The U-boats did use hydrophone based speed evaluation

[Subnuts]

Yeah, but was it as easy as dividing the RPM by a fixed number for a particular ship type? I'm looking at the "Anatomy of the Ship" book for the Hood, and it lists some trials figures, including speed vs. RPM.

Speed/RPM
13.5/80
15.6/93
17.2/103
20.4/124
25.2/154
27.8/176
29.7/191
32.1/207

In this case, it's about 6 revolutions per knot throughout the whole speed range. However, I've also looked at trials data on older ships with direct, rather than, geared propulsion. Dreadnought's turns-per-knot seemed to vary from anywhere between 12 and 16. Also, the difference in RPMs between the outboard and inboard shafts could be as much as 100.

Of course, most of the warships you're liable to encounter in SH3 will have geared drive, and therefore, easier to calculate their speed by propeller rate alone. Still, there are plenty of merchant ships, and smaller combatants which didn't have geared drive, which would make their speed harder to calculate if the sim didn't use fixed turns-per-minute.