At great personal risk an agent has taken these rare photos of a complex propeller used to power one of the latest German Type 212A submarines. Natürlich the agent's modus operandi remains forever secret. However, so as not to "blow his cover" he may well have used a button camera to take these incriminating photos. The strange small propeller at the end of the 212A's more conventional 7 blade propeller is called a Propeller Boss Cap Fins (PBCF).
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Up to a point the greater the number of propeller blades the slower the propeller needs to rotate to allow the submarine to reach the average patrol speed of around 5 knots. Slower rotation is one way to delay any onset of noisy, propeller damaging cavitation.
The strange small propeller at the end of the German Type 212A submarine's more conventional 7 blade propeller is called a Propeller Boss Cap Fins (PBCF).
An open Japanese website explains:
"PBCF device was developed in 1987 by Mitsui O.S.K. Lines, West Japan Fluid Engineering Laboratory, and Mikado Propeller (Nakashima Mitsuwa Propeller) and it has been adopted by installed on more than 3,000 vessels worldwide. Its fundamental mechanism and effects have been repeatedly verified through numerous series of model tank tests and actual ship measurements from the first stage of the development."
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Figure 1 courtesy PBCF website.
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PBCFs are small fins fitted to a propeller’s boss cap and are made of the same material as the boss cap as shown on the righthand diagram of Figure 1.
Tests on over 60 ships (and preumably Type 212A submarines) have shown benefits of 4-5% in fuel savings and an increase in speed of about 2% (at the same rotation rate). Without the fins, the flow of water around the propeller generates a hub vortex that wastes almost 10% of the engine's energy. The PBCF may also reduce a submarine's stern vibrations, hence reducing underwater noise. A quiet propeller, of course, adds to submarine stealth.
Tests on over 60 ships (and preumably Type 212A submarines) have shown benefits of 4-5% in fuel savings and an increase in speed of about 2% (at the same rotation rate). Without the fins, the flow of water around the propeller generates a hub vortex that wastes almost 10% of the engine's energy. The PBCF may also reduce a submarine's stern vibrations, hence reducing underwater noise. A quiet propeller, of course, adds to submarine stealth.
This June 2015 PBCF promotional video indicates at:
0:27 - the Propeller Boss Cap Fins developers
01:10 - why PBCF is successful?
02:10 - testing concept it in a cavitation tank
03:35 - adding PBC Fins efficiency advantages
06:45 - PBCF effects graphs
An effective submarine is made more efficient using a wide range of small improvements. The Propeller Boss Cap Fin is just one improvement that might be present on many more submarines than the Type 212A.
Pete