It depends on how fast it moves, and how far away his track is from you. His slow speed makes the bearing move slowly. Him being far away makes the bearing move slowly aswel. However, the further out his track is, the bearing also cuts out a thinner pie/pizza-slice while it is in your hydrophone coverage. So he stays less time inside your hydrophone range.
Hopefully this explains it a bit better:
http://www.subsim.com/radioroom/show...7&postcount=27