Shrouded submarine propulsors - conflicting claims?


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Jan 11, 2011
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I tried looking into the whole issue of pump jets; shrouded propellers; shrouded propulsors and open skewback props for submarines. What I expected to find is not a lot of data. But what I did not expect to find is conflicting claims.

First of all, there seems to be a pretty blurred line between shrouded propulsors and pump jets. There are various claims around using both names for what essentially looks like the same thing. Basically a converging shroud of considerable diameter (rivaling the diameter of open props of previous subs) which has several stator blades and which then usually has around 10 or so rotor blades. They're basically almost touching the shroud and they're of constant chord, with a fairly gentle curve.

Now, most of the western subs seem to have that design. And perhaps even the only eastern sub (borei) may have something similar. Though, that one youtuber/ex submariner Jive did claim in one of his videos that borei uses a different design, where the shroud is smaller as the blades are shorter in the vertical axis but much longer in the horizontal axis. basically forming long vents. now.... i am not sure that actual photos of the borei shroud support that claim. Does anyone know more about it?
And if i am not mistaken, IF that's true, that'd be the only such example of a pumpjet on subs - as western subs, at least as per open source drawings, seem to use propulsors that are closer to regular short blade pumpjets.

What's also confusing is the fact there are papers and articles claiming that pumpjets are usually becoming more efficient (power wise) than open props somewhere in the double digit knot speeds. Anywhere from teens to 30 knots and above.

But if that was true - then why would a SSBN like Borei, which likely wants to be optimized for slow patrols - use a pumpjet?

I can understand pumpjet usage on SSNs like on US and British SSNs if it's true. Basically, slow speed power efficiency is sacrificed for medium/high speed efficiency, while possibly retaining noise advantage at any speed. Given that SSNs would usually operate at higher speeds than SSBNs on the average mission.

But then there are also articles which claim basically the opposite. And say pumpjets are actually noiser, at high speeds than at lower speeds, compared to open props. Which... I guess even makes some sense if we look at borei SSBN and severodvinsk SSN. SSBN would want to be quiet while going slow. While SSN might want to trade off some of that low speed quietness for more quietness at high speeds - since it's going to be using those high speeds more often.

To sum it up - is there an actual scientific paper or article of half decent credibility that talks about the issues of pumpjets/open props for various speeds of submarines? Or are we destined to use dubious google result articles written by unknowns?

Foo Fighter

I came, I saw, I drank some tea (and had a bun).
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Jul 19, 2016
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Interesting and thought provoking post, than you.

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