This is the system we have devised to securely attach a heavy ballast to the fin, and the fin into the hull, for an A Class yacht. A 8 mm diameter stainless steel rod has both ends drilled and tapped M6. A length of M6 stainless steel studding is screwed into the upper end of the rod and is long enough to pass out of the top of the fin and take the nut used to retain the fin in the boat. The rod is bonded into the fin with about 30-40 mm protruding from the bottom. The ballast is drilled 8 mm clearance and the underside is counterbored to recess the head of the M6 pan head ballast retaining screw. Whereas the normal method for small yachts is to cast the fin into a cavity of the ballast this is not so practical for an A Class. So the fin is just trimmed to match the upper profile of the ballast and a method of preventing the fin from rotating around the attachment rod is needed. We use two locating pins that locate in matched holes in the lower end of the fin and the upper side of the ballast.