The Physics Of Sailing

[Yantlet]

A great film on why a boat can sail to windward
http://www.youtube.com/watch?v=yqwb4HIrORM&feature=youtube_gdata_player

 

[Lakesailor]

Lost me. I didn't recognise that dark blue stuff behind the sails.

 

[Yantlet]

Lost me. I didn't recognise that dark blue stuff behind the sails.

Well if you must live in POM Rock!

 

[BobPrell]

A great film on why a boat can sail to windward
http://www.youtube.com/watch?v=yqwb4HIrORM&feature=youtube_gdata_player

What did you think was great about it?

I thought there was a lot of irrelevant fluff - the music, the colourful sails being gathered and stowed, the pretty girls going sailing. These did not contribute to imparting knowledge about the physics of sailing.

There were statements about sailing ships that were misleading - that they were basically only able to go downwind. The ships pictured, cargo windjammers which served California, were quite capable of sailing to windward in most wind conditions.

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 behaviour is closely analogous to wings. Like most sailing theory explanations, they go on about "Lift". Why is this term used? For newcomers to sailing, "lift" is a force that moves things upwards. Why use it for describing horizontal force?

I think something was said about sails but I couldn't understand it because of wind, sail flapping and other distractions. There was too much use of talking heads. Graphics should be used much more to convey the knowledge.

 

[Yantlet]

????????

What did you think was great about it?

I thought there was a lot of irrelevant fluff - the music, the colourful sails being gathered and stowed, the pretty girls going sailing. These did not contribute to imparting knowledge about the physics of sailing.

There were statements about sailing ships that were misleading - that they were basically only able to go downwind. The ships pictured, cargo windjammers which served California, were quite capable of sailing to windward in most wind conditions.

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 behaviour is closely analogous to wings. Like most sailing theory explanations, they go on about "Lift". Why is this term used? For newcomers to sailing, "lift" is a force that moves things upwards. Why use it for describing horizontal force?

I think something was said about sails but I couldn't understand it because of wind, sail flapping and other distractions. There was too much use of talking heads. Graphics should be used much more to convey the knowledge.

Bob I think you really need to get out more!

 

[BobPrell]

How Sailing Works.

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.

 

[DJE]

"lift" is a force that moves things upwards. Why use it for describing horizontal force?

Lift can be more generally described as a force at right angles to the direction of flow. Whereas drag is a force parallel to the direction of flow.

In an aircraft at the top of a loop the lift acts downwards!

 

[Deleted member 36384]

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.

This is a wind up, surely? You are telling your students untruths and it would make you look ignorant if any of your students had some basic knowledge of the subject.

In your rocket thrust model an equal amount of wind force is being applied on each side of the sail, on the windward side its is deflected back which is supposed to thrust the yacht forward due to the keel resistance. However, there is also a thrust on the leeward side as well from the wind which is trying to push the yacht backwards. Where does the extra force come from to move the boat forward? Magic or fantasy, in your theory, because its impossible according to Newton's 3rd Law.

However Brounelli developed a theory that explained flow over curved surfaces and subsequent pressure loss on one side of the surface: http://en.wikipedia.org/wiki/Bernoulli's_principle You can test this very easily using a spoon. Hold the handle of the spoon between you finger tips such that gravity allows the spoon to hang down. Move the back of the spoon into a flowing stream of water from a faucet. The spoon is sucked into the water flow, not pushed away. This is the exact same principle being applied to a yacht when the wind blows on a sail.

You are being unfair to the "learners" if you assume that they can not understand sail theory. The aeroplane analogy is quite an easy comparison for students to visualise the concept and the spoon test is a practical demonstration that can easily be shown.

The fact that they have come for tuition mean that they want to learn; please don't lie to them.

 

[ctva]

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.

Utter rubbish!

Exhaust thrust out the back of the sail, where? Any kid or student would put their hand at the back of the sail and say "what thrust?". All kids and adults know about paper planes, and wings and so have a basic grasp of the concepts which can easily be demonstrated with either a spoon ond water flow or my preference is a sheet of paper and blow over the top to lift it. In 30 years of teaching I've never failed to pass the concept on.

 

[Yantlet]

Utter rubbish!

Exhaust thrust out the back of the sail, where? Any kid or student would put their hand at the back of the sail and say "what thrust?". All kids and adults know about paper planes, and wings and so have a basic grasp of the concepts which can easily be demonstrated with either a spoon ond water flow or my preference is a sheet of paper and blow over the top to lift it. In 30 years of teaching I've never failed to pass the concept on.

Exactly! The illustration with the paper is in the film.

 

[DJE]

Utter rubbish!

That's a bit harsh. The sails change the momentum of the flow. Momentum is a vector so a change in direction at constant speed gives a change in momentum. Force is equal to rate of change of momentum.
A lot of science teaching amounts to lies to children. Starting them off with a gross simplification then refinining it later.

 

[mjcoon]

That's a bit harsh. The sails change the momentum of the flow. Momentum is a vector so a change in direction at constant speed gives a change in momentum. Force is equal to rate of change of momentum.
A lot of science teaching amounts to lies to children. Starting them off with a gross simplification then refinining it later.

Of course, once one has learnt vector calculus such things become obvious. Not to mention (Clerk) Maxwell's equations for electromagnetic radiation (not that I can remember anything about them except that they are beautiful!)

Maybe the sideways "lift" that an airplane gets from its rudder is a better (more easily imagined) analogy to a yacht's sail?

I also like the mental image of a child's windmill which rotates a-c-w when blown on. If swung on a nut on a threaded rod it would screw its way towards you when blown on. Trouble is, it's quite a big step to substitute a yacht as a single windmill blade screwing its way through the water on an axle the diameter of the Earth...

Mike.

 

[BobPrell]

This is a wind up, surely?

Not intended as one.

You are telling your students untruths

What is truth? Be Careful.

In your rocket thrust model an equal amount of wind force is being applied on each side of the sail,

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

on the windward side its is deflected back which is supposed to thrust the yacht forward due to the keel resistance.

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

However, there is also a thrust on the leeward side as well from the wind which is trying to push the yacht backwards. Where does the extra force come from to move the boat forward? Magic or fantasy, in your theory, because its impossible according to Newton's 3rd Law.

I don't understand that either.

However Brounelli developed a theory that explained flow over curved surfaces and subsequent pressure loss on one side of the surface: http://en.wikipedia.org/wiki/Bernoulli's_principle You can test this very easily using a spoon. Hold the handle of the spoon between you finger tips such that gravity allows the spoon to hang down. Move the back of the spoon into a flowing stream of water from a faucet. The spoon is sucked into the water flow, not pushed away. This is the exact same principle being applied to a yacht when the wind blows on a sail.

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

You are being unfair to the "learners" if you assume that they can not understand sail theory. The aeroplane analogy is quite an easy comparison for students to visualise the concept and the spoon test is a practical demonstration that can easily be shown.

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.

The fact that they have come for tuition mean that they want to learn; please don't lie to them.

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.

 

[Fire99]

Blimey, it takes about 30 seconds for a thread to descend into an argument....

I liked the video and with a bit of 'hands on' you soon get to see the whole Aeroplane Wing idea.

 

[SimonP85]

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

Wind and therefore flow is around the entire boat, as already discussed it's the air flowing faster over one side of the sail than the other that results in a pressure difference.

 

[BobPrell]

Wind and therefore flow is around the entire boat, as already discussed it's the air flowing faster over one side of the sail than the other that results in a pressure difference.

That is certainly what happens with an aircraft wing, in accordance with Bernoulli, but an aircraft wing has different curves on the bottom and the top. A sail does not. The difference in speed around the two sides must be very small.

Has anyone ever measured the difference in speed and in pressure on the two sides of a sail?

 

[Deleted member 36384]

..... 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.

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 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.

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

lie 2 ||
noun
an intentionally false statement: ........
• used with reference to a situation involving deception or founded on a mistaken impression: ......

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.

 

[AngusMcDoon]

Wind and therefore flow is around the entire boat, as already discussed it's the air flowing faster over one side of the sail than the other that results in a pressure difference.

Lift is a combination of Bernouilli and the flow directed in a direction opposing lift. I remember the smoke trails in the wind tunnel from my mech eng degree. Got a bit lost when things went supersonic. Not me physically. I knew that I was still in the fluids lab. Just mentally.

 

[BobPrell]

I liked the video and with a bit of 'hands on' you soon get to see the whole Aeroplane Wing idea.

Fire99, I really would like to know what you liked about the video.

And did what you liked help your understanding of the physics of sailing?

It may be hard to answer that if you already had a good understanding.

My problem with the Wing Theory is that nobody ever questions it, or addresses the shortcomings I have mentioned.

It's pretty much "No, you're wrong, it's a wing! Everybody knows that!"

 

[nigelmercier]

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.