I think the Germans and the Americans both looked at the question "How do you get a torpedo to sink a boat when they put armor on the sides where the torpedo wi

I think the Germans and the Americans both looked at the question "How do you get a torpedo to sink a boat when they put armor on the sides where the torpedo will strike?" and came up with answer: Have it explode under the boat not on the side. The Japanese just said - have a larger warhead. The problem for them was that you then need a more powerful motor to drive that larger warhead but they didn't screw that up. The odd thing is that the Germans and Americans (but not the Japanese) mainly used submarines to sink lightly armored commerce ships not heavily armored warships.