Agreed, it's currently quite silly.
Twice, I've put 2 torpedoes into a merchant, set the whole thing ablaze, and it sailed on merrily with the rest of the ships (but DDs chased me off).
Twice, I've put a single torpedo into a ship and the same was true.
However, then I had the opportunity to surface inside the convoy of 9 unescorted ships...
... and with the deck gun I sank 7 of those 9 - the first kills of my patrol, because the torpedoes are rather useless!
I don't mind a ship taking 2-3 torpedoes to sink since it would be more realistic in some cases, but the deck gun seems a far better choice at the moment, when possible
