Theory of Sailing

[Loren Beach]

This video is dated 2008, so you may have seen it before. I just wandered onto it on the web.
I thought it was pretty well done.
I hope you like it too.

https://www.youtube.com/watch?v=yqwb4HIrORM

 

[Teranodon]

I'll be honest Loren: I didn't like this video.

What it says about square-rigged ships is downright slander. Even square sails could be braced to go to windward. Not to mention the headsails and staysails. How do the KQED folks figure that those big downeasters and clipper ships got to San Francisco?

The old Bernoulli Principle explanation is always too slick. Who says that "increased velocity means lower pressure"? Yes, it's true, but is it supposed to be obvious?

The tank demo is dishonest. They show a shape that is flat on one side, curved on the other. Even a fool can see that a sail doesn't have that shape, so the effect that they demonstrate is irrelevant.

They wave their hands when they try to apply the same lift concept to the keel and the sail. One is symmetrical, the other isn't!

Blowing on the piece of paper tells you nothing about lift in a fluid (i.e., where inter-molecular forces have to be considered in detail). All that is happening there is that the air molecules are bouncing off the paper and producing a Newton's Law reaction. That works fine for airplanes (where you have an engine pushing the air backwards) but not for boats.

IMHO it is impossible to give an intuitively-believable and scientifically-accurate description of the physics of sailing. It is sort of like trying to give a simple explanation of why the moon produces TWO high tides each day. The simple explanation is easy to understand, but it's wrong (produces just ONE tide). The correct explanation requires an hour-long lecture on gravitation, Keplerian orbits, inertial frames of reference, pseudo-forces, etc. Too much for the average listener.

 

[Teranodon]

....My aerobatics instructor said: forget it, the wind pushes against the bottom of the wing.

Your instructor was a wise man, Christian. Anyone can confirm his explanation by sticking his/her palm out the window of a car at some high speed. Holding the palm horizontal is easy, but as soon as the palm is tilted, the force of the wind moves it strongly up or down. Deflecting the air flow causes a simple Newtonian reaction. Nothing to do with the Bernoulli Principle, which is about something else.

 

[Loren Beach]

Your instructor was a wise man, Christian. Anyone can confirm his explanation by sticking his/her palm out the window of a car at some high speed. Holding the palm horizontal is easy, but as soon as the palm is tilted, the force of the wind moves it strongly up or down. Deflecting the air flow causes a simple Newtonian reaction. Nothing to do with the Bernoulli Principle, which is about something else.

I used to hold my hand out of the car window all the time when I was very young. It was fun deflecting up and down.
Not being a physicist, I admit to not knowing much about the principles of sailing.
(I was a 'business major' at the small college, so I know I little about a lot of things and nothing of value about anything.)

If someone has a better sailing theory video to post I would like to see it.

 

[HerbertFriedman]

The standard explanation for how lift is produced by an airfoil certainly works for high speed, i.e. aircraft, but is inadequate for slow speed like sailing. Clearly the standard explanation that a curved wing causes the velocity to increase on the top, curved surface and then invoking Bernoulli's principle fails for sails where the distance around the windward side is essentially the same as the distance around the leeward side, the sail is very thin. And invoking the argument that once the flow separates above and below the sail, the elements of the flow have to meet at the trailing edge, is incorrect for slow speeds.

The difference between high and low speed flows is the effect of viscosity which plays a much more important role in fluid dynamics of low speed flows. There is series of papers by Arvel Gentry, a fluid dynamics expert and sailing enthusiast at Hugh Aircraft in the 1970's who documented these ideas in great detail. Later, other fluid dynamics experts using Computational Fluid Dynamics, CFD, detailed these effects. Angle of attack being the most important factor in all these studies.

I have a collection of Gentry's papers, not easy reading but very informative but just Google Arvel Gentry for a complete list.

 

[Kenneth K]

IMHO it is impossible to give an intuitively-believable and scientifically-accurate description of the physics of sailing.

Easy to say, maybe, for the person who understands enough about the concepts to know that all is not really quite so simple. But what's the beginner to do? Sure, you can tell them it's like holding your hand out the window, but then they'll later ask, "why would it matter what the leward telltales do? All of a sudden, Bernoulli's not such a bad guy anymore, even if often misunderstood.

Despite the fact that a biologist would correctly claim I don't understand the meaning of the word, I continue to regularly osmose new information from this forum. The information is not all strictly technically correct, but it still continues to add to my body of knowlege.

And so it is with learning....

 

[Christian Williams]

I think the idea of "attached flow" helps people the most, as a practical matter. The telltales illustrate it. And everybody can feel and see when the wing stalls and the boat wallows. For those who teach sailing (which I don't and never have), a useful instruction must be, "let the sail out 'till it luffs." I see casual sailors overtrimmed all the time, and not realizing it. Yet you never see beginners luffing. They seem to fear it, whereas in fact luffing would give them the information they need.

 

[debonAir]

Its Complex

The standard explanation for how lift is produced by an airfoil certainly works for high speed, i.e. aircraft, but is inadequate for slow speed like sailing. Clearly the standard explanation that a curved wing causes the velocity to increase on the top, curved surface and then invoking Bernoulli's principle fails for sails where the distance around the windward side is essentially the same as the distance around the leeward side, the sail is very thin. And invoking the argument that once the flow separates above and below the sail, the elements of the flow have to meet at the trailing edge, is incorrect for slow speeds.

The textbook "Bernoulli" explanation kind-of fails for high speed aircraft too. Yes there are always Bernoulli effects in any moving fluid, and there are viscous forces, and turbulence, and pressures, etc. Fluid are complex beasts. The best way to I've found to discuss the "wing is more curved on top" argument is to ask "So how do airplanes fly upside down then?". Pause. "And how about those balsa gliders? Wing is a flat sheet.". The reason many aircraft wings are curved more on the (normal) top is to allow for higher angles of attack before flow detaches, to generate more lift at low speed. Just a way to make takeoff and landing easier.

Even the part of the old Bernoulli explanation stating "the air has to travel further along the top to meet the bottom air so it has to go faster" is not really true. You can google videos where little smoke puffs are added to a wind tunnel air stream over a wing and you can clearly see the smoke puffs splitting at the leading edge not really meeting back up at the trailing edge (in fact the top air gets to the trailing edge well before the bottom air)

https://www.cam.ac.uk/research/news/how-wings-really-work

A lot of the lift is just the air bunching up and bouncing off.

Christian's advice to "luff and trim in a bit" is probably 99% of sail setting.

 

[Alan Gomes]

The textbook "Bernoulli" explanation kind-of fails for high speed aircraft too. Yes there are always Bernoulli effects in any moving fluid, and there are viscous forces, and turbulence, and pressures, etc. Fluid are complex beasts. The best way to I've found to discuss the "wing is more curved on top" argument is to ask "So how do airplanes fly upside down then?". Pause. "And how about those balsa gliders? Wing is a flat sheet.". The reason many aircraft wings are curved more on the (normal) top is to allow for higher angles of attack before flow detaches, to generate more lift at low speed. Just a way to make takeoff and landing easier.

Even the part of the old Bernoulli explanation stating "the air has to travel further along the top to meet the bottom air so it has to go faster" is not really true. You can google videos where little smoke puffs are added to a wind tunnel air stream over a wing and you can clearly see the smoke puffs splitting at the leading edge not really meeting back up at the trailing edge (in fact the top air gets to the trailing edge well before the bottom air)

https://www.cam.ac.uk/research/news/how-wings-really-work

A lot of the lift is just the air bunching up and bouncing off.

Christian's advice to "luff and trim in a bit" is probably 99% of sail setting.

Interesting video. But what accounts for the fact that the air on top is sped up vs. the air on the bottom?

 

[debonAir]

Interesting video. But what accounts for the fact that the air on top is sped up vs. the air on the bottom?

Sorry for the long reply. This is one of my other hobbies

The way I think of it is as air molecules bouncing off each other and the wing. Think of the air molecules as little marbles all traveling at about the same speed and bouncing off at random directions and angles. It is these collisions which we call "pressure". You put more marbles in the can you get more collisions per unit time which means more pressure. You raise the average speed of the marbles (increase temperature) you get harder collisions which means higher pressure. If you make the can smaller, you get more collisions per unit area which means higher pressure.

If you look at the wing (or sail) from the perspective of the air molecules, the air on the bottom side is seeing a (temporary) reduction in the volume of space it occupies so has more collisions going forward and more collisions on average with the wing so increases pressure and pushes the wing up (and back). Eventually the molecules expand downward and back and regain equilibrium. Conversely, The air going over the top sees an increase in the volume of space it now occupies and thus has fewer collisions with the wing which lowers average pressure against the top surface. Also since there are fewer average collisions with molecules in the forward direction, the molecules velocity increases on average in that forward direction (towards the trailing edge) and the air is now moving much faster (on average) forward than the bottom air.

Interestingly, that forward velocity change has to come from somewhere, you can't just increase speed without energy. In this case it comes from a reduction in the overall average velocity. The air is moving faster forward, but at the expense of moving slower in all the other directions. And since this happened because the volume increased, the overall pressure is now reduced like in the can of marbles case. Technically, if you think of increasing the volume as, say, retracing a piston in a cylinder, you can see that the "marbles" hitting the piston will rebound a bit slower than they hit, since the piston is moving away. This has the effect of reducing the average velocity of all the molecules since they are all hitting each other, which means you are reducing the temperature. The faster you move the piston, the more temperature change you can make. This is the reason you see water condensing in the air over wings in very high lift scenarios. It gets really cold really quick.

 

[Loren Beach]

Mooring close to the final approach to PDX, we see vapor trails often from wings of landing aircraft. In cold and wet weather, I should add.

 

[HerbertFriedman]

All these discussions about simple models trying to describe complex phenomena remind me of the simple model of the atom being much like the planets orbiting the sun. Electrons orbiting the nucleus of an atom have nothing to do with planets, electrons are more like waves than particles so the solar system model is completely irrelevant. But it does help visualize some aspects so the analogy lingers on, and so will the Bernoulli effect for sails. I would suggest to "go with the flow".

 

[Alan Gomes]

FWIW, I sent the link to an engineer friend of mine and this was his reaction:

It looks like the air on the very top is going faster than the air on the very bottom, even though the wing is in the middle…that says that the wind tunnel height is too small…the wing is influencing the air by compressing the flow below the wing. Need a smaller wing or a bigger tunnel. The air outside the influence of the wing should all go the same speed…

 

[Kenneth K]

The Bermuda or Marconi rig (and the resultant triangular, wing-like sail) was develop sometime around 1650.

In 1738, Bernoulli published Hydrodynamica, where he stated that “an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy.”

In 1899 the Wright brothers “put wing warping to the test by building and flying a biplane kite with a five-foot (1.5m) wingspan. When the wings were warped, or twisted, one end of the wings produced more lift and the other end less lift.”

100 years later, new Air Force student pilots were still being taught that the Bernoulli equation, the “warped wing,” a couple of JP4-fired turbine engines, and some basic stick-and-rudder skills were all that was keeping their shiny-but-dated jet trainers out of the terra firma.

While this entry-level of knowledge about airfoils may be incorrect from the point of view of today's aeronautical engineer or a naval architect, it’s all that (or possibly even more than) the “operator”--the pilot or the sailor--really ever needs to know. It’s not for lack of scientific knowledge that pilots crash airplanes or sailors lose races; it’s lack of attention, poor execution, lack of procedural adherence, faulty equipment, poor judgement & decision-making, etc.

So why all the concern about the purely technical correctness of the Theory of Sailing?

source = Wikipedia

 

[csoule13]

While this entry-level of knowledge about airfoils may be incorrect from the point of view of today's aeronautical engineer or a naval architect, it’s all that (or possibly even more than) the “operator”--the pilot or the sailor--really ever needs to know. It’s not for lack of scientific knowledge that pilots crash airplanes or sailors lose races; it’s lack of attention, poor execution, lack of procedural adherence, faulty equipment, poor judgement & decision-making, etc.

So why all the concern about the purely technical correctness of the Theory of Sailing?

source = Wikipedia

As we used to joke around the hangar during my flying days, the weakest part of the plane is the nut that connects the seat and the yoke.

 

[footrope]

The Wright's Flyer wings were more like sails than today's typical airplane wings (And I've heard of hang gliders, too). Bird wing influence I suppose. Modern cloth sailors carry on the wing-warping concept long after it has been discarded by modern and more efficient airplane design. That saves weight aloft.

Cool thread.

 

[Christian Williams]

Yes, hang gliding good example. Here's a video from my home site.

https://www.youtube.com/watch?v=MjFbomr5Z54

The string trailing from the right-hand side of crossbar is called the "vg," for variable geometry. When it is long, the wing is flat and efficient, "in the grove" as a yacht night be in smooth water and steady wind. When the pilot lets off teh vg, and it looks short, he wants less speed and more control--the equivalent of flaps.

The beeping sound is the variometer. The faster it beeps, the more the pilot is ascending. If it squeals, he is descending. Typical rate of altitude gain in a thermal is maybe 200-300 fpm. In a very strong thermal, it can be 1,000 feet per minute--more than a standard general aviation plane dreams of. Just hang on tight.

Thermals are invisible. You feel them lift a wing and turn into them, circling to try to stay in the column of rising air. The variometer signals success.

All very much like sailing, and sailors take to it fast. Landing much like sailing into a slip. Don't hit the dock.

 

[Mark F]

That was some textured air in that hang gliding video! In reference to Christian's comment on variable geometry on a hang glider, here is a video of a flight at Yosemite, at the one minute mark the VG cord is pulled, flattening the wing. Its subtle but you can see the trailing edge get lower in relation to the tail fin. This air (early morning) is the opposite of textured ;-)
https://www.youtube.com/watch?v=WODJIN8Eymw

 

[bgary]

All very much like sailing, and sailors take to it fast. Landing much like sailing into a slip. Don't hit the dock.

(laughing) I learned a valuable tidbit on this through a sequence of hobbies.

Had a friend who was into remote-control gliders. Looked like fun. Spent dozens of hours building a glider that was billed as "forgiving flight characteristics for the beginner". Balsa frame, paper-and-dope for the wings and control surfaces, static-balanced and all. Took it to the nearest big field (the high-school sports field) and did a few hand-launched glides. Did great, very graceful in the air and soft landings in the grass. So I set up for the first "real flight" which, with the technology of the day, meant anchoring a long bungy-cord at one end of the field, connecting the hook on the bottom of the glider, stretching it as far as I could and then letting go.

Did all that, and, as if drawn by the Strong Atomic Force, my beautiful glider bee-lined for the left-side pillar of the goal post and converted itself into a big pile of small splinters.



Decided to switch the RC gear into (what became) a series of RC sailboats. Not just because I have some affinity for sailing, but because it is far less stressful. When an RC sailboat has some sort of problem, it happily bobs around in the middle of the pond, waiting for you to swim out and retrieve it.

Deeply metaphorical lesson. One which has (so far) convinced me not to make the leap from sailing to hang-gliding....

_/)_

 

[Teranodon]

I just think that airplane wings are poor analogies to sails.

The airplane has a propellor or jet engine to push the wing forward through the air. Then all you need is hydrodynamics to generate lift (via Bernoulii, etc.) that keeps the plane up in the air. A vertical force is not so hard to derive this way. A glider has no engine, but it moves forward by falling in the Earth's gravitational field (except when it exploits thermals).

But the sailboat situation is very different. Not only are you trying to generate lift from the fluid (air) moving past the sail, but you are trying to generate the forward pushing force as well. That's a lot tougher. Sure, you can angle the force vector forward in the standard diagrams, and make it long enough so that it has an even bigger forward component, but it smacks of hand-waving and cheating. It gets worse when you sheet in the sail, which rotates the force vector in the wrong direction.

Frustrating. Especially if you are a physicist, who is supposed to understand this stuff.