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This is a good start, but the physics of sailing is much more complex than presented in your app. For instance, a sailboat is not powered by the wind pushing o

This is a good start, but the physics of sailing is much more complex than presented in your app. For instance, a sailboat is not powered by the wind pushing on the sail. It works like this. The wind is split by the leading edge of the sail (the mast, headstay, or luff). It is split into two turbulent streams, one on each side of the sail. One stream has slightly less force on the sail so the sail begins to move in that direction. As the flow stabilizes over that side, the wind moves faster on that side than on the turbulent side. This creates lift like an airplane wing. So the wind is actually pulling the sail, not pushing it. Lift on the sail causes the boat to heel (lean over). Some of the lifting force is counterbalanced by the keel underwater keeping the boat upright. The remainder of the force pushes the boat downwind. However, the flat keel underwater prevents the boat from sliding downwind, with the result that the boat is forced forward. I recommend you read this book for your next version: http://www.amazon.com/Sail-Power-Complete-Guide-Handling/dp/0394727150 http://www.amazon.com/Sail-Power-Complete-Guide-Handling/dp/...

arnarbi 12y ago
Like an airplane wing, a sail generates most of its lift from deflection of air (air is accelerated towards "aft"), not the Bernoulli effect (pressure difference). http://amasci.com/wing/airfoil.html http://amasci.com/wing/airfoil.html > As the flow stabilizes over that side, the wind moves faster on that side than on the turbulent side. The most important rule of sail trim is to avoid turbulence on either side, __especially__ the leeward side. Laminar flow on the leeward side means the sail is deflecting air on that side as well, generating a "pull" in addition to the "push" on the windward side. The keel generates its righting moment through lift also and not only its wieght btw, since a sailboat moves through the water at a slight angle.
- theothermkn 12y ago
  > The keel generates its righting moment through lift also and not only its wieght btw, since a sailboat moves through the water at a slight angle. Actually, with the boat heeled over, the lift produced by the keel is both below the hull's center of pressure and producing lift to windward. Its lift would therefore contribute to heeling. > ...generating a "pull" in addition to the "push" on the windward side. I tried to overlook this because of the scare quotes around 'pull', but you put them around 'push,' too! :o) Probably the easiest thing to say is that the 'push' is real, and the 'pull' is just 'less push.' That is, air is still pushing on the leeward side of the sail, but with less pressure than on the windward side, and that this reduction in pressure is greater when the flow is attached, as you imply.
  - arnarbi 12y ago
    Thanks, I stand corrected! I was hoping for an actual fluid dynamics person to comment (I'm just a sailor), and your comment above is excellent. Your explanation of the keels lift countering drift, but not heeling, makes perfect sense, and resolves a long-time confusion of mine. As for "pull", I have a question: Assuming the flow is attached on the leeward side, does that side also turn flow and generate force? FWIW the words pull and push are used in sailing training to emphasize the importance of keeping the leeward flow attached when going to weather.
    - theothermkn 12y ago
      > FWIW the words pull and push are used in sailing training to emphasize the importance of keeping the leeward flow attached when going to weather. Ding ding ding! Sailing is a practical matter, for nearly all sailors. It's nearly a folk activity. The models that are used to transmit and retain that information have more to do with achieving results than with achieving theoretical understanding. The trouble only really comes in when we try to use these pragmatic models as scientific models! > Assuming the flow is attached on the leeward side, does that side also turn flow and generate force? Notionally, yes. To be theoretical, attached flow on the leeward side is going to exert less pressure than detached flow would, due to irreversibilities in the detached turbulent flow. To be even stricter, because the equations of subsonic flow are elliptical and not hyperbolic, a mess of detached flow on the leeward side is going to jam up the whole works, even upstream and to windward. The difficulty comes in when we try to mentally divide the flow between the windward and leeward sides of the sail. (Additional difficulty comes in because the "adverse pressure gradient" is essential to understanding this, but is tricky enough that it would probably do more harm than good to talk about it to sailing students. The first rule of Sailing Club is that you do not talk about the adverse pressure gradient. The second rule of....) In practice, "[k]eeping the leeward flow attached" really amounts to "not stalling the sail." This is probably more important when sailing to windward because you're running as close to a stall as you can (at as high an angle of attack as you can) in order to point as high as you can. Also, with the sheets reefed in and with the higher apparent wind, the sail is more likely to be distorted. This can result in one part of the sail being stalled. Once one part stalls, the increased pressure in that area is likely to inject turbulent flow into neighboring areas that might otherwise not be stalled. Another thing to consider is that there's something filling in that area of detached flow. Guess where it comes from? The windward side of the sail via the trailing edge. While the flow has to come off the leech in the direction of the leech, nature is a bit lest strict about what it does right after; It can get turned completely around, in short order, to fill that void behind the sail. The only thing it requires is that its turning radius not be unreasonably small. Congratulations! You've just (1) injected higher-pressure air into the lee, and equivalently (2) stopped turning the flow on the windward side. Because of (2), you can bet that there's an equivalent drop in pressure on the windward side as well. That was the pragmatic way of understanding things. Now, I'm going to break the first rule of Sailing Club. When a flow is adjacent to a surface, the flow stops at the surface. That is, a 10 knot breeze blowing over your sail comes to a halt at the part of the breeze that contacts the sail. Because the adjacent flow hasn't touched the surface, it tries to maintain its 10 knots. However, nature is sticky. Any time there are adjacent regions of flow traveling at different velocities--any time there is a velocity gradient--viscosity tries to average things out: The flow right next to the sail is nearly stopped; The bit next to that is traveling a bit faster; And so on. (Incidentally, on a larger scale, this is why the wind is stronger aloft than on the deck. You're in the boundary layer, as it's called.) All of this viscous transfer is doing work on the flow. Doing work raises pressure. Because this happens as the flow moves downstream, more work is done on the flow the farther you get from the luff, and the higher the pressure gets. I don't think I need to convince you that fluids like to go away from high pressure to lower pressure. That is, fluids like to follow pressure gradients. And adverse pressure gradient can stop, and even reverse a flow. Note that this is likely to happen at the surface in question. Typically, the flow on the windward side has a stronger favorable pressure gradient to start with, and so can tolerate the adverse contributions of the viscous boundary layer without becoming detached. Because the point of a sail is to have lower pressure on the leeward side, there's less total pressure available to start with to overcome the adverse pressure gradient, and the flow can become detached on the back side of the sail. This detached condition is called, in aeronautical terms, a "stall". Just sheet in until the telltales flutter, and then back off until they don't. If that doesn't work, play with the vang until it does. ;)
- chaotic-good 12y ago
  The most important rule of sail trim is to avoid turbulence on either side, __especially__ the leeward side. Laminar flow on the leeward side means the sail is deflecting air on that side as well, generating a "pull" in addition to the "push" on the windward side. This is the reason why sails usualy have short strips of canvas on the leeward side - to see if there some turbulence or not. Turbulence depends on sail convexity, it can be changed too, not only sail angle!
theothermkn 12y ago
This is the strangest folk non-explanation of lift I've ever encountered. > So the wind is actually pulling the sail, not pushing it. A handy distillation of fluid dynamics is, "Just like you can't push string, you can't suck fluids." That is, fluids don't resist in tension except in very particular very dynamic conditions that are wholly outside of the realms where our intuitions are developed. It looks like you've inadvertently modified the popular "Bernoulli's Principle" theory of lift, which was wrong even before you modified it. There's no real point in doing anything like a rebuttal, so let me do the best I can, instead, to explain how lift is actually developed and how that applies to the case of a sailing vessel. The fundamental thing that makes lift possible is the Kutta condition. This states that a real fluid has to leave a cusp in the direction that the cusp faces. There are complicated reasons for this having to do with velocity gradients, viscosity, radius of curvature of the flow, and even relativity, but the layperson can observe, feel, or take on faith that a flow leaves an edge in a direction parallel to the edge. This fact literally turns the flow and results in lift. (Historically and physically, we say that the Kutta condition resolves D'Lambert's Paradox via the Kutta-Jukowski theory of lift, if you want some keywords to look up more information.) So, the direction of the trailing edge of the flow turns the flow. There are two ways that we can examine the result of this: momentum or pressure. It turns out that they give the exact same result for the lift of an airfoil, but are "intuitive" in different ways. The momentum theory just says, "Lift is just a force that is accounted for by the mass of the air that you're pushing aside. This is what you typically might call 'downwash'," but a bit more mathematically. The pressure theory says, "Look, that's all fine, but that force is experienced as a pressure distribution on a surface, because the only part of the wing that's in the flow is the surface. Let's keep track of (integrate) the pressure over the surface and see what the resulting forces are," but also a bit more mathematically. So, take your pick of intuitions: Imagine a wing throwing air or fluid aside, or imagine it experiencing a pressure difference from top to bottom. They're both true, and are just different ways of accounting for the same phenomenon, different procedures for accounting for forces and masses as they interact with "control volumes," as engineers and physicists like to say. And remember that both intuitive explanations are driven by the Kutta condition, which is far more interesting, I think, than the erroneous Bernoulli explanation. So, that's lift. I turn a flow and I get a reaction force. Peachy. How does it apply to sailboats? Other than the buoyancy force and gravity, every force that we care about on a sailboat is due to the operation of a foil (hydrofoil or airfoil) within a fluid (water or air). The hull of the boat, the rudder, and the keel act like wings of (typically very low) efficiency. Their total lift acts to resist the downwind drift of the boat in the water. The sail acts as a wing. Depending on the point of sail, the lift of the wing acts more or less, literally, in the direction of travel of the boat. (Close to the wind, the majority of the lift is actually wasted in heeling the boat over and trying to slide it downwind.) I know I said that a rebuttal would be unproductive, but there are a few things I suddenly can't resist. > However, the flat keel underwater prevents the boat from sliding downwind, with the result that the boat is forced forward. The "flat keel" is a wing. It prevents this "slide" via lift. It's also not helpful to split the analysis into 'forces' and 'constraints'. The keel produces lift that is a fore that contributes to the balance of forces on the vessel, resulting in motion. > [The wind] is split into two turbulent streams, one on each side of the sail. Turbulence is a complete red herring, here. Turbulence has nothing to do with lift (other than causing stalls and complicating the analysis). > Some of the lifting force is counterbalanced by the keel underwater keeping the boat upright. Well...the center of mass of the entire boat levered, as it is, upwind of the center of buoyancy, produces a moment that counteracts the moment of the sail's lift. > For instance, a sailboat is not powered by the wind pushing on the sail. It kind of really is. It's just more complicated because analyzing fluids requires that we analyze continua, and not just discrete particles. In the end, though, sailboats sail because they exchange momentum with the atmosphere and the water through distributed forces, or pressures.
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AReallyGoodName 12y ago
the wind moves faster on that side than on the turbulent side. This creates lift like an airplane wing. There's no reason to complicate it like this. It's simply Newtons laws at work. You accelerate a mass of air in one direction, you get pushed the other. The over complicated theory you've stated there is actually completely incorrect btw. http://en.wikipedia.org/wiki/Lift_(force)#False_explanation_based_on_equal_transit-time http://en.wikipedia.org/wiki/Lift_(force)#False_explanation_...

arnarbi 12y ago

Like an airplane wing, a sail generates most of its lift from deflection of air (air is accelerated towards "aft"), not the Bernoulli effect (pressure difference). http://amasci.com/wing/airfoil.html http://amasci.com/wing/airfoil.html > As the flow stabilizes over that side, the wind moves faster on that side than on the turbulent side. The most important rule of sail trim is to avoid turbulence on either side, __especially__ the leeward side. Laminar flow on the leeward side means the sail is deflecting air on that side as well, generating a "pull" in addition to the "push" on the windward side. The keel generates its righting moment through lift also and not only its wieght btw, since a sailboat moves through the water at a slight angle.

theothermkn 12y ago
> The keel generates its righting moment through lift also and not only its wieght btw, since a sailboat moves through the water at a slight angle. Actually, with the boat heeled over, the lift produced by the keel is both below the hull's center of pressure and producing lift to windward. Its lift would therefore contribute to heeling. > ...generating a "pull" in addition to the "push" on the windward side. I tried to overlook this because of the scare quotes around 'pull', but you put them around 'push,' too! :o) Probably the easiest thing to say is that the 'push' is real, and the 'pull' is just 'less push.' That is, air is still pushing on the leeward side of the sail, but with less pressure than on the windward side, and that this reduction in pressure is greater when the flow is attached, as you imply.
- arnarbi 12y ago
  Thanks, I stand corrected! I was hoping for an actual fluid dynamics person to comment (I'm just a sailor), and your comment above is excellent. Your explanation of the keels lift countering drift, but not heeling, makes perfect sense, and resolves a long-time confusion of mine. As for "pull", I have a question: Assuming the flow is attached on the leeward side, does that side also turn flow and generate force? FWIW the words pull and push are used in sailing training to emphasize the importance of keeping the leeward flow attached when going to weather.
  - theothermkn 12y ago
    > FWIW the words pull and push are used in sailing training to emphasize the importance of keeping the leeward flow attached when going to weather. Ding ding ding! Sailing is a practical matter, for nearly all sailors. It's nearly a folk activity. The models that are used to transmit and retain that information have more to do with achieving results than with achieving theoretical understanding. The trouble only really comes in when we try to use these pragmatic models as scientific models! > Assuming the flow is attached on the leeward side, does that side also turn flow and generate force? Notionally, yes. To be theoretical, attached flow on the leeward side is going to exert less pressure than detached flow would, due to irreversibilities in the detached turbulent flow. To be even stricter, because the equations of subsonic flow are elliptical and not hyperbolic, a mess of detached flow on the leeward side is going to jam up the whole works, even upstream and to windward. The difficulty comes in when we try to mentally divide the flow between the windward and leeward sides of the sail. (Additional difficulty comes in because the "adverse pressure gradient" is essential to understanding this, but is tricky enough that it would probably do more harm than good to talk about it to sailing students. The first rule of Sailing Club is that you do not talk about the adverse pressure gradient. The second rule of....) In practice, "[k]eeping the leeward flow attached" really amounts to "not stalling the sail." This is probably more important when sailing to windward because you're running as close to a stall as you can (at as high an angle of attack as you can) in order to point as high as you can. Also, with the sheets reefed in and with the higher apparent wind, the sail is more likely to be distorted. This can result in one part of the sail being stalled. Once one part stalls, the increased pressure in that area is likely to inject turbulent flow into neighboring areas that might otherwise not be stalled. Another thing to consider is that there's something filling in that area of detached flow. Guess where it comes from? The windward side of the sail via the trailing edge. While the flow has to come off the leech in the direction of the leech, nature is a bit lest strict about what it does right after; It can get turned completely around, in short order, to fill that void behind the sail. The only thing it requires is that its turning radius not be unreasonably small. Congratulations! You've just (1) injected higher-pressure air into the lee, and equivalently (2) stopped turning the flow on the windward side. Because of (2), you can bet that there's an equivalent drop in pressure on the windward side as well. That was the pragmatic way of understanding things. Now, I'm going to break the first rule of Sailing Club. When a flow is adjacent to a surface, the flow stops at the surface. That is, a 10 knot breeze blowing over your sail comes to a halt at the part of the breeze that contacts the sail. Because the adjacent flow hasn't touched the surface, it tries to maintain its 10 knots. However, nature is sticky. Any time there are adjacent regions of flow traveling at different velocities--any time there is a velocity gradient--viscosity tries to average things out: The flow right next to the sail is nearly stopped; The bit next to that is traveling a bit faster; And so on. (Incidentally, on a larger scale, this is why the wind is stronger aloft than on the deck. You're in the boundary layer, as it's called.) All of this viscous transfer is doing work on the flow. Doing work raises pressure. Because this happens as the flow moves downstream, more work is done on the flow the farther you get from the luff, and the higher the pressure gets. I don't think I need to convince you that fluids like to go away from high pressure to lower pressure. That is, fluids like to follow pressure gradients. And adverse pressure gradient can stop, and even reverse a flow. Note that this is likely to happen at the surface in question. Typically, the flow on the windward side has a stronger favorable pressure gradient to start with, and so can tolerate the adverse contributions of the viscous boundary layer without becoming detached. Because the point of a sail is to have lower pressure on the leeward side, there's less total pressure available to start with to overcome the adverse pressure gradient, and the flow can become detached on the back side of the sail. This detached condition is called, in aeronautical terms, a "stall". Just sheet in until the telltales flutter, and then back off until they don't. If that doesn't work, play with the vang until it does. ;)
chaotic-good 12y ago
The most important rule of sail trim is to avoid turbulence on either side, __especially__ the leeward side. Laminar flow on the leeward side means the sail is deflecting air on that side as well, generating a "pull" in addition to the "push" on the windward side. This is the reason why sails usualy have short strips of canvas on the leeward side - to see if there some turbulence or not. Turbulence depends on sail convexity, it can be changed too, not only sail angle!

theothermkn 12y ago

This is the strangest folk non-explanation of lift I've ever encountered. > So the wind is actually pulling the sail, not pushing it. A handy distillation of fluid dynamics is, "Just like you can't push string, you can't suck fluids." That is, fluids don't resist in tension except in very particular very dynamic conditions that are wholly outside of the realms where our intuitions are developed. It looks like you've inadvertently modified the popular "Bernoulli's Principle" theory of lift, which was wrong even before you modified it. There's no real point in doing anything like a rebuttal, so let me do the best I can, instead, to explain how lift is actually developed and how that applies to the case of a sailing vessel. The fundamental thing that makes lift possible is the Kutta condition. This states that a real fluid has to leave a cusp in the direction that the cusp faces. There are complicated reasons for this having to do with velocity gradients, viscosity, radius of curvature of the flow, and even relativity, but the layperson can observe, feel, or take on faith that a flow leaves an edge in a direction parallel to the edge. This fact literally turns the flow and results in lift. (Historically and physically, we say that the Kutta condition resolves D'Lambert's Paradox via the Kutta-Jukowski theory of lift, if you want some keywords to look up more information.) So, the direction of the trailing edge of the flow turns the flow. There are two ways that we can examine the result of this: momentum or pressure. It turns out that they give the exact same result for the lift of an airfoil, but are "intuitive" in different ways. The momentum theory just says, "Lift is just a force that is accounted for by the mass of the air that you're pushing aside. This is what you typically might call 'downwash'," but a bit more mathematically. The pressure theory says, "Look, that's all fine, but that force is experienced as a pressure distribution on a surface, because the only part of the wing that's in the flow is the surface. Let's keep track of (integrate) the pressure over the surface and see what the resulting forces are," but also a bit more mathematically. So, take your pick of intuitions: Imagine a wing throwing air or fluid aside, or imagine it experiencing a pressure difference from top to bottom. They're both true, and are just different ways of accounting for the same phenomenon, different procedures for accounting for forces and masses as they interact with "control volumes," as engineers and physicists like to say. And remember that both intuitive explanations are driven by the Kutta condition, which is far more interesting, I think, than the erroneous Bernoulli explanation. So, that's lift. I turn a flow and I get a reaction force. Peachy. How does it apply to sailboats? Other than the buoyancy force and gravity, every force that we care about on a sailboat is due to the operation of a foil (hydrofoil or airfoil) within a fluid (water or air). The hull of the boat, the rudder, and the keel act like wings of (typically very low) efficiency. Their total lift acts to resist the downwind drift of the boat in the water. The sail acts as a wing. Depending on the point of sail, the lift of the wing acts more or less, literally, in the direction of travel of the boat. (Close to the wind, the majority of the lift is actually wasted in heeling the boat over and trying to slide it downwind.) I know I said that a rebuttal would be unproductive, but there are a few things I suddenly can't resist. > However, the flat keel underwater prevents the boat from sliding downwind, with the result that the boat is forced forward. The "flat keel" is a wing. It prevents this "slide" via lift. It's also not helpful to split the analysis into 'forces' and 'constraints'. The keel produces lift that is a fore that contributes to the balance of forces on the vessel, resulting in motion. > [The wind] is split into two turbulent streams, one on each side of the sail. Turbulence is a complete red herring, here. Turbulence has nothing to do with lift (other than causing stalls and complicating the analysis). > Some of the lifting force is counterbalanced by the keel underwater keeping the boat upright. Well...the center of mass of the entire boat levered, as it is, upwind of the center of buoyancy, produces a moment that counteracts the moment of the sail's lift. > For instance, a sailboat is not powered by the wind pushing on the sail. It kind of really is. It's just more complicated because analyzing fluids requires that we analyze continua, and not just discrete particles. In the end, though, sailboats sail because they exchange momentum with the atmosphere and the water through distributed forces, or pressures.

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AReallyGoodName 12y ago

the wind moves faster on that side than on the turbulent side. This creates lift like an airplane wing. There's no reason to complicate it like this. It's simply Newtons laws at work. You accelerate a mass of air in one direction, you get pushed the other. The over complicated theory you've stated there is actually completely incorrect btw. http://en.wikipedia.org/wiki/Lift_(force)#False_explanation_based_on_equal_transit-time http://en.wikipedia.org/wiki/Lift_(force)#False_explanation_...