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Barquito
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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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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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Discussion Starter #5 (Edited)
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? :confused:

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

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



Happy New Year

Enjoyed the video --- was surprised about this thread.... I used to read a lot of cycling forums and if there was a 'physics of cycling' thread started it would go on for ever... with knock down drag out arguments with engineeers... so I was kind of surprised at this one..

I have a question, related to physics of sailing

When the wind blows people go to the high side to try to bring the sail more vertical...in smaller boats they even hang out on a trapeze

I am wondering is that better than a) easing out the sails; b) reefing; c) heading more to the wind...

and why
 

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...When the wind blows people go to the high side to try to bring the sail more vertical...in smaller boats they even hang out on a trapeze

I am wondering is that better than a) easing out the sails; b) reefing; c) heading more to the wind...
There are so many dimensions to this answer that others who know more than I can go on for hours.

But one thing that I will point out is that the answer may be totally different depending on whether it is a planing hull (small boat), a displacement monohull (larger boat) or a multi-hull.

As usual, the only universal answer is "it depends."
 

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There are so many dimensions to this answer that others who know more than I can go on for hours.

But one thing that I will point out is that the answer may be totally different depending on whether it is a planing hull (small boat), a displacement monohull (larger boat) or a multi-hull.

As usual, the only universal answer is "it depends."
Planing... I thought that only worked when the wind was behind you?

A new thought --- what if you put more weight in the keel to keep it upright
 

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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.
Actually sort of backwards. An airplane wing or rotor blade will lose lift when it stalls. However I can make an airplane wing lose lift or a rotor blade lose lift just by unloading (think reducing angle of attack) without stalling. Also stalling is not a direct function of indicated (actually it is calibrated) airspeed. The stall angle can be exceeded at any airspeed (within the aircraft structural limits of course).' Google acccelerated/high speed stall. I am sure the pictures they can draw you will far exceed my poor grasp at math. It is all about the exceeding the critical angle of attack gentlemen, not about speed. Remember you can change the critical angle of attack somewhat by changing wing shape such as adding leading, trailing edge devices such as slats and flaps. The sail is a wing.
 

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The analogy of wind blowing over the top of the paper (a curved surface) to cause lift is incorrect.
See why here:
experiments @ sailtheory.com
That is sort of true since most lift is formed when an angle of attack is present. However lift is relative and is formed when there are two sides of unequal pressure. As you know some aircraft have laminar flow (think symmetric) and with zero AOA (angle of attack) no lift is present. Look at the keel of your boat. Symmetric, no unequal lift until you introduce it. Like when you raise your sails and now your keel tries to resist leeway.
http://www.scienceclarified.com/everyday/Real-Life-Chemistry-Vol-3/Bernoulli-s-Principle.html
 

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I have a question, related to physics of sailing

When the wind blows people go to the high side to try to bring the sail more vertical...in smaller boats they even hang out on a trapeze

I am wondering is that better than a) easing out the sails; b) reefing; c) heading more to the wind...

and why
The main reason why people move to the high side is to keep the boat flat, so that the shape of the hull that is in contact with the water is symmetrical. As the boat heels, the underwater shape becomes asymmetrical, and that creates drag. Coincidentally, keeping the boat upright also affects the direction from which the wind strikes the sails, but that isn't a principal concern.

Efficient sailing is not about maximizing sail power all the time. Efficient sailing is about using the available power efficiently, while minimizing drag. In light air, the boat is capable of efficiently using every bit of power that the sails can generate. In strong winds, the sails are capable of generating more power than the boat can use efficiently. In the latter case, the objective is to bleed off some of that excess power. Easing out the sails, reefing and heading more into the wind are techniques that you can use to reduce excess power.
 

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...Efficient sailing is not about maximizing sail power all the time. Efficient sailing is about using the available power efficiently, while minimizing drag. In light air, the boat is capable of efficiently using every bit of power that the sails can generate. In strong winds, the sails are capable of generating more power than the boat can use efficiently. In the latter case, the objective is to bleed off some of that excess power. Easing out the sails, reefing and heading more into the wind are techniques that you can use to reduce excess power.
This is traditional theory for displacement hulls, where any power in excess of what's needed to achieve "hull speed" results in loss of comfort and safety that most cruisers find unacceptable (but which racers may tolerate in the interest of getting a few hundredths of a knot extra).

However, your explanation does not necessarily apply for planing hulls, for which all power, if handled properly, could translate to more speed.

I believe the person who raised this question may have a planing boat.

As I said before, the correct answer depends on the boat, and the "correct" answer for displacement hulls may not apply to planing hulls.
 

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The main reason why people move to the high side is to keep the boat flat, so that the shape of the hull that is in contact with the water is symmetrical. As the boat heels, the underwater shape becomes asymmetrical, and that creates drag. Coincidentally, keeping the boat upright also affects the direction from which the wind strikes the sails, but that isn't a principal concern.

Efficient sailing is not about maximizing sail power all the time. Efficient sailing is about using the available power efficiently, while minimizing drag. In light air, the boat is capable of efficiently using every bit of power that the sails can generate. In strong winds, the sails are capable of generating more power than the boat can use efficiently. In the latter case, the objective is to bleed off some of that excess power. Easing out the sails, reefing and heading more into the wind are techniques that you can use to reduce excess power.
Easing out the sheet and or heading more into the wind reduces angle of attack thus reducing lift (thrust)
 
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