Quote History Quoted:
Do those second ones turn? I have no idea but guessing I'd say they are to direct the thrust of the main propellers to make them more efficient.
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Second ones don't look like they turn. I am betting they are to prevent cavitation or minimize it so the water flowing behind the ship is smoother and reduces vibration on the hull.
ETA:
Or maybe they do:
Cunard ordered two five bladed and adjustable propellers, which each would have a diameter 5.8 meters. Attached to the propellers would be two Grimm-wheels (I don’t know if that’s also the name in English but that’s what it’s called in Dutch). These Grimm-wheels are 15% to 20% larger in diameter then the propellers and are attached behind the propellers. These Grimm-wheels rotate separately from the propellers and use the slip streams created by the propellers to gain 3,5% of propulsion energy which else would be lost.
The propellers were designed to function with Grimm Wheels. The seven-bladed vane wheels were fitted behind the propellers and freely rotate on the same shaft. These wheels are in use on other ships to reclaim part of the energy normally lost in the slip stream of the propeller and convert it into additional thrust. They were predicted to do the same on the Queen Elizabeth 2 and to save 2 to 4 percent in fuel costs. Unfortunately, some of the vanes were lost during trials, so the remainder of the blades on both shafts were removed and not replaced.