In the Sail Trim Users Guide, you describe and issultrate the angle of attack as the angle between the apparent wind direction and the sail chord line. From the diagram, the sail chord line is something other than the straight line between the clew and the tack, but I can't find what it is or how it's determined. Would you please provide some details? BTW, I've found the book and the chart most helpful and useful in learning a lot about sail trimming. Now all I need is to practice it.
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Paul
Chord is a straight line
I don't know how it's depicted in your reference, but the chord of an airfoil is precisely the straight line between the leading and trailing edges. Paul
I don't know how it's depicted in your reference, but the chord of an airfoil is precisely the straight line between the leading and trailing edges. Paul
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Don Guillette
Definition Of Chord line
ED: Thank you for the nice words about my book and chart. I'm glad they are helping you. The actual definition of chord line is "the measurement of a foil or sail front to back or horizontally". Like most sailing definitions , what's all that mean in simple English? Here's a simple way to figure it out and you'll be real close for what the average sailor needs to know in order to set draft depth and draft position. Measure the foot of your sail and use that as your reference. So assume the foot of your sail is 10'. Mark the halfway point (5')on the boom with a piece of black tape as a point of reference. Now, assume you want to set the DRAFT POSITION position at 50%, and the DRAFT DEPTH (belly) at 20%. 50% of 10' is 5' so that is the point where the the DP would fall. 20% of 10' is 2' so that is the amount of DD (belly) you'd have in your sail. Draft depth (belly)is pretty easy to see and understand. Draft position is a bit more difficult to see. You should review the defintion of draft position section in my book. What your looking for to determine the draft postion, is the point on the boom where the imaginary line that runs down the deepest part of the belly of the sail and intersects the boom at some point. Normally it will fall in the 40% to 60% range. With a 10' chord that would be the 4' to 6'point. If your chord was 12', the range would be 4'8" to 7'2". A purist (which I'm not - I'm just a common seaman) would use a more precise measurement because the line changes as the shape of the sail changes but the system I outline will get you into the ballpark.
ED: Thank you for the nice words about my book and chart. I'm glad they are helping you. The actual definition of chord line is "the measurement of a foil or sail front to back or horizontally". Like most sailing definitions , what's all that mean in simple English? Here's a simple way to figure it out and you'll be real close for what the average sailor needs to know in order to set draft depth and draft position. Measure the foot of your sail and use that as your reference. So assume the foot of your sail is 10'. Mark the halfway point (5')on the boom with a piece of black tape as a point of reference. Now, assume you want to set the DRAFT POSITION position at 50%, and the DRAFT DEPTH (belly) at 20%. 50% of 10' is 5' so that is the point where the the DP would fall. 20% of 10' is 2' so that is the amount of DD (belly) you'd have in your sail. Draft depth (belly)is pretty easy to see and understand. Draft position is a bit more difficult to see. You should review the defintion of draft position section in my book. What your looking for to determine the draft postion, is the point on the boom where the imaginary line that runs down the deepest part of the belly of the sail and intersects the boom at some point. Normally it will fall in the 40% to 60% range. With a 10' chord that would be the 4' to 6'point. If your chord was 12', the range would be 4'8" to 7'2". A purist (which I'm not - I'm just a common seaman) would use a more precise measurement because the line changes as the shape of the sail changes but the system I outline will get you into the ballpark.
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Rick Macdonald
Ed, this may help...
I think Paul and Don missed the point of your question. Let me give it a try... The chord line is certainly "the straight line between the leading and trailing edges", but the sail has a twist to it. This means there isn't one chord line for the sail; it changes as you go up from the boom. This leads to the definition of TWIST on page 15: "The change in the angle of the chord line of the sail to the centerline of the boat at various heights up and down the sail". So, the diagram on page 18 that you refer to has arbitrarily chosen some point partway up the sail, perhaps as an average position or maybe as the center of effort point. Even many airplane wings have twist. This is so the angle of attack is greater at the wing root and less at the wing tips. Thus when the angle of attack is too great, the wing stalls first at the root, dropping the nose. If the wing _tip_ were to stall first causing a wing to drop, you can enter a spin. On a sailboat, the reduced angle of attack at the top of the sail to used to "spill the wind" to depower the upper portion of the sail.
I think Paul and Don missed the point of your question. Let me give it a try... The chord line is certainly "the straight line between the leading and trailing edges", but the sail has a twist to it. This means there isn't one chord line for the sail; it changes as you go up from the boom. This leads to the definition of TWIST on page 15: "The change in the angle of the chord line of the sail to the centerline of the boat at various heights up and down the sail". So, the diagram on page 18 that you refer to has arbitrarily chosen some point partway up the sail, perhaps as an average position or maybe as the center of effort point. Even many airplane wings have twist. This is so the angle of attack is greater at the wing root and less at the wing tips. Thus when the angle of attack is too great, the wing stalls first at the root, dropping the nose. If the wing _tip_ were to stall first causing a wing to drop, you can enter a spin. On a sailboat, the reduced angle of attack at the top of the sail to used to "spill the wind" to depower the upper portion of the sail.
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Alan
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Don Guillette
Definition Of Chord Line
Paul: The way I depicted it is exactly as you describe as a straigth line between the leading edge to the trailing edge but it is hard for beginners to intermediates to see it that way so that is why I just use the length of the foot of the sail. They have the frame of reference along the boom and I get them to adjust the draft position and draft depth using that. Later on, when they gain more experience and see sail trim more clearly, I would tell them what I told you originally was not exactly correct. The draft position should be a bit more here than there.
Paul: The way I depicted it is exactly as you describe as a straigth line between the leading edge to the trailing edge but it is hard for beginners to intermediates to see it that way so that is why I just use the length of the foot of the sail. They have the frame of reference along the boom and I get them to adjust the draft position and draft depth using that. Later on, when they gain more experience and see sail trim more clearly, I would tell them what I told you originally was not exactly correct. The draft position should be a bit more here than there.
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