Why didn't an escort spend all day using active sonar?

[Erich dem Roten]

Meaning, why wouldn't, say, the lead escort of a convoy just blast actively all over the place 24/7? The others could stay passive, and the active might encourage a sitting U-Boat to move and get picked up, or even be picked up by the active sonar itself. Was there some sort of detriment to using active sonar so much, aside from broadcasting your location?

Also, I seem to recall reading something about how active sonar was far less effective when the target was very close (<5m) to the bottom. Something about reflecting the sonar waves back at the boat and giving lots of false positives.

I'm just posting these as they come to me, so forgive my rambling please. I just get curious, and sonar isn't exactly something you can just Wiki and understand.

[superfecta]

I believe because listening passively had a MUCH greater range than the active sonar which was better for pinpointing a passive contact.

[GoldenRivet]

Why didn't Marine and Army patrols in Vietnam walk through the jungle screaming "Charlie! Where are you!?!?"

it broadcasts the position of the escort to the U-Boat, and it only effectively scans the area ahead of the escort on about a 45 degree arc.

banging away with active sonar wouldnt do you a great deal of good if the u-boat was 2500 yards off your port or starboard beam. and besides, unless he heard it on hydrophones, a u-boat crew wouldnt hear the pings with the naked ear except in certain situations. so a U-boat crew that heard a faint ping wouldnt jump into flank speed evasive mode automatically.

furthermore active sonar (depending on the time of the war) had varying degrees of effectiveness (consider thermal layers etc).

passive sonar (hydrophones) scans the entire area around the escort for any sounds of submerged prop beats. (except of course immediately behind the escort)

so...

hydrophones = broad focus to locate the u-boats approximate position directionally.

ASDIC = pinpointing the U-boat's position with a focused "beam"

[badwolf]

I would imagine also that using active sonar could drown the very faint distant signature for the passive sonars.

[Erich dem Roten]

Thanks for the responses.

a u-boat crew wouldnt hear the pings with the naked ear except in certain situations.
Are saying that all the movies I've ever seen have been lies?!?

Incidentally, what circumstances would allow a crew to hear the pings without aid.

[GoldenRivet]

what i mean is they could hear it most of the time... but not always. for example, late war sonar is ultrasonic... meaning it is beyond the range of human hearing. also, if you are beyond the range of the ASDIC transmitter you wont hear anything.

unless you were being pinged from close range with late war ASDIC you wouldnt hear it... in fact all you would really hear is the pressure wave of the sonar ping hitting your boat, you wouldnt actually hear the ping.

Furthermore... im pretty sure that when ASDIC pings reflect off of a thermal layer you cant hear them if you are under the layer.

certain conditions DO exist in which you will never hear pinging.

[GoldenRivet]

aside from the above post ill add the following.

Imagine your in a completely empty, totally dark 747 hangar.

I have set a bottle cap somewhere on the floor inside this hangar, and the bottle cap has been painted to blend in with the floor's color.

you can look for it one of two ways.

either

1. I can turn the hangar lights on

or

2. I can give you a small flash light which you can turn on and off for a few seconds at a time and it can only be shined directly in front of you.

which way would you choose?

[nirwana]

Well it is possible to track the pinging ship in the sub without being detected itself. The acoustic ping could have enough power to hit the sub but dies on the way back and as further away the pinging ship from the sub is, as less reflected pings will be detected by the receiver. The reliability of the signal decreases due to longer being influenced by the dynamics of the medium its using to travel.

[Pisces]

Yes, active signals take twice as long to reach the 'microphone' as do passive sounds. So they would weaken more. But I would guess an active signal is alot stronger when it has just been emited, as opposed to silent uboot noises.

Maybe it's because asdic is aspect sensitive (a smart kaleun would turn his bow or stern to it) and passive doesn't care about aspect so would work to counter any Kaleun IQ level.

But another important point would be the 'canon' of ping returns the asdic opperator would have to listen too when more than one asdic-vessel is sending out pings. Which return belongs to your vessel? It would require alot of organisation amoung multiple asdic capable vessels.

[Brag]

Active pinging had a very narrow beam. So unless the operator knew approximately where the U-boat was, the chance of detection was remote. Even when knowing the approximate position of the sub, the ASDIC operator could lose his target and fail to find it again.

[Hitman]

First of all let me clarify something: Pinging is giving away your position, true, but that's in fact what most escorts in WW2 would prefer, since U-Boats would need to be more cautious with a protected convoy and probably lose some chances of intercepting. Anyway, the reason they did not ping constantly is because the escorts patrolled around the convoy in patterns, accelerating to keep up with the convoy and then slowing to listen with passive array (Hydrophones). Chnaces of detecting a submarine with a WW2 sonar are slim UNLESS you know more or less where the submarine is, and that you can only get with passive array first, as its range is much larger than the active one.

Active sonar is actually the way to pinpoint the enemy's position when you are already on your attack run. Other than that, it was of not great use in WW2.

[Letum]

The amount of false contacts a destroyer would get would be a good enough reason not to use it all the time.

[predavolk]

It WAS used all the time! Maybe not 24/7, but certainly it was heavily used. Especially later in the war when escorts could be freed up to chase subs while others remained with the convoy. Reasons not to use it:

1- It gives your position away. You most absolutely, definitely, can hear some pings with the naked ear inside of a sub. But in this case, they would be faint pings detected by the operator. Any wave-type detection (SONAR, RADAR) can be detected from farther away that it can detect. So the detection advantage goes to the u-Boat.

2- Fatigue. This was perhaps the next biggest issue. It's hard to stay sharp and focused for long periods of time.

3- False echoes. Combines with above.

Reasons to use it:

1- Others can still hunt passively! Especially those far away.

2- It is much more effective in finding quiet subs.

3- Most importantly, it scared the hell out of most Kahleuns and kept them away from the convoy. It suggested that the destroyers were actively hunting for a sub near them. Only the bravest, or most reckless Kahleun would march into the middle of escorts who were already apparently alerted and hunting for contacts.

So, ironically, active pinging tended to be used more often in conjunction with convoys than by hunter-killer groups. The former wanted to use it as intimidation, the later wanted to avoid using it to maintain surprise for as long as possible.

[ridgewayranger]

Hi,
Sorry Pisces, you've got it wrong. Each ship used a different frequency so was not bothered by others.
When escorting a convoy Asdic would be used constantly. Disposition of escorts would be one ahead, one on each bow, one on each beam and a tail end charlie.
Each ship had an arc to cover with an overlap ensuring cover all round. If two ships had the same frequency, within the range 16k/cs to 24 k/cs, one would tune up or down. ALL transmissions were ultrasonic and to enable the operators to hear echos a process known as heterodyning (mixing two frequencies 1k apart so that the result was a 1k audible note) was used. The sound heard in the submarine was like a click. The constant ping ping ping so loved by movie makers, cannot be heard in a submarine. The subs sonar operator could detect those transmissions usually,as with radar, about 3 times the range at which they could detect you. Hence the advent of search recievers. The main use of passive sonar in surface vessels was to give warning of approaching torpedoes. That sound is
unmistakeable! I speak of R.N. practise, I know not about the U.S.N.
RR

[ridgewayranger]

Hi, There seem to be a lot of misconceptions about Sonar.
It was used at all times when escorting a convoy. It could not be used over about 16 knots because of water noise and the dome could be damaged by rough seas. Under those conditions it would be raised until speed was decreased or weather abated. A screen would, if escort was large enough be one ahead, one on each bow, one on each beam and one astern. Normal sweep was in 5 degree steps from red 80 to green 80, but in a screen like this each ship had an arc which overlapped his neighbours to give best coverage. A uboat abaft the beam was not considered to be much of a threat. There was minimal intership interference because each used a different frequency, between 16k/cs & 24k/cs. If two had the same frequency one would tune up or down. ALL Asdic transmissions were ultrasonic, obviously, but were heterodyned to give the operator an audible sound, the ping, much loved by movie makers but inaudible in a submarine, unless using the sonar headset. If the escort was in contact the sound in the boat was more like a click as his transmission hit your hull. Passive sonar in R.N. vessels was not very good in those days, it was mainly used to give early warning of approaching torpedoes, the sound of which was unmistakeable!
RR

[Sailor Steve]

I remember reading that from inside the u-boat the ping sounded like gravel being thrown against the hull. But it's been awhile since I read anything, so someone who's been reading the old books more recently might be able to give a reference.

[Letum]

. ALL Asdic transmissions were ultrasonic

Are you 100% sure about that?

[Letum]

*edit*

[Randomizer]

One of the other factors here is the poor communications between escorts for much of the war. The USN had line-of-sight VHF Talk Between Ships (TBS) but it was not universally fitted on Atlantic escorts until later in the war. Until late in the war, the RN and RCN mainly used flags, semiphore and short-signal HF morse all of which had a time-lag (sometimes considerable) between the origin of a signal and it's understanding on the recieving ship(s). Even TBS generally passed through a communicator and was seldom directly captain to captain.

An aspect of the lead ship banging away on active while the others listened would have been the ability to quickly pass the changing tactical picture to other escorts. Before the development of real time inter-ship data links, multi-ship ASW attacks were usually closely coordinated by the officer in tactical command, normally the escort group commander. Other captain's would be reacting closely to orders and might have lacked significant parts of the tactical situation. This is one reason why ramming was a frequent option, if the opportunity presented itself the need to await instructions vanished and the escort could act decisively against a target of opportunity.

High levels of training and teamwork coupled with comprehensive standerd operating procedures helped a lot and these were hallmarks of the successful escort commanders like Donald McIntyre, Johnny Walker and Daniel Gallery.

Active sonar/ASDIC was but one tool in the box of the tactical commander but ASW was (and is) an incredibly complex problem even when the enemy is seeking you out as in a convoy escort scenario.

[Letum]

If two ships had the same frequency, within the range 16k/cs to 24 k/cs, one would tune up or down.
ALL Asdic transmissions were ultrasonic, obviously, but were heterodyned to give the operator an audible sound, the ping, much loved by movie makers but inaudible in a submarine, unless using the sonar headset.

OK...I've been doing a little more reading and here is what I have come up with.

ASDIC was indeed usually between 16khz and 24khz, but sometimes as low as 4khz.

I found a program that can play sounds at any frequency and I can hear up to
18k/cs. Apparently it is possible for some to hear a little above 20k/cs (k/cs = khz).

I would guess all the water and steel involved would produce a lot of lower
frequency sounds from the Ping, but not like in the movies.

[Erich dem Roten]

Lots of really interesting stuff here. I had completely forgotten that ship-to-ship communications were still rather primitive compared to the radio networks we can set up today. Seems I learn something new every time I come here (in this case, a bunch of new stuff) :up:.

Just to clarify my thinking behind this question, the idea behind it is not to actually detect a specific sub but to act as a deterrent. If a lead escort is burning up the ocean with sonar waves a sub might be more reluctant to place itself in the path, not to mention there would be 3 or more others listening passively for any sound. I would think any kaleun would be aware of the passive escorts, and be wary about approaching. I suppose I'm placing too much emphasis on the idea that escorts only needed to keep subs away to achieve their objective. I can definitely see why a hunter-killer group would use passive sonar almost exclusively.

[Randomizer]

I believe that the technique was known as 'herding' but needlessly putting any additional noise in the water would make the hydrophone operators jobs more difficult. Although oscilloscopes and paper traces had supplimented the human ear on active and passive acoustic systems, the techniques were rudimentary compared to the computer generated visual waterfall displays of modern sonar.

Would secondary echos from a target be identified as such on third-party ships?

What to do when contacts with fish, thermal anomalies, flotsam etc cause multiple false alerts during the course of a crossing? Eventually one can see that it would be easy to cry wolf once too often with the inevitable slower and less effective escort reactions this might cause.

Much more is known about underwater sound propagation today and still research continues. The herding technique may have scared off timid captains but also may have given the bold and the skilled CO valuable insight into the convoy's defences and actually facilitated his approach and attacks.

Just $0.02

[Pisces]

It has allready been said that passive sounds carry further than active sounds. I forgot why, but now Randomizer mentioned 'waterfall displays' I get a flash of insight again (seeing 'narrowband in Dangerous Waters' before my mind's eye). Active sounds are high frequency (ultrasound) as others allready have said, but passive sounds are predominantly (very) low noises, like low rumble. The really low frequencies are made by vibrations of the sub's long hull and screwshaft and carry much energy, but those are also in-audible for the human ear. The somewhat higherfrequencies like fans or pumps or just rattle of smaller hull-sections (a bit less energy but still much) can be heard by the human ear. These low frequencies can carry much further than higher frequencies. As higher frequencies get absorbed more easily because they carry less energy to begin with. Anyway, that's what I learned from below links and community written manuals for DW.


http://en.wikipedia.org/wiki/Sonar

http://www.fas.org/man/dod-101/navy/docs/fun/part08.htm

@ ridgewayranger : Ok, I must admit was underestimating the sonar equipment and thinking too simple. They may not have had transistors or signal processing computers like they do have now, but such techniques like filtering for specific frequencies (or that heterodyne stuff whatever it is... 'voodoo magic', that's what it is!) can be done with vacuum tubes and simple electronics.