Many materials have been used to harness the power of the wind to propel boats through the water – wool, leather, flax and cotton being the most common historically.
For larger sailing ships (square rig naval and merchant ships, clippers etc), flax was the most used being tougher and heavier. For yachts cotton became known as the best yarn to make sailcloth – it was lighter and could hold its shape better than flax. Cotton and flax both have short fibre length (staples – flax about 8 to 10cm and cotton about 5 to 8cm), so these staples had to be spun and twisted to make the yarns long enough to be woven. Stretch was a continual problem, and a sail would have to be re-cut many times throughout its life. Water would eventually rot the fibres, and shrink the sail.Much care and attention had to be lavished on these sails to keep them at their best.
The upsurgence in the plastics industry from the time of World War II lead to the production of synthetic fibres. First was the silk replacement called Rayon and although spinnakers were made and tested, it was a little weak for this kind of use. However in the 1950s Nylon was invented and used very quickly in spinnakers and then Polyester (Dacron, Terylene, etc) found a extraordinarily quick adoption into for and aft sails. Not only were these yarns stronger than the natural fibres, but they were of infinite length, so the yarn filamants needed spun only for appearance, not to give strength.
More recently Aramid, Dyneema and Carbon fibre yarns have been used in woven form. This woven form has been found not to be the best way of getting the most from these very strong and resilient yarns. Woven fabric, whilst strong enough along its length and width, has a weakness in its diagonal direction. Since sails are triangular, some loads somewhere must go diagonally across the fabric. The way to minimise this is to cut the sail radially directing the fabric along the lines of stress, so reducing the strain. But weaving is less suited to have the yarns that run along the length of the fabric being the ones taking the largest loads.