http://www.fleetsubmarine.com/sonar.html
Mentions U-boats being able to 'hear' up to 50 miles by end of war.
http://www.ibiblio.org/hyperwar/USN/...SW-PART-I.html
more of a history of anti-sub warfare during the war.
http://www.uboataces.com/hydrophones.shtml
This one states U-boat detection ranges as 20km for vessels and up to 100km for convoys.