The U-boats did use hydrophone based speed evaluation

Subnuts · 09-01-13, 07:28 AM

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.

sublynx · 09-01-13, 07:36 AM

Quote:

Originally Posted by Subnuts

Yeah, but was it as easy as dividing the RPM by a fixed number for a particular ship type?

Incredible the amount of expertise in this forum

Great info, thanks!

I also have a faint memory of reading some convoy reports some year ago that mentioned that the ships in the convoy changed their RPM's in an unpredictable pattern to confuse the enemy. Speed is also a factor that is dependent on sea state too. So it's not as simple as in the game, but it is something that could be used in giving a rough estimate.

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, 07:28 AM	#1
Subnuts The Old Man Join Date: Feb 2005 Location: Connecticut Posts: 1,658 Downloads: 14 Uploads: 0	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. __________________ My Amazon.com reviews

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09-01-13, 11:40 AM	#3
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.