"Ring-tail" is a good question. I had to look it up myself. It is apparently a triangular sail that is set for extra speed above the quadrangular "gaff" sail on the mizen or rear mast of an older sailing ship.

There are also modern versions of this sail which are on small separate masts or posts at the extreme end of a boats that are there primarily for stability rather than hard-drivin'.

Cheerily,
Charley Noble