You guys are killing me. How can I get any work done?
Found some more info on the subject
This image illustrates the change in wind speed as a function of height in different conditions. The 1st, nearly linear line is for wind speeds less than about 5kts.
Point is that optimal trim angle at the bottom of the sail would be different than the top - thus the need to allow twist and open the top portion of the leach.
Once the breeze goes turbulent, then you get the curves to the right in the above image, and no need for twist until you're overpowered.
Some good articles here:http://www.ockam.com/2013/06/02/wind-shear/http://www.skysailtraining.co.uk/sail_trim.htm
(the bottom section)
Again, these are my theories that I've assembled based on tidbits of information from various sources - including Bethwaite's book, coaches, and "common sailor knowledge":
I don't think twist and light air are as directly related as many believe. In smooth water and light air, I don't see any reason to put a significant amount of twist in the main. There are some apparent wind effects due to the difference in velocity that will want to see a little twist but it's not much. You might also want a little twist on a jib/main boat because the jib will change the angle of the breeze that flows around the bottom of the main while the top needs to realize a different, unaffected, breeze angle. However, there are differences in the shape of the mainsail cut shape from top to bottom that help accommodate the affect of the jib - so that difference needed in trim angle from top to bottom due to the jib effect is pretty small. While the velocity may be different from top to bottom, the angle of the wind isn't different by much - so you are not necessarily looking for a lot of twist in the main just because the wind is light.
However, when you introduce wave chop onto the water surface, the boat starts to swing back and forth like a pendulum (aka on my boat as "tomahawking"), THIS is where twist really starts to become important. As the rig swings back and forth, it's seeing large changes in velocity and apparent wind angle. You couldn't possibly trim fast or accurately enough to account for this so introducing a little more twist than usual will at least ensure that some portion of the sail is properly trimmed at any point in that swinging motion.
The tradeoff to more twist, however, is that the fuller sail creates more drag and reduces pointing angle. As the chop increases at higher wind speeds, a fuller sail, over normal trim shape, is beneficial to help provide a little more driving force to keep speed up through the waves and you can make up for the loss in efficiency.