Ignored by Destroyers

[Berinhardt]

Last night I came across a large convoy at night, and had to set up my approach from the surface starting a few miles out. I thought for sure the destroyers would make a run for me sooner or later, but they didn't.

The destroyer patrolling the perimeter close to my location ignored my at 2,000 feet, and I dove to periscope depth and turned away from her instead of pushing my luck. At 800 feet she was still sweeping back and forth using passive sonar. I opened my stern tube doors - no response. At 600 feet, I launched a torpedo - the destroyer made no attempt to evade - just kept her pattern at medium speed. I hit her right under the keel and sunk her. The fleet started to break up and scatter - but the other two escorts did nothing!

Over the next hour I sank seven more merchants, emptying my forward tubes and half of my aft with the sub at periscope depth moving ahead 2/3rds, and my crew at battle stations. The escorts still didnn't respond.
As fast as my crew reloaded the tubes, I continued to sink the convoy until I only had four topedoes left and four ships - two escorts and two old freighters.

Since its a long way back to Pearl from the South China Sea, I thought I'd save the last four fish until I was closer to home. So I turned away from the convoy and at 7,000 meters distance and 3am I surfaced. Now, the final two escorts suddenly turned sharply and headed staight for me at high speed.

What gives? Is this common?