Since I use realistic tactics (no deck-gunning of escorts, deck gun only to finish off single merchants, etc...) and (lately) the NYGM mod I've never completely annihilated a convoy.
This is actually one of my biggest beefs with SHIII: back in AoD the greatest achievment was shadowing a convoy until the wolfpack was assembled. One of my favourite moments in gaming is still a concentrated attack by 5 U-Boats at night against a well-escorted convoy in September 1942.
The lack of wolfpacks pretty much killed SH3 for me as far as "classic games" go.
To this day I can't understand why the devs implemented Luftwaffe attacks but didn't manage to include wolfpacks...
