In a similar sort of situation, in stock SH3, I was loitering underneath a small merchant doing 4 knots (very slow convoy) matching its course then shooting my rear and front eels straight forward and back at the merchies there, almost a guarantueed hit...
And so it turned out, they both sunk, what I hadn't counted on was the zig-zag behaviour after being hit which resulted in a C3 from an entirely different lane plowing straight into my conning tower while I was trying to find out how to manually reload torpedoes...
Worst non-combat mishap however was a convoy I attacked when it had just started a general course change, when a ship got hit they all started zigzagging while changing course, complete mayhem, then apparently another C3 decided it was way off course, stepped on the gas to get back in line and plowed into my poor boat going 20 knots
lots of screeching metal on metal noises but no survivors
oh and had I mentioned being sunk by a tug?