Non veni pacem mittere, sed gladium.
Interesting, I sew a lot, but not sail, so I looked it up: "There are a number of advantages to a zig-zag stitch for sails. Essentially the biggest advantage to a zig-zag stitch is its strength, because we can get 3 stitches in a small length of cloth, while keeping the holes far apart from each other, where as a straight stitch has to punch holes very close to each other in order to obtain the same amount of stitches in the same amount of length. This distribution of stitches allows for the entire sail and stitch to stretch evenly. Whereas with a straight stitch the stretching at the stitch would cause puckers all along the seam."