Its a ballsy move, but you could try running at P depth as somoene mentioned earlier. The ASDIC has a certain angle it and activly search for which limits it when you are closer to the surface. Its really hard to explain, and its not a textbook manuver either. I recommend keeping the P down and just listening via sonar and keep track with the little buggar till its away, from what you said its the active sonar thats biting you, so maybe you can sneak away with the passive?
Could try that next time, you might get away, you might bit the dust?
