From the feedback - a Star special

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From: Steve Haarstick
Would Mylar be a step in the right direction? Having read some of the comments regarding the potential cost and durability of Mylar laminate Star sails, I am in doubt as to the objectivity of the authors. As very few subjects are 100 per cent black or white, this topic should carefully consider the available data, and conduct controlled on the water experiments, before any final decision is made. The relative pricing of Star sails made from Mylar laminates versus Dacron sails in part depends on the cost of the cloth used. This cost is equal to the manufacturers' price/yard of the cloth used and the required yardage to cut the sail. However, comparisons between the cost of Dacron fabrics used in current Star sails versus the cost of laminate fabrics is not simply a matter of comparing their relative cost per yard.

The types of Dacron cloth used in Star sails are manufactured in 36" widths, while Laminates come in 54" widths. Moreover, most Dacron fabrics over 3.8 oz are stronger in the "fill" (cross panel direction) than in the "warp" (parallel to the cloth edge). As a result. most current Star sails are made with a combination of radial panels in the head and foot areas only. The body of the sails are laid out as cross cut panels, that is, the cloth is rolled from leech to luff, with the fill threads approximately aligned with the leech of the sail. The strength advantages of warp versus fill layouts are not as pronounced in Dacron fabrics as they are in laminates. Aside for the differences in panel widths (cutting 36' panels from 54" cloth is not cost effective), it would not be advisable to use the current Dacron panel layouts to build Mylar laminate Star sails.

Because of this basic difference between Dacron and Mylar, the current "cross cut" designs in the body of the Star main would have to be redesigned to use a vertical panel layout if the strength properties of Mylar laminates are to be properly aligned with the major loads in the sail. Mylar laminate Star sails would have to be cut with a vertical or tri-radial panel array, similar to offshore sails. This change of panel layout would require completely new designs resulting in development costs that may not be trivial.

A vertical, or tri-radial panel layout uses more cloth per square foot of sail than a cross cut panel layout. As long as we allow the use of 'Polyester' laminates only (as other classes have done), it is true that these laminates are cheaper than woven Dacron, if we prorate the cost per yard of 54" laminates down to 36" Dacron panel widths. However, it takes more cloth to cut a tri-radial panelled sail. To compare the relative cloth cost of a 54" wide laminate sail versus a cross cut 36" wide Dacron, it is necessary to know how many yards of the relative fabrics are necessary to build the same size sail, but with two very different panel layouts.

We have some data on tri-radial panel cloth costs, as we have been building tri-radial panelled Star mains for some time, using 36" Dacron woven cloth. As I am not privy to the exact yardage required to build the other Radial head and foot, with cross cut bodies, I have estimated this yardage. I have listed this data below using either Contender's 3.8 "Polykote", or 3.8 "Pentex Polykote", and compared the cloth cost for a 4.0 oz Pentex laminate:

1.) Tri-radial panelled 3.8 oz "Polykote": from Contender's 2000 price catalogue, the price per yard at 36" width = $8.55. Yardage required to cut sail with 36" cloth= 43.5 yds. Total cloth cost = $372.

2.) Tri-radial panelled 3.8 oz Pentex Polykote: from Contender, price per yard at 36" width = $10.25. Same yardage as above, total cloth cost = $446.

3.) Tri-radial panelled 4.0 oz Pentex Mylar: from Contender, price per yard at 54" width = $11.50. However at 54" width, yardage to cut sail drops to 32 yds, Total cloth cost = $368. 4.) Radial head and Foot with Cross Cut body: estimated
yards at 36" cloth= 36.5 yards. Total cloth cost with 3.8 oz "Polykote" = $312. Same sail with 3.8 Pentex Polykote = $ 374.

As you can see, there is considerable variation in the relative cloth cost in a Star main depending on cloth used and panel layout. Beyond cloth costs, tri-radial mains would require more labour to build, as it takes considerably more time to assemble the panels in a tri-radial main than the panels in a cross cut main. (Our current tri-radial Star main has 55 panels in the sail). If we combine the relative cloth and labour costs, I have estimated below the relative change in the price of the finished sails: cross-cut Star Main using 3.8 oz Contender Polykote = 1.00, tri-radial Star Main using Pentex Mylar = + 23%=1.23, tri-radial Star Main using 3.8 Pentex Polykote =+ 32%= 1.32.

In short, the most expensive positive sailcloth option and panel layout is currently legal, and allowing the use of Polyester laminates would not necessarily increase the current cost of sails. The second issue of durability is not so clear cut. Durability is dependent on a variety of factors, some of which have nothing to do with the choice of fabric. I have listed what I believe are the most important of these:

1.) The level at which the skipper competes: If you are trying to win the Worlds, or the Olympics, you will not take any chances on the potential loss of speed with use. I don't think anyone would compete at this level without new sails, regardless of how durable the fabric is.

2.) How the sail is designed: If the cut of the sail is such that any slight loss of strength will begin to show, the durability would not be considered good. For example, if the leech of the main has a very straight exit, any increase of stretch as the fabric ages will show flutter sooner than a design with a tighter exit. If the designer wants the sail to be fastest the first time out of the bag, then any change in stretch will probably have a negative impact on speed. In my opinion, I have not found Star sailors to be very tolerant with any design concepts that require "breaking in" the sail for a few races to reach its peak performance.

3.) Finally, the most important question: will a laminate sail retain its performance longer than a Dacron sail? Again, there is no simple answer to this. It depends on the type of Dacron or laminate. Even more importantly, it also depends on selecting a batch of fabric that exhibits good retention of strength when flogged. There is enough variation in stretch properties after flogging between batches of the same style cloth from the same manufacturer, that the relative comparison between Mylar and Dacron fabrics can be misleading. Testing a "poor" batch of Dacron against a "good" batch of Mylar can lead to the conclusion that Mylar is more durable.

However, reverse the situation, and the result can be the opposite conclusion. The relative stretch of the "new" (unfluttered) fabric versus the stretch after flutter is a reasonable estimate of the fabrics' durability. If the stretch after flutter is a many times higher than the stretch when new, it is fair to say the fabric looses more strength when fluttered and would have a greater change in shape with use. After testing cloth for the past 30+ years, and without getting into the details of this testing, there is a significant difference in the stretch properties of any type of fabric after it is flogged. Many fabrics lose more than 50% of their initial strength after relatively short periods of severe flogging, and this is not limited to only Dacron cloth.

The variation within one type of fabric from the same manufacturer after flutter can be as dramatic as the variations between different fabrics! If the fabric when fluttered loses more of its strength on the thread line than it does off thread line, this shape change will be different than the opposite case where the fabric loses more strength on the bias. In general, Dacron woven fabrics loose more strength off thread line as they flutter. In general, the opposite is true with Mylar laminates. Many Dacron or Pentex woven fabrics more than double their original stretch at the same test loads at 30 degrees off thread line after flutter.

The only way to determine the relative merits of which type of cloth maintains its performance over a longer period of time is to conduct a controlled experiment. This requires at least two identically cut sails to be built - one from Mylar laminates, one from woven Dacron. Both sails would have to use a vertical or tri-radial panel layout, or the Mylar laminate would be at a significant disadvantage, as it's strength is in the warp or panel direction. Contender's 3.8 oz Polykote is also stronger in the warp than the fill, and I would suggest this Dacron be used as the Dacron example.

You can not make a valid conclusion but simply sailing around with a Mylar suit of sails that are different in design and panel layout to their Dacron counterparts. Two suits of sails must be built as per above. These two suits of sails must be always be used together, under identical conditions, for identical periods of time, and most importantly, equally flogged when the wind is over 15 knots. It's not important if the test sails are competitive with current sails. What is important, is how the differences that develop in their sail shapes with use affect their relative speed over the course of a season's sailing. This type of objective experiment is necessary to remove the current misleading and sometimes erroneous conclusions now being proposed.

Finally, it would be possible is increase the racing life of Star sails by simply allowing transverse battens in both main and jib. As the class has adopted the use of a transverse top batten in the main, this step alone has enhanced the durability of the sail. A second transverse batten, or even more would further enhance durability - for both Mylar and Dacron fabrics. Ditto for the Jib, which does not yet allow a transverse top batten- this should have be adopted at the same time as the main. I think there is enough justification for laminates to go ahead with a controlled experiment. One that is objective, and unbiased. .