Has a smooth shank to facilitate withdrawal when, inevitably, the copper sheathing plates have to be removed.  Being silicon bronze there will be essentially no electrolysis when fastening copper plates to the hull.  These are harder than copper, so it  can be easier to drive them in and they have a smaller diameter head than the copper sheathing nails.  Some of our traditional shipwright friends in Sydney prefer these to the copper sheathing nails.