Shipbucket

I found this pic on a other forum and wondered Why a 2nd propellor on the big one?



The photo is of the new Israëli Dolphin II class submarine build by HDW.

Dolphin II Class Submarine
Length: 68,6m
Tonnage : 2400 ton submerged

4x 650 mm Torpedo tube
6x 533 mm Torpedo tube

Photo Source: http://intercepts.defensenews.com/2014/ ... rly-ready/
Just one more for the beauty

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I bet is an new way to brake propeller noise but perhaps help on the effective on the propeller at low speed.... Just an bet, noting else.

Were I to speculate, I would say that it reduces the rotational element of the flow coming off the tip, which helps cut drag. Fluid dynamics looks easy, but getting it right is hard (this is why I'm better at the math for fixed installations than moving ones ), and to the layman looks like a dark art (you could make the case prior to CFD that it was).

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I found that the german Type 212's and the export variant Type 214's also have the same little ''propellor thing'' attached to the other prop.

Could it be somthing to reduce the noise comming from the propellor ?

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Not really for the type 212

or this small propellor is "retractable" ?

Retractable? no, covered thats the word. This thing probly was top-secret when the U-31 came in service in 2005, and maybe the german variant is different from the ''export'' variant because on the U-35 that came in to service this year also had the ''little prop'' covered up during its period on land.

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It's used for vortex shedding. It helps reduce the amount of propeller blade cavitation/acoustic signature when operating at or near the surface. More important for diesel boats that have to remain on top for hours at a time to recharge batteries.

http://en.wikipedia.org/wiki/Vortex_shedding

yup just what I believed it was.