David Flynn

Typically, woven polyester (commonly referred to by DuPont’s trade name "Dacron") makes more sense for cross-cut construction (panels running from luff to leech in conventional fashion), than in tri-radial oriented sails where the material thread line is placed to align with primary stress paths. This is because most woven material is "fill" oriented, or has their primary strength on the short axis of the fabric. In tri-radial construction, the material is oriented to the load path with the long axis (warp) bearing the primary loading. Since woven materials are weaker on the warp, it would not make sense to use traditional woven poly in tri-radial construction. 

Making a woven material with its strength in the warp axis is difficult due to the nature of the weaving process. When you try to weave a warp oriented woven material you end up with a relatively loose weave which exhibits a lot of "bias" stretch (off axis, or anything not in the warp or fill direction). It is impossible to get the tight weave of a fill-oriented construction. Some woven materials have a more balanced construction with the warp and fill closer in size and strength. In order to accomplish this, both the warp and fill threads are typically somewhat smaller, so the ultimate strength in the primary load bearing direction (warp or fill), is not as great as a typical fill-oriented construction. 

These "balanced" materials are usually used in smaller boat applications where the loads aren't as great so the extra strength of an oriented (unbalanced) material is not as critical. Balanced materials can be used in small tri-radial sails where the loads are not too high. There is a relatively new generation of woven materials designed for radial construction which do offer some real gain in warp strength without giving away too much bias strength. However, these so called "Radial Warp Wovens" are still not as strong as a polyester composite along warp or bias, so they should be limited to smaller boats with lower loads. In practical terms, to take full advantage of the structural benefits of tri-radial construction, specially-designed composites are the proper materials for the job.