Contact reports work really great in this game. SitRep on the other hand....
Just last night I had made an attack on a convoy and sunk one ship. After the DDs left, I set ahead flank to go after them again. Once I could see them, I sent in a contact report so other subs in the area could help out.
COMSUBPAC told me that destroyers were primary targets so that other subs could have a turkey shoot. So I steamed up all ahead flank right up their whazoo until the escorts realized they picked up another ship in their formation. Once they started firing at me I crash dove and headed right towards one of them at flank. The other was madly steaming up at my 6, but he wouldn't make it in time.
Long story short, 2 dds to the bottom and a clear convoy for other subs in the area!