Which ships am I allowed to attack?

[U-96]

From 1939 on, Anything with a British or Canadian flag on it's mast is open game. Merchants and warships, though tackle with a destroyer at your own risk. From 1940 on French and Norweigian ships are open game as well.

From 1942, U.S. ships are open to attack. You won't encounter many ships of other nations so you don't have much to worry about.

Save your Torpedoes for Merchants or especially tankers and Troop ships, they are very valuble targets and add handsomely to both your tonnage and renown. For warships, anything smaller then a cruiser is a waste. Destroyers are very hard to hit (Don't even think of going after one with T1 Torpedoes, all you're doing is giving away your position. If you MUST attack destroyers go for any torpedo that's electric powered as these are wakeless and very hard to spot, if they do and your fish is on the mark, it's too late for the destroyer to make evasive manuvers.)