There is an other way to get the exact distance from any target and keep it there when ever you need it..
In the set key program (Get at
http://u-boot.realsimulation.com/ in the general section) to assign your own custom shortcut keys look for (Sonar Ping) and set it to {Crew and what ever key you want}. Then in game when you come across a ship go to the hydrophone and find the target you want then use the shortcut you made for (Sonar Ping) and it will give you the exact distance from the target every time you hit your Short cut key,
However it wont give you the bearing and if you go back to the Hydrophone you will lose the target, but you can go deep and use the ping to find out how far the target is from you at any time as long as you are not on the hydrophone. This is real nice trick to know if you are being pursued by a Destroyer, but you can only ping 1 target at a time so if 2 or more are after you then you can only keep an eye on one