Happy times are closing to an end, but yesterday I probably had the best SHIII patrol ever. Over 20k total sunk tonnage and aside from that destroyer I spent no torps. Guess how did I manage it.
Deck gun. My dinky lil' Type VIIB comes with a no less dinky 88mm naval cannon. I ain't winning shootouts with destroyers with it and can't use it in high waves, but it's more reliable than torps.