How realistic is hydrophone hunting?
According to the war diary of U-853 in:
http://www.uboatnet.de/Artikel/Krieg...3_Seite013.htm
the GHG operator counted the revolutions per minute of a ship audible in GHG ("120 U/min") and estimated a fast speed.
According to the US Navy's Submarine Sonar Operator's Manual dated in June 1944, chapter 5:
http://www.hnsa.org/doc/fleetsub/sonar/chap5.htm
the sonarman was supposed to report the revolutions per minute and a verbal speed estimation.
The USS Enterprise's deck logs mention the speed of the ship and the RPM's of the ship together.
http://www.cv6.org/ship/logs/log19421026.htm
Technically I think it would have been possible for a U-boat to listen to for example Ark Royal's or Nelson's RPM's and evaluate it's speed simultaneously with visual methods giving it an estimation of the RPM's the ship needed to achieve a certain speed. In peace time this kind of intelligence gathering could have been done quite freely and safely.
What I am interested in is the question: are there any known instances of this being done? Did WWII submarines ever evaluate more exact speeds just by RPM counting? Do modern submarines do that?
I would expect that hydrophone hunting would not have been used, as the war diaries, POW reports and captured U-boat equipment don't mention that kind of activities reported or charts found. Has anybody heard or read otherwise?