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Old 11-02-2013, 07:54   #1
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Settle an argument

Ok, had a friendly argument with my father in law this weekend. Last time we rented a sailboat for a day the instructor commented on the importance of sail trimming due to the fact the sail is basically a wing. Later on that day my father in law brings up that he believes this is all mumbo jumbo and that the "wing effect" is basically oversold. He claims that a perfectly workable sail could be made out of a flat solid sheet of material with no curve and you would get decent performance out of it. Not as good a a curved sail, but almost as good. I told him it might work but would basically be crap and give little to no forward thrust. Who is right?

Tom
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Old 11-02-2013, 08:04   #2
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Re: Settle an argument

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Ok, had a friendly argument with my father in law this weekend. Last time we rented a sailboat for a day the instructor commented on the importance of sail trimming due to the fact the sail is basically a wing. Later on that day my father in law brings up that he believes this is all mumbo jumbo and that the "wing effect" is basically oversold. He claims that a perfectly workable sail could be made out of a flat solid sheet of material with no curve and you would get decent performance out of it. Not as good a a curved sail, but almost as good. I told him it might work but would basically be crap and give little to no forward thrust. Who is right?

Tom
There are always two sides to a story .
Beating to windward, FIL is 100% wrong
Sailing dead downwind, FIL is 100% right
Anywhere in between, well he is a bit right and a bit wrong.
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Old 11-02-2013, 08:32   #3
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Re: Settle an argument

Tom,

2/3 of a sail's force is created by the leeward side, 1/3 by the windward side. That is, if you have laminar flow on both sides. Hence on anything from 40 to about 100 degrees apparent wind it will make a huge difference. Anything above that angle - as the previous poster said, it does not matter as much.

If you are out there you can experience it first hand: Let your mainsail out with wind in the 130 to 150 degrees quadrant. Now if you slowly turn to windward you will experience a very nice kick in the butt the moment the leeward wind goes laminar, and your boat will accelerate a lot (at least mine does). This effect you would be missing.

Another way to convince him: Look at the speed polars of boats. On most of them, you will see a significant increase in speed at the angles listed above.

...if he's still not convinced, ask him to take his next flight in a plane that has flat wings without profile. Unless you power it with a rocket it won't get off the ground...

Oliver
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Old 11-02-2013, 08:48   #4
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Re: Settle an argument

Its a wing.
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Old 11-02-2013, 08:50   #5
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Re: Settle an argument

Hold a teaspoon upside down with a faucet running, then let the outer curve of the teaspoon contact the flow of water. There will be no mistaking the effect, the spoon will be pulled towards the flow. Now try it with a butter knife.
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Old 11-02-2013, 09:14   #6
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Re: Settle an argument

A flat solid sheet with no curve will still work as an (inefficient) wing, developing lift due to the angle of attack.

Directly downwind there is no "wing" effect, but on other points of sail the lift developed by the sail is important.

Its worth rembering that sailboat foils ( keel and rudder) also work primarily as "wings" and many unfortunatly are simple flat plates.
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Old 11-02-2013, 09:23   #7
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Re: Settle an argument

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Hold a teaspoon upside down with a faucet running, then let the outer curve of the teaspoon contact the flow of water. There will be no mistaking the effect, the spoon will be pulled towards the flow. Now try it with a butter knife.

THAT SIMPLE TEST IS IMPRESSIVE!

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Old 11-02-2013, 09:26   #8
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Re: Settle an argument

My FIL is a pilot and we debate many things. This would not be one of them.
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Old 11-02-2013, 09:28   #9
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Re: Settle an argument

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Hold a teaspoon upside down with a faucet running, then let the outer curve of the teaspoon contact the flow of water. There will be no mistaking the effect, the spoon will be pulled towards the flow. Now try it with a butter knife.
That is at 0 angle of attack.
A flat, or symmetrical, plate will still generate lift at a positive angle of attack. If if did not our sailboat rudders would not work.
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Old 11-02-2013, 09:35   #10
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Re: Settle an argument

To those who think the leeward side does work, think again, it goes along with the urban legend that a vacuum sucks the sails along, well sorry a vaccum cannot exert a mechanical force on an object, the pressure difference is what does the work and creates the force.
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Old 11-02-2013, 09:49   #11
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Re: Settle an argument

**** SIGH****

Life is too short for silly arguments...
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Old 11-02-2013, 09:51   #12
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Re: Settle an argument

Not quite flat but the guys on mythbusters once built a sailboat entirely from duct tape. They welded up an aluminum frame, covered it with duct tape ..result it was waterproof. Then they raised an aluminum mast and boom and made a sail from duct tape.

Damn thing sailed along quite nicely.

But the rest of the posters are correct. The closer you sail to the wind, the more profile you need
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Old 11-02-2013, 10:04   #13
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Re: Settle an argument

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Originally Posted by stevensuf View Post
...............with the urban legend that a vacuum sucks the sails along, well sorry a vaccum cannot exert a mechanical force on an object, the pressure difference is what does the work and creates the force.
Sure it's an airfoil or "wing". .....it's also the partial vacuum that is responsible for a degree of the pressure difference dependng upon the point of sail. There's nothing inappropriate with the "suck" description as it identifies this pressure difference,- no urban legend, just semantics.
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Old 11-02-2013, 10:22   #14
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Re: Settle an argument

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Sure it's an airfoil or "wing". .....it's also the partial vacuum that is responsible for a degree of the pressure difference dependng upon the point of sail. There's nothing inappropriate with the "suck" description as it identifies this pressure difference,- no urban legend, just semantics.
The minute you start talking about a partial vacuum, you've entered the rhelm of physics. It doesn't take a great intellectual leap to go from being a creationist or a climate science denier to claiming that a flat sail would work aerodynamically.

The real question isn't science--it's engineering: just how are you going to keep a flat sail flat?
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Old 11-02-2013, 10:24   #15
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Re: Settle an argument

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There are always two sides to a story .
Beating to windward, FIL is 100% wrong
Sailing dead downwind, FIL is 100% right
Anywhere in between, well he is a bit right and a bit wrong.
Sailing dead downwind, FIL is 70% right, a cupped shape will provide more drive downwind than a flat plate.
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