Fighting the U-boats
A hunter-killer group would consist of one escort carrier and several escorts. In the US Navy, the escorts initially were destroyers, with emphasis on the old "Flush Deckers" (or "Four-Pipers") left over from the post-World War I construction program. As the destroyer escorts, or DE, became available, these came to dominate the screen of the hunter-killer groups. However, these escorts were tasked with more than just assuring the safety of the escort carrier. They were frequently sent many miles to hunt down u-boats that had been sighted and driven under by aircraft.
One of the tactics was to use allied code-breaking information, HF/DF (high-frequency direction finding, commonly "Huff Duff"), asdic and visual contacts to hunt the same U-boat for days until they either lost it or the boat was forced to surface when its batteries were exhausted and scuttle. This was radically different from the way the convoy escorts fought the U-boats earlier in the war as their main task was to fend the U-boat away from the convoy and then the escort returned to its regular position.