Hi Brett:
I have found that in SHIII your best bet is to attack escorts rather than try and evade them. You can evade them if you have a 100% undamaged sub so you can go to 210 or so meters depth (if you have the depth), and if you have rough seas. Otherwise, you are going to take a pounding that just wears you down until you are finished.
When a convoy is steaming unmolested, they usually drive in a straight line, including the escorts. So hitting the first escort is easy.
After that, you need to get their attention so that they will chase you. Put up your scope (pull it down if they start shooting at it though!). Ping away on your depth finder. Run at flank speed. Suspend silent running. Then turn to point your stern EXACTLY at them. When you look at the destroyer through your periscope it should be lined up exactly on 180 degrees.
Identify the class of warship in your ship identification manual. Then set your torpedo to run 1m deeper than their keel depth. Set for magnetic pistol. Open your outer door and be ready to fire. When they get to around 600m range, fire your torpedo. Assuming you don't have a premature detonation from a faulty magnetic pistol (and they almost premature at such short ranges), you will blow the bottom out of them.
If you have enough time to set it up, you can also run this scenaior "in reverse", by driving your sub at "back emergency" and pointing your BOW directly at an oncoming destroyer, and getting them to drive in exactly on 000.
If the escort chasing you is zig-zagging, when they close to 200m drop a noise maker. As the escort goes over the noise maker, they will reverse engines and "hover" over the noisemaker for as long as it is active. That is your chance to bag them.
Note that this type of game play is "gaming" the system and taking advantage of the way escorts and such work in the game. But if you want to see May 1945, it works!
Steve