From the wiki:
Hydrophones are, in essence, underwater microphones. They allow you to detect targets from approximately 20km away. In order to use the hydrophones, your sub must be submerged. Note that your hydrophone range is affected by several factors:
* Speed of the U-Boat - Ideally, you should be running at Ahead Slow or even All Stop to maximise your hydrophone range.
* Depth of the U-Boat - Ideally, you should be between periscope deep (11-12 meters) and 25 meters; but also you can obtain best results at 50 meters & 70 meters; maybe thermal layers?
Everyone complains about the lack of prey in the assigned patrol zones. Most will say to do your 24 hours in your assigned patrol zone, then go to one of the hunting grounds described in the wiki to use up the rest of your torps:
http://www.communitymanuals.com/shii...unting_Grounds
Although this may not apply to the type II with its limited range, but a couple more patrols and the type VII will become available. Also transferring flotillas can open up new U-boats to upgrade to, different flotillas will accomodate different U-Boats.