Sailcloth is woven in two forms: balanced and unbalanced. The yarns in balanced cloth are the same diameter and weight lengthwise (the ‘warp’) and across the width of the cloth (the ‘fill’ or “weft”). Unbalanced means a heavier yarn is used in one direction. Most modern sails are “crosscut”, which needs an unbalanced fabric – where the heavier yarn is in the fill.
The Warp Yarns are wound onto large “beams” to form the length wise threads, and are under some tension. The warps get threaded in alternates to one of two heads, each head moving up or down alternately – with the fill yarn being fired across the width. The heads then move to the alternate position, and the fill yarn being fired back across means that in two cycles the fill yarn is under one warp thread and over the next. This process is an extremely fast and efficient process. Weaving can be done to all yarns, and the resulting fabric is known as a taffeta if the weave is tight, and a scrim if loose, with clearly visible spaces between the yarns.
The quality and weight of the weave can be more critical than the choice of fibres, since a poor weave can lead to high stretch and poor sail form.
Weight is often described in ounces, for example “an 8 oz. cloth”. This means that an area of 28.5 inches x 36 inches weighs 8 ounces. Grams per square metre is gaining acceptance. To go from one to the other, multiply the oz weight by 42.8.
This allows greater loads to radiate up from the clew along the leech. Woven sail cloths have an inherent problem with stretch resistance. In a weave the warp and fill yarns pass over and under one another. As load is applied the yarns attempt to straighten out, this results in the fabric stretching, commonly referred to as ‘crimp”. To prevent this, the sailcloth is “finished” by heat setting and resins being applied before the final rolling and flattening processes.