That's a pretty big can of worms you opened up just there...
It's a very complicated subject to tackle effectively, because convoy engagements have so many variables.
One of the first things I did, was start reading up on the subject from historical accounts, be they the personal memoirs of the captains, or more general information about the battle of the Atlantic.
I, myself am not terribly great at shooting multiple ships at the same time, but I can offer what I have learned.
One of the most important aspects of a successful convoy engagement is the angle your boat approaches at. The ideal position is 90 degrees abeam, so that if they are sailing right to left (or left to right) the view from your scope will be like that in a shooting gallery. Almost never happens. Count yourself lucky if it does. 45 degrees off the target is good too, as long as you are ahead of it. This way, the target has fewer chances to take evasive action if the torpedo or your scope were spotted, and in all probability, will run right over it anyway.
Once you have your angle of approach worked out, the next step is to get into range. Varies with the types of torpedoes you have on board. Best chance for hits is 2 - 1 Km away. Not always attainable. This is where your experience will help you. If you are too far away at the outset, you may have to shadow the convoy on the surface, using your higher diesel engine output to run ahead of the ships, and then dive, to lie in wait...
That's if everything works out all right. When it doesn't (and it won't) you may have to deal with escort ships or aircraft, outfitted with Radar. They will pick you up, move in to investigate, and hopefully your watch crew or radar set will pick them up in time, forcing you to dive and give up your speed advantage. Very dicey, this convoy hunting business.
In a typical convoy deployment, the most valuable ships will be in the center, or as close to the center column as the formation allows. Tankers, troopships, passenger liners acting as troopships, and heavy cargo, of course. You won't get too many attacks right away so you have to make your shots count. This is where knowing how to set your torpedoes comes in. Do you fire one at a time, or spreading salvo? All at the same depth, or varying depth? Are they G7as and will leave a wake to be spotted by the target? In any case, once you have your targets picked out you have to begin the firing process quickly because these things can change for any reason, at any second. The longer you stay undetected, the better your chances to make multiple hits. Fire at the targets farthest away first. Try not to wait longer than 15 to 25 seconds the latest before sending your next fish on the way. Repeat as many times as targets worth shooting at, but you must do it quickly. If you are successful, your torpedoes should all impact more or less at the same time. It's a nice sight to see really...
But if you wait too long, you may hit one or two out of your four torpedo spread. Once those explosions start to happen, the herd starts to panic. Hard to port, hard to starboard, all stop, all full ahead, anything can happen. One thing is sure to happen though and that is, you missed with tubes 3 and 4.
Some Captains, the daring ones, will try to get as close as possible. Slip down to 20m and all stop or ahead 1 knot at silent running stations, to get through the escort screen. Once the escorts are comfortably away, up to periscope depth, secure from silent running, and prepare to attack. In some cases it's possible to attack on the surface or even from inside the convoy itself! Only time and experience will tell you when to try that.
I hope that helps somewhat, like I said, my method was to read historical accounts and try to apply what they taught me, if nothing else, use the exterior camera to help plan your attacks. And if you did all the torpedo settings manually, well, this would become a nightmare. I make most of my hits with automatic aiming.
