Advances in modern sailmaking began with the implementation of polyester and nylon fibre being made into woven sailcloth...

For its weight, polyester fibre has more than ten times the stretch resistance of cotton. This gives sailmakers the ability to produce light-weight, high-strength sails - while nylon's major benefit is its significantly high tenacity, or resistance to tear strength - allowing light-weight, durable spinnakers.

One, seldom mentioned, real benefit of Dacron® and nylon is the ability of both these fibres to shrink with the application of heat and also their ability to shrink by up to 20 per cent after weaving, which tightens the woven construction. After the weaving process, the woven fabrics are know as "taffetas" in their unshrunken state. These woven taffetas are put through a series of processes, commonly known throughout the sailcloth industry as finishing. Combinations of heat, tension and pressure are utilised to shrink the fabric to stabilise the woven construction.

As a further effort for lower stretch and improving stability, resins are sometimes added to fill the spaces in an attempt to further minimise stretch. The problem is that they break down when the sailcloth is subject to loadings that the resin cannot overcome - breaking the resins loose, and resulting in a permanent change in the sail shape. Experience has shown that resins are effective for sailcloths that have a small range of wind loadings and are suitable for small, one-off designs. With larger boat sails, which have a higher range of wind loadings and require a much longer life, we find that resins break down quickly; after this occurs, the sails are reliant on the stability of the woven construction to maintain shape. Research and development has been continuing to find methods of producing fabrics that will yield stability at as light a weight as possible.

Laminates In the mid 1970s, Mylar® (or polyester film) was laminated to woven polyester to lock or stabilise woven constructions. This early sailcloth was either heavy and stiff to handle for its intended wind range or light weight and subject to low tear strength and low durability.

For sailcloth purposes, Mylar® films are less than 3 mils in thickness. [A mil is roughly equivalent to less than one ounce per sailmaker's yard (SMY)]. Films above this thickness become very stiff, and when folded, develop a permanent crease. Most laminated sailcloths use 1.5 or 2 mils in various combinations. The problem with Mylar® in these thicknesses is that, although stable at low loads; if subjected to more, they yield - which creates permanent distortion in the sail. As the Mylar® begins to yield, more of the load is shifted to the woven polyester which is usually of insufficient weight or stretch resistance on its own to prevent further distortion, usually resulting in delamination as the Mylar® fails.

Mega-yachts Until the early 1980's there were very few sailing yachts over 80ft. With advent of Masts, electric hydraulic furling systems, and winches, sails could be made larger without the need for large crews. Yacht owners the world over have been building increasingly larger sailing vessels which have continued to push the technology of sailcloth. Today, yachts over 90ft have become fairly common, and large ones now exceed 160ft. Most are sloop rigged because of the basic benefit of simplicity. However, this means that sails are now larger than ever. Owners are also demanding that yachts perform better on all points of sail while wanting to minimise the number of sails and crew they can carry. A typical 135ft mega-yacht will have a 130 per cent roller-furling genoa that is capable of being roller reefed down to 90 per cent and must set properly when full set or reefed. It is also expected to provide at least 3-5 years service before needing replacement depending on itinerary. These sails typically have sheet loads exceeding 20 000 pounds - over five times the sheet loads on an 80-footer. To date, radially cut Spectra® laminates have proven the best compromise between weight, stretch resistance, and durability. With the sails getting ever larger, new problems have developed. All is relatively fine when the sails are sheeted, fully set, three corners out. But, when reefing, the rolling of these built-up layers creates a tremendous disparity of load between the outer layers and the inner one. The layers on the outside of the roll are in tension, taking all the load while the inner ones are actually in compression, due to the smaller circumference under which they are forced to travel. This action causes two problems. First, the sail is reduced in strength to only that of the stretch resistance and load carrying capacity of the layers which are under load. Unless they can stretch there will be no transmission or contribution of strength from the other layers. Secondly, the Mylar®, being forced to share the load, yields - causing delamination. Another problem with laminates is that because they are build of combinations and layers of different materials (each having a glue joint to adhere one component to the next) the building of high strength, low stretch makes them heavy because each glue joint weighs approximately an ounce per yard. The increased stiffness created by making them thin and layering aggravates delamination.

VECTRAN® In an effort to find a solution for the problems that could not be overcome with laminated sailcloth, Hood began work with VECTRAN® fibre in 1992. VECTRAN® has similar flex fatigue characteristics to Spectra®, but unlike Spectra® has high heat resistance. This allows it to be woven with polyester, and through a special patented process, using a much higher tenacity (stronger polyester stretch resistant) polyester yarn can be shrunken around the VECTRAN® fibre. Achieving a very low stretch non-laminated woven sailcloth, as the stretch resistance is so improved, the true weight of the sailcloth can be reduced for a given job compared to a simple polyester version and, in some cases, compete with laminates on a weight-for-weight basis.

As the combination of VECTRAN® and high tenacity polyester allows for a simple woven product, so can the simplicity of construction revert back to crosscut horizontal panelling: a significant point when viewing roller furling sails - whether it be furling headstays in mast or more importantly "in boom" furling mainsails - where partial reefing of the sail is common place. A real problem with the aforementioned warp-oriented fabrics with radial construction is the off-thread-line loads applied once the sail has been reduced in area from three corners out. With simple crosscut sails, the VECTRAN® fibres in the fill or weft direction of the fabric continue to take the loads from the most highly stressed corner of the sail - the clew. This applies even after partial reefing.

By reverting to weaving, a low stretch, hard wearing sailcloth can be achieved with unique recovery properties which enables the sail to perform, be light for a given weight aloft, but more important obtain enhanced longevity. Early results have shown VEKTRON® woven sailcloth to outlive polyester sailcloths with all the advantages of a high-tech fabric.

Other sailcloth manufacturers have developments underway with a goal of providing enhanced durability. The trend for sailcloth and sail-making will depend on the demands of yacht owners. If the yachts continue to get larger, with ever higher loads, then sailcloth manufacturers will need to continue developments that provide increased durability.

These products are registered trademarks: Dacron® and Kevlar®: Dupont. Spectra®: Allied Signal Corporation. VECTRAN®: CNA Holdings, Inc.

Warp Laminates In the late 1970s, in an effort to reduce weight and improve the stretch resistance and wind range, or simply performance, Kevlar® was first woven in the fill (or weft) direction with polyester in the warp. These sails showed significant strength improvements in the fill direction, but relied almost totally on the Mylar® for off-threadline and warp strength. Beginning in 1983, sailcloth manufacturers began making laminated sailcloth with woven Kevlar® in the warp direction, and cutting the sails in a radial configuration, both problems were solved. The radial configuration minimises off-threadline loading. Unfortunately, Kevlar® is very prone to flex fatigue. When folded or flexed repeatedly, it quickly looses its strength as the individual Kevlar® fibres begin to break with use. Contemporaneously, Hood began laminating trials with Spectra® fibre in an effort to enhance the longevity of laminated fabric. Spectra® fibre is impervious to flex fatigue as well as having a modulus 3.5-times that of Kevlar®. A major advantage of a Spectra®-Mylar® laminate is a longer lasting sail. Instead of the Kevlar® breaking down and leading to failure, the Mylar® becomes the least durable component in the laminate. Spectra® laminates were the first to be recognised by the sailmaking industry to be suitable for large cruising yachts.

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