Hi mate! A good tool to use to train yourself before getting into the hardcore manual targeting world, is an intercept calculator. Here is one made by David Sandberg:
http://files.filefront.com/intercalk.../fileinfo.html
And remember: Take in account that if you fire your torpedoes with the torpedo tube doors locked, this will affect the calculations you have made. Always open the tube doors before firing, to get the best result! Good luck, matey, and happy hunting!
Edit: Timing a ship:
The 3:15 rule:
When you spot a ship, mark its position on the map with an x. Start the stopwatch, and when the clock reaches 3 minutes and 15 seconds, mark the ships position again. The distance between those two points indicated the speed of the ship. IE: If the ship has travelled 1200 meters, its speed is 12 knots. If it has travelled 800 meters, its speed is 8 knots.
The 90 degree way:
Position yourself at an 90 degree angle of the target. When the bow of the ship passes your crosshair (scope/uzo) start the stopwatch. let the watch run until the ships stern passes your crosshair, then stop the clock. Now: Take the length of the ship (see in rec.manual) and DIVIDE it with the time, then multiply with 1,946. If the ship is 91 meters, and the time it took to pass with one shiplength is 32 seconds, the figures will look like this:
91 divided with 32 = 2,84... Then multiply 2,84 with 1,946 = 5.53 knots!