Sail design tools these days are the computer and laser, but still require a good deal of instinctive skill. The introduction of new strong, lightweight materials has played a crucial part in the rapid development of sails. Major racing events force the development of new materials and sailmaking techniques, which benefit the rest of the sailing world.
It the past, sails were made from natural materials such as hide, flax, cotton, bamboo, coconut fibre and jute. These materials suffered from shrinkage or stretch, decayed over time or lost some of the wind's energy as the air passed through them. During the 19th century, sails made from thick and heavy flax canvas gave way to the smoother and finer fibres of cotton.
The need for better performance led sailmakers to create better sail shapes, and in turn improved construction. The old gaff mainsails were made with vertical panels whose seams provided the sail with much of its strength. When Bermudan mainsails and larger headsails evolved, this layout proved unsatisfactory.
Woven cloth is strongest along the axis of the warp' threads, or exactly at right angles, on the axis of the 'weft' or 'fill' threads. The 'bias' is weak and stretchy, when the load falls diagonally and the small squares formed by the weave turn into diamond shapes on the application of force.
To overcome this, the cloth maker tries to make the sailcloth stable by tightening the weave and by using fillers to lock the threads in position then the sailmaker has to design panel layouts in which the cloth takes the greatest strain along its strongest axis.
In the 1950s the sailmaking evolution started when synthetic cloth became available, known as Terylene in Britain and Dacron in the USA. It proved stretchy at first, but gradually became stable as makers coated it with polyurethane filler. This had effect of giving a hard finish to sails, which were difficult to handle and did not remain in good condition.
Kevlar introduced in the 1970s, was a much stronger and stiffer fibre at which time sailmakers began utilize computers to produce efficient sail shapes and layouts. This encompassed some very complex sails, which had seams radiating from the stress points.
Experiments began in the 1980s with composite sails where the basic shapes were created using panels of a non-woven plastic, such as Mylar, where reinforcing threads of glass fibre or Kevlar are bonded and applied in a complex pattern that reflects the stresses that the sail undergoes in use.
The basic aerofoil shape was produced previously by rounding the luff (leading edge) of the sail and by tapering the seams between panels where today the outline of each panel is calculated by computer and cut by a laser knife, which produces exactly the intended shape.
The sailmaking evolution continued into the 1995 America’s cup series whereby the whole sail is constructed in one piece over a mould, the same as glass fibre is laid up in a mould to produce the hull of a boat. Called 3DL sails, they are made by laying threads of synthetic yarn over a mould and then securing them in place by bending on a layer of non-woven plastic. There are no seams, no cloth and the shape is totally 'built-in'.
These sails are of interest to the top echelon of yacht racers, but new materials such as Spectra and Vectran have resulted in cruising sails that are strong, light, soft handling and last for many years. This is important when many cruising yachts have sails that roller-furl and are never removed from the boat.
Lightweight nylon woven into rip stop' cloth coming in a variety of bright colours is the basis of spinnaker construction. Cloths range from the lightest 'gossamer' cloths that weigh as little as half an ounce per yard to a typical all-purpose spinnaker for a 30-footer using a 1.5 ounce cloth. Different shapes and spinnaker construction form a spinnaker designed for running, reaching, or all-purpose use.
Newer racing classes use an asymmetric design, which combine the features of a genoa and a spinnaker and are very effective for high-speed reaching. Asymmetric spinnakers do away with the traditional spinnaker pole and have their forward corner (the tack) attached to a bowsprit with sheets attaching to the clew to either side of the boat, like a foresail.
This method works very well on lightweight dinghies that efficiently sail downwind on a series of reaches but larger boats are required by tactics to sail a square run where a conventional spinnaker and pole is found to be superior.