If the hydro-operator says "long range" when the target crosses the exact 20km radius it would be a big spoiler and far away from reality. Range based on listening to soundlevel is everything but exact science. Too many variables. I allready have a problem with the hardcapped max range of 34km. If you hear it yourself, you know it is within that. If not, you can be sure nothing is within that range. The max range should extend out beyond the range a screw would be audible.
I disagree though that you would need to be in the hydrophone-shack all the time if you do the listening yourself. Just a quick listen and go about your business again. If you found something just head towards it or lead it some in the direction of the drifting sound. Checking again some 15-30 minutes later for a changing bearing and correct your course.
But if you want to know a bit more about it's motion before sprinting towards it try this:
http://www.subsim.com/radioroom/show...260#post875260