Bias Stretch

While the bobbins holding the warp and the shuttle holding the fill are important, it's actually the beater that plays the most important role in weaving sailcloth. As noted above, the finished fabric has strength both along and across the cloth, since pulling in those directions means pulling along the length of the yarns. But it is weak along the diagonal since forces working in this direction are pulling at an angle to the yarns and can distort the weave. To minimize this "bias stretch" as it's called, the weave has to be tight, and the harder the beater slams into each fill yarn, the tighter the weave.

In fact, much of the weaving process is designed to eliminate, or at least minimize the bias stretch of the finished fabric, so let's study this aspect of fabric design in more detail. By way of illustration, take any household fabric and look closely at the warp and fill fibers. Assuming that it's a conventionally woven fabric, try stretching it by pulling in either the warp direction or the fill direction. Chances are you will meet with some pretty substantial resistance as you tug directly against the dozens of tiny yarns. Now take that same fabric and pull it from corner to corner, in other words on the bias (Figure 3.3). Immediately, the fabric will stretch and distort. Now imagine what can happen to sail shape in a

Figure 3.3

Try stretching any woven fabric by pulling in either the warp or fill direction. You will meet with some pretty substantial resistance as you tug directly against the dozens of tiny yarns. Now take that same fabric and pull it from corner to corner, in other words on the bias. Immediately, the fabric will stretch and distort.

BIAS STRETCH

Loads across and along the fabric pull directly on the yarns.

When pulled on a bias the fabric elongates

When pulled on a bias the fabric elongates

Figure 3.4

The fill yarns are shot across the fabric by the shuttle, therefore, they tend to be laid down absolutely straight. But since the warp yarns go over and under the fill there is a lot of room for elongation once a load is exerted on them.

As part of the manufacturing process, woven Dacron is passed through a huge heater where temperatures of around 400 degrees Fahrenheit shrink the yarns in order to create a tighter weave.

heavy breeze when the load no longer travels along the engineered load paths, but rather comes onto the bias. The sail will stretch and distort and the carefully crafted shape will no longer resemble anything the sail designer had in mind.

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