capteric36 wrote: > First, no sailboat I've ever heard of can sail at > an angle less than about 35 degrees 'off' of the > wind. > I think you will find that modern racing sailboats can do a great deal better than that! Even my 1973 S&S boat can get 27 deg either side of the wind, wind generator and all! Of course, she WAS a state of the art racer in '73!
> Second, if you look at the sails they are angled > in such a way as to be more than 90 but less than > 180 degrees 'off the wind( i.e. not across to the > wind and not in line with the wind. > This creates a situation where the wind travels a > further distance in 'front' of the sail than > 'behind' it. Thus the wind in 'front' must travel > faster thus producing less pressure on the 'front' > of the sail then on the 'back' of the sail. Exactly. > This > velocity related pressure difference is what makes > carburetor, airplane wings and sails work. > At least, for a boat, when she is going upwind...downwind is another story. > > still NO SUCTION INVOLVED, NO MAGIC, JUST COMMON > EVERYDAY PHYSICS. > Yes, ""suction IS involved...it is implied by the 'pressure difference' you > noted front to back! The front of the sail has a 'positive' vector, that is > the air pressure is exeriancing a force which tends to 'increase' the air > pressure at or very near the sail, and the back of the sail has a 'negative' > vector, air pressure is tending to try to 'decrease' at or very near the > sail. > > If you could run a tube from the surface of the back of the sail to ambeint > pressure still air, it would pull air from there and inject it along the back > of the sail. In other words, it would have a 'suction'. > This guy here http://www.sailtheory.com/sail.html does a good job of explaining it, he attributes it to the "Coanda" effect and says; "As explained above deflecting the wind results in the sailforce. This force is given to the sail by pressures, A lower pressure at the leeward side and a higher pressure at the windward side. Air has the tendency to be sucked to the lower pressure area from the higher pressure area. The air in front of the sail is also sucked to the lower pressure area on the leeward side. This means that there will be more air flowing on the leeward side as on the windward side. This results in that deflection on the leeward side is more important as on the windward side, because there is more air to deflect." Notice his use of the word "sucked". He goes on to say this applies more to boats going upwind, just as I said. For more on the coanda effect, see http://en.wikipedia.org/wiki/Coanda_effect and http://jnaudin.free.fr/html/coanda.htm and http://user.uni-frankfurt.de/~weltner/Mis6/mis6.html and this one, the best of the lot, http://jef.raskincenter.org/published/coanda_effect.html where they say; "The experiments with the miniature wind tunnel described earlier are readily understood in terms of the Coanda effect: the downward-curved wing entrained the airflow to move downward, and a force upward is developed in reaction. The upward-curved (concave) airfoil entrained the airflow to move upwards, and a force downward was the result. The lumpy wing generates a lot of drag by moving air molecules up and down repeatedly. This eats up energy (by generating frictional heat) but doesn't create a net downward motion of the air and therefore doesn't create a net upward movement of the wing. It is easy, based on the Coanda effect, to visualize why angle of attack (the fore-and-aft tilt of the wing, as illustrated earlier) is crucially important to a symmetrical airfoil, why planes can fly inverted, why flat and thin wings work, and why Experiment 1 with its convex and concave strips of paper works as it does." In sum, I think it is fair to say that a sailboat going upwind gets it lift, as do wings, from the Coanda Effect, that is air is entrained along the surface of the sail or wing, and the lower pressure on the back of the sail makes the air move faster, and deflects it, the deflection vectors front and back add up to create the force driving the boat up wind. Coanda Effect, not magic. But, suction IS involved!-Ken _______________________________________________ Liveaboard mailing list [email protected] To adjust your membership settings over the web http://www.liveaboardnow.org/mailman/listinfo/liveaboard To subscribe send an email to [EMAIL PROTECTED] To unsubscribe send an email to [EMAIL PROTECTED] The archives are at http://www.liveaboardnow.org/pipermail/liveaboard/ To search the archives http://www.mail-archive.com/[email protected] The Mailman Users Guide can be found here http://www.gnu.org/software/mailman/mailman-member/index.html
