They don't. The lift is orthogonal to the wind direction, so it will be most efficient when the wind is directly from the side, with the forward force falling o

They don't. The lift is orthogonal to the wind direction, so it will be most efficient when the wind is directly from the side, with the forward force falling of with the cosine of the wind angle. There's really no way to change that with these types of sails. This is actually quite an important limitation for cargo ships, as wind directions on the large oceans are somewhat constant and mostly east-west. Hence these sails would be better for north-south routes.