Does a propeller being pushed through the air have more friction if free to spin? I just passed a boat being pulled on a trailer and noticed the propeller spinning in the wind. I began considering the possibility that a propeller free to spin might cause less friction against the forward motion of a force pushing the propellar forward. 
If that were the case, we could create a race car where the passenger area is free to spin as a propeller, shaped as a screw sort of, with the inside spinning at a constantly opposite speed so the driver himself didnt spin, but the vehicle's body gained improved aerodynamic performance.
So, what's the science behind this? Does a spinning device like a propeller experience less friction than an identical shape unable to spin? 
 A: The net drag force on the surface of the propeller is a function of the relative velocity of the fluid against the propeller's surface.
The simple model for drag force that can be (loosely) applied to the question regarding the propeller is
$$F_d=C_dA\frac{1}{2}\rho v^2$$
where $v$ is the relative velocity, $A$ is the projected area normal to the direction of $v$, $\rho$ is the fluid density, and $C_d$ is a linearized drag coefficient particular to the geometry considered.
In either case (propeller fixed or rotating under the wind's force) receives the same incoming velocity from the the trailered boat's speed. But at the surface of the propeller the relative velocity is reduced in the case of the spinning propeller which modes away from the direction of the incoming wind, thus reducing the relative velocity.
Therefore the moving propeller you observed spinning indeed reduces friction over what would occur if the propeller remained stationary.
NOTE: although drag forces were reduced, nothing was really gained in terms of energy expenditure. In other words by having the propeller spinning you don't take any less load of the engine that is pulling the boat trailer. Initially the higher wind frictional forces were required to accelerate the propeller into a spin, and at steady state there is momentum exchange to maintain the spin. The terminal (spinning) velocity is an equilibrium of forces - that which the wind supplies, and that what is consumed in the bearing friction of the propeller shaft. So the shaft frictional losses see some of the energy flow that was once all in the energy lost to drag over the still propeller. The total energy losses are the same. Sorry- no perpetual motion machine.
