[CaptainD]

Thanks for the answer!
I think that this urban legend comes from movies when you can see the sonar man giving a speed in knots abouts targets or DD.
I am talking about WWII movies of course and about the sonar/computer system I remember having seen something about a system like that in the movie "The Hunt for Red October".
For the same use in WWII, may be it was possible if the target has been precisely identified.
In that case, knowing the performances of the target, the screw speed could be interpreted to give a target speed... or not.
It also depend on the ability and skills of the sonar man.
I remember having read that a sonar man of the HMS Unbroken, Lieutnant Command A. Mars, had spécial capacities.
One times, jailed in the bow torpedoe room for some blunder, he eared foe DD asdic without any equipment.
Of cours, nobody aboard the sub believed him.
As the man continued to talk about the pings he eared, L. C. Mars ordered the sub some feets up and then the sonar man on duty eared in his headphones the pings coming from an Italian or German DD somewhere over the sub.
The man was immediately extract from is cell and thrown at his duty place.
A friend who had been on duty on french submarine told me that people on charge of sonar service were able to ear "The grass growing".
I think that L.C. Mars sonar man was close to that.