[aschar]

Umm... are you trying to command a U-boat, or a battleship? A U-boat's deckgun is not only useless against warships, but realistically impractical, to the point where doing so is akin to ordering the same gun crew to jump overboard in high seas. For one, it'll do very little structural damage if any to a destroyer, all the while the same warship is armed to the teeth and will sink you before you sink it. And most importantly, deckgun crews are wide open without cover and completely vulnerable to even small arms fire, something that a warship will be more than happy to dish out. Torpedoes are the only way to go with these guys.

I'll only ever attack destroyers if conditions are ideal: if I'm submerged and/or can't easily be seen, if the destroyer is stationary or moving at an angle where a torpedo doesn't have to be led so much, if it's at close range, if it's not moving too fast, if I can predict its movements, if I have ample torpedoes, if I'm only engaging "a few" escorts rather than many, and/or if I can dive below 90 meters where enemy sonar starts to become ineffective. It's a lot of "if's", that's why U-boat commanders seldom attacked escorts openly. Even "Silent Otto" Kretschmer found himself on the receiving end of destroyers; he was lucky, only getting captured while other uboat aces like Prien and Schepke went down with their boats.