issue of Physics Today.

[jjablonowski]

Anderson's article in Physics Today elicited a couple of letters to that publication, which are presented here: letters

[tomaz_423]

Answer is in the vortex and is nicely described in a book I have read, but can't remember the author or the title.
But this on line article is in sync with the book's theory.

Try this one: The Origins of Lift

[tomaz_423]

This one is useful as well: http://www.arvelgentry.com/techs/A%2...l%20Theory.pdf
It shows a lot of old theories and why they are wrong.

[chucklesR]

Sorry I can't answer any of the OP's questions

I love the way the author of the linked article closes out::
"
A more practical alternative is the catamaran—a double-hulled sailboat. Catamarans are being developed to provide relatively stable, fast sailing. Although they are more expensive than traditional single-hull sailboats for a given amount of living space, catamarans are becoming increasingly popular.
"

Gee, wonder why I like that?

I also wonder what would happen if I put a series of 3 meter wide sails on my boat, tall and thin should do it.

Would the turbulence be less if I had a properly designed and cut sheet of no stretch spectra that essentially closed the windward side of the sail off into a flat surface (less concave at least) - that would in IMHO make the pressure difference even greater and provide more lift - and decrease the turbulence caused by air detaching from the sail.
Maybe I just don't get it.

[hellosailor]

"But the air goes the same distance on the top and bottom of a sail."
I don't think so. The boundary layer on the concave side of the sail is a dead air pocket, the main airflow is further away from the cloth, not right on it. Take that and any vortex...and the real "surface" of the sail is not ON the cloth, but located some distance away from it, with a different shape and length on each side of the sail.
I think that's all it takes.

Remember that engineers have proven the bumblebee cannot fly, it lacks sufficient horsepower, thrust, and lift.