ASDIC and sonar have been available at the beginning of the second world war not in this perfection.
With all destroyer types the type 123A is inserted from 1.1.1938 and looks magnificent.
Nevertheless, the reality was in such a way:
http://jproc.ca/sari/asd_et1.html
http://en.wikipedia.org/wiki/Sonar
I consider what can be done, so that the ASDIC of the destroyers 100% does not work reliably. As well as it is solved now, there cannot remain.