dustoff wrote:I have one question for Judy B.
Shouldn't you want to decrease mainsail twist in heavy winds? Meaning you would move the traveler to LEEWARD to have the mainsheet pull more vertically on the boom.
That's a great question. I think the answer is" it depends"..... on whether you're underpower or overpowered at the time the gust hits.
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When overpowered....
In this thread we're focused on how to sail when you're overpowered. When you've already reefed, and you're still overpowered, you can depowering the top of the sail by luffing it. (twisting it off excessively to decrease the angle of attack at the top of the sail)
When a gust hits, the apparent wind shifts aft and increases in velocity, and powers up the upper section. That may sound like the wrong thing to do, but.... if you hadn't twisted it off to luff it, it would have a too-sharp angle of attack and be overtrimmed. This causes increases drag vs lift, and heeling rather than driving the boat forward.
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When Underpowered....
However, if you were underpowered, with normal twist in the sail (2nd batten parallel to the boom) at the time the gust hit, you could profitably harness the extra power of the increased wind velocity. As the apparent wind angle moved aft with the gust, iIt would be appropriate to ease the traveller to adjust the angle of attack while maintaing the same amount of twist (due to constant leech tension )
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Twist is controlled by changing the tension on the leech. If you increase leech tension by pulling down on the clew more, you reduce twist
The traveller controlls the angle of attack on the mainsail, without changing the twist. Steering is also a way to control angle of attack without changing the twist.
The easing the mainsheet changes both angle of attack and twist.
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Here's another point to consider, which I oversimplified in my previous post:
Jib and mainsail: two foils or one big foil?
IF THE MAINSAIL *IS* IN THE DOWNWIND AIRFLOW OF THE JIB, THE TWO SAILS ACTS AS ONE FOIL
When you're pointing close hauled, the two foils act as one big foil . The jib acts as the leading edge and the aft part of the mainsail acts as back part of the foil.
In this case, easing the mainsail via traveller has the effect of reducing camber in the whole foil, and changing the angle of attack of the whole foil, from the leading edge of the jib to the leech on the mainsail. If you ease the traveller just a little in a gust (I don't mean dumping it!), the airflow stays attached and the boat harnesses the increase in wind velocity and turns it into lift.
IF THE MAINSAIL **IS NOT** IN THE DOWN WIND AIRFLOW OF THE JIB, THE TWO SAILS BEHAVE INDEPENDENTLY
In this scenario, easing the traveller has the effect of reducing the angle of attack of the mainsail. And doesn't have a great effect on the action of the jib. On my Catalina, I can control both the traveller and the mainsheet from the helm. On a reach, I play the traveller in the gusts, not the mainsheet. On a point, I steer higher and ride the lift.
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There are lots of combinations of ways to use your trim controls: some affect angle of attack and twist, and that will affects lift and drag. Others affect draft position and depth. We cruisers only have two hands so when we get overpowered, so we have to choose judiciously amongst all the available options. (or four hands if you have knowledgeable crew)
Racing crews tend to have lots of hands working together as single organism. The helmsman steers and reads the wind. somebody plays the traveller, somebody trims the mainsheet, somebody trims the jib (and adjusts the cars on the jib to affect twist in the jib).... etc.
I like to think of cruising trim being the same as racing trim, just with fewer hands available, more time to respond to the wind, and a whole less adrenalin...... the theory is same...
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DISCLAIMER:
As always, I am writing this off the top of my head, and greatly over-simplifying theory (as far as I understand it) to make it "learnable". I've probably overlooked a few points, oversimplified a few, and maybe made an outright mistake. I'm sure if I've overlooked or mis-stated something, some one will notice. Everybody: PLEASE feel free to correct what I've written or add to it. I'm open to corrections!
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Fair winds,
Judy B