Sailcloth is the material from which a sail, on any of the various types of sailboats, is constructed. Modern sails are typically made with synthetic fibers, unlike the naturally derived cloth canvases that were employed earlier in history. Modern sailors and sail makers can choose from a wide selection of material, including nylon, polyester, and various polymers. Fabric properties, including elasticity and strength, are considered in each selection.
Historically, sailcloth was woven from linen, a substance produced by the flax plant. Linen is strong but also very heavy, and by the end of the 19th century, cotton was used in its place. The United States was one of the first countries to incorporate cotton into sail making. The difficulty of importing linen during wartime and the abundance of cotton within the country this change necessary.
Cotton is substantially lighter and more flexible than linen, but linen is stronger. Neither cotton nor linen, however, is very resistant to ultraviolet (UV) light. Both are also quite susceptible to water absorption and subsequent rot. These materials were not expected to have a long life, especially on the open seas, where environmental forces were largely unpredictable and the sun’s UV light was constant.
Synthetic materials emerged after the age of automotive watercraft had already begun and have become the most popular for sail making. Nylons, most commonly used in spinnaker sails that travel with the wind, are strong, lightweight, and have a high stretch factor. Two types of polyester are used, polyethylene terephthalate (PET) and polyethylene naphthalate (PEN). PET is durable and cost efficient, while PEN offers less stretch than PET. Polyethylene terephthalate is the most popular fiber choice for sailcloth, and its composition can be slightly altered to add more strength and prevent breakage.
Kevlar® is the most common sailcloth choice for racing craft, as it is stronger than PET and has about five times the stretch resistance. P-phenylene-2, 6-benzobisoxazole, also known as PBO or Zylon®, is a strong polymer that offers a substantial increase in performance over most Kevlar® sailcloths. It is a liquid crystal polymer (LCP), which are known for their superior strength and inherent heat and flame resistance. PBO, however, has a worse UV durability rating than Kevlar® and is one of the most expensive sailcloth fibers.
Spectra, an ultra high molecular weight polyethylene, that improves upon PBO’s UV resistance and price, is slightly less favorable in its stretch resistance and strength. Dyneema® is similar in composition to Spectra. It is also slightly more resistant to creep, the tendency of a sail to stretch and lose its elasticity over time.
Carbon fiber is relatively new in the sailcloth market and has become a popular choice despite its high cost. Its desirable properties include great strength, complete resistance to the effects of UV light, and low stretch. Carbon fiber can be crafted into numerous compositions, maximizing a specific property to fit each vessel's needs. It is, however, still susceptible to loss of flexibility over time.