The Physics Of Sailing

[Lakesailor]

A lot of science teaching amounts to lies to children.

That's quite right. Teachers have been telling children for quite a few years now that leaving lights on and driving 4x4s will damage the planet.

 

[BobPrell]

If you understand how a sail works and you choose to tell a student that it works a different way, then you are lying, not mistaken, lying.

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.

I assume from your descriptions that you understand how a sail works, but that you choose to lie, because you think your students would not benefit from the correct explanation. That is my opinion of you from your posts so far.

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

I accuse you of lying in the context of the definition of the word, not as a slur on your character: -

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

I am not interested in duelling with you and won't participate in a dual. I think you do your students a disservice using the thrust model as you described using the rocket analogy.

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.

 

[BobPrell]

But Bernoulli's principle doesn't explain how aircraft are able to fly with symmetrical wings, with the same profile top and bottom. For this you need Newton, the air deflected downwards causes an equal force upwards.

Thank you Nigel, I think your post supports my idea. Likewise, most fans just have blades with curve, and they work well enough for most purposes. If a really powerful efficient fan is needed, it is usually given blades with different profiles top and bottom.

 

[AngusMcDoon]

But Bernoulli's principle doesn't explain how aircraft are able to fly with symmetrical wings, with the same profile top and bottom. For this you need Newton, the air deflected downwards causes an equal force upwards.

Aerofoils that use both Bernouilli and deflection (like standard aeroplane wings) are more eficient than those that don't (like helicopter blades). Similarly you could make a sail from a flat sheet of plywood, but it wouldn't be very efficient. Nor would it look quite right on Valsheda.

 

[SimonP85]

A foresail has fairly significant curvature cut into it to produce the different curves. The spoon example works well here too, the same curved surface has very different flows on either side. The sail shape and draft position is adjusted by changing the lead in angle

A flatter sail will produce less power and this technique can be used to de-power the sail in increasing winds. The sheet is also used to control the draft position and twist in the sail.

 

[blackknight]

...They continued with misleading, by showing solid, rigid wing sections in the flow tank. Sails are not solid or rigid, and I don't believe their behavior is closely analogous to wings.....

My very first post, I know. But I could not let this slide. The understanding of how a wing/sail works is very well known. It is only personal ignorance that makes people believe anything other. (Incidentally just like Biological Evolution but that is way off topic.)
Do people forget the pioneers of flight with their "sail wings" so quickly?

 

[DJE]

Aerofoils that use both Bernouilli and deflection (like standard aeroplane wings) are more eficient than those that don't (like helicopter blades). Similarly you could make a sail from a flat sheet of plywood, but it wouldn't be very efficient. Nor would it look quite right on Valsheda.

All aerofoils deflect the flow passing over them in order to generate lift. If the flow exerts a force on the foil then the foil exerts an equal and oposite force on the flow. This force changes the momentum of the flow by changing its direction.
The Bernoulli effect produces differing pressures on the two sides of the foil and the sum of these pressures is the force on the foil or the flow.

 

[Skysail]

Symmetrical airfoils do work, but only when given an angle of attack; in which case they become unsymmetrical relative to the airflow.

Try looking at wikipedia http://en.wikipedia.org/wiki/Lift_(force)

which basically says you are all correct depending on what level of complexity you want to go for - Newton, Coanda, Bernoulli, Kutta-Joukowski.

The pressure on the leeward side of a sail has been measured as lower than the windward side - 'Sails' by Jeremy Howard-Williams, and 'Sailing Theory and Practice' by A. C. Marchaj.

The 'Newton' approach is OK if you are happy with a very basic approach, but this does not explain the full effect, and in my experience most students are very happy with the aircraft wing analogy.

 

[deuc02]

Some interesting thoughts and theories here so I thought i might as well stick my 2p in the pot.

Sails basically work exactly the same way as an aeroplane wing. They induce lift by creating a pressure differential between (for simplicity) the top and bottom of the sails. For a sail to work efficiently you need to create laminar flow overthe 2 surfaces as remember the lift is created across the whole top surface. This laminar flow is what makes tell tales on sails work. All they are doing is letting you know if you are creating a boundry layer ie smooth airflow over the 2 sides of the sail. And just like aircraft wings on slow planes and faster planes the chord angle will dictate how efficient the sail will be at differing wind speeds. Hence why when the draft is to excessive the boundry layer will start pulling away from the sail surface inducing turbulent airflow ie drag (noticeable on a main for example when areas of the leech start fluttering). So part of the sail (the forward part) will provide a bit of forward motion to the boat and the lift that creates sideways lift is partly transferred to forward motion by the hull shape and keel. And allthe excess sideways lift is what makes you heel

Thats how i understand it anyway. And 1 final point. I was always under the assumption that draft position was altered using halyard tension but maybe someone can correct me if ive got that 1 wrong

 

[AngusMcDoon]

I was always under the assumption that draft position was altered using halyard tension but maybe someone can correct me if ive got that 1 wrong

You can quite effectively alter the maximum draft position of your sail my using it long after its useful life as well.

 

[Frankie-H]

Just to show I more than cranky and critical, here is the explanation I have used with learners. It is simple, so they are not overloaded with complex ideas that don't contribute to the activity.

The sails deflect the wind over the stern of the boat, so it is acting like a rocket exhaust out the back. Like a rocket, the reaction force pushes the boat forward. The flexibility of the sail allows the sailor to adjust it so the deflection is as efficient as possible.

The wind exerts a force on the sails as it is being deflected. This moves the boat sideways in the water. The shape of the boat, especially its keel, allows it to move forward easily in response to the "rocket thrust", but only slowly sideways. This slight sideways movement is called leeway.

B0ll0X

And you are an instructor

From memory, the theory of lift was discovered at the begining of the last century. The shape of aerowings should give you some help.

Oh. I get it - sorry. You are a Troll and the whole thing was a wind up.
The alternative is too sad to contemplate.

 

[jimbaerselman]

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!

 

[Yantlet]

Now now gentlemen, and here I am thinking I was doing something useful in posting the link - never expected WW3 to break out, isn't science wonderful?

 

[chewi]

I suggest a few folk here read Arvel Gentrys articles, published in Sail magazine and found on google. He was a research Aerodynamicist at Boeing who learnt to sail but found the books make explanations of airflow that made no sense, especially the slot.

He explains how sails really work, including the fabric, unfixed nature of a sail, how the main affects the jib, the jib affects the main. stalling, and circulation. All based on bernoulli, Reynolds and other giants of aerodynamics. He became aero/hydronamic consultant to the Americas cup team.

his "gentry tufts" are now known as telltales.

It may take a few reads to take them all in, but its worth the effort.

 

[BobPrell]

B0ll0X

And you are an instructor

From memory, the theory of lift was discovered at the begining of the last century. The shape of aerowings should give you some help.

Oh. I get it - sorry. You are a Troll and the whole thing was a wind up.
The alternative is too sad to contemplate.

This is an example of a response I have had to my explanation a number of times. It amounts to "No you are wrong " (put more or less rudely), "It's a wing. Everybody knows that."

This goes a little further. Nobody could possibly be that ignorant when it's in all the sailing books, so I must be saying it out of malice. Troll!!!!

 

[BobPrell]

Now now gentlemen, and here I am thinking I was doing something useful in posting the link - never expected WW3 to break out, isn't science wonderful?

Yes, thank you for the link. My post #4 was critical of the style of the film rather than the content. I looked at my post after I published it and thought now if I have been critical of something for being overcomplicated I should offer an alternative. So I did, thinking I would look less grumpy that way.

I did not anticipate the level of flak it would attract.

I'm glad you mentioned science. To uncritically accept conventional wisdom is not scientific.

 

[mjcoon]

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.

Just to stir things up a bit more, that reminds me that I think there was a proposal to use vertical rotating cylinders to get sailing propulsion. Anyone remember what happened to that (or what it was called)?

Mike.

 

[Lakesailor]

Was it a bit too much weight, too high up?

Just a guess.

 

[Deleted member 36384]

.... Anyone remember what happened to that (or what it was called)?
.

There was a few articles about it a couple of years ago, maybe a bit more, a trimaran was rigged up with rotating drums. It worked, but not that well. If I remember correctly the original ship was not that efficient either. You would need to check the various articles for the facts, but that is what I remember, not efficient.

Here you go a link: http://www.grc.nasa.gov/WWW/k-12/airplane/cyl.html

HISTORICAL NOTE: In the early 1920's the force from a rotating cylinder was used to power a sailing ship. The idea, proposed by Anton Flettner of Germany, was to replace the mast and cloth sails with a large cylinder rotated by an engine below deck. The idea worked, but the propulsion force generated was less than the motor would have generated if it had been connected to a standard marine propeller! Here's a picture of the ship provided by Brian Adkins, BAE, Georgia Tech, 1993

 

[BobPrell]

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.

..... 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.

...... 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.

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.