If you want to think about how it works, think about the fact that the roach is supported by the battens. The battens are supported by the line of tension that runs from the clue of the sail to the head of the sail — from the end of the boom to the top of the mast. Picture stretching a tight line, a tight rope, between those two points, and the battens push against that rope. That’s what supports everything aft of that line.
The more stuff you get outside of that line, the more tendency there is for the sail to twist and open up. Square-top sails have so much area out there, the challenge is minimizing twist so that you have an appropriate profile up the sail. As you go higher off the water, the wind velocity is higher, so you want a certain amount of twist in a leech. On the triangular-sail boats, we’re working hard to introduce that twist. In light air especially it can be quite challenging to get enough twist in the leech of a triangular sail. But with a square-top main that’s not the challenge at all. The challenge is actually managing the twist, keeping the sail from twisting too much.