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Old 12-29-2013
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Physics of Sailing

Found this on a surfing expedition today. Thought it might be of interest to some in present company.


Happy New Year
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Re: Physics of Sailing

Good stuff, thanks for sharing.
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Old 12-29-2013
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Re: Physics of Sailing

I really don't know anything about sail physics, but, that was pretty simplified. I think they got the bit about luffing all wrong (7:43). They said luffing is like an airplane wing stalling. I would think that would happen at a high angle of attack, not at zero angle of attack. Regardless, those are some nice shots of sailboats doing their stuff. Any boats we know in there?
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Re: Physics of Sailing

An airplane wing stalls when it looses its lift. So, from that perspective a sail luffing is a good analogy. But your right that an airplane wing looses it's lift due to insufficient wind speed over it's leading edge which is not what a sail does. Most sails, unlike airplane wings, are not rigid so it's a challenge to compare the two in all circumstances.
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Old 12-30-2013
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Re: Physics of Sailing

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Originally Posted by Barquito View Post
I think they got the bit about luffing all wrong (7:43). They said luffing is like an airplane wing stalling. I would think that would happen at a high angle of attack, not at zero angle of attack.
I think you're right. Good catch. I think the situation they were showing in the tank was the equivalent of an over trimmed main sail, which is like a stall and nothing like a luff...where you're decreasing the angle of attack. Right?

I did like the bit about not having to understand all the physics to make the boat go. It helps, but is not essential.

Last edited by billyruffn; 12-30-2013 at 01:05 AM.
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Old 12-30-2013
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Re: Physics of Sailing

Good stuff, thanks
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Old 12-30-2013
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Re: Physics of Sailing

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Originally Posted by Torch View Post
An airplane wing stalls when it looses its lift. So, from that perspective a sail luffing is a good analogy. But your right that an airplane wing looses it's lift due to insufficient wind speed over it's leading edge which is not what a sail does. Most sails, unlike airplane wings, are not rigid so it's a challenge to compare the two in all circumstances.
Stalling of a wing has little to do with wind speed as you described it...
An airplane wing stalls when the angle of attack gets to big for the speed...
What happens essentially is that the airflow separates from the upper wing, creating turbulences and thus eliminating lift, because lift is only produced in a laminar flow situation...
So the insufficient wind speed at the leading edge has nothing to do with it... Speed only factors in in terms of that the maximum angle of attack decreases with speed for a given, solid foil or wing...
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Old 12-30-2013
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Re: Physics of Sailing

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Originally Posted by capt vimes View Post
Stalling of a wing has little to do with wind speed as you described it...
An airplane wing stalls when the angle of attack gets to big for the speed...
What happens essentially is that the airflow separates from the upper wing, creating turbulences and thus eliminating lift, because lift is only produced in a laminar flow situation...
So the insufficient wind speed at the leading edge has nothing to do with it... Speed only factors in in terms of that the maximum angle of attack decreases with speed for a given, solid foil or wing...
Hence the old Saw: "When in Doubt, Let it Out".

Over trimming can also lead to excessive weather helm requiring a lot of rudder which can lead to stalling the rudder/keel.

N'any case, I think the film is quite interesting and informative.
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Re: Physics of Sailing

I too think of stall on, say, a jib as having too steep an angle of attack despite a nice curved airfoil, so I have to ease the airfoil out to give the wind an easier job of turning the "outside corner" of the foil just aft of the headstay.

Easing out too far and creating a true luff isn't 'stall' to me, but rather the sudden nonexistence of the "floppy foil" itself, which instead becomes a "flag/rag/drag" and no longer a foil/wing at all.
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Old 12-31-2013
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Re: Physics of Sailing

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I did like the bit about not having to understand all the physics to make the boat go. It helps, but is not essential.
Especially since the physicists don't even agree (or know) how sail aerodynamics work. From a pedagogical perspective I think it depends on your learning style and background. It is OK with me to describe sailing with force vectors. But, for some this might result in needless blank stares.
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