Also be sure to use a fitting that fits tightly to the tang...that shackle has a lot of room for the pin to slide side-to-side on the tang which exacerbates the side loading and eventual stress fatigue of the tang. If the shackle spends most of it's time with the tang slid all the way to one end of the pin, it's putting a lot of twisting load on the hull tang. At least put some spacers on the pin of the tang so it stays centered or use a shackle made for the width of the tang (I've got a couple of stamped ones that are made for soft lines but I can't recall where I got them).

The boat has a lot of age on it, so it's unlikely this caused the problem...but it didn't help.