By the 1770s, Royal Navy warships were sheathed with copper plates below the waterline as protection against attack by gribble worms. This necessitated the change from iron to copper for the fastening bolts below the waterline (because of electrolytic corrosion of the iron bolts). Ships were still fastened with iron bolts above the waterline, where there was no copper sheathing. Copper bolts have been found on the wreck of Colossus, but are not present on the older wrecks of the Association and the Eagle.