Originally Posted by Skipper995
I have to disagree with sailingdog on one issue however. In very very light conditions the main needs to be made as flat as possible to try and squeeze as much power out of the breeze as possible. In these conditions air flow has a very hard time staying attached to the sail. Detached air flow means loss of efficiency and power. A flat sail allows the breeze to maintain nearly complete flow attachment for best sailing performance.
... the problem is, though, that the weight of the boom will close the leech of the sail off and you'll lose that attached flow because, essentially 'the flaps are on'. The topping lift's support will allow the leech to maintain its proper shape without the force of a reasonable breeze.
Today's rigid vangs can often provide the same function.