Propeller simulation in "moving flow"

Hello! I have a little problem:

I need to simulate boat propeller in a moving water (like real life, when boat moves), and i've been thying to solve this problem for a couple days now.

The question is how to make initial flow speed equal to 4 m/s? Boundary conditions wont work, because it slows down the propeller to match initial speed. I tried using fans, but couldn't understand how to set a flow speed there...

Thanks!

SolidworksFlow Simulation