The U-boats did use hydrophone based speed evaluation

[sublynx]

Ever since Raduz's Hydrophone hunting tutorial there's been controversy among us reality buffs about whether the Germans used hydrophone for assessing ship speeds.

Well... Have a look at these pics I took from U-552's 6th war patrol KTB:

http://i.imgur.com/tWDIsku.jpg
Topp's description: "200 RPM = 20 knots"

http://i.imgur.com/OCEvCLX.jpg
The torpedo firing report

To me this looks clear. Topp was submerged, the hydrophone operator counted the 4-stacked destroyer's engine revolutions per minute and Topp estimated that 200 RPM corresponded to a speed of 20 knots.

I haven't seen this in other war diaries I've read, so I believe hydrophone speed estimating was a rare occurrence. But I also believe that estimating the speed of ship by a hydrophone was not something that Topp just invented then and there. If he had, he had written more about it - now it looks just like a standard practice that needs no explanations whatsoever. Just the "200 RPM = 20 knots".

Here's the link to the war diary:

http://www.uboatarchive.net/KTB552-6.htm

Anyone interested in giving back to the SH3 community? There would be a need for someone to make the RPM / speed charts for the Warships in SH3. The charts for merchants have already been done for GWX and NYGM by Canovaro and CapSea - now we would need to get the info for those pesky Black Swans and Flower corvettes etc :ping:

[Sailor Steve]

Good job! New information is always welcome. :sunny:

[Ligne Maginot]

In this article written by Hitman, you can read that not only the U-552 implement this tactic. There are also references in logbooks of U-505.
Determining the speed from the revolutions account provided only a general idea about the speed of a ship.

http://www.u-historia.com/uhistoria/tecnico/intecnico.htm

The article is called "Adquisición de datos para las soluciones de tiro en los U-boot", in the technical section. The page does not let me give the direct link.

Best Regards. :salute:

[Borgneface]

S! Very interesting document. I really appreciated reading it.
and thanks for sharing it and the link to the diary.

Great investigation work

Merci:yeah:

[Leandros]

Wikipedia: "Propeller RPM: The speed of a ship is roughly proportional to the rotation rate of the propeller. This approach is most useful when the propeller has constant pitch. It is less useful on ships with propellers that have variable pitch propellers or no propellers (like sailing ships). A variation of this approach was used by submarines during World War II to determine the speed of targets. The sonar operator would listen to the sound of a target's propeller and would determine its rotation rate. Knowing the propeller rotation rate and visually identifying the type of ship, the target's speed could be estimated".

This would be a nice mission for one of the excellent modders here: A solution for deciding propeller RPM's taken from the sonar and, consequently, estimating the ship's speed compared with the ship type as observed in the periscope....

Fred

[BigWalleye]

American boats certainly used hydrophone RPM counts to determine speed. Dick O'Kane has a couple of lengthy passages describing how he rigged a mic and a shaving brush so the sound man could broadcast the simulated propeller sound over the 1MC. The entire approach party trained in determining RPMs from the beat count. And Paul Schratz (IIRC) describes the sound man counting out the beats to get Rs. I have no evidence, but I find it likely that the KM did the same. I always assumed that the "Moving slow." portion of the sound man's report was based on R counts. Certainly couldn't get it from BRA at first contact.

[Leandros]

Would have been interesting to know if Topp was actually correct in his speed estimate of 20 knots. First off I react a little, considering that the Town class top speed was around 30 knots. Having a deplacement hull I should think it would then need almost 400 RPM's to achieve 30 knots, given that 200 RPM's gave 20 knots.

That is a lot of RPM's on such big propellers in water. Just my opinion.

Fred

[sublynx]

This would be a nice mission for one of the excellent modders here: A solution for deciding propeller RPM's taken from the sonar and, consequently, estimating the ship's speed compared with the ship type as observed in the periscope....

Here you go: a version of NYGM merchants
http://www.subsim.com/radioroom/showthread.php?t=193770

and GWX merchants
http://www.subsim.com/radioroom/showthread.php?t=186854

The warships haven't been done and now with all the info you others have provided I'm thinking a warship version should also be done.

It should not be totally accurate maybe, as the Kriegsmarine would have had no easy way of measuring what the precise values were. They could have measured the precise values of their own destroyers, cruisers and battleships easily though and use those values as proxy for a crude estimation of what the Royal Navy warships would have been able to achieve

[Missing Name]

American boats certainly used hydrophone RPM counts to determine speed. Dick O'Kane has a couple of lengthy passages describing how he rigged a mic and a shaving brush so the sound man could broadcast the simulated propeller sound over the 1MC. The entire approach party trained in determining RPMs from the beat count. And Paul Schratz (IIRC) describes the sound man counting out the beats to get Rs. I have no evidence, but I find it likely that the KM did the same. I always assumed that the "Moving slow." portion of the sound man's report was based on R counts. Certainly couldn't get it from BRA at first contact.

US sonar operators received training in techniques and had equipment to facilitate accurate prop counts.

Have some training records from WWII. (http://www.hnsa.org/sound/#jp) Fascinating stuff, really. (Note: recordings contain typical wartime anti-Japanese slurs, may offend some)

[BigWalleye]

Don't know about the equipment part. O'Kane claims he bought a metronome in a music store in San Francisco, just for his sound man to use. Accounts I've read made it seem like it was widely used, but not doctrine.

[sckallst]

Don't know about the equipment part. O'Kane claims he bought a metronome in a music store in San Francisco, just for his sound man to use. Accounts I've read made it seem like it was widely used, but not doctrine.

That's my impression as well. Lots of times things are done in the fleet in times of war that are not fully signed off on/developed by the doctrine guys in the shore establishment. The "what works" lessons often are fully implemented and bureaucratized until after the shooting is done.

[Missing Name]

It was mainly in the form of special filters to isolate all noise except for the high-frequency components of prop beats, making speed estimates easier and more accurate.

[Canovaro]

Very interesting find!

[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.

[sublynx]

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 :know: 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]

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.