In an earlier post to this discussion I mentioned that I purchased a new main for this season. In that discussion I offered to photograh it at the end of the next few seasons and have a sail maker evaluate its shape degragation over time. My offer was to do that if someone else with a new Dacron main would be willing to do the same (it doesn't cost anything - the sail makers will do that at no charge). So far, no one has agreed to do the comparison.
Further, I offer this feedback from my local sail maker. While it is not cost specific it does offer some intesting technical points.
From the sailmaker:
For further analysis we asked our sail designer, how changing to Aramid would affect the performance, durability, and ease of use. Part of the process was also to analyze the flying shape of the sail design and how it would have to be changed to accommodate the stronger fiber. From earlier work, we know that there are critical changes that need to be made when changing to a stronger fiber. As part of that analysis, we utilize design programs that analyze flying shape in various wind conditions as sails stretch. One of the outputs of our program is "yarn strain". This indicates the amount of stress on the structural yarns in the sail as the sail is loaded. Areas of the output that appear in red are areas in which the sail is loaded to the point where excessive strain is being put on the yarns. As the sail is stressed to the red areas, the yarns and then eventually the Mylar film, are taking excessive load. When a sail is loaded excessively, the yarns can reach the point where they do not recover to their original length and the Mylar film becomes loaded. When Mylar film becomes loaded, it tries to fight the load by aligning itself against it. When this happen the film recoils permanently, which is what is commonly known as film shrinkage. Once yarns are overstretched and film has shrunken, the original design shape of the sail has been lost.
The image below shows the yarn strain analysis of an Aramid (right) and a Pentex sail (right) in 12 knots of true wind speed. Note the significant amount of red in the Pentex sail and the absence of it in the Aramid sail. This simply means that the Aramid yarns are not seeing nearly the strain as the Pentex yarns, meaning the shape distorts less under load. The end result is going to be that the Aramid sail is being less damaged with normal use. This simply translates into a more durable sail.
Figure 1 - Yarn Strain
I'm sorry, the diagram would not paste into this forum. Send your e-mail address if you would like it.
Common concerns that people have with Aramid sails when they have become used to polyester or Pentex sails are UV degradation and flex degradation. Frankly most of these concerns stem from the original Kevlar sails built back in the 1980's. The original Kevlar sails were woven like Dacron, with Mylar film on one side of the fabric. The size of the yarns in the weave was very small and nearly all of the fibers were exposed. Modern laminates use larger bundles of yarns that are unwoven. The yarns on the outside protect a large portion of the yarns in the bundles, so a very small percentage of the yarns are actually exposed to UV. When a racing sails reach the end of their performance life, it is the changed shape of the sail (primarily from film shrinkage) that is the reason, not that the fibers have lost strength. Aramid yarns do break down faster than Pentex yarns from flex degradation, but what you see from flex degradation is more a breakdown in breaking strength of the yarn, not stretch resistance. But as stated above, most racing sails are retired because they no longer have a fast shape, not because they have broken.
The bottom line is that Aramid sails provide the best value in performance racing sails. The only classes that use Pentex as a fiber are those that originally use polyester fiber in their sails because at the time polyester was considerably less expensive than Aramid. When a customer comes to a sailmaker looking for a top value laminate racing sail for boat under 40' without fabric restriction, sailmakers nearly always choose Aramid, but never Pentex. In fact, sailmakers are more likely to recommend a carbon Aramid blend than Pentex.
In addition to being more cost effective in the long run, trimmers will find the Aramid sails easier to set up properly. Because the sails will hold their shape better in heavy air, they will need less aggressive movement of the halyard and lead positions to get the proper flying shape. Having to use less halyard tension to get the proper flat shape in heavy air will also increase the sail's longevity.