That's certainly good information in this link & discussion, but I still think this was more of a game error. Five direct torpedo hits seems a little unbelievable. Plus, the fourth shot did cause that secondary detonation in the engine compartment. This was the first time a ship didn't sink after having this happen.
While I think it's very realistic to have a ship filled with dry cargo to take longer, why don't other ships with similar cargo take >5 shots to sink?
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