Hello and welcome ragupasta Cool name btw
So to answer your questions. A hydrophone is not the same as a sonar. A hydrophone can only listen. A sonar can send a sound, or "ping". The time it takes for the "ping" to bounce off the target ship and come back to your sub is range. Well U-boats can only listen. So your sound guy can only estimate the range based on propeller loudness of the target. The louder the propeller, the closer the ship is to you.
Now this leads to the sound guy being able t only estimate long, medium, or short range. No exact distance.
Now for the patrol methods. I think most people use surface search, just cruising around till you spot something and the occasional dive and listen for a few hours, then surface again, new location, dive and listen. And so on and so on
Although I do suggest more mods. The game experience will chenge dramatically, for the better ofcourse.
At least get the OPEN HORIZONS II mod.
Happy hunting.