Originally Posted by deltaten
OK... think I got it!
I can get my head around main sheet angle vis boom position being better purchase and tension w/a traveler.
I (vaguely, soon to be better) understand main sail shaping , re; keeping the boom at a given (lower) position to point closer.
I like the idea and prospect of "Travelers let you point higher, reduce weather helm, heel less and - when the main / jib is balanced even steer the boat."
Absolutely clear now,thanks all
The mainsheet controls the twist of the mainsail. Once you have set your desired twist, so that the sail has good flow, and is not stalled, you still want to be able to adjust the angle of attack of the mainsail, either to alter course, or to depower the sail by dumping a bit of wind. That is where the traveler comes in. For example, when I am sailing upwind, I get my sheet tension set, and then I have the traveler line in my hand. When a gust hits, I can ease the traveler quickly to keep the boat from heeling too much, and when it passes, a quick yank on the traveler brings the boom back to centerline again. The whole time I was in the gust, I still had optimum sail shape. Without a traveler you are messing with your sail shape every time you ease it. (in the absence of a traveler you can use a vang to try to maintain twist, since you need the sheet to control angle, but it doesn't work quite as well.)
So you are probably thinking "what is this 'twist' he speaks of?" Check out this LINK
and it might help. I would cut and paste it, but there is some great animations that illustrate really well,and I don't think they will transfer.