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post #8 of Old 02-23-2013
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Re: Mainsail Design Question...

Tri-radial Dacron sails usually benefit from a variety of things. First of all Dacron tri-radial sails are usually made from warp oriented cloth. Most cloth has an equal fiber in warp and fill threads, but warp oriented cloth has heavier threads running the length of the fabric, and between the heavier weight and higher tension on the warp, the thread lie straighter in the cloth. When woven cloth stretches the biggest initial portion of that stretch is the threads pulling straighter so that alone reduces stretch.

Because warp oriented cloth is a premium product it is made with higher quality Dacron, and usually goes through a more rigorous inspection program than the commodity Dacron cloth which is often for cross cut sails. That alone helps increase the life of the fabric.

If you look closely at a fabric, there are little squares formed between the vertical and horizontal threads. The cloth is strongest and stretches less in the direction of the threads. The diagonals of the square holes between the threads is the 'bias' of the fabric. These square holes distort into parallelegrams when the stress does not align with the threads. This results in greater stretch than a similar stress would cause parallel with the threads. In a cross cut sail, the stress pattern in the sail is such that it causes bias loading a throughout a larger portion of the sail and therefore greater stretch in the sail.

Radial cutting of a sail allows the panels to be better oriented to the stress patterns in the sail. This means that there is less bias stretch. And because the cloth is generally higher quality and the stress follows the path of the fibers, and the fibers are less distorted there is much less stretch.

Stretch of course is a bad thing in almost all ways. To a racer it means poorer upwind ability and a less stable flying shape, but to a cruiser, it means a sail which is powering up, just when you want the sail flat. The problem with a powered up sail is that it causes more heeling and weather helm, so you end up reeling much sooner than you might with a lower stretch sail.

But also,when threads stretch they spring back to something close to their original length. That ability to remain elastic is one of the nice things about Dacron. But every time it stretches, it does not return perfectly. Instead it grows ever so slightly longer and never returns to its original length. That is called creep. Cumulatively it is creep which ultimately spells the end of the useful lifespan of a Dacron sail, before it has deteriorated into the realm of a white triangle pretending to be a sail. The more the cloth stretches, the more creep occurs. Since cross cut sails stretch more, they tend to experience more creep. Creep means a fuller sail and for the cruiser that means more heeling and weather helm as well.

The reality is that your boat's performance will always be limited by the configuration of the hull and rig. You can easily argue that the performance gain of a radial sail would be minimal. On the other hand, you can easily argue that if anyone could benefit from a bit more performance, it might be your boat. You can also argue that the reduced heeling of a radially cut mainsail with warp oriented fibers would reduce heeling, weather helm and the need to reef, all really good things. And lastly, the greater lifespan of the radial sail may pay for itself over the life cycle of the sail.

I also think that none of us can specifically answer the question of what is right for you. These are preferential questions. Only you can evaluate your priorities, who's much sailing you do in gusty, or windy conditions, first cost or life cost and so on. There is no single, universally correct answer here.



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