The Physics Of Sailing

jimbaerselman said:
Some of you do like to complicate things by looking at the detail.

Put any shape into an airflow, and it will change the momentum of the airflow; so a force will be created.

If it's a cylinder, it'll slow the airflow down (drag); the pressure at the front of the cylinder will be greater than that at the back (bless Bernoulli if you wish; whether it's turbulent or laminar, friction slows the air down . . .)

If it's longer than it is wide, it may be a plank, a symmmetrical aerofoil, an asymmetrical airofoil, a thin curved sheet, a sail, I don't care. But any of these shapes could be rotated relative to the airflow, and (unlike the cylinder) this will create changes of direction of the arflow as well speed.

Sure, with a bit of twizzling, you can find an angle at which they're just a bit of a drag.

But, at any other angle, they will not only slow things down a bit, they'll also change airflow direction. That's a change in momentum - which will be matched by a force. Change in momentum=force.

Do vectors to find how much and in exactly what direction the resultant force acts. But crudely, it's at right angles to the mean flow - half way between input and output (ignoring the additional element of drag). The force manifests itself as a difference in pressure each side of the aerofoil, explained by Bernoulli, flow being faster on the "outside" than on the "inside"

You don't have to have think "inside" vs "outside" either. Water and spoons have been mentioned. Flow direction will change whether the flow is just around the convex, or just inside the concave, and the force developed will be the same if the change in flow angle is the same. Take a pause and consider how many water sprinklers work - changing water direction through a right angle.

That's how any shape creates forces in an airflow. Changing momentum of the airflow. Over simplifying, direction change=lift, speed change=drag (or thrust if it's had energy added by going thru a device!)

For doubters, aircraft with asymmetric aerofoils can still be made to fly level upside down - if they're strong enough and the engine doesn't stop.

So all arguments about "what shape" are not very relevant. These shapes are just variations of machines which change air momentum. Parachutes are great at drag, but poor at lift. Gliders are brilliant at lift, and create very little drag indeed. Sailboats have two airfoils pushing against each other, one under water, and one above which flips to match the airflow direction. This incidentally creates a heel angle, but the net result is a bit like sqeezing a damp lemon pip - there's only one way left to go.

Not exactly the rocket out of the back; more like the only remaining option!
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as ever from JimB, well expressed sense.
 
BobPrell said:
Jim you explained at greater length than I did, I am a bit lazy at composition.

I have quoted the parts which are critical to my explanation.

High school physics students are taught very early that velocity consists of speed and direction. Acceleration is to change velocity, and requires a force. If something has velocity and you change its direction only, that is still acceleration and requires a force.
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Very concise compared to my windy effort!

. . . the tension in a string when you're swinging a weight around in a circle . . . the sideways scrub of tyres on a turning car . . . the extra lift in an airplane which has to bank to turn . . . all side forces creating change in direction.

And now the fundamentals are clear of Bernoulli fog (OK, he explains how the sideforce is generated by pressure changes for those who need to know), we can get down to efficiencies . . . how do you change direction of the wind without creating too much drag?

Depends how big an angle change you're going for.

Going to windward? Minimise the induced drag. Aspect ratio matters - very long, very deep, very narrow keels with little right angle plates on the end, just like the average airliner wing tip. Slight twist at the top of a sail so there's less pressure change there to create vortices.

Always, encourage laminar flow. Slots help, and tell-tales to give you a clue.

Downwind? Parachutes? . . . but modern efficent parachutes look more like wings, and don't go straight down. They glide, keeping laminar flow. No wonder these racing guys gybe downwind
 
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It might be easier on your mind to hang the spoon off your nose and make bird noises.

BobPrell said:
Not intended as one.

What is truth? Be Careful.

I don't understand that. How did the wind get around the other side of the sail to act on it?

I did not say keel resistance caused the boat to go forward.

I don't understand that either.

Yes I have seen that many times and read Bernoulli lots of times. I don't think they are needed here.

I made no such assumption. I just think a much lighter theory will suffice. To show wing sections with rigidity and thickness also misleads the student.


At the beginning you implied, and now have openly said that I am lying. You must retract that or I will have to call you out for a virtual duel.
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jimbaerselman said:
Indeed. Very concise. You avoided "lies to children" by starting with the big picture: Momentum.

It's a pity some are taught how sails work with cod-explanations of how pressure changes are created . . .
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What does "cod-explantaions" mean, just out of interest as I have never heard that before?
 
If I understand what is being said its the change in velocity (acceleration) of the wind (which has mass), that causes a force to act relative to the sail, F=ma. This is what drives the boat up wind.

Bernoulli's Principle is just an explanation why a pressure drop exists over a surface that intersects a fluid stream but is not the cause of the force.
 
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BobPrell said:
I do not accept the conventional explanation of how a sail works. I have developed an explanation which is good enough for me at this point. I will discard my explanation if a critical flaw in it is explained to me.


That is a stupid assumption. I have not chosen to lie. You cannot possibly know what I think unless I tell you.

I did not intentionally make a false statement, nor did I intend to deceive.

You are in a duel because you persist in accusing me of lying.

My reply is to accuse you of being stupid for making a dumb assumption about my motive, and making the stupid statement that lying is not a slur on my character.
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I dont understand how you live, upside down all the time! What happened when you sailed off the end of the world? is that when you went upside down?
Stu
 
I was in Dubrovnik a couple of years ago. There was a young couple discussing the cruse ships visible in the distance.

Girl - I would love to go one of those
Guy- They are not safe made of metal apparently
Girl-but metal doesnt float
Guy -exactly
Girl-but that ones floating
Guy-they let off bubbles of air under the ship which stops it sinking

Some of the theories and explanations on this topic have been of a similar standard. :)
 
How sails Provide lift and interact with each other to increase efficiency is well understood and has been for a very long time. It has got nothing to do with "jets" coming off the back of the sail. NOTHING!
Only one contributor has mentioned Arvil Gentry. No-one has mentioned circulation theory.
You can't have differences of opinion and expect to be taken seriously. There is one explanation and one only and it has been common knowledge for "ever!"
Just because you don't like the explanation or you don't understand it, doesn't mean that you can make one up and put it out there as a viable alternative.
"Intelligent Creation" might be easier for some people to understand, but it's still a lie!
sam :-)
 
SamSalter said:
How sails Provide lift and interact with each other to increase efficiency is well understood and has been for a very long time.
You can't have differences of opinion and expect to be taken seriously. There is one explanation and one only and it has been common knowledge for "ever!"
Just because you don't like the explanation or you don't understand it, doesn't mean that you can make one up and put it out there as a viable alternative.

sam :-)
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With attitudes like that, Science would never have gotten past the Phlogiston Theory, let alone developed the Theory of Relativity.

I will expand on a statement I made before. To uncritically accept conventional, established knowledge is not scientific.

This post is another that essentially says "You are wrong. Everybody knows that it behaves exactly as described in [authoritative source]". The critic does not point out a fallacy in my explanation, he just flatly contradicts it.
 
BobPrell said:
..... This post is another that essentially says "You are wrong. Everybody knows that it behaves exactly as described in [authoritative source]". .....
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It would appear so. My assumption that you understood how sails work based on the the popular Bernoulli's Principle was wrong, so my assertion that you lied to your students is also wrong. You are not a liar.

The change in momentum as explained by DJE, jimbaerselman and yourself (plus Wikipedia) is an easy explanation to understand, much easier than Bernoulli's. I certainly took it on faith from text books that Bernoulli's was the reason for the force.
 
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