It is actually a "chain tensioner" used to hold the anchor
tight in its chocks. If I could ever figure out how to attach a photo
on this forum, I would gladly do so. Looks like I am not the only one who has not figured it out, as I have not seen any photos so far.
Good luck on your future crossings of Bass Straits. We opted to fly from Sydney
to Hobart after friends went up on the rocks at Eden.