Your idea of aiming for the bow sounds good.
I just got finished sinking a C2 Cargo ship using up *four* (!) torpedoes.

I'd disabled it with a T-IV acoustic torpedo which destroyed its propeller. I came back later to find it motionless, but afloat and, being that I happened upon it at an AOB of 180°, sent 1, then 2, torpedoes into its stern. These stern shots didn't seem to bother the ship at all except for starting a harmless fire. I had to re-position my sub and send the final torpedo into its port side to finish it off.
I pr'y could've saved a couple of torpedoes if I'd hit it closer to the bow.:hmm: