As you may have read in Edwards article on Austin Farrar, modern sailcloth was born out of the enthusiasm of small boat racing in the post war era. The search for more stable materials to enable better sail shape control became the key development area.

Sailcloth can be divided into several specific groups and within those groupings there are many variations on the theme.

Below I try to describe some properties of different yarn types, but there are also different methods of constructing these yarns into sail cloth.

Woven method is nearly as old as time and needs no explanation, but the quality of the weave, the finish used and yarn used of course dictates its properties. Two further basic categories can be explained as, coated finish and impregnated finish.

Impregnated finish, is effectively bathing the woven yarns in a type of glue, this bonds the yarns together. This typically gives us a Fill to Bias ratio of 1:6 so the bias stretches 6 times more than the fill.

Coated finish cloth, has a chemical finish applied to the surface of the woven yarns. This allows less movement between the yarns. Some manufactures impregnate the cloth before coating to give less movement. This locks in the crimp as well as stabilising the bias. Typical coated ratio can be as low as 1:2

Woven technology mainly centres on polyester and nylon.

Laminated cloths are formed by taking a scrim or yarn ‘set’ and sandwiching them between two layers of film. Further to that you can add to the lamination, woven “taffetas” to add to the durability. Full on race laminates tend to be film on both sides. Cruise and cruise/race laminates have taffetas on one or both sides to add durability but this also adds weight with little increase to initial performance.

Laminates have and still use many scrim/insert material types. The film is nearly always polyester but Pentex and Capton have been tried. Capton is extruded Kevlar and is so expensive NASA thinks twice before using it!

I will try to explain some different applications for the materials another time.

Below, starting from the highest stretch to the lowest stretch materials:

Nylon

Sub categories -- coated and impregnated.

Used mainly for spinnakers, generally available in weights from 30 grams/meter squared up to 150 g/m2

Polyester – often referred to as Dacron®.

Sub categories -- coated, impregnated and laminated. Polyester is also known as Mylar when extruded as a film.

Available in many forms, from 32 g/m2 for spinnakers, and up to 500g/m2 for large cruising boats mains and genoas.

Dinghy sailors will be very familiar with the coated styles known as ‘Yarn Tempered’, ‘HTP’® and ‘Polykote’®. These are very hard finished polyesters and are very noisy on windy days.

Pentex® – similar to polyester but has some difficult technical properties that don’t lend themselves to weaving. It’s more usual to see this yarn in a laminate. Pentex® or ‘Pen’ has a slightly lower stretch compared to polyester.

Aramids – often referred to as Kevlar® come in several types although there is less available now than 5 years ago. K49® and K edge® were at the lowest stretch end of Aramids used in sail cloth but now you are more likely to see K29®, HMT® or Technora® black, due to the lack of availability of others. This actually represents a backward step in terms of ultimate strength to weight ratio. I must say however, in our Instron tests, we found the quality of lamination to have a bigger effect than the difference between K49® and HMT® fibres. Technora®, although not having the best initial stretch results, is affected less by UV and flexing, so deteriorates less quickly and is my favourite aramid yarn.

Vectran® is not really an aramid but behaves like one and is an alternative, but shows no real advantage in practice. I have had some bad experiences with it splitting, so I am wary although, I do use it in rope form on my International 14 forestay!

Dyneema® / Spectra® – Hard wearing, low stretch material but is flawed by creep. This is a process by which the material will stretch a little but not recover. So the sail shape permanently changes as soon as it’s loaded. Other than that it’s perfect for sailcloth!

It’s usually used in high-end cruising applications where ultimate strength and longevity are more important than fine shape control. It’s very light and strong, takes flexing well and has excellent UV properties.

PBO ZYLON® – When this stuff hit the scene I thought Kevlar® would be ousted as the racers material of choice. It has stretch comparable to some better quality carbons with flex life equivalent to Kevlar®... BUT... It hates light – any kind of light and UV kills it!!! It can loose 25% of its initial strength with 1 hour of exposure to day light.

The solution was to cover it in an Orange film which cut out the light wave causing the most damage. The other downside was cost, being more expensive and less trendy than carbon saw the end of this wonder fibre in sails. It’s now mainly used in our sport for high end standing rigging and is coated to protect it from light.

Carbon – This is the term that conjures up images of race cars and space shuttles, the hull of my 14 and its mast are made from it and it has transformed our sport in many areas. However, in sailcloth I’m not so convinced the reality is in step with the hype.

Low stretch laminated sailcloth’s have in the past had wetted out scrim fibres to lock the scrim to itself and the film. This reduced “crimp,” the name given to yarn that is not 100% straight – all woven materials are crimped and is the description of bend forced into a yarn where it crosses another.

If you use flat, untwisted yarn and glue it together rather than weave it, you get the lowest stretch lay up for any given fibre. The glue stiffens the yarn and in brittle materials causes early flex failures. This was typical in the early days of using carbon in sailcloth. If you want some life from your sail, the solution has been dry lay ups and fat yarn bundles. If the yarn bundle is big the outer layer of fibre protects the inner from both UV and flex. The down side is a lower cover factor, so big gaps between the yarns. This can cause some unevenness in load distribution.

So as carbon appears in more sailcloth it tends to be dry, so we are actually seeing the stretch results worsen despite the yarns been much lower stretch. But what we do see is all this “unused modulus” been held in reserve and as the sail ages the undamaged yarn takes on the load. So strangely some carbon lay ups are outlasting Kevlar cloths, having more initial stretch but less ultimate stretch...!

Carbons are available in many types and the choice for sailcloth at the commercial end is critical to life as well as the construction method.

At the moment Carbon represents the lowest modulus material commercially available to sailmakers, but there will be others in the future that’s for sure.

In some areas we have come along way since Austin Farrah painted PU varnish onto parachute nylon after the war. But in spinnaker cloth technology we have simply refined what he did.