If, when you say "convoy", you mean groups of more than one ship sailing together then I would have to say that I don't attack them. That would be due to the fact that in all of my patrols I've seen dozens of single merchants and ONE pair of small cargo ships sailing together. I only had one stern eel left at the time, and in rough weather I could never get to a position to fire at either of them.
I think this whole convoy thing is just another urban legend perpetuated on the interwebs.
Tony
PS: There are highly detailed accounts of American skippers destroying entire Japanese convoys during the war. The general tactic was to make the first attack the biggest. They would calculate solutions for a different target for each forward tube. They would fire farthest to nearest with enough lag between shots that all torpedos would hit at approximately the same time. I used to have a nomograph for calculating the lag when I played SH1. They would then sprint out to the edge of visual range (or beyond once they got radar), and make course for a new intercept. At that point they could generally only get one or two shots per attack due to the convoy being allerted. In some cases the process went on for two or three days until all targets were sunk.