This is what I learned from the sound trainer. It is quite helpful.
Once I make sonar contact I set my Sonar Operator to track nearest contact.
Then I make a mark, with the map at 500m, and start my stopwatch. (I use a real one)
After 3 minutes & 15 sec. I make anouther mark. Measure the distance and that is their speed. (600m = 6kts, 900 = 9kts, ect.) It's called the 3min rule.
Now you also have the course. line up at a 90 degree angle to that course.
Come to a complete stop. I try to get to about 500m.
All this time your SO is calling out the targets bearing. this will be vital soon.
Make sure your periscope is set to 0. It does not need to be raised above water, just enough to see the numbers.
Set the speed on the TDC, AOB is 90 either Port or Stbd, and then the distance (ie 500m).
Look at the TDC view, hit the question mark (can't remember what it is called), the torpedo line will be offset from zero. That is your lead. move the periscope that many degrees in the oposite direction. Go back and it will line up with zero ( it will be 345 or 15 or something like that depending on how fast they are going). Make note of that bearing, that is when you will fire. Good time to open your tube/s.
You are ready for action. When you SO calls out that bearing FIRE.
I have not missed with this method.
Some things to think about.
You wont see this target, so you wont know whos side they are on. Make sure you are somewhere at least that is not likely to have axis merchants around. Also if you have downloaded the sound trainer the documentation comes with charts that list vessel types and prop rotation. you know the speed so if you count the prop rotations you can have an idea of which type of ship it is. This will tell you if you will need one torpedo or four to make the kill.
I hope this helps it sure is fun and so far I have not sank any friendlies
(blame it on a mine if you sink a nuetral) Oh and avoid their capital ships after that too.
So good luck and happy fog hunting.