Tuesday, February 28, 2006

Bernie and Arvel

I have to confess I was mildly disappointed in the response to my two recent posts questioning why or, more precisely, how sails work. I was hoping to trigger a discussion on the topic on a number of levels.

On the most straightforward scientific level I wanted to explore the question I was specifically posing: how is it that wind flowing past our sails generates the force that makes our boats move. None of the initial respondents came up with what, to me at least, is a completely satisfactory explanation. More on this later.

But I was also hoping to explore some mental aspects of this. What are the models that we sailors carry in our heads to explain this phenomenon? After all we spend countless hours adjusting our sails to try and extract another smidgen of boat speed from the wind; surely we must each have some explanation in our head, perhaps even an unconscious one, as to what is happening. I wanted to find out whether all we have the same or different theories.

I'm also interested in the question of whether it matters if our mental picture is the "correct" one as currently accepted by the scientific establishment. When a fielder runs to catch a ball in cricket or baseball I suspect he's not consciously thinking of Newton's explanations of motion and gravity to help him. So why should sailors care about the physics behind the generation of lift? I'm not sure of the answer to this one. Part of me wants to believe that the better we understand the physics the better sailors we will be. But I could be wrong. I often am. Especially when sailing.

The thing that set me off on this quest was that I am fairly certain that the model I carry in my head for everyday use for how sails generate lift is actually wrong. I'm deceiving myself. My "everyday" model is the one I advanced in my first post - the one about wind being accelerated on the leeward side of the sail because it has to go further to catch up with the wind on the windward side and because of the Bernoulli effect this generates a pressure difference which causes lift. This is, as they say in the best scientific circles, a load of old cobblers. Some of the reasons are discussed here. (Warning 453k pdf file). But if one of my sailing students asks me how sails work I will mutter something along those lines - airplane wing, air going faster, lower pressure blah blah blah.

Dan from Adrift at Sea responded that he's not a physics major so can't comment on the physics directly. At first I was annoyed at this ducking of the question. But then I realized that the lack of curiosity by most of my readers to these questions was telling me more about myself than about them. Yup - I was that little nerdy kid who was always driving his parents and teachers crazy asking why, why, why? Why is it possible to balance on a bicycle? Why does the moon get larger when it's nearer the horizon? What happens if I poke this fork into the electrical outlet? Why shouldn't I eat the yellow snow? Why do I have to learn Latin? (Never did get a satisfactory question to that last one.)

Yup. I was a little scientist from as early as I can remember. Indeed my earliest memory of boating was the day I conducted an experiment in flotation using a water-soaked log and another family member in the river next to the house where we lived. I must have been around four or five years old at the time. I fondly recall that day as The Day Of My First Scientific Experiment In Which I Independently Discovered Archimedes' Principle. Eureka! My mother (who sadly did not encourage my early experiments) recalls it as The Day You Tried To Drown Your Baby Sister. Aaah - the sacrifices we make for science.

So, yes, I realize now. I have an unnaturally developed curiosity about how things work. And I mustn't expect all my readers to share it. Except litoralis of course who apparently has the same recessive gene.

But today I discovered another reply to my questions -- in a comment appended inadvertently to another post. And it's actually closer to the kind of answer I was looking for than any of the other replies.

Here is part of what Sailingaray has to say ..

Oh, a very timely question. I'm not sure how we ended up talking so quickly about sailing boats on a reach when trying to answer such a fundamental question. Although the bloggers do mention other very increasing factoids, they play a role much later in understanding the concept. There are two camps these days. In my opinion both camps play a role in helping us to understand the ways of wings and/or sails. Why should we keep two theories around? Maybe an analogy is handy. Einstein proposed that light acted both as a particle and as a wave. So far, no one has proved him wrong and both are used.

The first aerodynamic explanation is the good 'ole Bernoulli concept. You nailed the description. Lift is generated by the change in the speed of air as it moves across the two sides of the sail or wing. On one side, the pressure lowers and the other side sees ambient pressure, or slightly higher. Differentially, a force is created in the direction of the low pressure on the leeward side.

The second theory is the Circulation Theory. In this theory the apparent wind approaching the lifting body (sail or wing) sees a low pressure area and bends dramatically to get to that low pressure. It is this bending motion that transfers energy to the sail -- conservation of energy. Circulation theory seems to have it biggest value in explaining slot effect. Slot effect? On a wing, slots are found as either leading or trailing surfaces. On sails, the jib forms a slot with the main sail. Forming a good, efficient slot is apparently a black art. However, the circulation theorist will show you wind tunnel results to prove that the Bernoulli (Bernie) Law falls apart when used to describe a slot. Typically Bernie says the air speeds up even more in the slot, producing more lift. Circulation theory predicts, "Nope, it slows down." Wind tunnel results say, "It slows down."
I especially like his or her explanation of why having two opposing theories may have practical value (even when one of them actually fits much better with the experimental data.)

The circulation theory of lift is explained at more length in Arvel Gentry's article on The Origins of Lift. Not all that simple to follow for those of us, including myself and Dan, who are not physics majors. And I suspect that even after you've read it you may not be any faster on the racecourse. But are you prepared to risk it? I'm not.


Adrift at Sea said...

I generally don't comment on areas where I have no solid foundation to speak from, and the last physics course I took was almost twenty years ago. I too, am cursed/blessed with a strong sense of curiousity and will often go out of my way to find out the why of something.

As far as I know, the Bernoulli principle is the major force acting on sails, provided you're not sailing directly downwind, in which case it's just the wind pushing against the surface area of the sails.

The change in wind speed, caused by apparent wind, and the movement of the boat, is what allows a multihull or planing hull sailboat to sail faster than wind speed. This also applies to ice boats. The fastest design boats, like iceboats, are almost always sailing close-hauled or on a close reach, as the generate a good deal of their own wind due to their speed.

The newer generation of asymetric spinnakers takes advantage of the physics of airflow over sails, and most are cut flatter than their predecessors, which were strictly downwind sails. Some of the newer designs are optimized for upwind sailing, as high as a close reach.

Even though I generally don't race, I abhor going slow in a sailboat, if at all avoidable. I also find using a diesel or outboard repellent, unless absolutely necessary. Seriously, why would one motor when one can sail instead??

Jeremy said...

Meh, so many people explaining sails with Bernoulli.. I am a physicist, but I'll cop out with "I am not a fluid dynamics expert" instead. That said, I think it is so much simpler to think of it as a simple change of momentum. The air is flowing in a more or less laminar flow. When it encounters the sail, it is deflected and this change in direction is a huge change of momentum of all that air. Start by thinking of a plane (completely flat sail) at angle to the wind.. the air will push on that plane. Now if the plane is not allowed to move in any direction, but is constrained to move along one path (by the keel or centerboard), the plane will be pushed forward along that path. The problem is that a flat plane will produce turbulent flow at the luff and leach, so the curve of the sail provides a smooth transition from the wind traveling in one direction to the wind traveling in a deflected direction. Less turbulent flow, less friction, more push.

To me, this is all very much like the centrifugal force.. we've heard of it, but it's not real (it's really just momentum as viewed from a rotating reference frame).. it's just a way to explain things from a different perspective.

Three cheers to all of you who discuss this though. I just started sailing a month and a half ago, and this is exactly the kind of discussion that for me adds to the fun of it. As to whether it helps us sail faster, well.. :-)

Full sails,

Jeremy said...

Ok, one last thing for anyone who might happen across this thread.. I found a couple pages at University of New South Whales that really explain all of this very well in my opinion. The first page, physics of sailing, really does a nice job of summarizing the concepts:

In that page, you may notice a link to a page discussing the Bernoulli effect:
That page is really a great read. It seems that Bernoulli had it right and the popularization of his explanation was distorted and wrong (a little cart before the horse).


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