When you encounter a zig zaging DD have your sonar man fallow the nearest warship, If he suddenly picks up speed to fast, he's got you pegged dive and get out of there or be prepared to fight. If he's heading at you but at the same speed he's most likely fishing. Don't leave the fallow nearest WS on too long as the sonar won't update the rest of the convoy.
It is rewarding to sink escorts so enjoy it while you can "cause as you go to manual targeting it is much more difficult to do. That's why I suggested you play go slow and enjoy each new thing you add.
Take it easy on the ships and save a few for me.
Magic