This is what I do.
It depends on how you find the convoy. If your watch crew sees it, you dont have much time to think out a plan. Upon getting the warning, I go straight to the conning tower (key F4). I look to see what the situation is. Where is the convoy. Is there any escorts and where. What is the heading of the convoy. Where am I in relation to the enemy. If I am far enough away (5k +), I stay on the surface and race to get ahead of the convoy. I want to be abeam (90° left or right of the ships) when I am at my firing postion so I can get good hits. If I am already abeam of the ships but far away, I turn to run a parrallel course and speed ahead at flank. I want to get right in the path of the convoy, basically I want them to pass me in front and astern. When I get far enough ahead and then I position myself so the convoy is coming at me. I just lay at periscope depth with either no speed or slow speed. I keep an eye on any escorts with the pereiscope. The scope should just be above the waves. I let the escort pass me and I wait to pick out the ships I want to attack. I go for the C2, C3, T1, T2 first. Small merchants and such are not worth my time......at the moment. I do my TDC on the ships I want to attack. When the convoy comes through I am sitting between the rows of ships perpendicular to their course. I shoot two eels at the large targets but since I have only one rear tube, I shoot that one fish anyway. Hopefully it will take out the engine or damage the ship so that it drops out of formation and I can attack it later.
I pre-occupy the torperdo rooms with crew ahead of time so that when the fish are fired, the reloading begins immeditaly. I try to have at least one or two officers qualified for torpedoes. It cuts the loading time.
Fire One! Fire Two! Fire.......well you get the idea.
Afterall all the tubes are empty, I turn hard left or right to follow the same course as the convoy. Of course, I have my sonar man listening to the escorts. I head down deep. 120-140m if there is enough water. When you get below 20m turn again perpendicular to the convoy course. You want to make as much distance from them as soon as possible.
Now you wait..............BOOM!........BOOM!..........'tor pedo missed, sir!' (not every shot works). Keep your sonar man on those escorts. Keep track of them. Dont worry about the convoy, you will never keep pace with them submerged. Just save your batteries. By 1941, convoys have at least three escorts. They might not be too good but they know how to find you.
Go to the sonar room (key H) and play sonar man for a while. Spin the the wheel and pick out all the escorts. Try to keep track of them. You should hear the merchants as well. If you see a 'unknown' show up, it means that a escort is heading your way but is far off or you disabled a merchant. If your officers tell you that you may have been detected, go to silent running (key Z). You wont be able to load torpedoes while youre in this mode so move your crew out of the torpedo rooms so they wont tire out.
Now you have two things that will happen. The escorts will not find you but drop their charges elsewhere while you make your escape or they will find you and dump on you. If the escorts are really getting your location, you may have to go deeper still. Dont always stay on the same course. Listen to your sonarman. If he says the escorts are to your left, turn right, you need to get some distance away from them.
They will depth charge for about thirty minutes to one hour before they race back to the convoy. By then you should be far enough away where you can go back to periscope depth (key P). If youre lucky, that damaged merchant should still be there. You can use the deck gun or put ONE more fish into her.
Now if you are a good U-boat captain, you should have recorded the course on your navigation map. Now you want to run about 7-8k perpendicular to the convoys course and then turn to parrellel it. Run at flank speed. Convoys only run at 7-9 knots. This is the time to be reloaded your fish into the tubes as well as from the exterior racks. With the escort attacks on you, you are probably about thirty minutes to one hour behind the convoy. This should give you enough time to recharge your batteries and reload. When you believe you are far enough ahead of the convoy, turn back to the course it was on in the beginning. After reaching the spot where the convoy should be, you can submerge and see if you can hear the convoy. If not, surface and progress back towards the convoy (head on collision course). Your watch crew should warn you before the lead escort sees you. I usually dont run time skip at this time.
Now repeat what you did the last time. You should be able to take out two more ships and disable another.
Repeat and then rinse.
Of course, this is just one way of doing it. It may not work for you due to your setup.
I run SHIII with GW mod only so your results may vary.
