Generally odd numbers of blades are better for vibration. I know that in wind turbines they are favored because it reduces the effect of the blade crossing the

Generally odd numbers of blades are better for vibration. I know that in wind turbines they are favored because it reduces the effect of the blade crossing the tower. When one of the blades in a three blade wind turbine crosses over the tower the force on it is reduced (since the wind is already slowed by the presence of the tower). The other two blades are in maximum loading, but balance much of their forces against each other reducing the bending moment on the turbine shaft.