Oy vey! :bang: (They don't have a smiley that curls up into a fetal position.)
Don,... you still got some books for sale?
Don,... you still got some books for sale?
Let's talk twist ...
- Thread starter Scott T-Bird
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Scott,
Just to expand on my definitions.
Chord line is the line from the luff ( leading edge ) to the leach ( trailing edge ) in the forward-aft direction.
Draft depth is the distance from the chord line to the sail ( at the deepest spot ).
Draft location is the where that deepest spot is, forward or aft along the sail.
Flat = less draft depth
Full = more draft depth
I have never seen a sail with wind it it, that did not have draft in the upper sections. Unless you completely dump the wind, you should have draft. With full battens, you should consider the batten tension ( and stiffness ) as an partially adjustable outhaul for the upper sections of the sail. When we race, we try to set that tension for the expected wind strength.
The main sheet might decrease upper sail draft a little bit, because it increases the leech tension, but mostly it would pull the draft aft. This requires halyard or cuningham to pull the draft back forward.
I just reviewed my North U trim book. It makes the point that for close hauled work, keeping an untwisted main preserves pointing ability, on a reach it suggests twisting off for spilling power.
Reducing the angle of attack, reduces drag, not increases it.
My final argument ( and I am about out of thoughts here ) is that all the fast keel boats play the traveler, so it must be the right way. For fast keel boats trying to go to windward.
Just to expand on my definitions.
Chord line is the line from the luff ( leading edge ) to the leach ( trailing edge ) in the forward-aft direction.
Draft depth is the distance from the chord line to the sail ( at the deepest spot ).
Draft location is the where that deepest spot is, forward or aft along the sail.
Flat = less draft depth
Full = more draft depth
I have never seen a sail with wind it it, that did not have draft in the upper sections. Unless you completely dump the wind, you should have draft. With full battens, you should consider the batten tension ( and stiffness ) as an partially adjustable outhaul for the upper sections of the sail. When we race, we try to set that tension for the expected wind strength.
The main sheet might decrease upper sail draft a little bit, because it increases the leech tension, but mostly it would pull the draft aft. This requires halyard or cuningham to pull the draft back forward.
I just reviewed my North U trim book. It makes the point that for close hauled work, keeping an untwisted main preserves pointing ability, on a reach it suggests twisting off for spilling power.
Reducing the angle of attack, reduces drag, not increases it.
My final argument ( and I am about out of thoughts here ) is that all the fast keel boats play the traveler, so it must be the right way. For fast keel boats trying to go to windward.
Twist ..... or why allowing the boom to rise to depower a sail is not the 'best'
Twist is the constant change of needed angle of attack in a sail due to the differing wind speeds (gradient wind) immediately above the water. The gradient wind is caused by the action of friction as the airflow gets nearer to the water, the wind will be ‘faster’ further away from the water. But hold on – its not going to be ‘all that faster’ at 50-60' feet off the water. The higher the windspeed the less the ‘gradient wind’. Only when the windspeeds are ‘light’ will twist need to be exaggerated and ‘enhanced’ by the trimmer; in the higher windspeeds twist is not that important to keep a boat moving because there is LESS gradient wind effect. Typical sail design, and for ‘non-precise’ cruising sails will have a ‘inbuilt’ twist in the neighborhood of 5-10 degrees of twist 'already' designed into the sail; usually in a well designed sail to get the twist correct usually requires ‘little’ adjustment (traveler position, etc.) if the sail is otherwise properly raised and otherwise 'shaped' by the sailor.
The chief ‘control’ of twist is ‘how hard’ one pulls **DOWN** with the MAINSHEET. This puts strain along the leech and ‘twist’ is reduced, the overall sail camber increases with increasing MAINSHEET strain and the resultant is that the sail ‘powers-up’; conversely, easing the mainsheet tension ‘opens’ the leech, allows more twist aloft, can allow flow separation (flow no longer attached) at the head panels which depowers the sail especially in higher windranges which causes less heel, etc. Looking at ‘overtwisting’ in another way: the top of a sail that is ‘overtwisted’ is akin to putting in drastic ‘overdraft’ or so much draft aloft that the flow separates at the area of ‘overdraft’ .... a separation stall under high speed wind flow because the shape is ‘too round’ and the draft ‘way too much’ !!!!
The LIMIT of how much the sail can be twisted is set by the VANG and the Traveler ..... because the traveler can set to be almost directly UNDER the sail which can ‘substitute’ for the amount of mainSHEET tension. Using the traveler is faster and vastly more precise to control the amount of twist.
Setting up the CORRECT amount of twist is to get precise MAXIMUM speed/power out of a sail, is NOT a long term substitute for reefing nor flattening nor for ‘blading-out’ a sail to correct/trim for too much ‘heeling’ moment, etc.
In ‘blading-out’, an adjustment done with solely the traveler – sometimes called “‘playing’ the traveler”, the already set angle of attack ‘all the way up’ through the panels stay essentially the same or unchanged, and the sail begins to ‘stall’ under a uniform manner and at a consistent (and usually predictable) manner along the WHOLE sail.
In contrast, by allowing the boom to rise, when ‘dumping the mainsheet’ to allow more twist at the top, the leech does ‘open’ but also at the same time the lower and MIDDLE panels of the sail become MORE DEEPLY drafted (powered-up). Yes, the top panels are now ‘essentially flogging’ (reducing the heeling moment); but, the center panels are now set up to be in SECOND GEAR deeper draft and thus ‘progressively vulnerable’ to drive the boat further over onto her beam ends, if the helmsman isnt that quick or experienced ..... all set up for a ‘dumping’ if the wind is more powerful than the boat/helmsman can handle ... OR if there is a significant windshift .... because the twist (overdraft) is now down into the middle and lower portions of the panels and they are now at MORE draft than when the 'twist' was set correctly.
“Blading-out” with the traveler doesnt allow the draft to change to a more ‘powered up’ shape, the sail stays flat and is restrained from changing to a ‘powered-up’ shape .... because the boom can’t rise (even ‘rigid vangs’ will have some amount of ‘stretch’).
Twist is the constant change of needed angle of attack in a sail due to the differing wind speeds (gradient wind) immediately above the water. The gradient wind is caused by the action of friction as the airflow gets nearer to the water, the wind will be ‘faster’ further away from the water. But hold on – its not going to be ‘all that faster’ at 50-60' feet off the water. The higher the windspeed the less the ‘gradient wind’. Only when the windspeeds are ‘light’ will twist need to be exaggerated and ‘enhanced’ by the trimmer; in the higher windspeeds twist is not that important to keep a boat moving because there is LESS gradient wind effect. Typical sail design, and for ‘non-precise’ cruising sails will have a ‘inbuilt’ twist in the neighborhood of 5-10 degrees of twist 'already' designed into the sail; usually in a well designed sail to get the twist correct usually requires ‘little’ adjustment (traveler position, etc.) if the sail is otherwise properly raised and otherwise 'shaped' by the sailor.
The chief ‘control’ of twist is ‘how hard’ one pulls **DOWN** with the MAINSHEET. This puts strain along the leech and ‘twist’ is reduced, the overall sail camber increases with increasing MAINSHEET strain and the resultant is that the sail ‘powers-up’; conversely, easing the mainsheet tension ‘opens’ the leech, allows more twist aloft, can allow flow separation (flow no longer attached) at the head panels which depowers the sail especially in higher windranges which causes less heel, etc. Looking at ‘overtwisting’ in another way: the top of a sail that is ‘overtwisted’ is akin to putting in drastic ‘overdraft’ or so much draft aloft that the flow separates at the area of ‘overdraft’ .... a separation stall under high speed wind flow because the shape is ‘too round’ and the draft ‘way too much’ !!!!
The LIMIT of how much the sail can be twisted is set by the VANG and the Traveler ..... because the traveler can set to be almost directly UNDER the sail which can ‘substitute’ for the amount of mainSHEET tension. Using the traveler is faster and vastly more precise to control the amount of twist.
Setting up the CORRECT amount of twist is to get precise MAXIMUM speed/power out of a sail, is NOT a long term substitute for reefing nor flattening nor for ‘blading-out’ a sail to correct/trim for too much ‘heeling’ moment, etc.
In ‘blading-out’, an adjustment done with solely the traveler – sometimes called “‘playing’ the traveler”, the already set angle of attack ‘all the way up’ through the panels stay essentially the same or unchanged, and the sail begins to ‘stall’ under a uniform manner and at a consistent (and usually predictable) manner along the WHOLE sail.
In contrast, by allowing the boom to rise, when ‘dumping the mainsheet’ to allow more twist at the top, the leech does ‘open’ but also at the same time the lower and MIDDLE panels of the sail become MORE DEEPLY drafted (powered-up). Yes, the top panels are now ‘essentially flogging’ (reducing the heeling moment); but, the center panels are now set up to be in SECOND GEAR deeper draft and thus ‘progressively vulnerable’ to drive the boat further over onto her beam ends, if the helmsman isnt that quick or experienced ..... all set up for a ‘dumping’ if the wind is more powerful than the boat/helmsman can handle ... OR if there is a significant windshift .... because the twist (overdraft) is now down into the middle and lower portions of the panels and they are now at MORE draft than when the 'twist' was set correctly.
“Blading-out” with the traveler doesnt allow the draft to change to a more ‘powered up’ shape, the sail stays flat and is restrained from changing to a ‘powered-up’ shape .... because the boom can’t rise (even ‘rigid vangs’ will have some amount of ‘stretch’).
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Although some of these posts have errors in them these same posts do have some correct information. In looking at them from a novice prospective it's hard to know which info is correct or believable and which is wrong. This thread in particular has some wildly differing points of view and I'm hoping that by the time it comes to an end all of the misinformation can be addressed so as not to leave beginning sailors with incorrect information to use on the water.
I have to say that this discussion is an enlightening way to sort out and correct the various sailing techniques that are employed to conquer the wind. Don, I don't envy you trying to sort it all out but it is for a good cause.
I have to say that this discussion is an enlightening way to sort out and correct the various sailing techniques that are employed to conquer the wind. Don, I don't envy you trying to sort it all out but it is for a good cause.
Check out videos of wing masts on C Cats. Once they have the proper camber on the sail. (draft), the only control used is the traveler. Good video in today's Sailing Anarchy. Same goes for other catamarans. In a gust, point higher and or drop traveller. It does not increase drag.
Here's what Rich said:
Fundamentally, the traveler allows or controls the proper amount of 'twist' in the upper portions of the sail.
The probable main benefit of a traveler for a 'newbie' sailor is: It allows one to better 'head up' or 'feather' the boat in gusty or moderate/strong wind conditions ..... and without the mainsail from 'powering up' when releasing or easing the mainsheet.
Without a traveler (AND without a good vang) and when hit by a gust, one will normally ease or release the mainsheet to 'spill wind'. Easing the mainsheet will allow the boom to rise and that can radically 'power-up' the sail - increasing heel, etc. ... just the opposite reaction of what you are trying to do when releasing the mainsheet.
With a traveler (AND vang), once can simply momentarily ease or 'drop' the traveler, instead of and/or releasing the mainsheet, and keep the mainsail 'flat', without 'powering up'.
I was thinking as I was driving last night ...
I knew that Rich could not have been saying that the amount of twist is established by the traveler. So I came to the conclusion that what he meant by control is exactly how he just explained it and it became more clear. I also realized that my comments about draft at the upper portions of the sail and twist at the lower portions are not accurate and probably not pertinent. I came to the conclusion, then, that Todd's discussion with me regarding draft didn't really have much bearing and that we were splitting hairs. I came to this conclusion because Don did not include draft depth as being influenced by the mainsheet. Don says that the mainsheet affects 3 of the 4 factors ... angle of attack, twist and draft position. I was thinking that we had come to agreement over the influence of angle of attack and twist, and that draft position just wasn't very important in this discussion.
Now, though, RichH seems to be saying that draft depth is significantly impacted by loosening the leach and that powering up with the deeper draft in the mid portions overrides the depowering that occurs when twisting off at the head of the sail. I can understand this argument. I wonder if we can reconcile this between Don and Rich. Alan and Todd seem to be intuitively saying the same thing as Rich.
I am also somewhat struck with the notion that Rich, Alan, & Todd are influenced by what they say because of their experience with larger, fractional-rigged boats where the main is larger and heeling is far more influenced by the shape of the main. On my boat, with a mere 150 sf main sail and a 200+ sf genny, I don't think that the difference between playing the traveler and playing the main makes enough difference for me to distinguish. In both cases, we are still primarily adjusting the angle of attack. I have experienced that when I establish twist efficiently via positions on the traveler, the mainsheet, and the vang, we sail on our feet far more effectively under gusting conditions and we have far less need to adjust for knockdowns and far more ability to take advantage of gusts for increased speed and higher pointing.
A few other things ... Don, do you really reach for a tinney at 6:30 in the morning? I think I envy you! And Joe, why not stop being a fly-on-the-wall and join in here!
I knew that Rich could not have been saying that the amount of twist is established by the traveler. So I came to the conclusion that what he meant by control is exactly how he just explained it and it became more clear. I also realized that my comments about draft at the upper portions of the sail and twist at the lower portions are not accurate and probably not pertinent. I came to the conclusion, then, that Todd's discussion with me regarding draft didn't really have much bearing and that we were splitting hairs. I came to this conclusion because Don did not include draft depth as being influenced by the mainsheet. Don says that the mainsheet affects 3 of the 4 factors ... angle of attack, twist and draft position. I was thinking that we had come to agreement over the influence of angle of attack and twist, and that draft position just wasn't very important in this discussion.Now, though, RichH seems to be saying that draft depth is significantly impacted by loosening the leach and that powering up with the deeper draft in the mid portions overrides the depowering that occurs when twisting off at the head of the sail. I can understand this argument. I wonder if we can reconcile this between Don and Rich. Alan and Todd seem to be intuitively saying the same thing as Rich.
I am also somewhat struck with the notion that Rich, Alan, & Todd are influenced by what they say because of their experience with larger, fractional-rigged boats where the main is larger and heeling is far more influenced by the shape of the main. On my boat, with a mere 150 sf main sail and a 200+ sf genny, I don't think that the difference between playing the traveler and playing the main makes enough difference for me to distinguish. In both cases, we are still primarily adjusting the angle of attack. I have experienced that when I establish twist efficiently via positions on the traveler, the mainsheet, and the vang, we sail on our feet far more effectively under gusting conditions and we have far less need to adjust for knockdowns and far more ability to take advantage of gusts for increased speed and higher pointing.
A few other things ... Don, do you really reach for a tinney at 6:30 in the morning? I think I envy you! And Joe, why not stop being a fly-on-the-wall and join in here!
Scott, just to clarify something. The use of a traveler to address puffs applies to small and big boats alike. I race everything from Laser to Match 40's and the same trim controls apply all the way along. Admittedly a Laser doesn't have a traveler but it does have a very effective vang.
I'm willing to bet if you had a good effective traveler on your boat you would quickly adjust to using it in a puff in place of the mainsheet and discover why it is a better control.
I'm willing to bet if you had a good effective traveler on your boat you would quickly adjust to using it in a puff in place of the mainsheet and discover why it is a better control.
Scott - not to belabor the point but I think the differences of 'opinion' emanates from your sailing venue - a semi protected lake that doesnt have quite the 'fetch' of large bodies of water where the waves can build to be quite high and quite steep.
In more open fetch venues the action of the waves promote an 'oscilatory' motion of the top of the mast .... mast rocking back and forth as it travels through high waves where the boat is aggressively pitching and one minute the velocity of the top of the sail is much much higher than the wind speed, and immediately followed by the top of the mast rotating towards the stern & causing the apparent wind especially at the head to radically decelerate. With the head over-twisted when the top of the mast is oscilating 'backwards' the 'twist' gets to be non-constant due to the rapidly changing wind velocities AT near the top of the mast. In relatively FLAT water you dont get such mast top oscilations.
In more open fetch venues the action of the waves promote an 'oscilatory' motion of the top of the mast .... mast rocking back and forth as it travels through high waves where the boat is aggressively pitching and one minute the velocity of the top of the sail is much much higher than the wind speed, and immediately followed by the top of the mast rotating towards the stern & causing the apparent wind especially at the head to radically decelerate. With the head over-twisted when the top of the mast is oscilating 'backwards' the 'twist' gets to be non-constant due to the rapidly changing wind velocities AT near the top of the mast. In relatively FLAT water you dont get such mast top oscilations.
Scott
1. I only reposted what Rich had written earlier he didn't do that I did for him, because I think you got his "almost a quote" wrong in your original question on this topic. That's all.
2. If we agree on the meaning of draft depth (and I think we all do), then it is geometrically, trigonometrically and really impossible to NOT not {double nots!!} affect draft when you raise the boom, which is what loosening the mainsheet does. Raise boom, sail material goes UP with the boom, more sail material above boom, draft increases. Where does all that sail material go? Please help me understand how this cannot be so.
3. Nope - a main sail is a main sail, and the actions do the same thing. Yes, the results on a fractional rig may be more extreme because of the driver on the boat being the main, not the jib. But this argument is just as specious as claims that we dump our traveler because it's easier on big boats. Simply not the case.
4. Yes, I agree that you are changing the angle of attack, but just like the old curved C30 travelers that Don and I explained earlier, you are also changing something else that in gusts you don't want to change when you loosen the mainsheet instead of dumping the traveler.
I think we all agree that because of your traveler what works for you does so, and we're glad you've found a solution. Earlier points about the implications on the mainsail and definitions of terms still apply.
This has been fun, and I've learned a lot. Good topic, Scott, thanks.
If you raise the boom via the mainsheet, the interior part of the main goes up, forms a belly, an up and down curvature, right? But the leach of the sail also "bellies" out (up and down curvature). Therefore the chord line (luff to leach) has shifted angle, so the draft depth, which is a luff to leach curvature, not an up and down curvature, has not changed.
But when the boom goes up and the belly gets formed, doesn't the belly (up and down curvature) move towards the mast thereby changing the position of the draft depth on the chord line?
Is that not the "powering up" part?
Frank
One point that I believe has been overlooked is that each control appears to have it's own effect, but that generally the adjustment of one control necessitates adjusting others. If you let out the mainsheet and bring up the traveller, then you've induced twist and brought the angle of attack back to where it was. In that case, you have powered up. If all you do is play the sheet out, any power gain from twist is likely matched or exceeded by a loss of power from angle of attack. No?
The discussion here has been about temporarily dumping power or whether a particular adjustment would power up or power down. I think I've asked this before, but here it is again. (I was going to start a new thread to avoid forking this one up, but decided instead that it would fit here.) How do you view the difference between trimming for 'power' and trimming for 'speed'. It seems to me that both are a function of sail force as discussed in this thread. So what is the difference, if you see any at all?
The discussion here has been about temporarily dumping power or whether a particular adjustment would power up or power down. I think I've asked this before, but here it is again. (I was going to start a new thread to avoid forking this one up, but decided instead that it would fit here.) How do you view the difference between trimming for 'power' and trimming for 'speed'. It seems to me that both are a function of sail force as discussed in this thread. So what is the difference, if you see any at all?
Phil, trimming for power is done in a seaway where waves cause the boat slow down and change direction. In this case cracking open for more power will help drive the boat through. In flatter water where higher speed can be attained a flatter more efficient shape develops a greater ultimate speed.
Alan, that's about what I've heard before. Could you elaborate? Is it the change in attitude experienced by the sails as the boat pitches and crashes through waves that requires a 'power' setting? Let me put it another way. If the power setting is powerful because a large force is generated, then wouldn't that same large force lead to greater speed in flat water? On the other hand, thinking about your comment and mine together, for a given sail trim if speed increased in flat water, then apparent wind would increase and necessitate a different trim. Yes?
All told, for a given sea condition, is there any difference in trimming for a large force to power through waves versus trimming for a large force to go fast? Getting back to the thread topic, do you play your traveller in a longish period, large wave situation as the pitch of the boat changes? If you indulge me a little further I think I might understand how this particular topic appears in discussions just a little better. Thanks.
All told, for a given sea condition, is there any difference in trimming for a large force to power through waves versus trimming for a large force to go fast? Getting back to the thread topic, do you play your traveller in a longish period, large wave situation as the pitch of the boat changes? If you indulge me a little further I think I might understand how this particular topic appears in discussions just a little better. Thanks.
Phil,
Think of it as gear ratios in a car. You get more torque ( but slower speeds ) in lower gear. High gear gives more speed but lower torque.
Waves require the acceleration ability of powered up sails to punch through the waves and to keep moving.
Think of it as gear ratios in a car. You get more torque ( but slower speeds ) in lower gear. High gear gives more speed but lower torque.
Waves require the acceleration ability of powered up sails to punch through the waves and to keep moving.
Powering up and speed
When you power up the boat goes faster until you have to retrim to account for the greater speed as the angle of the apparent wind comes forward. That will result in a less powerfull trim which will either stop the speed increase or slow you down. Or you bear off to keep the same wind angle.
Nothing is free.
This is very pronounced in light airs where you always seem to be on a beat.
When you power up the boat goes faster until you have to retrim to account for the greater speed as the angle of the apparent wind comes forward. That will result in a less powerfull trim which will either stop the speed increase or slow you down. Or you bear off to keep the same wind angle.
Nothing is free.
This is very pronounced in light airs where you always seem to be on a beat.
Good points. Power because acceleration because waves keeps pushing back. I still get it, power for waves, retrim for speed when wind comes forward. However, I'm thinking of power as in physics, Work per time, or Force*dist per time. It all comes back to force. You overcome the force of pounding (decelerating) waves with a power trim. Still, you need force to overcome drag to generate speed too. That's power. You can push a skiff with a 5HP motor and putt-putt across the flat water, or you can rocket across with an 80HP drive. So really, one wants to trim for maximum force. In sea and wind conditions demanding a power trim, no doubt that trim also produces maximum force. In sea and wind conditions calling for a speed trim, no doubt that trim produces max force too, that is, max power (100HP instead of 5HP). It's silly semantics, I suppose, or maybe I'm still just blind to some aspect of the problem.
I think that most are agreeing on the effect of twist on power. What do you say about the traveller (angle of attack only) on power? I would say (based on a proper polar) that there is an angle of attack that is max power/force. If you're at that angle and bring the traveler up or down (or steer the boat up or down), then you're losing power/force, unless you change other sail controls at the same time.
I think that most are agreeing on the effect of twist on power. What do you say about the traveller (angle of attack only) on power? I would say (based on a proper polar) that there is an angle of attack that is max power/force. If you're at that angle and bring the traveler up or down (or steer the boat up or down), then you're losing power/force, unless you change other sail controls at the same time.
Phil,
I see what you are asking now. I think the thing you are missing is the shape of the force curve. A flat fine entry sail loses more force if you are not trimmed to the ideal angle of attack. A deeper sail with a rounder entry will have less force at the ideal angle of attack, but will have more force when you are just off that ideal angle.
I see what you are asking now. I think the thing you are missing is the shape of the force curve. A flat fine entry sail loses more force if you are not trimmed to the ideal angle of attack. A deeper sail with a rounder entry will have less force at the ideal angle of attack, but will have more force when you are just off that ideal angle.
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