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qprdave
04-07-2009, 03:06
To the stokers out there.

During C.A.S.E.X's (Combined Anti Submarine Exercises) on the Undaunted it was often piped over the Main Broadcast "Mask the Turncount" I know that it was carried out to stop the submarine from "counting" the revs and getting the speed. But how was it done? Was it by adjusting the revs individually of both props up or down?

jbryce1437
09-07-2009, 19:43
Seems everyone is Baffled by the term;)

qprdave
09-07-2009, 19:44
Yes Jim. I wrote "Baffle" first then realised it was "Mask". But couldn't change it. Do you remember it being piped on the Unwanted?

astraltrader
09-07-2009, 19:54
By magic it has now been changed!!

qprdave
09-07-2009, 19:59
Thanks for your time Terry

jbryce1437
11-07-2009, 19:44
Yes Jim. I wrote "Baffle" first then realised it was "Mask". But couldn't change it. Do you remember it being piped on the Unwanted?

Sorry Dave, presume it was piped after tot time, after which time I can't remember much:D

Macadian
12-07-2009, 02:05
Can't say I remember it either...but then again I rarely watch kept in the Engine Room....being a Boiler Rm Killick/PO during my time aboard. So no first hand knowledge. At a wild guess, I would think the Chief of the watch staggered the revs on both shafts a little...the effect of one shaft with higher revs than the other being compensated for by the helmsman.....as to what effect such an adjustment would have on a listening Sub trying to determine the speed/revs being produced by the target vessel.....you would know that better than I ;)

Ok, Ok, I did say it was a WILD guess.....:cool:

P.S....been really busy of late and have lost the 'thread' of things so to speak and have just had a chuckle at the postings......

qprdave
12-07-2009, 02:22
That sounds right Mac

If you listen to the Prop noise (HE) there is the sound like sshh, sshh, sshh. Count these over 1 min and you have the R.P.M. hence the speed. What you are saying would break up the sshh sequence and make it dificult to get speed. It is important to the submarine as you have to aim off the target, fire the torpedo and let the target move into position. So Speed and Bearing is very important. Obviously, you can only Mask the turncount on multi screwed vessels.

oldsalt
12-07-2009, 13:58
Years ago I remember one ship had been fitted with underwater propeller viewing windows, she was carrying out trials on a system called Agouti, it was I think to do with air being expelled through holes in the propeller blades. I know this is a bit sketchy, anyone know anything about this system?

Macadian
12-07-2009, 15:23
Yes Old salt, the memory banks do recall said system. I seem to remember that compressed air was forced down the centre of the prop shafts, to the screws. Again if I remember correctly, propeller shafts were manufactured hollow in the centre to better absorb 'torque' under extreme thrust etc, This feature lent it self to the system you allude too.. As to how successful it was I am not sure...I suspect that I was on a ship at some time that had the system however as to whether it was operational I am not sure.

I may have this all wrong of course...some of my grey cells retain all sorts of fragments from my time in the mob....:)

jbryce1437
12-07-2009, 17:22
Years ago I remember one ship had been fitted with underwater propeller viewing windows, she was carrying out trials on a system called Agouti, it was I think to do with air being expelled through holes in the propeller blades. I know this is a bit sketchy, anyone know anything about this system?

Not sure if we had such a system on Exmouth, after her conversion to gas turbine. There was a lot of innovative machinery installed, such as the joystick steering system, waste heat generator, etc. I know she had a variable pitch propellor fitted and seem to recall that air could be emitted through the blades. As for the viewing window, not sure about that, was it alongside the golden rivet?:D

oldsalt
13-07-2009, 17:15
Not sure if we had such a system on Exmouth, after her conversion to gas turbine. There was a lot of innovative machinery installed, such as the joystick steering system, waste heat generator, etc. I know she had a variable pitch propellor fitted and seem to recall that air could be emitted through the blades. As for the viewing window, not sure about that, was it alongside the golden rivet?:D

Just a bit aft.:eek::eek:

harry.gibbon
13-07-2009, 18:20
See if anything here helps resolve the question:-

http://www.maritime.org/fleetsub/sonar/chap4.htm

I am continueing my sweep search whilst trying not to become baffled myself

Little h

jbryce1437
13-07-2009, 19:53
See if anything here helps resolve the question:-

http://www.maritime.org/fleetsub/sonar/chap4.htm

I am continueing my sweep search whilst trying not to become baffled myself

Little h

Quite an enlightening document Harry. Not sure if altering the shaft revolutions would change a chug, chug, chug, to a chug, chug, chug, chug.
What do others think?

qprdave
13-07-2009, 20:59
It certainly would Jim. There is always something wrong with a blade of a prop. Like a small nick or dent. This gives it a slightly different noise. As the Document says CHUG, chug, chug. This is the one that you count. First you have a stop watch in your hand and then you start pumping your arm up and down (No comments form the back row). When your arm is fully down with the CHUG then you start the stop watch. Count how many CHUGS in 15 secs and then x 4. This will give the R.P.M.

Now if you start messing about with the Revs in the Engine room i.e. 5 revs up in one screw and 5 revs down on the second and then reverse the action with random rev changes. Then it will upset the CHUG, chug, chug rhythm and make it difficult to get a rev count

harry.gibbon
13-07-2009, 23:21
Again I have the duty to place this link here since this is where the original Q was put... I have dropped it into the Ping itty ping thread first by mistake... sorry n all that:-

http://www.hnsa.org/doc/sonar/chap13.htm

Little h

Steamshy
04-09-2009, 07:34
Years ago I remember one ship had been fitted with underwater propeller viewing windows, she was carrying out trials on a system called Agouti, it was I think to do with air being expelled through holes in the propeller blades. I know this is a bit sketchy, anyone know anything about this system?

The ship you refer to was the Undaunted. The year 1962/63. The study on the effect of Cavitation was being done using various prop types by the boffins. One of the mods required was to install viewing ports in the hull above the Props. From memory they were standard scuttles complete with deadlights. The lack of water clarity around the UK made a trip to Gibraltar and the Med necessary
Most of the trials were done at full power and at night so the old machinery got a thrashing. A system of illumination was rigged , like a engine timing light, that rapidly rotating propllers could be held to appear at rest.
The experience of raw power was unforgetable and worth laying prone in the tiller flat bilge. Another good part was the Spanish border was open at the time.

jbryce1437
04-09-2009, 19:10
Don't know if they were still fitted to Undaunted during my time on her - 1968 - 1971ish, but we did have Agouti air compressors on the Exmouth, following her conversion to Gas Turbine. She had several innovative pieces of equipment, including the variable pitch propellor, which didn't sound very good when it hit the bottom of the canal on the way up to Brussels.
I remember a gaggle of Boffins arriving on the Undaunted one summers day, complete with a mountain of washing up liquid and foam generators, etc. These were set up on the flight deck and a trail of foam laid astern, while we were chasing the duty sub off Portland. Don't know if the trials were to see if the foam bubbles "made us invincible" or whether it was just a ploy to clean up the seas:)

Tony Fitt
03-11-2010, 14:51
Don't know if they were still fitted to Undaunted during my time on her - 1968 - 1971ish, but we did have Agouti air compressors on the Exmouth, following her conversion to Gas Turbine. She had several innovative pieces of equipment, including the variable pitch propellor, which didn't sound very good when it hit the bottom of the canal on the way up to Brussels.
I remember a gaggle of Boffins arriving on the Undaunted one summers day, complete with a mountain of washing up liquid and foam generators, etc. These were set up on the flight deck and a trail of foam laid astern, while we were chasing the duty sub off Portland. Don't know if the trials were to see if the foam bubbles "made us invincible" or whether it was just a ploy to clean up the seas:)

It was probably the Agouti,it was designed to blow air over the propellers to mask the cavitation hence confusing the acoustic signature and making rev counting (which using TPK can be converted to speed) diffucult. Unfortunately it also make the ship more noisy thus doubling the range at which the ship could be detected by the submarine!

Destroyerman
03-11-2010, 15:29
Qprdave,

never heard of the term 'Mask the Turncount' and certainly never heard it piped whilst a member of HMS UNDAUNTED ships company. All instructions to machinery spaces were given either by telegraph or telephone. The main broadcast domain was reserved specifically for other departments to broadcast their evolutions to all and sundry.:D

I can however confirm that UNDAUNTED had two sighting scuttles in the tiller flat from which to observe the effects of air flow over the propeller bosses. "Steamshy" is quite correct in what he says. My experience was between 1962 and 1964. UNDAUNTED was carrying out trials at this time with the AGOUTI system and the small compressor was located in the Gearing Room beside the Lubricating Oil Separator.

When I designed a very basic Gas Turbine marine propulsion system in 1964 I included an AGOUTI air cushioning system around the variable pitch propeller boss. Variable pitch propellers were necessary because it was almost impossible to drive a gas turbine engine backwards.:)

In conclusion, I feel that the order to 'Mask the Turncount' would have been relayed verbally to the Chief of the Watch in the Engine Room so that he, by virtue of an already existing 'mean revs count' could vary the shaft speed up or down within given parameters.;)

qprdave
03-11-2010, 15:47
Thanks Destroyerman

I do remember it being passed over the main broadcast during C.A.S.E.X.'s (Combined Anti Submarine Exercise) or I wouldn't have heard it. I have no idea why. As you say if it was specifically for the engine room, it would have been passed over the Bridge/engine room comms.

This was quite a bit later than when you were on the Unwanted (1967-69). Being Portland Squadron and doing more than our fair share of C.A.S.E.X's it was used very often.

Dave

ap1
12-03-2012, 20:05
Qprdave,

never heard of the term 'Mask the Turncount' and certainly never heard it piped whilst a member of HMS UNDAUNTED ships company. All instructions to machinery spaces were given either by telegraph or telephone. The main broadcast domain was reserved specifically for other departments to broadcast their evolutions to all and sundry.:D

I can however confirm that UNDAUNTED had two sighting scuttles in the tiller flat from which to observe the effects of air flow over the propeller bosses. "Steamshy" is quite correct in what he says. My experience was between 1962 and 1964. UNDAUNTED was carrying out trials at this time with the AGOUTI system and the small compressor was located in the Gearing Room beside the Lubricating Oil Separator.

When I designed a very basic Gas Turbine marine propulsion system in 1964 I included an AGOUTI air cushioning system around the variable pitch propeller boss. Variable pitch propellers were necessary because it was almost impossible to drive a gas turbine engine backwards.:)

In conclusion, I feel that the order to 'Mask the Turncount' would have been relayed verbally to the Chief of the Watch in the Engine Room so that he, by virtue of an already existing 'mean revs count' could vary the shaft speed up or down within given parameters.;)

Any more info on this lot, Sandy?

Bonzo
12-03-2012, 20:24
My recollection of Mask the Turncount was that the speed of the port and starboard shafts were to be set different by a number of revs?? which I assume disguised the average speed. I vaguelly remember doing it, a simple call from the bridge to the Chief of the Watch??

Destroyerman
12-03-2012, 20:29
Not really Andy, as I remarked on another thread, it's all getting a little confusing ................ ;)

However, "Mask the Turncount" thankfully comes from UNDAUNTED and QPR Dave's and Steamshy's comments verify this.

I have absolutely no documentation from my RN days and all recollections are in my head.

If I can, I will reminisce with other engineroom rates and see what I can dig up.

Peter Ash (Pash) ex CCMEAP was on UNDAUNTED with me and may recall more. He is a member of this Forum.

Sandy.

TCC
13-03-2012, 12:22
I was once shown 1 or 2 'photos' of the stern of a RN escort in drydock, the ship was recent... say 1970s or 80s... I can't tell you where or when or I'd have to kill you, as he passed me the sheets with the words "This is a bit secret". But it was the props that was 'special'.

The blades looked like a sword Saladin would wield but the tip of the blade was bent round on itself like a 'U'. So you have 3 'U' shaped blades around a normal variable pitch? hub. Or 2.. the ship had 2!

I thought the idea was clever as you got twice the surface area of prop for the same external diameter of prop.

So as the blade went outwards as normal and then it curved back round and in towards the hub again. If you looked at the top of the prop shaft, you'd see something like this. 2 props on their shafts ->


......n___
......u................... Bow ---->
......n___
......u


Clever, eh? Each blade has twice the area!

ap1
13-03-2012, 16:22
I was once shown 1 or 2 'photos' of the stern of a RN escort in drydock, the ship was recent... say 1970s or 80s... I can't tell you where or when or I'd have to kill you, as he passed me the sheets with the words "This is a bit secret". But it was the props that was 'special'.

The blades looked like a sword Saladin would wield but the tip of the blade was bent round on itself like a 'U'. So you have 3 'U' shaped blades around a normal variable pitch? hub. Or 2.. the ship had 2!

I thought the idea was clever as you got twice the surface area of prop for the same external diameter of prop.

So as the blade went outwards as normal and then it curved back round and in towards the hub again. If you looked at the top of the prop shaft, you'd see something like this. 2 props on their shafts ->


......n___
......u................... Bow ---->
......n___
......u


Clever, eh? Each blade has twice the area!

TCC, Very interesting; can you not give us a slightly better sketch though, please?;):cool:

TCC
14-03-2012, 03:16
TCC, Very interesting; can you not give us a slightly better sketch though, please?;):cool:

Yeah sure... but the guys from 'Room 40' will be looking for us! ;)

Non of you salt-encrusted shellbacks ever seen the like? The prop blades shaped like a schimiter.

The bubbles introduced around the prop: Cavitation is a fault when the blades create air... the air breaks up the waters composition and the thrust from the blades deteriorates.

So how is introducing bubbles around the blades a goood thing? Props need 'solid' water to push against!

Alan D
14-03-2012, 04:14
Normally the OOW would call down to the controlling engineroom to mask or reduce one or two RPM on one shaft. The different blade count was to change the ships signature and hopefully throw the submarine off track to what their estimate of the vessel was.

buster185
14-03-2012, 10:04
Someone commented earlier in this thread that you needed two screws to mask the turn count. I recall being in company with RFA Stromness when our skipper gave the order to mask the turn count and said to signal the Stromness to do likewise. Someone on the bridge queried this with him because Stromness was single screw and he replied that they would know what to do. I presumed from his response that there was a procedure for masking the count on a single screw. Anyone else had experience of this??
Buster

ap1
14-03-2012, 10:20
Yeah sure... but the guys from 'Room 40' will be looking for us! ;)

The prop blades shaped like a schimiter.

The bubbles introduced around the prop: Cavitation is a fault when the blades create air... the air breaks up the waters composition and the thrust from the blades deteriorates.

So how is introducing bubbles around the blades a goood thing? Props need 'solid' water to push against!

Hi TCC. I think you have got a few things wrong here.(O.K. written at 0416hrs.):D

a) The blades do not create air. Cavitation is a cavity with a vapour in it, i.e. like a partial vacuum. When this space collapses it uses energy, some of which creates powerful sound waves.
b) The composition of the water is H2O. It remains so.
c) Props do not need 'solid' water to push against. That's why Spitfires can move in air alone, a low density fluid. Water with bubbles in it, is still a high density fluid.
d) Introducing bubbles from the prop (Agouti system) reduces high frequency sounds, and helps to disguise the ship's sound profile.

I pinned a photo of Scimitar-like blades, is this the kind of shape you mean?

TCC
14-03-2012, 15:02
Hi TCC. I think you have got a few things wrong here.(O.K. written at 0416hrs.):D

Andy, I'm loathe to google 'cavitation' as I have enough trouble reading the things I want to read. If you make me google cavitation I swear I'll track you down and kill you myself!!! :)

a) The blades do not create air. Cavitation is a cavity with a vapour in it, i.e. like a partial vacuum. When this space collapses it uses energy, some of which creates powerful sound waves.
Water with 'vapour' in it? What do you call a volume of 'vapour' in water?
b) The composition of the water is H2O. It remains so.
And the 'O' from H2O is what? I'll come back to that.
c) Props do not need 'solid' water to push against. That's why Spitfires can move in air alone, a low density fluid. Water with bubbles in it, is still a high density fluid.
Water is uncompressable. Bubbles in water create voids of air that the thrust from the props gets wasted on. Air bubbles are compressable! Am I wrong?

If a ship sails into a imaginary area of bubbling water, it's speed will get cut down as the props won't be able to push the boat thru the water as efficiently as in 'clean' water. Why? Because the water has lost it's cohesiveness and it's 'solidity. The props are as likely to start pushing bubbles around as they would the water they are in.

I'm not tallking about the cause of cavitation, I think I'm explaining why it's bad. No?

I didn't get the Spit reference.

d) Introducing bubbles from the prop (Agouti system) reduces high frequency sounds, and helps to disguise the ship's sound profile.

IIRC, when a prop spins too fast for its design, it drags the oxygen out of the h20. (like boiling water has air bubbles) I'm not saying the props heat the water, I think this is a mechanical process.

I pinned a photo of Scimitar-like blades, is this the kind of shape you mean?

No mate those in the image are normal blades. Imagine you take those blades to the blacksmith shop and he heats them and bends the the top third back round and back towards the hub. The result is the same area for less dia. Or the same dia but more area.

I guess it didn't work out as they're not the the Type 45s now... or are they? :)

...........

medway508
16-03-2012, 11:02
Re Scimitar props, this is a 23 sitting in dry dock from Navy News. Not so secret now.

ap1
16-03-2012, 13:00
Re Scimitar props, this is a 23 sitting in dry dock from Navy News. Not so secret now.

Thanks for that great picture, medway508.
This is still one of my favourite topics;... propellers, that is.;)

TCC
16-03-2012, 18:18
Re Scimitar props, this is a 23 sitting in dry dock from Navy News. Not so secret now.

I did a quick (took blxxdy ages) edit of this random image. This is the idea of them... but 3 bladed... and the curve at the end was more gentle. But the length of the return is about right.

As it's the end of the prop blade that imparts the thrust to the water, do you think this idea would work? If you think about it, the blade is pushing water against it's own tip!

I'm now thinking there's a chance he was pulling my leg with an off-hand remark, and the props was from a ship that had grounded. No damage on the tips though!

I don't know. Thoughts?

medway508
16-03-2012, 21:19
Probably get into trouble with the moderators about posting in the wrong place but this is an Arleigh Burke prop.

On the same subject, I once asked the guys in Chatham why Ocelot had no props. Answer, she had them when she went into dry dock but the Navy turned up just afterwards & took them off in a covered low loader. The description was of a scimitar type prop. Normally they had a 3 blade "petal" prop version of the type 12 props.

Rumour has it that the batch 3 Type 22s had the same prop as the 23s at the end of their lives but I've never seen a photo of one in dry dock to confirm this.

Prom
16-03-2012, 22:07
There seem to be 3 options here.
1) the design you saw is now not classified, in which case I am sure you can find a picture of it, such as that in Navy News.

2) Your friend was selling you skyhooks, in which case you might prefer not to mention it

3) Or, it is still classified. In which case I am at a loss as to why you would betray the confidence of the person who showed you, and indeed those still serving.

It may seem an irrelevancy, but knowledge of the design of a prop can indirectly assist a sonar operator a great deal.

medway508
18-03-2012, 02:33
One thought, propellers used in Kort nozzels quite often have squared off tips, so could this be a merchant prop not a warship one?

ap1
21-03-2012, 20:41
Not sure if we had such a system on Exmouth, after her conversion to gas turbine. There was a lot of innovative machinery installed, such as the joystick steering system, waste heat generator, etc. I know she had a variable pitch propellor fitted and seem to recall that air could be emitted through the blades. As for the viewing window, not sure about that, was it alongside the golden rivet?:D

You are spot on here Jim.

H.M.S. Exmouth did have the Agouti air system fitted on conversion to a gas turbine; and here is a sketch of the supply down part of the propeller shaft.

I know there may be some psuedo scientific views on the forum, who think this is a 'no-go'; (because a ship can't push against bubbles?), but I'm afraid it is a well proven concept, and was/is used extensively, in many different, but important ways.

TCC
25-03-2012, 03:57
You are spot on here Jim.

H.M.S. Exmouth did have the Agouti air system fitted on conversion to a gas turbine; and here is a sketch of the supply down part of the propeller shaft.

I know there may be some psuedo scientific views on the forum, who think this is a 'no-go'; (because a ship can't push against bubbles?), but I'm afraid it is a well proven concept, and was/is used extensively, in many different, but important ways.

Well fella, you've twisted that one... and in an unpleasant way. I never said this system didn't work!

As you've seen, it's hard to explain technical concepts in words over the internet. All I said was bubbles in water reduce the desity of the body of water and then I asked if adding bubbles behind the propellors reduced their efficiency. It was a question, not a statment of fact.

http://www.youtube.com/watch?v=MSmAXp_BHcQ

The shop-owner who pushed a photostat in front of me? It was probably a ship that had touched ground and bent the props! This may have been the 'confidential' bit?

ASSAIL
25-03-2012, 07:51
Agouti was a compressed air system as members have correctly posted however, it was not used to hide the speed from boats, it was used to increase the cavitation inception speed by filling the space immediately behind the blades to stop the water imploding into the gap. These were standard in the RAN Type 12/leanders but I can't recall if we had it in Achilles or Bacchante during my time on exchange as PWO (Tas)
Cheers

ap1
25-03-2012, 10:27
Agouti was a compressed air system as members have correctly posted however, it was not used to hide the speed from boats, it was used to increase the cavitation inception speed by filling the space immediately behind the blades to stop the water imploding into the gap. These were standard in the RAN Type 12/leanders but I can't recall if we had it in Achilles or Bacchante during my time on exchange as PWO (Tas)
Cheers

Yes, Assail. You are absolutely correct.

Have you read the post "Quitening ships' propellers" on the 'Papers from Naval Engineering History' thread?
It gives a detailed description of the physical mechanisms involved in cavitation, and how it was reduced, over the years.