Saturday, February 18, 2006

How Do Sails Work

A couple of regular readers took the bait offered by my post asking Why Do Sails Work? (On second thoughts it should really have had the title of this post: the question is how not why.) How is that air blowing at, over or around a sail generates lift? After all it's the forwards component of the lift that drives our boat through the water on a reach or a beat, and without it there would be no sport of sailing. We ought to be able to understand how it works, don't you think?

I raise the question because I think all sailors have some idea of how their sails work. But, as I suspected when I wrote the original post, we don't all have the same idea. And the explanations in sailing books and websites are sometimes different from each other too.

Dan from Adrift at Sea made some other observations on downwind sailing but addresses my key question by reference to a site on the physics of sailing. The author of this site Joe Wolfe, a professor at the University of New South Wales, offers an explanation of how wind generates lift in a sail by using simple mechanics and Newton's Laws of Motion. According to Wolfe, the curved sail changes the direction of the wind. This change in velocity can be considered as an acceleration which is caused by a force that the sail exerts on the air. By Newton's Third Law, there is an equal and opposite force on the sail: the lift.

, once a student of the legendary Jerry Milgram, has a different answer to my question. He says that a "sailboat on a reach and especially upwind generates lift more like a paper airplane than a conventional airplane. What I mean is that most of the lift is generated as a result of the angle of attack between the sail and the wind." If this were anyone other than Litoralis I would take this to imply that even a perfectly flat sail will generate lift. I'm sure he's not saying this. Or is he? For the sake of argument let's assume that's what he means.

So now we have three different explanations of how sails generate lift.

1. The curved shape of the sail causes the air on the leeward surface to travel faster than the air on the windward surface which generates a pressure difference because of the Bernoulli effect.

2. The curved shape of the sail exerts a force on the air that changes the wind's direction and through simple Newtonian mechanics the air exerts an equal an opposite force on the sail.

3. A sailboat works like a paper airplane so a flat sail would generate lift because of its angle to the wind.

So which of these are right and which are wrong? Are two -- or perhaps all three -- of these explanations actually equivalent to each other? Or are all three, as they appear to be on the surface, totally different explanations?

I think I could poke holes in all of these three theories but I'm no expert on this subject so I'd be interested to hear some other views. But please, no calculus. What I'm looking for here is a simple physical explanation that would make sense to, say, an average intelligent thirteen-year-old.


Anonymous said...

Not being a physics major, I can't comment on the physics specifically. Assuming that we are not discussing sailing directly downwind, on a run, in which case none of the three apply, I don't think that number three is quite correct.

The reasons I have for believing this are purely based on my sailing experience, and that a fuller shape to the sail generates more power than does a flatter shape. If the sails generated lift like a paper airplane, then the cut of the sail shouldn't be as significant to the performance of the boat—aging sails should not have as detrimental an effect on the boat's performance as they lose shape.

This is also true of the airfoil shape of airplane wings, as higher speed planes, like jets will have flatter profile wings than slower planes, like propeller-driven planes, as the lower airspeed of the propeller-driven planes requires a higher profile or fuller profile wing to generate sufficient lift to fly safely than the higher speed jets. Also, high speed boat designs, like iceboats often have much flatter cut sails than slower designs.

Whether the sail generates lift via Bernoulli's principle and due to the further distance the wind has to travel over the leeward surface generating lift, as seen in an airplane wing, or whether the force is generated by attached flow, due to the sail changing the direction of the air flow, I can't say.

The keel can generate some lift due to the movement of the water over its surfaces, but this is mostly determined by the underbody design of the boat's hull, and is immaterial to the movement generated by the sails.

Litoralis said...

I think the keel/centerboard is an even better example of lift as a result of angle of attack because keels/centerboards are symmetrical foils.

Anonymous said...

True, most centerboards, daggerboards and some fin keels are shaped to work as proper foils.

Killer Whale said...

How would bernoulli's theory work if a sail has pretty much no thickness and therefore the distance the air travelling over the "top" of the sail and that travelling over the "bottom" is roughly the same?

Killer Whale

Anonymous said...

Hi, the concept that air travels faster over the "longer" side of a sail is commonly taught but incorrect. Air particles that start next to each other at the front of the sail do not match up again at the rear of the curved sail, contrary to what is taught. The air on the longer (lee) side in fact trails the air on the shorter (windward) side. Therefore, the air on one side of the sail is not less dense than the air on the other side. Forget about pressure differences.

Instead, air passing over the inner curve of the sail (windward) is pushed in a new direction by the shape of the sail. The air passing over the outer curve of the sail (lee) is pulled in a new direction (in the same way that water flowing from a tap and down the outside of a tapering drinking glass is pulled by the shape of the glass). Of course, any pulling or pushing of air means there is pulling or pushing of the sail owing to Newton's law stating that every action has an equal and opposite reaction (i.e., I can't pull you without being pulled toward you, and I can't push you without pushing myself away). Hence, the sail (and boat) accelerate.

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