Traveller vs. mainsheet
What has never been clear to me is why pulling the traveler to windward would not lessen the downward pull of the mainsheet and therefore increase twist while moving the traveler to leeward would not do the opposite. If so, is the traveler controlling twist??
As pointed out by another poster, the mainsheet and traveller are independent controls, and for close reaches to close hauled points of sail, do not affect each other.
If you have a mainsheet and traveller, when you're close hauled, and you pull on a lot of mainsheet, it will pull the boom down, and tighten the leech, and this will reduce twist at the top of the sail. As the boom end and the mainsheet attachment point to the traveller are closer together (through the arc of the boom), the primary component of force of the mainsheet is to pull the boom DOWN.
But at the same time this is happening, the boom will still angle out, and the mainsheet will be at an angle. It still won't be straight up and down from mainsheet boom attachment point to mainsheet traveller attachment point, because there is a vector of force blowing the sail sideways. And because the mainsheet isn't rigid. So it essentially blows off of the 90º of the mainsheet to horizontal reference ON THE BOAT not the water (can't reference the water as horizontal because of boat heeling. The traveller track itself is the horizontal reference point.) Maybe the mainsheet makes a 95-100º angle to horizontal. Consequently, if the traveller is centered, the boom is also able to blow off the centerline of the boat, due to the mainsheet angling sideways. (Ok, for boats like Hobie 16s that are sailed mainsheet tight block to block, this doesn't apply. But on your big monohull boat, is your traveller as high as the boom, or boom as low as the traveller? I doubt it. No block to block there...)
So, you can use the traveller to accommodate for this angle offset, and pull the traveller to windward, thus allowing the boom to be centered along the boat centerline. Because the mainsheet is controlling downwards pressure on the leech, it is setting the amount of twist, and because you have NOT touched the mainsheet adjustment while moving the traveller, you have not affected twist. Though you have changed the angle of attack using the traveller.
This is why, for close hauled, anyway, the mainsheet controls the leech tension and thus the amount of twist, and the traveller controls the angle of attack.
If you are further from the wind than, say, a close reach, yes, both the mainsheet and the traveller can affect the angle of attack. This is because as you let the mainsheet out, and the boom arc swings further away from the traveller (greater mainsheet angle to horizontal) there is less of a component of force pulling the boom down, and more of a component of force pulling the end of the boom through its arc. Of course, in situations like a beam or broad reach, due to limited traveller "throw" if you will, it won't affect angle of attack AS MUCH as letting out the mainsheet will.
Further to this, one could make the argument that the boom swings in an arc (and the mainsheet attachment point to the boom swings in an arc) but the traveller is a straight piece of track, and the mainsheet attachment point to the traveller does not swing an arc. Thus, the more you move the traveller off of centerline, it actually diverges aftwards of the attachment point of mainsheet to boom, and thus applies more tension to the mainsheet. Ok, yes, this would be true. Some very high performance racing boats do indeed use a curved traveller track to mimic the arc of the boom. But honestly, for most boats, especially cruising boats or cruiser/racers, sail trim just doesn't need to be that precise, and straight travelers are more than adequate to assist in proper twist adjustment and angle of attack adjustment. Seriously, most people are using double braid polyester as a mainsheet, and the amount of stretch in polyester makes this hairsplitting a moot point.
Hope this helps,
Brian
What has never been clear to me is why pulling the traveler to windward would not lessen the downward pull of the mainsheet and therefore increase twist while moving the traveler to leeward would not do the opposite. If so, is the traveler controlling twist??