Authentic speed measurement in SH4

[Rockin Robbins]

Unfortunately, in SH4 the tpm does not correlate to the speed of the target well enough to use. Same with SH3, and anyone who has tried it has abandoned the attempt. As a practical matter, only relative speeds, "target is speeding up" or "target is slowing down" could be determined because turns per knot was not a known quantity like it is today.

A large part of what modern subs do is monitoring known ships and determining turns per knot for that individual ship. Now these are in a database allowing sonar to identify thousands of ships by name and pretty precisely peg their speed from the accumulated database. That wasn't possible in WWII.