Detection of torpedoes by hydrophones/sonar

[Commodore Sandurz]

Reading the Designing Warships blog post, the following caught my eye:

Quote

As technology progresses, towers may include Hydrophone or Sonar Stations which increase the detection range of underwater objects (Torpedoes, Submarines and Mines).

Specifically the torpedoes part. Does anyone have any info about non-visual torpedo detections in WW1 or WW2 that resulted in a warship successfully avoiding a torpedo attack? To my knowledge, torpedo detection by a moving warship at any meaningful range by hydrophones or asdic in the WW1 to WW2 era was improbable. For hydrophones, while there were systems with a "torpedo detection" mode I'm not aware of any actual successful use of this to detect, identify and avoid wartime torpedo attacks. I suspect that these were "brochure" capabilities; perhaps a lone escort moving at slow to moderate speed in sea state 1 or 2 could detect and identify an inbound torpedo, though whether it would have sufficient time to accelerate and maneuver to avoid it I would question. But I doubt this being realistic for a fleet moving at speed.

For WW2 active sonar with it's limited fov this would be even worse; it would need to be pointed in the right direction, detection ranges would be extremely short for torpedoes, and the operator would need to identify the contact as a torpedo.

From my readings, even in the Falklands war 40 years after WW2, none of the submarine launched torpedoes from either side were detected by their targets or escorts.

So I'm hoping that this isn't really an effective capability in the game as I would find it highly immersion breaking if ships are regularly able to detect and avoid torpedo attacks by just equipping some hydrophone/sonar equipment. Or at least, that there is some way to minimize or remove it by modding. I admit that the blog post doesn't outright say that the detection range would be sufficient to successfully evade so perhaps I am worrying about nothing. But I thought I'd bring this up as perhaps someone does have information about the successful use of hydrophones or sonar to detect torpedo attacks in the WW1-WW2 era.

 

[TBRSIM]

On 5/5/2019 at 9:13 PM, Commodore Sandurz said:

Reading the Designing Warships blog post, the following caught my eye:

Specifically the torpedoes part. Does anyone have any info about non-visual torpedo detections in WW1 or WW2 that resulted in a warship successfully avoiding a torpedo attack? To my knowledge, torpedo detection by a moving warship at any meaningful range by hydrophones or asdic in the WW1 to WW2 era was improbable. For hydrophones, while there were systems with a "torpedo detection" mode I'm not aware of any actual successful use of this to detect, identify and avoid wartime torpedo attacks. I suspect that these were "brochure" capabilities; perhaps a lone escort moving at slow to moderate speed in sea state 1 or 2 could detect and identify an inbound torpedo, though whether it would have sufficient time to accelerate and maneuver to avoid it I would question. But I doubt this being realistic for a fleet moving at speed.

For WW2 active sonar with it's limited fov this would be even worse; it would need to be pointed in the right direction, detection ranges would be extremely short for torpedoes, and the operator would need to identify the contact as a torpedo.

From my readings, even in the Falklands war 40 years after WW2, none of the submarine launched torpedoes from either side were detected by their targets or escorts.

So I'm hoping that this isn't really an effective capability in the game as I would find it highly immersion breaking if ships are regularly able to detect and avoid torpedo attacks by just equipping some hydrophone/sonar equipment. Or at least, that there is some way to minimize or remove it by modding. I admit that the blog post doesn't outright say that the detection range would be sufficient to successfully evade so perhaps I am worrying about nothing. But I thought I'd bring this up as perhaps someone does have information about the successful use of hydrophones or sonar to detect torpedo attacks in the WW1-WW2 era.

 

The only regular and doctrinal torpedo detection in the WWI/II era by sonar was by the launching submarines, the sonar operator usually listened when a torpedo was launched to determine if it was "running hot, straight and normal". This was a safety measure to enable evasion if a circle run (e.g. due to a defective gyro) occured. But those submarines were at far lower speeds than any target with scant flow noise on the sonar when doing this and the torpedo's propulsion system blasted its noise directly at them at very close range.

Even today Torpedo Detection, Classification and Localisation (TDCL) is by far not an universal capability of naval ships. Outside of ideal conditions and low speeds passive sonar based torpedo detection systems encounter false alarm rates approaching unity, i.e. thery are worse than useless. More modern approaches incorporate active sonar but fielding is "in progress"...

Just have a look at the programs of topical conferences such as UDT and at the DOTE's annual reports of the last decade or so to get an idea, all this is public information if one knows where to look.

As described on the website the whole "torpedo detection" aspect is massively immersion breaking. And, as the game only goes to 1936, most if not all torpedoes will be wet heaters ("steam") anyway and come with wakes that are distinctly visible in all but the highest sea states. Visual torpedo detection and the quickness of evasive action could be influenced by "tower" design and the related research, more size for additional "torpedo/periscope" lookouts (looking at the sea, not the horizon), more and better binoculars, sound powered telephones for lookouts, a "lookout doctrine" such as quartered optical searches, "step aside" reporting/command language etc. could reflect increasing awareness of the torpedo threat and enable quicker "detection" of torpedo wakes. But much/most of this would be doctrinal and "soft" tech that would be introduced navy-wide regardless of the individual ship's design and without the need for redesigns/refits.

Now, if destroyer (and perhaps CL) "towers" would reflect submarine (as opposed to torpedo) detection capabilities (i.e. the anti-submarine sonar hardware fit) that could work, but tying sonar to the "tower" feels contraintuitive.

Edited May 11, 2019 by TBRSIM

[Nick Thomadis]

Hydro and Sonar will be only available to CL and DL classes. This equipment due to the thicker walls and high noise of capital ships would be highly ineffective. Hydro is something not fitted directly to towers but they will be just depended from them during combat in order to operate (reflecting the specialized personnel and reading equipment).

Towers have also limited but useful ability to detect torpedoes. So we expect battleship high towers to give valuable warning at an average of 900-1200 m for incoming torpedoes. For the rest, how everything works regarding detecting and avoiding torpedoes, how challenging and immersive will be, the game will show it to you better 

[akd]

Found a snippet on torpedo detection via hydrophone during WWII in the Canadian navy:

Quote

132 Hydrophone Set

This was the first defensive set and was especially suited for cruisers and other large warships such as aircraft carriers. For example, take a cruiser. Due to its size and speed, it could not alter course within the turning circle of a submarine so the installation of Asdic would be pointless. The cruiser's main defence was speed. In place of Asdic, the cruiser was fitted with a hydrophone listening set that continually swept the sea trying to locate the hydrophone effect (H/E) from a submarine. It had a large retracting dome that operated electrically with two transducers operating independently of one another. One was used on 15 Kcs for detection and the other was used for listening on 10 Kcs. Each had its own electronics and range recorders with scales of 5,000 and 2,500 yards respectively.

H/E, or cavitation noise is produced from the motion of a turning propeller regardless of whether it is a surface vessel, a submarine or a torpedo. Cavitation noise sounds like a pulsed note and can be heard at a good range depending on the size of the propeller and the speed of rotation. For example, a torpedo produced a very high pitched and fast rotation noise. When torpedo H/E was heard, the operator would sweep back and forth rapidly across the loudest H/E and report the centre bearing and movement. The Officer of the Watch (OOW) would then alter course in an effort to present the smallest possible target to the torpedo. On escorts, if a 'bow on' attack was detected, the OOW would increase speed and carry out a depth charge attack. If the assault was 'stern on,' he would alter away until the torpedo passed from astern, then alter course to carry out a depth charge attack.

On Cruisers, the procedure was slightly different. If H/E was detected, say on the forward beam, the typical tactic was to alter course, pass over the submarine, drop a pattern of depth charges, then continue on a course to clear the submarine. While doing this, information on the submarine's position would be passed to the escort who was more capable in dealing with the submarine. If the H/E was picked up aft of the beam, the cruiser would increase speed and alter away from the submarine while passing information about the submarine's position to their escort.

Bob Welland, relates some personal experience with torpedo detection. "The Asdic operators were always alert for torpedoes, and it became more important when the Germans introduced an electrically propelled model known as 'Gnat'. [These were the first sound-homing torpedos -AKD] Its propeller noise could be heard out to a mile and the Asdic was efficient at picking it up. I always had the operator do a 360 degree listening sweep every few minutes. This sweep, with no pinging, only took about ten seconds to execute. During Haida's last convoy run to Russia, this procedure proved itself. We dodged two torpedoes, having had time to turn the ship into the Asdic detected rush-noise." After the HSD reported that the torpedoes had passed, we went on to attack the U-boat."

http://jproc.ca/sari/asd_et1.html

A few notes on this relative to NA: Dreadnoughts:

1. These would have been sweeps conducted again submarine ambush, i.e. in fairly static situations where sectors that would otherwise be silent were known relative to fixed positions in a convoy.  Not in the dynamic, very noisy environment of a fleet action.

2. Speeds would need to be slow (thus the comment about a cruiser's best defense being maintaining high speeds, not listening on hydrophones) to detect hydrophone effects from submarines or torpedoes.  A slow target is much more vulnerable to torpedo attack.  Even less speed-dependent active sonar (sending out loud pings and listening for echos) was still speed-limited to 18-24 kn max.  Since escorts (at least those capable of much higher speeds like destroyers) would need to slow down to effectively search with active sonar, they could also conduct listening-only sweeps for torpedo / submarine sounds.  A cruiser or capital ship in most situations would be better off maintaining a high cruising speed (20 kn +) and thus making itself a very difficult target for submarine ambush, rather than moving slowly and conducting hydrophone sweeps. 

3.  These detections would be bearing-only (like radio direction finding in game) and would not reveal the actual distance and course of the torpedo.  

Taken altogether, I still find the contribution of sonar to torpedo spotting in game currently to be near fantastical.

Edited July 19, 2020 by akd

[SonicB]

On 5/11/2019 at 8:16 PM, Nick Thomadis said:

Hydro and Sonar will be only available to CL and DL classes. This equipment due to the thicker walls and high noise of capital ships would be highly ineffective. Hydro is something not fitted directly to towers but they will be just depended from them during combat in order to operate (reflecting the specialized personnel and reading equipment).

Towers have also limited but useful ability to detect torpedoes. So we expect battleship high towers to give valuable warning at an average of 900-1200 m for incoming torpedoes. For the rest, how everything works regarding detecting and avoiding torpedoes, how challenging and immersive will be, the game will show it to you better 

Reading this after over a year, a lot has changed...
I absolutely agree with @akd that torpedo spotting is pretty fantastical, but then, DDs and other fast torpedo-launching warships currently get a fantastical speed boost from the gunnery maths, and an equally fantastical reload time.. Here's hoping the devs can find a better balance for torpedo use that doesn't depart so far from historical facts.

Personally I would love it if, as in history, the surface-launched torpedo is most effective as a threat, not as a weapon. We can see this already when I can threaten the AI with light vessels in order to take the pressure off my heavy ships. Right now it doesn't work that way for the player, as my primary torpedo defence is to see it coming and dodge, not plan my maneuvres to minimise the threat in advance.

[akd]

The Battle of the Denmark Strait provides some insight into the utility and limitations of hydrophones in a surface action: the GHG hydrophone system (the most advanced during and immediately after WWII) aboard Prinz Eugen provided early notice of the approaching British ships, giving the German ships forewarning of the Brits expected bearing and the edge in visual acquisition.  However, the GHG system and its operators (now dealing with the ship steaming at full speed, probably severely degrading the sensitivity of the system) then went on to give repeated false torpedo alarms (one of which then became a visual false alarm) leading to series of maneuvers that may have thrown of the German gunnery enough to allow Prince of Wales to escape following Hood's destruction.

[madham82]

Someone had shared some time ago the log from Scharnhorst showing the sonar operator on board had detected British DD torpedoes fired at her. So the technology was definitely there and worked. Of course it was bearing only, but that's usually enough to evade most. 

[akd]

Some more detail on the GHG hydrophone system used aboard Prinz Eugen, from Koop and Schmolke’s book on the Hipper-class cruisers:

Quote

The Gruppenliorchgerät (GHG), a hydrophone system, was installed aboard Prinz Eugen, probably in August 1940. Whereas the three Deutschland class ‘pocket battleships were equipped with two groups of 54 receivers (lower platform-deck, Compartment X), Prinz Eugen carried two groups of 60 receivers (one group each port and starboard). The base length of each group was 4.5m with a slight elliptical curvature upwards. Each receiver group had an amplifier attended by an operator. On the morning of 24 May 1941 the British capital ships Hood and Prince of Wales were first reported by Prinz Eugen’s hydrophones telegraphist with the words ‘286 degrees, noise of two turbine-driven ships at high speed.’

The Americans were very interested in this system. In trials in the spring of 1946 run by the Philadelphia Navy Yard, underwater noise was detected at a distance of 27km despite the hammering of the Prinz Eugen’s steam turbines as she made 20 knots! It was found that all torpedoes fired from beyond 2km could be avoided in time (further trials at maximum speed were not possible because of the poor condition of the cruiser’s machinery). A link-up involving both amplifiers and their operators could provide a listening field 40km wide, although this was not carried out in practice. At long range, noise interference increased substantially and a very highly experienced operator was needed for reliable interpretation.

Whereas the German hydrophones—as with early Allied installations—used Rochelle salt, towards the end of the war the Allies switched to the synthetic ammonium dihydrogen phosphate crystal for reasons of economy. The German GHG came as a complete surprise to the Allies, since their research into the frequencies used by the Germans had been abandoned as hopeless—an error parallel to that of the German scientists respecting radar centimetre technology. In 1949 and 1950 the Americans built for their submarines two hydrophone systems of the German Balkan (Dress-Circle) type which led, in the mid-1950s, to the BQR-2 sonar and its many successors. They all have the same German ancestry.