3 Amazing Things I Learned From Sailing Blogs

It's amazing what you can learn from sailing blogs.

Only this week on three different sailing blogs I learned that some science that I thought I knew was totally wrong, I discovered some excellent marital guidance counseling, and I became aware of a solemn duty that nobody had told me of before.

1. How Tides Really Work

I thought I knew how the moon and the sun cause the tides on earth. But apparently the explanation I had heard (and which I suspect 99% of my readers believe) is actually totally wrong. The blogger known as Keep Reaching who writes the excellent Laser sailing blog Reaching Broadly has a link to a video that puts us straight.

WRONG!

CORRECT!

2. How Sailing Can Ruin a Marriage

I have learned all sorts of useful stuff about sailing on Damian's excellent blog The Final Beat. But I didn't expect to read that sailing might ruin my marriage. Don't be like Damian. Learn from his mistakes. Let this be an awful warning to you.

If you want this...

but she wants this...

then you will have too much of this...

and you might end up doing this...

and will never get to this.

3. Why It's Your Duty As A Sailor To Drink Rum.

I am always looking for a good excuse to drink rum. But I never knew before that it is our duty as amateur sailors to drink rum.

Thanks to John Vigor's excellent blog which is cleverly titled John Vigor: Blog I have learned a lot about the history of sailing and rum - not to mention futtock shrouds - and why this means we absolutely have to drink rum.

Futtock shrouds

A mug

Some rum

I think I'll have a tot of rum now and take a nap.

Fatheads and Pinheads

On Saturday of the weekend before last I did some solo practice in my RS Aero 9 at the club. One of the very satisfying things about sailing a new class is that almost every day is a learning experience and I can see improvement in my skills every time I sail the boat.

After sailing I was starting to de-rig my boat when another club member, whom I hadn't met before, came across to chat with me and check out my boat. It happens pretty much every time I sail the RS Aero there.

But he asked me a question which initially had me stumped…

Why is the top of the sail square?

Of course he is right to ask. The tops of the sails of the other classes sailed at the club look like this…

Whereas the top of the sail of the RS Aero looks like this…

Hmmm.

I vaguely recalled that I had read or heard somewhere why the top of the Aero sail is like that but in the moment my mind went blank.

I blurted out something like, "I have no idea. I'm not a sailmaker or an aerodynamicist."

Aerodynamicist? Where did that come from? Is it even a real thing? And even if it is, what on earth does it have to do with sailing?

And then in a desperate attempt to sound not completely ignorant about my new boat, I mumbled something along the lines of, "Well, I guess it's a way to add more sail area."

As soon as I said it I knew it couldn't be the whole story. There must be other ways to add more sail area.

But my inquisitor, who seemed to be a very nice chap conceded, "I think you could be right."

I scrambled to think of another reason the top of the sail might be square and vaguely remembered something someone had told me and hazarded a guess… "And a sail that shape spills wind well in a gust."

The very nice chap was either not detecting my total ignorance about sail design or just being polite as he said again, "I think you could be right." And he went off to do something on a boat with a pointy top to the sail.

---

As soon as I got home I logged on to the Google machine and did some research on why some boats have sails that are square at the top. Maybe I looked a bit of a fool the first time I was asked this question but I wasn't going to get fooled again.

I found this interesting paper by Damien Laffforgue which has a whole section on square head sails vs roached sails.

Among the advantages of the square head sail that Damien lists are these. (My comments in italics.)

1. The surface area of a square head sail will be bigger than a roached sail for the same mast length (luff), but the aspect ratio will be smaller.

2. For the same surface area, a square head sail will have a smaller mast than a roached sail, therefore the centre of gravity of the rigging, and the centre of effort of the sail will be lower, which increases the lateral stability of the boat.

I guess #1 and #2 are two sides of the same coin, so to speak.

3. The square head gives a better aerodynamic efficiency in the upper part where the wind is stronger (velocity gradient.)

Ahah. That's what I forgot when I was talking to the nice chap at the club. More sail area up high where the wind is stronger is obviously an advantage.

4. The square head does not increase the lift but reduces the drag.

I'm not sure I understand this one. Why would it be less drag? More research is indicated.

5. The square head allows a better control of the main sail twist, and self-regulates the sail shape during gusts.

I am not sure I really understand the first part of #5 but the second part is basically what I was telling the nice chap at the yacht club. The square sail top opens up in gusts and spills air.

6. The head of a square head sail is more tolerant for small angles of attack, and then produces less induced drag.

More tolerant for small angles of attack? What does that mean? And there he goes again saying that sailing a square head sail produces less drag. Why would that be?



A little more research discovered that the reasons a sail with a square top has less drag than a sail with a pointed top is something to do with the tip vortex. Here is a good explanation of this effect in an article from Sponberg Yacht Design...

In any given aerofoil planform, the airflow on both sides of the surface are at different static pressures—high pressure to windward, low pressure to leeward—and they would really like to equalize. In a triangular planform, the airflow on the high pressure side gets a chance to equalize sooner, by virtue of the shape, than on a rectangular planform for example, by skewing up toward the tip and off the surface.   

This skewing of flow from the high pressure side, mixing with the flow on the low pressure side, creates a vortex off the tip. The bigger the skew, the bigger the vortex, and the greater the induced drag. 

In his book Aero-Hydrodynamics of Sailing, C.A. Marchaj (pronounced MAR-ki) shows a photo of what the tip vortex looks like.

And I found a slightly less technical explanation of this issue in a comment from Rick White on the Cat Sailor Forum in a thread about the pros and cons of square top mainsails…

I believe it was Dave Calvert that started the concept for windsurfers, before he started designing multihull sails. 

The theory is you have a fuller, more powerful area of the sail up high for lighter winds. 

Now, when a puff hits, because of the length of the batten sticking out from the upper mast, the wind uses that leverage to allow the top of the sail to blow off to leeward, thus depowering in the puff, lowering the center of effort, and also reducing the heeling moment. 

Sort of an automatic transmission, it shifts to a high gear in the puffs, and when the puff subside, it shift down again for more power.  

And they definitely work much better than pinheads.

So now you know.

Next time I am asked why the top of my sail is square I will be able to confuse the questioner with aerodynamic gobbledygook about tip vortices and velocity gradients and angles of attack and planforms... and pinheads.

You have been warned.

Do the Math

Can anyone explain to me what this means?

Is this the secret formula for Laser boat speed?

Originally posted here.

Are Words of Wisdom a Waste of Time?

We racing sailors like "words of wisdom."

"Words of Wisdom" are what they call those reports by winners after a regatta or an afternoon of club racing.

Everyone who wasn't a winner wants to read them so they can pick up tips on how to be winners.

I'm as guilty as the rest of the non-winners.

But I have bad news for you other non-winners.

I think that words of wisdom might be a waste of time.

---

Why would I say that?

Well, a few weeks back I thought I would start posting some words of wisdom from various race winners and regatta winners on this blog. I thought that if I extracted the key points from some words of wisdom and rewrote them in my own words on my blog, then I might remember them, and then I would apply the words of wisdom in actual races.

The reaction from my readers surprised me at first...

I wrote some words of wisdom from Bill Brangiforte on sailing upwind in a large fleet and got a couple of comments that they were "pretty much right out of Stuart Walker's books."

I summarized some video interviews on the Improper Course blog which had words of wisdom from winners at the Florida Laser Masters Week. A commenter on the original post, remarked, "Well, nothing new here, but it's all about getting those basics right isn't it?"

What? These words of wisdom from top sailors who had won major regattas were "basics" and right out of some books?

Well, yes. I guess they were.

So what is going on? If regatta winners do so well by simply applying well-known principles about how to start, or what strategies to use in different wind conditions, or how to go fast downwind... then why aren't we all doing these things and winning regattas too?

---

I thought about it for a while, and came up with these seven possible reasons why words of wisdom might be a waste of time.

1. Perhaps these top sailors are not spilling the real secrets of how they won the races? Maybe they are just explaining how they won by repeating well-known "basics" or stuff from Stuart Walker's books? This is true as far as it goes, but really they also know some super secret go-faster techniques that they are never going to reveal to everyone on the Interwebs. This theory does appeal to the paranoid side of my personality, not to mention stoking my negative self-esteem and fueling my feeling that I will always be a crap sailor. But I do find it hard to believe that the top guys could be so mean.

2. On the other hand, perhaps these top sailors have some ways of going faster and winning races that are totally subconscious? They don't even know themselves why they are so fast. They are not deliberately hiding their go-faster secrets from us mere mortal mid-fleet mediocrities because they are not even really conscious of what they are doing themselves. I like this theory even better because it means the winners are not diabolically plotting to keep the rest of us down by deliberately misleading us. I can still think of them as nice guys. But their words of wisdom are still a waste of time.

3. Or perhaps these top sailors have magical powers and super sharp senses? For example, perhaps they can see what the wind is doing way better than those of us with average (or below average) eyesight? It's all very well to know that you should stay in the puffs but if you are not seeing the stronger patches of wind on the water as well as the eagle-eyed top sailors, then no amount of words of wisdom are going to help you. I think this is my favorite theory. The winners are super-heroes with magical powers. It's not so bad getting beaten every week by sailors with extraordinary superhuman powers.


On the other hand perhaps getting the basics right is all there is to winning sailboat races? There are no secrets. Everything worth knowing on the topic has already been written and talked about and published on the Interwebs. But for some reasons, those of us who are perpetual non-winners simply don't execute the basics right. Now why would that be?


4. Perhaps we are just too unfit to get the basics right and win races? This probably does apply to some of the words of wisdom I read about Laser racing. It's all very well to say that you should hike the whole beat with straight legs and your shoulders back, or sail the whole run balanced on the soles of your feet as if you were on a surfboard, but unless you have quads of iron you are not going to be able to do those things as well as the winners do.

5. Perhaps we know the right moves but we haven't practiced them enough to execute them properly? It's all very well to read some expert talking about catching waves and to always be sailing "downhill" on waves on a run, but it's a lot harder to do it right all the time unless you've spent an enormous amount of time sailing downwind in waves.

6. Perhaps we have bad habits and even though we know, for example, what we should be doing to get a great start, when the pressure is on we revert to our old habits designed to guarantee a mediocre start?

7. Or perhaps we don't have the right mental attitude to apply the words of wisdom we have heard or read? After all there are so many things to know. In the heat of the moment on the racecourse we forget all the words of wisdom about strategy and tactics and boatspeed and boat handling and boat tuning etc. etc. and hack around the course watching the winners disappear into the distance just like they always do.

---

So where does that leave us?

For sure you won't win races by spending every weekend by the fireside reading words of wisdom.

If you have been racing for a few years, you probably already know pretty much everything you need to know about winning sailboat races.

All it takes now is to improve your physical fitness; develop the right mental attitude; break all your bad habits; practice, practice, practice; and then go out and race a lot.

Oh good.

Who knew it could be so easy?

---

So what do you think?

Am I missing something?

Do you really learn new stuff from words of wisdom?

Does reading words of wisdom make you a better sailor?

DDWFTTW

DDFTTW

Is it possible to build a craft that will "sail" dead downwind faster than the wind (DDFTTW)?

Watch the video. It certainly looks like that cart is sailing DDFTTW. How can that be possible? Is this an illusion, a fake, a scam? If it's real, how does it work? What's the science behind it?

Second Date

On Wednesday of last week I took my new Laser sail out for another gentle breaking in-session. Mainly reaching around in medium winds, no excessive tightening of the sail controls, much along the lines of how I treated her on the First Date. At the end I concluded she was ready for use in a real regatta at the weekend.

Why is this necessary? I don't have a clue.

I asked another sailor about this topic of new sail care the previous weekend and he quoted no less an authority then Ed Adams as saying that such gentle treatment of a new sail extended its useful racing life by at least 50%. But my friend couldn't explain what this method actually does physically to the sail to boost its longevity.

Anyone know the answer? Why can't you just go and take out a new Laser sail in 25 knots? What harm would it actually do to the shape of the sail? What is actually happening to the sail fabric in the traditional breaking-in method? And do sailors of other classes care for their new sails the same way? Or is it just the crappy fabric used in Laser sails that needs this kid-glove treatment?

In desperation I turned to The Google and uncovered basically the same question posted in the archives of the Laser email list, from the good old days of the Laser on-line community when larger-than-life characters like drLaser and LaserBabee roamed the land. The question was in fact posted by the legendary LaserBabee in 1999...

I have a new sail I intend to start using in a few weeks. A couple years ago there was a letter from Dan Neri of North Sails posted on the list that explained all about the proper method for breaking in a new Laser sail. All I remember is the part about reaching back and forth for a couple hours in moderate winds.

Does anybody have a copy of the letter or does anybody know why reaching back and forth for two or three hours would help my new sail??

There were no replies to the post.

So here is an opportunity for all you expert sailors, armchair sailmakers, sailing geeks, and folk who just like to spout on topics on which you know little and understand less. What is the physics of all this "breaking in" of Laser sails? What does it actually do to the fibers in the sail? Or is it all some urban legend like the port tiller rule?

Beast of Burden

Sun Mar 30

I have a physics question for all you sailing geeks.

We all know the relationship between body weight and boatspeed in a small racing sailboat like a Laser, right? Or we think we do.

In heavy winds, let's say over 25 knots, the heavier guys do better in the races (other things like overall ability being equal). Generally speaking, in these conditions someone of 200 lbs will beat someone of 150 lbs in a Laser because the big guy can hike the boat flat upwind and will still be surfing downwind, while the little guy is struggling all the way upwind and would be better off in a Radial.

On the other hand in lighter conditions, let's say 5-10 knots, the little guy will probably be faster around the course. Upwind he may well be hiking flat out while the big guy will be simply sitting on the side deck and going slower, wallowing in the hole his heavier mass makes in the water. And downwind the lighter sailor will be soooo much faster. It's all to do with displacement and drag and Newton's Second Law, F=ma and all that.

So far so good. Please don't bother to argue with the above, because I know I'm right. I can even look at the results of the racing at my old frostbite fleet and make a very accurate guess of what the wind conditions were like based on the relative finishing positions of two (both excellent) sailors of very different body weights.

The question I have is what happens at the very light end of the wind spectrum, say 0-3 knots? I know that no race committee in their right minds would run races in these conditions... but sometimes they do. I've noticed on a number of occasions that the heavier sailors start doing well again in these conditions.

There was the time my (heavier than me) son beat me in the 2006 Laser regatta at Hunterdon Sailing Club in New Jersey, in a very light patchy easterly wind.

And it happened again on the last day of the Kurt Taulbee clinic in Florida last month. There was one other guy on the clinic who said he weighed 150 lbs while I owned up to 200. (OK, I rounded down, but who's counting?) On the Saturday in 5-10 knots he beat me easily in every practice race. But on the Sunday in a dying gradient breeze fighting the new sea breeze, shifty, patchy, and dying away to zero occasionally, I did a horizon job on him in every race.

So my question is, why? Is it a matter of these heavier sailors actually having very good light air skills (relative to the opposition that day) that overcome any weight disadvantage they may have? Or does weight actually become an advantage again in the very light stuff? Is there some kind of inertia effect which means that the heavier sailor somehow keeps coasting through the lulls, maintaining flow on the sails and foils, until the next little puff comes along?

Answers please. Do no attempt to write on both sides of the paper at once. Credit will be given for references to any physicist other than Newton and for the use of second order differential equations. Bonus points will be given for anyone quoting Stuart Walker and being able to explain the quote in less than 500 words. Anyone referencing Nietzsche, Sartre or Rilke will be awarded a failing grade.

Apparent Winds, Shear and Twist

I love the folk who comment on my confused posts on the theory of wind and sails, such as the one on Gradients, Tacking Angles and Coriolis, itself an attempt to clarify some questions raised in an email from a Dutch reader. It is pure joy to me to see how an apparently simple question can send my readers off in all directions with such a varied mix of answers.

Carol Anne and Ant shared with us their experiences of wind variation sailing on inland lakes, while JB and Litoralis gave us a technical briefing on wind shear, gradients, viscosity and friction - not to mention fake forces. Fred gave us an insight on how wind varies with height on open water and even managed to explain how this whole issue is related to why there was so little sailing in Valencia last week. Tim and Milo focused in on one area that I had hoped somebody would pick up: why the apparent wind direction at the top of the mast will be different from that at the bottom.

But wait. I see some disagreement here. Tim believes that "the wind will be more on the beam the higher up the sail you go" whereas Milo claims that "the apparent wind is more beamy at the bottom."

Hmmm. They can't both be right, can they? What do you think?

But none of these comments support the suggestion in the original email that the Coriolis effect (fake or otherwise) can cause an asymmetry between port and starboard tacks such as in the claim that "on starboard tack .... sailors experience more need for twist high in the main then on the port tack." It would seem to me that this would be true if there were some consistent change in true wind direction (as well as speed) between the top and bottom of the mast. Are we all sure that this is never seen? Are higher altitude winds generally shifted the same direction from low altitude winds? If so, does this effect apply at the scale of a boat's mast?

Gradients, Tacking Angles and Coriolis

Four of my distinguished and erudite readers have already commented on the question raised by my Dutch correspondent about the Coriolis effect and its part in sailing tactics. They all were sceptical about the suggestions in the email I quoted.

They may be right. But just ponder these questions before you dismiss the subject...

1. Can wind direction at the top of a mast be different from the wind direction at water level?

2. If so, and you measure wind direction with a mast-head instrument and boat direction with a deck-level compass, then won't the angles between the close-hauled course and measured wind direction (tacking angle) be different on port and starboard tacks?

3. What causes the difference in wind direction between the top of the mast and water level? Isn't it the shifting of the high altitude wind by the Coriolis effect? Isn't it always to the right in the northern hemisphere?

Just asking. I haven't a clue about this issue as it really doesn't come into play on a Laser. But if your mast is 292 ft high... what then?

Why?

In the absence of any real sailing (by me) to write about recently, please indulge me by helping me to answer the question below that was emailed to me by one of my readers. It seems to me that this issue has all the potential to create as much confusion, misinformation and just plain bad science that I managed to stir up with my posts on How Sails Work early last year.

Good day to you sir,

The last 6 months I asked many off the same questions to myself that you also asked yourself. I have red everything that is to be red in regards to articles from A Gentry and I bought the latest version of C.A. Marchaj "Sail Performance". All in English so for a young Dutch guy a bit off an obstacle .... but I'm getting there.

I'm not addressing you in regards to "how or why sails work" but about the "Coriolis effect" and it’s part in Sailing tactics.

Nexus made instruments that can make the effect off wind gradient and wind shifts visible for us sailors. Very handy in regatta sailing. They claim that flowing air is influenced by the Coriolis effect.

Example:
With an overall tacking angle off lets say 75 degrees to apparent wind it can happen that on starboard tack the tacking angle is consistent 35 degrees and the port tack 40 degrees. Also on starboard tack (beam to port) sailors experience more need for twist high in the main then on the port tack.

I explain “beam to port” because here in the Netherlands we call a starboard tack a tack were the beam is on the starboard side and the wind is coming from port, I think in some other places they use it the other way around.

Could you give me your perspective on this question …… Why is the Coriolis effect influencing the wind gradient/direction?

Best regards

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.

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.

Litoralis, 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.

Why Do Sails Work?

Why do sails work?

In particular why does the air flowing around a sail generate lift?

The usual answer given to this question is that a sail is like an airplane wing and that it generates lift for much the same reason. Because of the curvature of the upper surface of the wing, the air passing over that side has to travel a greater distance than that passing under the wing. Since it has to go farther, it has to go faster in order to reach the trailing edge at the same time as the air flowing past the underside of the wing. Because of the Bernoulli effect the faster flowing air on the upper surface has a lower pressure than the slower moving air on the underside of the wing; and the pressure difference generates the lift.

Does this makes sense? Is it the correct explanation? If not, why not?