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.