We don't see any activity from the land in this game. Presumably there would be military men stationed at each port, at every tower. Every time an unknown man o' war passed within sight it was likely marked down and sent as a letter to warn the admiralty. This is my suggestion. Ships within sight of a enemy port, forts, or towers, there's a % that they will be spotted within X time, percentage increasing with how many ships are spotted within that time. So say if 1 ship passes within s