I do know, and I also understand that hydrophones are, by necessity, part of any asdic set (you need a device to receive reflections of sound waves you've sent after all). I just wondered what he meant by that. It maybe just Brit humour that non-Brits like me don't get, or perhaps (that crossed my mind) they relied on active pinging around in hopes of getting a solid asdic echo instead of using receiver to listen for screw sounds. Maybe the second method was simply very ineffective by comparison so they skipped listening part and just pinged around?
But I'm no expert on asdic procedures so it's just my wild guessing
