Hydrophone hunting - German U-boat counting screws

[sublynx]

How realistic is hydrophone hunting?

According to the war diary of U-853 in:
http://www.uboatnet.de/Artikel/Krieg...3_Seite013.htm

the GHG operator counted the revolutions per minute of a ship audible in GHG ("120 U/min") and estimated a fast speed.

According to the US Navy's Submarine Sonar Operator's Manual dated in June 1944, chapter 5:

http://www.hnsa.org/doc/fleetsub/sonar/chap5.htm

the sonarman was supposed to report the revolutions per minute and a verbal speed estimation.

The USS Enterprise's deck logs mention the speed of the ship and the RPM's of the ship together.

http://www.cv6.org/ship/logs/log19421026.htm

Technically I think it would have been possible for a U-boat to listen to for example Ark Royal's or Nelson's RPM's and evaluate it's speed simultaneously with visual methods giving it an estimation of the RPM's the ship needed to achieve a certain speed. In peace time this kind of intelligence gathering could have been done quite freely and safely.

What I am interested in is the question: are there any known instances of this being done? Did WWII submarines ever evaluate more exact speeds just by RPM counting? Do modern submarines do that?

I would expect that hydrophone hunting would not have been used, as the war diaries, POW reports and captured U-boat equipment don't mention that kind of activities reported or charts found. Has anybody heard or read otherwise?

[JazzJR]

Well it was possible to the crew the get information about speed, range and bearing of the ship. Under water you will hear more then your watch-crew is seeing.

Thats what a read Hydrophone would sound at a freighter

If you are a trained crew and know what you are doing you would get a lot of informations from this sound.
In the war diary of U-853 is written "leaving port to listen around" Page 5 24.3. 0600
Also on Page 9 "Battle Sations! low sounds of a reciprocating engine in GHG. Nothing to see... Dismiss Battlestations!"
On Page 10 is written "Hydrophone reports at 2139 °Clock bearing 90-110 degree a novel sound like from a buzz saw. Which becomes louder and soften again and also turn its pitch. Bearing is at 290 degree. I presume it is a Noise buoy"
Page 13 "Screw sounds in GHG. (120U/min) traveling fast from starboard to port side ahead. Far away. Above , nothing to see"

Modern submarines doesn't use Hydrophones anymore. The sonars are so far developed, that a Hydrophone would be out of place. Today mordern submarines are using:
passive low-frequency towed array sonar,
passive low-, and medium-frequency hull-mounted flank array sonar
mine detection sonar

Even the Periscope is using a thermal sight with laser range-finder. So no need to get information like in the old times. The new ones can also dive more then 700meters and stay there for weeks.

The Informations are from the newest generation of german submarines "Klasse 212A"

[sublynx]

Thanks for the translation and the info

Wheww, 700 meters... Incredible depths

[JazzJR]

Originally Posted by sublynx

Thanks for the translation and the info

Wheww, 700 meters... Incredible depths

No problem

Yes 700 meters is the destruction depth of the 212A submarine
compared with the Virginia class (USA) 240 meters
Russian Typhoon class 400meters. Remember that the Typhoon is the largest sub"cruiser" ever build and has a swimminghall , a sauna and a lot of other nice things on board

[Sailor Steve]

Pay close attention to the words "estimate" and "fast". Do they ever give a precise speed? The US had charts that supposedly did that, but in fact no one was ever able to fire submerged and blind accurately enough to make it common practice. Yes, they could track ships underwater, and in fact SH3 already gives you those speed estimations, but a full underwater attack was not feasible at that time.

[SubConscious]

I'm a complete novice at topics like engineering and acoustics, so please pardon if I ask questions that may have answers that are obvious to others, but I can't help but wonder how one could ever determine even a good estimate of speed from RPMs.

Perhaps my problem is that I can't differentiate propeller sounds from the engine noise. I would think that even knowing the exact ship, engine noise could be impacted by wear, damage, refitting, lubrication levels, and how the draft of the ship. My guess is that propeller RPMs would be influenced by the size of the propellers, shape of the blades, wear/damage of the blades, the draft of the ship and the weather (e.g., an unloaded ship in heavy seas could experience the propellers breaching the surface of the water - I would guess the RPMs would be higher without water resistance and also difficult for a sub to hear).

I'm intrigued. How does one count RPMs and estimate speed?

[Canovaro]

Originally Posted by SubConscious

I'm a complete novice at topics like engineering and acoustics, so please pardon if I ask questions that may have answers that are obvious to others, but I can't help but wonder how one could ever determine even a good estimate of speed from RPMs.

Perhaps my problem is that I can't differentiate propeller sounds from the engine noise. I would think that even knowing the exact ship, engine noise could be impacted by wear, damage, refitting, lubrication levels, and how the draft of the ship. My guess is that propeller RPMs would be influenced by the size of the propellers, shape of the blades, wear/damage of the blades, the draft of the ship and the weather (e.g., an unloaded ship in heavy seas could experience the propellers breaching the surface of the water - I would guess the RPMs would be higher without water resistance and also difficult for a sub to hear).

I'm intrigued. How does one count RPMs and estimate speed?

I guess you can hear the engine sounds much better than the prop screws. The chugging sounds in SH3, isn't that the engine sound rather than the prop? I bet it is.

[sublynx]

Originally Posted by Sailor Steve

Pay close attention to the words "estimate" and "fast". Do they ever give a precise speed? The US had charts that supposedly did that, but in fact no one was ever able to fire submerged and blind accurately enough to make it common practice. Yes, they could track ships underwater, and in fact SH3 already gives you those speed estimations, but a full underwater attack was not feasible at that time.

I tend to agree with you here. I have never come across any specific speed information.