In WWII The Germans too had a lot of problems with dud and faulty torpedos at the beginning of the war. Also unless you hit a ship in a sweet spot it can take a long time for them to finally sink. 1-2 tops should be more than enough for a smaller freighter..
I usually send them one tin fish then wait and if they dont sink i surface and finish them off with the deck gun. Thats if they are unarmed and have no escorts
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