Well Tuee. The only way to be sure is to stay there untill you can hear / see her going down... Just like in the RL. One eel more for her can make the difference...

Or few rounds from your artillery.
I think the minimum range for the torpedoes is around 250 - 300 meters. Or at least it should be. And to be honest, it might not be a very good idea to drive closer... :rotfl: