from The Century Dictionary.
- noun A heavy piece of iron, rotating on a vertical axis, serving to close a port in a turret-ship.
Sorry, no etymologies found.
Nos. 3, 4, 5, and 6 man port-tackle: as muzzle approaches port-stopper, "Open Port!"
After the third shot from the 15-inch gun of the Nantuck a port-stopper became jammed, several shots striking very near the port and driving in the plating.