The U-boats did use hydrophone based speed evaluation - Page 2

BigWalleye · 09-01-13, 11:40 AM

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.

09-01-13, 11:40 AM	#16
BigWalleye Sea Lord Join Date: Jul 2012 Location: On the Eye-lond, mon! Posts: 1,987 Downloads: 465 Uploads: 0	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.