Stay outside of that grey circle, run paralell to them at flank speed (Large convoys travel at 4 or 5 knots in rough waters, 6 to 8 knots in clear water, never trust the map speed indicator), your sub should have no trouble overtaking them. When you've pulled in front of them flank speed in front of the grey circle at a 45 degree angle, submerge and wait for them to pass in front of you, try to get a 90 degree shot from less than 1000M if possible. Dont keep your scope above water during the whole approach, pop it up and down until youre ready to fire, use the hydrophones to keep track of the target ships. Launch all torpedos and immedietly dive and turn away to set up for another paralell run on the convoy after the crew reloads your torpedoes.
Attack at night whenever possible. i prefer close range submerged attacks but if you're afraid of the destroyers a surfice night attack will keep you away from the asdic.
April 7 1940 (Sitting in narvik...... waiting......) and a destroyer hasnt so much as pinged me.
I caught some damage once running away on the surface after being spotted, who knew those ships had a 8KM gun range!? And could do 40 knots.... ugh