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:
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.