Sandy Goodall - Sailmaker, Designer and Sailing Principles Instructor, Victoria, British Columbia, aboard the Sif, Discussing sail construction and shape
So in the old days, sails were made of natural fiber materials, woven cloth made out of cotton or flax. The resulting material was very stretchy and prone to mildew and rot. It was difficult if not impossible to control how much the sails stretched and they became fuller and fuller with more and more curvature. These days modern sails are made of synthetic materials – quite stiff – that don't stretch nearly as much and therefore sail design and sail making has become more of a process of introducing shape into the sail, three dimensional shape, by curving these seams, curving the edges of each of these panels. So, when you put them together they are much like the steel plates on the hull of a ship where because of the edge curvature you get a three dimensional shape when you're finished.