[UnderseaLcpl]

Some convoy-finding tips;

The GWX manual (in adobe format) contains maps of heavily trafficked shipping lanes but you can never go wrong with the areas north and south of ireland or the Strait of Gibraltar. The catch is that as the war progresses you will find an unbelieveable amount of escorts, and worse, aircraft. In late 44' I was catching convoys with (I believe) 8-12 escorts of varying classes as well as Bogue class escort carriers and seemingly immediate support from fighters and coastal ASW aircraft.
When you do get to the desired area spend as much time submerged and at slow speed as possible (all day and half the night). Stay at around 40m in the daytime to preclude aircraft attacks but come to periscope depth at night to get position reports. Late in the war, in the really HOT areas, submerge to 40m (even at night) for about an hour after surfacing/snorting to preclude attacks from any aircraft you have probably alerted somehow.
If you get a contact report the rest is easy;
Use the compass to establish an area to search for the convoy based on speed and contact time(remember the distances are in kilometers but the speed is in knots. Thank God for that conversion table thing they included). If the contact is NNE ENE WSW SSW or whatever, draw a line from the center of the convoy or ship box in that direction, and then 2 more 20 degrees left and right. This is your area most likely to produce contact as GWX convoys change course frequently but their hemispheric heading will remain the same (west/east north/south)
Once you have done this guess at an intercept point. Then query your navigator as to the time to course end. Adjust your guesses until you reason yourself to be able to reach the intercept an hour or two ahead of the convoy. Haul ass there and make a hydrophone check or wait for radar signals from the convoy escorts.
You should have plenty of time to maneuver into a good position and submerge before their radar picks you up. Then its just a matter of waiting around with the periscope up (use the O-periscope it seems to detect aircraft more readily than the attack periscope, but maybe that's just me) for a merchant to steam into view on your map. Plot his course and the course of all the other lanes by drawing a line through the center of the map icon or directly along it's side. Pick the best two adjacent lanes and maneuver into position at slow speed silent running staying at least 500m away from the escorts and as far away as possible from their forward arc. In the last minutes before the convoy reaches you 1/3 speed can still avoid detection if you are behind the lead escort(s). Pick a position in between the lanes most populated by fat ships and attack with a 5-10 degree oblique angle to perpendicular. Now you are set up for both bow and stern tubes.
The method I use for targeting is one degree lead angle for each knot of target speed with fire angle set at 0. If you got a contact report you already know the speed so this is easy. If you just run into them it's a bit harder as you need to either time their speed over 3:15 or (what I do) check the map contact for a general speed and then guess on the low end ( slow 5kts, medium 7kts, fast 10 kts) Never fire at the instant the prow of the ship crosses the degree angle you select as the torpedo will often forward by a hair. If the speed is unknown you can generally deduce the correct fire angle in successive attacks. The advantage to this method is that you can target both bow and stern tubes rapidly, especially if you forego identifying the targets and just use 2m for a small ship, 3 for medium and 4 for large. This will also prevent duds due to torpedoes striking the steeply angled lower parts of the ship, ships rising in swells, and misidentification.
Finally, if using the 1kt/1degree method you can target rapidly enough to bring a good attack to bear on almost any ship. I would recommend reducing the lead angle by at least one degree after the intial attack as zigzagging reduces their speed relative to you a bit and significantly if they are in middle or a course change at full rudder.

Phew, sorry about the verbosity but I hope all or some of this helps.

Happy hunting!

P.S. On my system at least a considerable stuttering of game performance indicates you are close to a convoy