Hello Seaveins!
I hope this advice can help you but these are just my personal methods. Some of the cleverer Kaleuns probably have better methods.
1) Location, location, location; I always hunt convoys in places I'm bound to run into them, principally grids am52 and bf 15. There isn't much room for them to slip past me in bottlenecks like that. Just west of the Gibraltar Strait is another good locale but I seem to run into single ships much more often than convoys there.
2) Patrol your killzone of choice at periscope depth by nightfall, going deeper in the daylight to preclude aircraft from happening across you and bombing you to bits. It doesn't happen often but it can happen. Keep the scope down and ABSOLUTELY NO SCHNORKEL! I am convinced that the schnorkel is not made of metal, but some strange magnetic substance that attracts every plane, destroyer, and patrol boat for miles around.
You most likely won't have too much trouble with enemies locating you while patrolling until later in the war but the above are good S.O.P.'s to form habits around.
3) If you are at periscope depth, in addition to having a much greater detection radius, you can still get contact reports. This helps immensely.
Once a contact develops pick the first "merchant" bearing line and track it at 8x tc
until you can develop a general heading for it. Just use your pencil and plot the end of the line whenever it moves. Make sure you are zoomed in on the map to a fairly close level as the bearing lines at lower zoom levels tend to be off a bit. Take as many plots as you need to be confident that you know the convoy's general heading.
4) Pick an intercept point. I'm afraid this requires a little practice and some brainwork. First thing to account for is convoy speed. If you get a contact report this has already been solved for you but your hydros will only give you slow, medium or fast, as you know. Assume slow is 6kts, medium is 9 and fast is at least 12. I don't think I've ever seen a fast convoy.
Next, plot a rough intercept course at full or flank speed on the surface and query the N.O. for "time till course end". Next, use the convoy's speed and the compass tool to see how far the convoy will move in the time for you to reach your destination. I use a trial and error process until I get a "course end" that is an hour or two ahead of the convoy. Don't get much farther ahead than that because there is a good chance they will change course and then you're back to square one.
5) Haul a$$ to the intercept and dive to periscope depth. Put the e-motors on slow ahead. Try to gauge from the "fan" of bearing lines if the convoy is still making way towards you. Obviously, the fan will get wider if they are approaching you head on.
Eventually, you will develop an eye for the contact lines and can more or less assess the layout of the convoy just by looking at the bearing fan.
Pick a side to attack from and try to line yourself up with the outermost edge of that side of the convoy.
Using the compass, check the distance between yourself and the lead edge of the convoy periodically. When they are about 4000m away begin doing brief periscope checks. Use the nav map and the ship plots that will appear on it to draw lines along each column's tracks. This way, when you have the scope down or are deep during the attack you will still have a good idea of where the ships are.
6) Pick two adjacent columns to attack and nominate target ships. If there are no desireable targets on your side of the convoy it is usually best to let it pass and then run around in front of it again rather than trying to zip over to the other side before the lead escort gets too close. He will almost always detect you unless the sea is very rough.
Also, calculate the speed of the convoy precisely if you don't have it already. If you don't know how to do this I'm sure someone will be happy to explain it to you in this thread or there are several other threads on that topic.
When you have decided which ships to attack, position yourself in between the target tracks, put the scope down and wait. I personally don't go "all stop" I just go ahead slow and back slow to station-keep but feel free to stop the motors if you wish. If you're not already in silent running, toggle it now.
7) The key at this point is good, quick shooting. Since all my torpedo deflection calculations are done in my head I never have a problem with this, but if you must use that newfangled TDC contraption, set it up now. I personally don't really know how it works but I assume the solution for a 90AOB shot at a target with a known speed from about 500 yards is probably pretty easy.
You can peek with the scope for short intervals but keep it as low as possible. If even one ship sees that thing you'll blow the whole attack and their zig-zagging will make things harder for you. Plus, the escorts will be headed your way, naturally.
8) As you develop your skills you shouldn't have to do as much pen-and paper crap to intercept and successfully attack, but it still helps on occassion.
9) Once the attack is completed, dive underneath one of the convoy lanes (this is one reason why we drew those lines along their tracks
)or pick an escape vector, go deep and stay quiet. Keep moving but only at ahead slow, that's the only speed that can fox their hydrophones. The escorts will have difficulty accquiring you or holding on to your contact as they try to avoid colliding with their merchantmen and each other. If you have BOLD, drop one every once in a while, but save at least five just in case a persistent escort manages to hang onto you. I won't discuss evasion tactics here but there is a lot of good information about this on the forums and some user websites. Try Brag's page, it's good.
10) Final word of caution. If you are compromised or in danger of being compromised it is best to just abandon the attack and try to run around again. If the convoy changes course, don't try to get into an attack position submerged or increase speed, you'll just get caught and blasted.
11) If you like, there are also some good threads on attacking escorts. Obviously, if they are underwater, you can do whatever you want with the convoy.
Whew! Sorry so long but I hope it helps a bit.
Happy Hunting!