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Between 1930 & 1970, the USN Sub Force suffered several really, really bad decisions. he Mark XIV torpedo I'm sure you all know. There were also the MAN & HOR diesels, which were nightmarish in WW2. And the postwar "pancake" diesels, in the the Tang class.
And there's Thresher, due to faulty welding....
Then there's the Scorpion accident, due to a problem with "deferred maintenance" (AIUI).
So, your challenge is to prevent it all. POD any time after 1900.