Let's consider the Dreadnought data and apply them to a merchant with a top speed of 15 kts. Assume that the deviation is linear. That is,
5 kts = 12 RPM/kt
10 kts = 14 RPM/kt
15 kts = 16 RPM/kt
That's a spread of +/-15%. If we use the median 14 RPM/kt to calculate speed, based on what we hear in the hydrophones, we get
5 kt actual = 4.3 kt measured
7.5 kt actual = 7.0 kt measured
10 kt actual = 10 kt measured
12.5 kt actual = 13.4 kt measured
15 kt actual = 17.1 kt measured
Operationally, that's probably as good as the fixed-wire method or a three-minute range and bearing plot. Not perfect, it but could certainly be used to aim a spread of two or three torpedoes. And, if time and circumstances allow, it is always good practice to use several methods to develop a firing solution.
Conclusion: a 15% variance in RPM/kt over the speed range of a ship should be good enough to allow the use of RPM-based speed estimates in developing a firing solution.
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