Contacts speed determining via turn count historically correct?

[raduz]

Did the american sonarmen get the speed of contact using some kind of RPM reference chart, counting the RPM? Or it was only an utopia that had no real use?

[FinnN]

I have a reprint of the US recognition manual for Japanese naval vessels (according to the intro the original was carried on a USN sub), and it does have a table of RPMs and speeds for most ships. The catch is that although the table is there, it only lists speeds for a handful of the ships, and then only for one or two values. I guess the data simply wasn't available and the sub commander might have been expected to fill in the blanks. Possibly for merchant ships more info was available.

Have fun
Finn

[MikeJW]

It was used IRL in WW2.