The line on the Nav map is just a little longer than the actual contact. If you use the track closest contact command for your Sonarman he will give you constant updates. Make a mark and time 3 min 15 sec measure between them and you have its speed. You can line up and fire torpedoes without ever raising your periscope if need be. This link is a step by step I use in dense fog. I learned it from the sound trainer by Greyrider and Captain Nautilus. If you don't put up your periscope you wont be temped to sit and watch your shot while all those hunters lock on to you.
http://www.subsim.com/radioroom/show...593#post715593