Over time man has used many materials to harness the power of the wind to propel boats through the water – wool, leather, flax and cotton being the most common. For larger ships, flax and cotton became known as the best yarns to make sailcloth – and they were used on Square Riggers, Clippers, and J-Class racing yachts.
Cotton and flax both have short fibre length (5-8cm), so had to be spun and twisted to make the yarns long enough to be woven. Stretch was a continual problem, and a sail may have been re-cut many times in its life. Water could rot the fibres, and shrink the sail. Much care and attention had to be lavished on these sails to keep them at their best.
Synthetic fibres were introduced in the 1950s with Nylon being introduced for spinnakers, and then Polyester (Dacron, Terylene, etc).
Most recently Aramid, Dyneema and carbon fibre yarns have been used in woven form.
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 in the diagonal across the fabric. The way to minimise this is to cut the sail in a radial fashion, directing the fabric along the lines of stress, so reducing the strain.