Nice work!
Just the other day I came across a convoy with only two escorts, and a S class sub!

Sank all three, sufaced and had my merry way with the convoy! All that was left was a small merchant from Sweden (Would be in my rights to sink him, but I need someone to tell Churchill the bad news!

) Bagged another ship on the way home, which brought me home with over 100,000 tons and some tin and ribbon, as well a a promotion or two!