While subs are delicate when compared to surface vessels, there is a thin-line balance point for sea-worthiness versus a realistic damage model. If you make the vessel too delicate, it sinks in a 5 knot wind in this game. Too strong, and it won't sink at all. All of the AI subs have been given a good going over several times, with ALOT of testing done on them. The Sen Taka Dai will sink with one hit, but not always. While it is a smaller submarine, it did have compartmentalization, and could therefore seal off damaged portions of the boat. Imagine that your torpedo detonated early or only partially, or you hit the target too low or high, too far forward or aft, or that the crew was at Battle Stations, and had the water-tight hatches closed. Pick your story. In the meantime, I'll look at it one more time to verify, and get back with you and pass on my findings to the team.
