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688 with Albacore props?

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Postby Feet Wet » Sun Sep 07, 2003 3:11 pm

Hi All,
On Tim Smalley's site is a photo,date 08/30/03 listed under CIS photos,of a 688 with what appears to be two props ala Albacore/Jack.Does anyone have any info of a 688 being so converted,or for that matter any info on just what this set-up may be.
Thanks
Jonathan Eno
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Postby AntoineL » Sun Sep 07, 2003 4:17 pm

Hi Jonathan,

It is not a C/R 2 props but a single prop with a vortex attenuator "duncecap".
You can see another example here

hope this helps

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Postby Dolphin » Sun Sep 07, 2003 5:12 pm

Me again. Photo of USS Los Angeles SSN-688 in Fremantle, Australia (HMAS Sterling) in 1999. A bottle shaped dunce cap, 7 blade prop with a 11 blade vortex attenuator aft, all turning as one in the same direction. Vortex attenuator blades stubby and rectangular in plan form. Other 688 subs use simular. Ringed props without attenuators common on 688I's.

Steve R.

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Postby Feet Wet » Mon Sep 08, 2003 12:11 pm

Many Thanks to you both, Antoine and Steve. I stand corrected and educated. Now for ignorant question #2: do these attenuators function as noise reducers or efficiency enhancers or both or something else entirely ? It would be an interesting modeling subject, for that matter props in general would fill an article.
Thanks
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Postby KOEZE » Mon Sep 08, 2003 3:29 pm

As far as I know the shape of the hub of a propellor determines part of the noise generated by the prop. The blades are milled to a near perfect shape for enhanced efficiency. In the last batch of 688 class subs a ring was added to prevent water from flowing over the tips for the high pressure side to the low pressure side causing noise and loss of efficiency. You can compare the ring with a winglet as seen on aircraft.

The hub of the propellor is another source of noise production. I don't know the exact physics behind it but the blades you see disperse the flow behind the bud this reducing the noise. Another method can be seen on the photo's of the Walrus class in drydock on my page.
Walrus prop
You can see that the hub of the prop is not pointed but flat. This also helps disperse the flow and helps the flow detach from the hub.

Hope this helps.

EJK
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Postby ThorDesign » Mon Sep 08, 2003 8:29 pm

Hint: Russian Wake Homing Torpedos...
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Postby FX Models » Mon Sep 08, 2003 8:35 pm

Dolphin wrote:Me again. Photo of USS Los Angeles SSN-688 in Fremantle, Australia (HMAS Sterling) in 1999. A bottle shaped dunce cap, 7 blade prop with a 11 blade vortex attenuator aft, all turning as one in the same direction. Vortex attenuator blades stubby and rectangular in plan form. Other 688 subs use simular. Ringed props without attenuators common on 688I's.

Steve R.

Image

Hi Steve,

I think thats the USS Chicago in that image... Yes? No?

Marc
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Postby Dolphin » Tue Sep 09, 2003 1:27 am

Hello Marc and everyone,

Likely the 'Chicago' might use these too.

See the USS Los Angeles Home Page - http://www.la-ex.org/
It is in the photo section of this page, see WestPac Cruise 1999. Photograph by Gregory Smith. WestPac 1999.

Image source - http://www.la-ex.org/View_Photos/view_photos.html


This image link has been posted here in the SC page message board in the past, and credit for finding this image goes to another SC member from that old message board which we cannot refer back to when searching messages here.

This is a rather sensitive and surprising image that has been on this web page for some length of time. What it does disclose is common knowledge within the 'intel' of our's and other Navies, both friend and possible foe. But is new relatively to us modelers as you would expect hopefully.

My speculation here only as a middle aged greengo civilian. :D These attenuators as I understand them in a very limited way have likely several purposes. As Matt Thor hinted above - wake homing torpedoes. Attenuators and blunt ends of propeller dunce caps try as I understand it to create a low pressure area behind the propeller. Not to dissipate the wake vortices, but to collapse them. Make them smaller or disappear.

The operating speed range of the submarine would likely (either for stealth and/or evasion from counter attack) determine what kind of hydrodynamic design of the attenuator is required to be effective.

These flat ends on dunce caps were very common in 637 Sturgeon class boats from the 1960's onward in some form. They often had a 'bell shaped' depression in the blunt end. Sturgeons during the cold war likely operated at their highest 'silent speeds' to shaddow Russian and Chinese SSBN's and SSB's. A bell shape end on their dunce cap may have been sufficent, neither a blunt end nor a elaborate multiblade 'thingy'.

A blunt end on ....say a Walrus diesel sub propeller operating at normally lower speeds may ....depending on the speed and frequency of the wake be sufficient to collapse this wake. The 688 class on the other hand are comparative 'Greyhounds', the attenuators seem to show that much more in the form of blades on the dunce cap (along with Russian SS, SSN, SSBN, SSGN's) is required to collapse the wake at those higher speeds. Collapsing this wake behind the sub to a slender thread would make the sensors of a wake homing torpedo crisscrossing back and forth through this wake as it homes in on it's target more difficult and tenuous.

Also imagine what it must be like (or have been like) to get a 'blade count' with passive sonar to determine speed and identity by the opposition when these first props first appeared! Could the analog and early digital computers then cope with not just 7 blades ..but twenty??! Nothing remains static in sub warfare that is for sure. Imagine what else they must be doing now that we could hardly imagine. Aren't these subs fascinating!

Steve Reichmuth
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