Captain as naval rank and shipboard command title

Could a real navy remove captain from the rank structure and use it only for whoever commands a vessel?

It could under a combined-service system, but naval custom already distinguishes the ship's captain from other captains by their appointments.

A navy has little need to abolish captain as a substantive rank merely because the commanding officer of any vessel is called captain. A combined armed-force rank structure might create reasons to separate title from rank, but naval custom already handles the ambiguity. On a flagship, several officers may hold captain's rank while only the commanding officer is addressed as the captain. Others are identified by their appointments, such as fleet captain or chief of staff. The language preserves both professional rank and a clear chain of command aboard ship.