Are you familiar with the experiment. A water float with an air propeller above the water, connected through a shaft, to a water propeller under the water. Put a fan on the air propeller, and the float moves towards the fan.
Can that be considered a demonstration of the efficiency of an air propeller versus a water propeller? The air propeller is less efficient than the water propeller, that's why the float moves towards the air propeller?
Thanks.