When I am pinned down as you discribe - I try and hide and wait it out.
This mod here
http://www.subsim.com/radioroom/showthread.php?t=181540 with mess with the DDs sonar after an depth attack.
So when depth charges explode - hit full speed - after a short while (30sec for me) kill engines and coast on silent.
If you can go more than (im guessing here) 30mins?? - the DDs will give up and move to look elswhere or rejoin the convoy if it has moved a certain distance away. With the later however, beware that one single DD may sit all stop above you - just waiting for you to think it is safe.
So for best results - get the mod above and update IRAI to the newest version. If you plan to change the AI setting in IRAI - do it before you install it.