I've been dancing, with the Austrian fleet supported, by the Turkish fleet and all the German ships that started scattered around the world, in and out of the Mediterranean shipping box on orange/orange. But, my repeated efforts with a well lead and powerful surface feet have not yet sunk a single merchant ship.
On the other hand, all of my subs are in the Mediterranean and have managed to sent a few merchants to the bottom.
What am I doing wrong or is the game designed to make sinking merchant ships next to impossible?