Where to aim torpedos on ships?

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I like to know how the "contact influence" is coded in to the game. I use it often, normally below a couple feet below the keel depth. I`m using TMO 1.7, not sure if that makes any different. How is the damage system coded in to the game? Normally it takes 2 torpedos two take out a small/medium target. Sometimes it takes a couple of hours to sink, sometimes 30 mins. It maybe depends where you hit the target, i`m not sure.