[John Pancoast]

Silent running may not help once they have you with asdic, but to not have it on anyway is a mistake.
I.e., if you are successful in evading them, after their depth charge run, etc. being on silent running can help prevent them from picking you up again.
It's easy to test; they can be nearby and not have you/no asdic being used with silent running on, but turn it off and immediately they have you and start pinging.
It should automatically turn off when one increases speed, etc. but it doesn't.