You are not silent when you launch torpedoes. The hydrophone/Asdic operator will hear a torpedo launch and know its approximate location.
Thus, it is important to get away from datum.
In well escorted convoy, it only takes an escort 3 - 4 minutes (or less) to arrive at datum.
My tactic is to go flank and dive rapidly after launch. After passing 60 meters, go silent and change heading.
