[laiocfar]

I dont belive it possible, the chance of sinking a ship is related to the number of ships in the game. If there are 40 ships, its more like next naval battle one ends with the fish; if there are only the 4 start ships + 1 combat ship it is near impossible to sink anyone. With a little adjustement of that equation and last ship of the royal fleet is invincible.
Plus you dont need to defeat the whole royal force, by controlling all cities and holding when last complete comboy arribes. You dont need to defeat the last two regulars that dont make a full comboy, the royal fleet, etc.