MAXIMUM SAIL POWER CHAPTER 3 FROM THREAD TO FINISHED FABRIC How sailcloth is made - Part 5

LAMINATED FABRICS In Part 4 we took a closer look at laminates and what goes into making good laminated sailcloth. This is the last section of Chapter 3 where we will continue to look at laminates and the various parts that comprise a well engineered laminate. Scrims As already noted, in addition to the adhesives that bond the layers together and the films that cure bias stretch, the other real breakthrough in laminated sails was the discovery that it was no longer necessary for the substrate to be woven, or at least not as tightly woven as had been the case with standard Dacron. The result

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has been laid-up scrims in which the warp and fill fibers are simply arranged in a loose grid pattern that is then held together by the Mylar film. The yarns used in scrims are extruded flat rather than round, and because they are simply placed on top of each other there is none of the over and under that causes crimp in a standard weave. When the scrims are laminated, they are held in tension to remove initial stretch from the fabric, and as soon as they are bonded to the film they remain in place and the initial stretch characteristics of laminated fabrics are always very good as a result. The resulting gaps between these yarns have been shown to be the key to good laminated sails since the adhesives used in the laminating process work better when they are allowed to adhere to film An example of a basic Scrim rather than the fibers. Fabric makers have also discovered that in many cases a third layer, for example, a taffeta, helps prevent tearing since the fact that the individual yarns do not interlock as in a woven fabric results in a lower tear strength. The taffetas are usually light, fairly loosely woven polyester fabrics. Their job is not specifically to help with diagonal stretch, although they do contribute a little. Rather the taffetas are there for abrasion resistance and overall strength. These days taffetas are treated with UV inhibitors and anti-fungal agents to prevent mildew.

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Diagonal Yarns The final piece of the laminate puzzle was the addition of diagonal yarns, which lend some strength to the film when it comes to handling off-threadline stretch. The angles at which the yarns are placed in the fabric have been part of patents, and their number and relative thicknesses are an integral part of the fabric engineering process. It is not as important to have as many diagonal yarns as warp or fill yarns, since if the sail is engineered properly the principal load-bearing yarns will manage most of the loads on the sail. The diagonal fibers come into play when the sail is eased out and the loads no longer travel along predictable load lines or “catenaries.” The good news is that once a sail is eased out, the loads are greatly reduced, therefore having as many fibers there to accept them is not that critical. Laminated Sails in Depth Laminated fabrics have come to play an increasingly larger part in the sailmaking industry and their continued growth is assured. There are a number of reasons for this: • Lamination is the most effective way of combining materials with different characteristics to maximize the advantages of each. • Laminates allow individual fibers to be placed in straight uninterrupted paths. This is the most effective way of getting the most from the fibers and the most efficient way of using a fiber. • When films like Mylar or PEN are introduced, the result is an effective way of minimizing off-threadline stretch. • As laminates have become more accepted and earlier problems of delamination and mildew have been dealt with, the demand has reduced the prices to a point where they are competitive with Dacron.

Laminate Styles Because the number of plies is virtually unlimited, and various combinations of fibers and film are readily available, there is no practical limit to the kinds of laminated fabrics that can be

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created. The only limits are engineering and cost. In the case of some of the giant SuperYacht’s, these fabrics are complex creations designed to handle the loads and abuse the sails are certain to undergo. For the rest of the sailmaking world, there are four main construction styles of note. Type 1: Woven/Film This is the most basic style of laminated sailcloth, and at the low end might consist of an inexpensive woven polyester laminated to a substrate or film. The film provides stretch resistance while the woven fabric provides resistance to abrasion and tears. The combination results in a relatively inexpensive sailcloth that more than adequately serves its purpose as, say, a performance cruising fabric. If a heavier weight or stronger fabric is required, a second layer in the form of a light taffeta or rugged woven material can be added to the other side of the film (woven-film-woven). A high-end version of this type of fabric might use a woven Spectra or Vectran layer laminated to a film, where the Spectra or Vectran yarns provide stretch resistance, and the film adds off-threadline stability. Again, this is an effective way of creating a fabric that is both rugged and offers good stretch resistance. A third layer can be added to the other side of the film in this case as well, possibly a light, balanced, tightly woven taffeta to help with durability and bias control. In all cases where a woven material is used, you need to remember that there will be crimp in the fabric and the problems associated with crimp, notably initial stretch, will be inherent in the fabric. If rugged durability and overall stretch resistance is required with a cheaper construction cost, these fabrics are a good choice. Type 2: Film/Scrim/Film This construction style results in a low-stretch, low-weight fabric that is excellent for racing sails. The structural fibers are sandwiched between two films and inserted into the laminate in a scrim. That way the load-bearing fibers are used to their fullest potential with no crimp,

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with the loads going directly onto the fibers. With the fabric laminated film-tofilm, the adhesive bonds between the fibers resulting in a strong bond that uses a minimal amount of glue. By eliminating layers of fiber and keeping the amount of adhesive to a minimum, the over- all weight of the fabric is kept as light as possible. The scrims for these fabrics are usually of a high modulus type like Kevlar, Spectra, Twaron, or a combination thereof. Fabric makers add UV inhibitors to the film so that the delicate load-bearing yarns are protected from the harmful rays of the sun. The drawback of these kinds of films is that they are not very rugged or abrasion resistant and get a lot of wear and tear, reducing the life of the sail. As a result only racing programs that replace their sails on a regular basis tend to choose this style of construction. Using the same engineering lay-up but different yarns and deniers for the scrim allows the fabric maker to have a range of fabrics. For example they can make a fabric for a No. 1 genoa for a 35-foot racing boat made up of a scrim of alternating 400 DPI Twaron and Spectra in the fill, and 1,100 DPI Twaron in the warp. To add some off-threadline performance the fabric maker might add 375 DPI Spectra on the diagonal. On the other hand, in a heavier fabric for a heavy No. 1 genoa on a 55-foot racing boat, the scrim might have the same fill and diagonal yarns as for the lighter fabric, but the warp would be 3,780 DPI Twaron, giving the fabric the strength it needs for the heavier sail. Type 3: Woven/Film/Scrim/Film/Woven This construction style is an obvious further development of Type 2 in which woven taffetas are added to the film/scrim/film center in an effort to create a rugged, low-stretch fabric. The “film–on-film” center, as Type 2 is known, includes lowstretch, load-bearing yarns that are then protected by the woven taffetas on the surface of the fabric. These woven taffetas protect the film from both flex fatigue and abrasion, and they protect the core fibers from UV degradation. You can even use light-sensitive fibers like Kevlar and PBO as the load-bear- ing yarns in the center since they will be protected by the outer layers.

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The taffetas can be made of either a lightweight polyester, or a woven Spectra or Kevlar. Note, however, that one problem when combining woven fabrics with scrims is that, because of the crimp in the wovens, their initial stretch is much more than that of the scrim and the scrim ends up by taking all the loads with very little being handled by the outer layers. As a result, for a rugged fabric created for long offshore races such as the Vendée Globe or the Around Alone, Spectra taffetas do help a little in terms of stretch, but their main function is to add durability. This kind of lay-up has been very successful, although the cost of producing the fabrics is quite high. The Type 3 fabric is also useful for the SuperYacht market where you need both strength and stretch resistance. Again, because the outer layers protect the inner layers, these inner scrims can be made from high-modulus fibers like Kevlar and Vectran, with the outer layers adding a measure of protection to the inner fibers. Type 4: Woven/Scrim/Woven This type of fabric is relatively new and eliminates film from the construction. In high-load applications like sails for SuperYacht the films have proven to be a liability in terms of longevity and durability. The difficulty, however, has been bond- ing two woven fabrics together since you get a better adhesion by bonding to a film. Development continues on this kind of engineering and new adhesives show some promise. Doyle Sailmakers, for example, is pioneering this technology specifically for an inventory of sails for the 248-foot Mirabella V, the latest generation of SuperYachts.

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In summary? There are a number of reasons why a laminate is a better option for paneled sails over Dacron. As we discussed with a cross-cut Dacron sail you have to engineer the sail for the highest loads which are along the leech of the sail. Unfortunately because of the way the sail is put together that same fabric weight is in the body of the sail and along the luff where it is overkill for the job. With a laminate you can make a tri-radial sail and as such you can put a heavier fabric in the high load area and a lighter fabric in the areas that see less load. It’s a much more efficient way of engineering a sail. This will all be covered in much more depth in Chapter 4 - A Primer on Panel layouts. Note: In Part 5 of From “Thread to Finished Fabric” we will continue to look at laminates and how they change the engineering of the fabric and improve the performance of the sail.

I hope that you enjoyed this blog. There are many more at my website www.greatcirclesails.com. If you need new sails for your boat just click this box and I will send you a no obligation quote.

BRIAN HANCOCK Owner Great Circle Sails

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