Really, really easy way to attack a convoy is from head on. Charge at them at full flank speed, then when you get close enough for a visual or at under 10nm dive as deep as you can and go as quiet as you can. Cruise towards the convoy dead silent and the escorts wont know your there. If possible get under a thermal layer, as it scatters sonar and machine noises.
Then once your sound guy lets you know you past the front escort pop up to periscope depth and let rip. Expend all torps in the tubes, and turn to the same course as the convoy. Once your in between the convoy ships the escorts cant really find you due to all the other ship noises, but just be aware of Sonar pings and try to keep yourself near convoy ships to use them as cover.
If you get "pinged" go under a convoy ship and tag along with it. The AI cannot work out wtf you are doing when you sit under another ship.
Happy hunting.
