Welcome aboard, Herr Kaleun!
Discovering tactics that work well for your command is half the fun of this game, although I do think out-turning a destroyer might be more due to the AI's shortcoming rather than the maneuverability of your u-boat, especially a type IX. They didn't nickname them "seekuh" for nothing!
S.O.P. for u-boats was to target merchant shipping. Your primary purpose is to cut supply lines. Capital ships are secondary targets of opportunity. Escorts are to be avoided but a commander can engage them at his discretion... in self-defense or to deprive a convoy of protection so it can be attacked at will. U-Boat commanders that made a habit out of going after escorts either wound up at the ocean bottom or in the brig.
Good sailing and sink 'em all!
