Here is my original post:

And it is confirmed by this post:

Like I said, I have had the best luck using them when close to the surface (usuallu around the 12-13m mark) seeing the DD go to flank to lay out DC's, go to flank myself and deploy decoys at the same time. I have seen DD's come to a complete stop and back up. While crusing the "Med," I did this once and even got a DD to back up over its own DC's and sink itself. Whats that island off the coast of Sicilicy? It was there that this happened.

Using the external view, watch for the DD to "gear up," or use the hydrophones and listen for the engines to pitch up high, go to flank speed and drop your decoys. Depending on which type you have BOLD 1-2-3, it can give you up to 8 minutes to reach safe distance.

This works very well for me.

Least wise, that has been my experience.

As a few have said before: the decoy is only good to "mask" the noise of your boot. It was not designed to fool Asdic but to mimic the sound of prop noise. As JB said if you are prentending to be a hole in the water, never use a decoy. If you are getting hammered by the Tommy DE's, flank speed, release decoy, knuckle hard left or right, wait till you are on a straight course, then order Kleine Fahrt.....all good!!!!

It definitely does not mimic prop noise - it simply creates a cloud of bubbles which create a false ASDIC return. In real life, an experienced ASDIC operator would immediately spot the deception as the false return, being static, doesn't exhibit any doppler shift. In game, the decoys will work whatever your speed, it's just that there's little point in using them except when you've already been detected and are making violent evasive manoeuvres.

Now you see, I must make the exception.

Whenever I have used them, I have never really manuvered, I've mostly used Flank speed for up to 3 minutes on a straight course then backed off to "Ahead Slow." This oftentimes allows me to have just enough distance to take a peek astern and hopefully be able to make a good stern shot of whatever is behind me. That is, unless there are three or more DD's there.

However, I have also found out that whenever I'm out of "fish," I can do the same thing and dive to 120 m "knuckle" right or left and sometimes avoid all detection. Not always, but sometimes this works.

And they definately have not worked when I've been running deep. Found that out also.

I would have thought this was accepted wisdom, but the fact it's been stated means it isn't. Still, as posted earlier U-100 rides high on a crest white horses thanks only to the timely intervention of decoys, a double knuckle and 190m!

Ya but U-boat Alka Seltzer is slightly more non-funny than U-boat farts. Also you only release one at a time, not two. Also I can't remember ever having expelled an Alka Seltzer.

Sorry, I should have expounded a little: the decoy is designed to mimic cavitation noises (vaporization of water into gas bubbles) , which a boot would produce at full or flank speed. Distance and deception were the only friends to a boot. Yes, the BOLD is static, and an experienced soundman could detect the ruse given time because as you point out there is no movement of the source of the noise. That is why it is not a be all end all. It needs to be used in conjunction with course, depth and speed change. Like a sleight of hand trick. If it can buy you some time and distance then it has done its job, the rest is up to the Kaleun.

1480, mate, it doesn't mimic cavitation, although that is an interesting concept for a decoy. All Bold/Pillenwerfer does is create a fine bubble cloud to create a false return for the poor Hostilities Only ASDIC operator to bash away at fruitlessly.

Assuming your principle, even a novice would notice that the cavitation source wouldn't be producing any "chop-chop" wash noise, neither would there be the otherwise expected sound of electric engines, or turbulence from water passing over openings and protusions in the casing usually associated with a submarine.

There was a German decoy (forget the name) which could actively emit noises and, within limits, move at slow speed, but it sadly isn't modelled in the game.

The name that eludes you is Sieglinde.

"Sieglinde"
Similar to BOLD, this device was a slight variation where it was designed to resemble a submerged U-boat moving at 6 knots. These two were normally used together.

http://www.uboataces.com/sonar-decoys.shtml

Interesting. I wonder if I could mod a T-1 to go that slow, and make noise.
Problem is then docking at a U-tanker to wind up with a boat full of Sieglinde.

OK, I think I get it now. The technique is to continue going ahead flank after the bold is deployed, or cut speed and use the drift from going ahead flank to put some distance between you. A few more questions to make sure I have things straight- after all, when I end up using these things I imagine screwing up will not be a pleasant option!

1- Does it work against active pinging as well as passive?
2- It doesn't work at deeper depths?
3- Does ordering a knuckle turn create the same effect?
4- Why would the destroyer stop instead of charge towards the decoy?
5- How many do you have?
6- Is there any point to using them multiple decoys at the same time to make a bigger disturbance?

Thanks.

1) it ONLY works against active pinging
2) why wouldn't it?
3) as a decoy? no.
4) idk, sometimes they haul off in some direction that's not towards you OR the decoy
5) i hazard a guess; 10?
6) yes, you can soak up a lot of escorts this way but I make a point of not dropping them on top of each other.

Cohaagen, you make some valid points but I want to break down my observations down to the basest level and explain why I have come up with the theory that: the BOLD or Bolde decoy, employed by the Ubootwaffen in the early 1940's mimic's a noise produced by submerged submarines. The noise being cavitation:

1. Cavitation is the noise produced by a properly functioning prop. The prop in and of itself, does not produce a "chop" sound in a submerged submarine.

"Propeller sounds are of two general kinds- (1) singing, due to vibrations of the
propeller blades, and (2) cavitation. Cavitation sounds are the most important of all submarine sounds. Vibrations of the propeller blades may be due to faulty design or manufacture and are generally not difficult to eliminate."

2. Cavitation is a distinct sound because it is the disruption of a medium.

"Cavitation results when the propellers turn so rapidly that the water does not close in behind the blades. Thus, a stream of bubbles resembling those in a boiling kettle is formed. These bubbles may be caused by reduced pressures on the backs of the propeller blades or by vortices at the tips of the propeller blades."

3. Other mechanical noises a submarine produces and the actual noise the water makes as it is disturbed by a moving submarine are of a secondary indicator to an experienced hydrophone operator.

"Besides these two main sources of submarine sounds, there are some minor sources, such as splashing of water at the bow and in the wake when the submarine is at the surface; when submerged, the fittings of the vessel, such as handrails, may be set into vibration by the turbulent flow of water past them. These sounds are considered to be of small significance compared with those due to cavitation. "

4. A passive listening device is only a tool. The actual interpretation of the noise it picks up depends on the person listening.

"Any listening system must consist of (1) a hydrophone, (2) an electronic receiver, (3) a bearing indicator, and (4) a speaker or headphones. The sound-listening problem for the operator consists primarily of learning to distinguish between (1)

sounds emitted by another ship's machinery through the hull and from the propeller and (2) the multitude of other sounds that exist in the ocean."

5. When the BOLD was 1st employed, it worked because hydrophone operators were attracted to the similar noise that a submarine made.

" ...sea water would enter the canister and react with the calcium hydroxide, releasing
a dense volume of hydrogen bubbles. The result would give a sonar 'signature' very similar to the U-boat itself." Gordon Williamson pg 85, WOLF PACK.

So I will take a great leap in logic and still contend that the BOLD in it's original form mimics the sound of cavitation. Cavitation = noise made by bubbles. BOLD= noise made by bubbles. Yes, as the operators got better with experience, so did the decoy as danurve and UScpl point out.

The other cited source is the NAVPERS 10884 circa 1953.

Not going to quote 1480's whole post here but I have to repectfully disagree with some points made.

As has been mentioned and you state in your own post from Wolfpack, the bubbles produce a sonar contact, not a hydrophone contact. Yes the cannister probably does make a noise too but if it does it is not intended to mimic a submarine.
I have yet to see any evidence that BOLD, intentionally or unintentionally, had any effect on hydrophonic detection. Certainly a sub turning its props fast enough to cavitate water would be immediately distinguishable from a BOLD, because of the doppler shift if nothing else.

I see where you are coming from. Lets get a few things straight: SONAR generically is used when talking about active & passive varities. Active sonar or back to another term thrown around out of context: ASDIC, is a ranging and bearing device that will tell you a large enough object is over there at 100 yards. Won't tell you what that object is. The object needs to be of a solid nature to bounce the "ping" back to the device and using a formula for the speed of sound wha-la. Passive "sonar" correctly known as hydrophones are water proof microphones attached to a highly evolved and expensive home theater unit. Sonar signature, (signature not contact) is a term used for a set of distinct noise characteristics that are used to identify a moving mechanical object ie warship, merchant or submarine. An active sonar would cut right through a cloud of bubbles and get a solid but somewhat muddled return because of the effects of density, pressure etc . And since we now know that Active sonar/ASDIC has no capability to listen to anything but the ping it sends out, why on Earth do they call decoys, NOISEMAKERS? Other then to be picked up by "passive sonar"/hydrophone heard by the operator, processed by his gray matter and interpreted.