BACKGROUND

A tandem propeller utilizes a pair of conventional propellers fitted on the same shaft in series and rotated in the same direction.

Tandem propellers should not be confused with previously discussed counter-rotating propellers which have two shaft lines, one within the other, to drive two propellers in opposite direction at nearly the same rpm. Examples: USS JACK (SSN-605) and the USS ALBACORE (SS 569) in the last of its many modifications.

Open sources indicate VICTOR, CHARLIE, OSCAR I and the single MIKE Class Soviet/Russian nuclear submarines employed tandem 4-bladed propellers (total of eight blades) on some or all units of those classes. Most of those tandem propellers had a 45 degree angular separation between the eight blades; however, at least one CHARLIE Class SSGN had a 20 degree separation which produced a 70 degree gap to the next set of blades.

The link below shows an image (click to enlarge) of the KOMSOMOLETS (MIKE/Project 685) Soviet nuclear submarine) tandem, 4-glad)ed propeller resting on the bottom in the far northeastern Norwegian Sea at a depth of 5530 feet (1685m). It sank on 7 Apr 1989. Note the four-vaned vortex dissipator on the shaft hub (far left).

DISCUSSION

Increasing the number of blades on a submarine propeller while retaining the same diameter increases the thrust that can be developed at a given rpm or, alternately, permits a propeller of smaller diameter to develop the same (original) thrust at that given rpm.

Decreasing the diameter of a submarine propeller reduces the rotational velocity of the blade tips, the location where cavitation first occurs; hence, the use of tandem propellers appears to have been an effort to raise the cavitation inception point of the above noted classes of Soviet nuclear submarines for given combinations of depth and speed.

This conclusion is based on the assumption that measurements more refined than those the writer can make ffrom open sources will confirm the diameter (and rotational velocity at a given speed) of a single-plane, 5-bladed VICTOR propeller is greater than the diameter (and rotational velocity) of tandem propellers used by other VICTOR Class units operating a the same shaft rpm.

The extensive use of tandem propellers indicates that, at least as late as the 1970s, the Soviets were most concerned with the detectability of cavitation in a submarine versus submarine environment to the exclusion of other considerations. An independent assessment states: (quote) A tandem propeller on a single shaft creates less noise compared to a single propeller; therefore, the sonar system of a submarine, limited by self-noise at high submarine speeds, can perform better. (end quote)

CONCLUSION

So, we have - apparently from the Soviet perspective - a two-fold benefit from the use of tandem propellers: (1) reduce the high frequency detectability of the submarine (cavitation) while also improving its capability to detect other submarines. This extensive use of tandem propellers appears to have been another example of "Better is the enemy of good enough." where better later became the ability to design and fabricate the skewed, seven -bladed propeller first employed by the AKULA Class SSN in the mid-1980s..

"where better later became the ability to design and fabricate the skewed, seven -bladed propeller first employed by the AKULA Class SSN in the mid-1980s...."

Courtesy of commercial companies from 2 of our close allies (Japan and Norway)!!!

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Submarined by Japan and Norway, Published: June 22, 1987, NY Times

Western security has been undercut by the avarice of two companies, Toshiba of Japan and Kongsberg Vaapenfabrikk of Norway, and by their Governments' lackadaisical supervision of militarily sensitive exports. Under false pretenses, the companies shipped machinery to the Soviet Union that will enable the Russians to fabricate quieter submarine propellers. Soviet submarines will now become much harder to detect, all so that Toshiba and Kongsberg could turn some extra profit........

Entire article can be read at < http://www.nytimes.com/1987/06/22/opinion/submarined-by-japan-and-norway.html >

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Rick Greatting

Rick:

It takes more than Japanese and Norwegian technology to successfully produce advanced propeller designs. It takes engineers who know what they are doing. See my archived posting on the misadventures the Soviets/Russians had with the TYPHOON screws.

Ten years ago, a knee surgeon, who asked what I had done during my career, responded that he had told a Toshiba representative who was trying to sell him X-ray equipment that he would not buy anything from Toshiba because of their high technology sale to Russia. So, even in Louisville, KY, there are those who are informed about what goes on in the world of submarine technology.

Bruce

Bruce,

It is my understanding that the Soviets had the engineering knowledge but lacked the precision equipment required to implement their propeller designs which is where the Japanese and Norwegian (although the Norwegian involvement in this always seemed to have been marginalized) sale came in. I've heard this sale probably saved the Soviets something on the order of 10 years of effort. Don't know how accurate the 10 year # is but it sounds reasonable.

Rick

Rick:

Years ago, an Electric Boat engineer told me that propeller design - and especially the finishing of propellers - was as mich an art as a science with the final steps often involving hand-working the trailing edges with a file or its powered equivalent.

Bruce

Sounds somewhat like passive acoustic analysis.

Rick:

You got that right - which is exactly why those with technical degrees - and no operational experience - seldom develop advanced levels of analytical expertise; they quickly realize that although there may be rationality to analysis, it also requires knowledge of an incredible number of variables - and they go on to more easily mastered disciplines.

ONI learned this 50 years ago when - as I discussed previously - they hired two mathematicians whom they hoped George Miller and I could train. It was a hopeless endeavor. There neither was then nor is there now any substitute for years of "pounding the mets" - or its modern equivalent - at the right locations.

OBTW, I posted those ALROSA specs to provide the basis for others to calculate the propulsive efficiency of that pump-jet system.

Bruce

PS for Russ. I assume the verification text/numbers have been changed because software had become sophisticated enough to read the old script. Well, the new verification may be better but its also a test for at least this writer who often has to make four or more attempts before getting it right.

Bruce, Rick,
I always referred to it as "Time Behind The Eyes" !!!
An amazing job we all performed. What great fun and excitement for my 21 years. From the Green doors to the retirement ceremony what a trip.
Chuck

Hey Bruce,

I am posting this so I can see what you are talking about.

I am unaware of any changes to the captcha but the forums are hosted on bravenet and they do change things occasionally.

Looks like they have gone to a photo captcha. I do not think there will be anything I can do about it but I will log into the dashboard and take a look to see what options there might be.

Russ

Looked, the only options I have to work with are on or off. For now it will stay on I think.

I am not sure what issue you ran into. The two captchas I have been presented with have been easy to read and enter. So, not sure what to reccommend to you on it.

Russ:

Thanks for checking. I'll just pull out my 8X magnifier and forge ahead.

Bruce