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[quote=Edchris177][quote]completely irrelevant but true is the fact that this low pressure actually sucks you forward, you are not being pushed upwind... but being sucked :) sounds dirty and i like it that way :~p[/quote]

Sorry Andrew, you have it mixed up. Nothing is sucked, or pulled. We don't actually "pull" a wagon, it is pushed. If you analyze the loads involved you realize that  the palm of your hand is pushing on the rear edge of the wagon handle. This increased pressure forces the wagon forward.
Our shrouds are under tension, follow the loads from the hound to the pin at the hull & you find the same thing happens. There is no sucking that shroud downwards.
A sail is much the same as an aircraft wing, which operates on Bernoullis Therom. Airflow meeting the leading edge of a wing, or sail, is forced to split. To simplify greatly, lets think of two air "packets", "A" is forced to travel around the front side, "B" the rear. The two packets want to meet at the same time at the trailing edge. 
A wing generally has a degree of camber,(again simplifying, the greater the camber, the greater the potential lift) this camber results in a greater distance along the top edge. "A" is forced to speed up to go the greater distance in the same time. We never added any energy to packet "A" or "B", so the total energy must not change. To achieve this, the pressure of faster moving packet A decreases. We now have a low pressure on top of the wing, & a relatively higher pressure underneath. You have heard since grade 4 science that Nature abhors a vacumn. The higher pressure area tries to get into the lower pressure & equalize it. Result, the wing is forced upwards,(lift) by this higher pressure. We need the higher pressure to obtain results.
Our cars engine does not "suck" air, it is pushed in by higher outside pressure, hence the decrease in power at higher altitudes as atmospheric pressure decreases. To regain that power we turbo charge, or super charge. TC or SC do not create more "suction", they create more pressure on the intake side.  It is important to understand this principle, it is not just a matter of semantics. 
This differential of pressure is responsible for quite an amazing repertoire in our everyday lives, carburetors,  forcing solvent up the tube (notice I didn't say "suck") of the gizmo we attach to our air compressor for cleaning things, & most importantly breathing. Take a breath, our diaphragm drops creating a lower pressure in the lung cavity. Higher atmospheric pressure pushes air into our lungs. Jump off your Cat with a piece of hose in your mouth, then use it as a snorkel. Floating on the surface it is easy to breathe, about 16" down, you die. At that point the water pressure squeezing your chest cavity has equaled the air pressure trying to push into your lungs. No amount of "sucking" can refute the laws of physics and allow you to draw breath. If you could increase the atmospheric pressure it could overcome water pressure to a greater depth & allow you to breathe. This is one reason a SCUBA tank must be pressurized.
Back to sails, now that we understand it is a higher pressure driving the sail,(and skegs or boards converting this push into forward motion). To get more lift(power)we need greater total pressure,which can be obtained by more area, higher wind velocity, denser air, co efficient of lift that is determined by wing shape (CL), or some combination of these. Air velocity has a "squared" function in the equation, & so has the greatest effect. We can't do much with air density, but we can alter the CL by changing sail shape.
 A wing,(aircraft or sail) can increase/decrease camber several ways, flaps being the most common way to increase. As they move rearwards & down, they increase both chord & camber. Same is true for leading edge devices,(slats). Increasing chord results in an increased wing area. If you couple the larger area with an increase in CL ie greater camber, the result is greater lift, (power). Of course nothing comes free, greater lift results in greater induced drag. As long as we increase lift more than the resultant increase in drag, we are ahead of the game. A tall narrow sail results in what is called a high aspect ratio,(chord vs span) very efficient...think of a gliders wing, or any soaring bird. This is where the sail trimmer comes in. Modern rigs allow us to change the sail shape to suit the conditions. A hard wing sail will be more tunable, & powerful, but the cost & complexity will also be greater.
And you thought college physics was a waste of time8-)<em>Edited by Edchris177 on Dec 19, 2010 - 01:54 PM.</em> [/quote]

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