I do some sail repair as well. In fact, that is why I have the zig zag machine. The sailors, all the books and sailrite, always suggest a zig zag for sail repair. As far as I know, the zig zag stitch was invented as a stretch stitch. It gives as the fabric stretches so the threads don't break. It was also used to prevent raveling at the fabric edge. A precursor to the serger. With all that being said, I am not sure in this application it would be all that important. It would probably look better to restitch with a zig zag, between the double rows of straight stitch. Other than looks, I don't really see any benefit. Thanks everyone.