Easing the mainsheet in a gust has a cross-purpose. When the clew moves up and leeward three things happen. 1 Power down: overall angle of attack decreases 2 Power down: sail twists, lowering angle of attack at the top 3 Power up: Slackened clew increases depth.
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The depth increase is negated by the change
in Angle of Attack. Depth has no power if the viewed angle is reduced.
in Angle of Attack. Depth has no power if the viewed angle is reduced.
Not necessarily .....
When the clew raises, the maximum depth (in the bottom panels) increases and the luff takes on a more rounded (powered-up) shape ... and if the sail is twisted more vulnerable to instabilities of airflow attachment (stagnation stalls as well as separation stalls). Its a certainty that ANY adjustment along one edge of a sail (here, the clew rising which produces a slack leech) ALWAYS results in the other edges have some shape response..... Inotherwords, if you want 'perfect'/good sail shape you cant simply make ONE adjustment - you always have to make two or three adjustments (of edge tension) to get the target shape. So, with a vang system to keep the clew at the same level the sail will keep essentially the same shape and not 'power-up' as when without a vang..... and then just the angle of attack changes (as you 'blade-out' the main).
When the clew raises, the maximum depth (in the bottom panels) increases and the luff takes on a more rounded (powered-up) shape ... and if the sail is twisted more vulnerable to instabilities of airflow attachment (stagnation stalls as well as separation stalls). Its a certainty that ANY adjustment along one edge of a sail (here, the clew rising which produces a slack leech) ALWAYS results in the other edges have some shape response..... Inotherwords, if you want 'perfect'/good sail shape you cant simply make ONE adjustment - you always have to make two or three adjustments (of edge tension) to get the target shape. So, with a vang system to keep the clew at the same level the sail will keep essentially the same shape and not 'power-up' as when without a vang..... and then just the angle of attack changes (as you 'blade-out' the main).
Thoughts
1: Decrease inangle of attack. Obviously power-down IF you are trimmed correctly - if over-trimmed you may find yourself heading into the sweet spot? 2: Twist: same concept. This seems the be the one the vang would apply to, not #3. There's flat (no twist), not enough to just enough twist, or too much twist. Flattening the sail or letting the leech twist off are both ways of depowering - I don't think you can do both without changing the mast bend relative to the sail's luff curve. If this case, we're easing the mainsheet to depower, but whether or not the boom actually rises is a factor of lots of other stuff (traveler and vang settings + surely more) 3: This sounds to me like the sail as a whole stretching in response to the wind pressure, including stretching the outhaul and halyard (mainly outhaul). The whole sail gets baggier, which powers up at some angle of attacks and powers down in other angle of attacks. This is just me thinking it over- I'm no expert!
1: Decrease inangle of attack. Obviously power-down IF you are trimmed correctly - if over-trimmed you may find yourself heading into the sweet spot? 2: Twist: same concept. This seems the be the one the vang would apply to, not #3. There's flat (no twist), not enough to just enough twist, or too much twist. Flattening the sail or letting the leech twist off are both ways of depowering - I don't think you can do both without changing the mast bend relative to the sail's luff curve. If this case, we're easing the mainsheet to depower, but whether or not the boom actually rises is a factor of lots of other stuff (traveler and vang settings + surely more) 3: This sounds to me like the sail as a whole stretching in response to the wind pressure, including stretching the outhaul and halyard (mainly outhaul). The whole sail gets baggier, which powers up at some angle of attacks and powers down in other angle of attacks. This is just me thinking it over- I'm no expert!
Another factor you can add to this discussion is..
apparent wind. As the gust hits the boat may accelerate, changing the apparent wind angle, which affects all the other variables discussed already. Not to mention, that the gust itself may indeed hit from a different angle. Oscillating winds are the norm.
apparent wind. As the gust hits the boat may accelerate, changing the apparent wind angle, which affects all the other variables discussed already. Not to mention, that the gust itself may indeed hit from a different angle. Oscillating winds are the norm.
Another factor to consider
Another factor to consider as decreasing power of the main is related to fluid dynamics. As you ease the main, and begin to luff, the flow of air becomes "disturbed" over the air foil reducing "lift".
Another factor to consider as decreasing power of the main is related to fluid dynamics. As you ease the main, and begin to luff, the flow of air becomes "disturbed" over the air foil reducing "lift".
yes and no (in relation to the main question)
Introducing twist won't necesarily power down the sail, especially during a wind shift. Which is why it's better to ease the travelor than the mainsheet in a gust. Keeps the sail flat. Not sure what you mean by "slackening the clew," but when the clew/boom rises it depowers the sail. In pre-vang rigs, in other words in rigs with heavy wooden booms that didn't need a vang, the boom would raise in a gust and naturally de-power the sail. I've seen this happen on an old Cheoy Lee ketch, and it's really something to watch because both booms depower themselves simultaneously. Not that I'd want to go back to wooden booms.
Introducing twist won't necesarily power down the sail, especially during a wind shift. Which is why it's better to ease the travelor than the mainsheet in a gust. Keeps the sail flat. Not sure what you mean by "slackening the clew," but when the clew/boom rises it depowers the sail. In pre-vang rigs, in other words in rigs with heavy wooden booms that didn't need a vang, the boom would raise in a gust and naturally de-power the sail. I've seen this happen on an old Cheoy Lee ketch, and it's really something to watch because both booms depower themselves simultaneously. Not that I'd want to go back to wooden booms.
Sail response
Thanks for all the great input to my question. “Slackening the clew "meant, as the clew rises the distance between the clew and the luff becomes shorter, lowering tension. The effect becomes less relevant as you move down the mast. I am trying to get the basics of how this thing works. Assuming all parts of the sail are pulling, when won’t increasing the variation of angle of attack top to bottom (twist) power down a sail during a gust? Higher winds have even less gradient
Thanks for all the great input to my question. “Slackening the clew "meant, as the clew rises the distance between the clew and the luff becomes shorter, lowering tension. The effect becomes less relevant as you move down the mast. I am trying to get the basics of how this thing works. Assuming all parts of the sail are pulling, when won’t increasing the variation of angle of attack top to bottom (twist) power down a sail during a gust? Higher winds have even less gradient
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