I always try to get ahead of the convoy and let it come to me. I hover a periscope while silent running. When the juicy targets sail by I let loose my little friends.
Of course, I may have to adjust my boat a little, but when all is said and done, I barely moved. It's like a buffet that comes to you. When all my fish are expended I sneak away and reload. I usually only make one pass on a convoy. I don't like to stalk.