For very fast moving objects, like a propeller blade, sometime you get better results by using a fake motion blur effect instead of trying to simulate it by using rendered motion blur.  I've used a disc with an appropriate texture to produce very nice looking moving aircraft  propellers in stills and animations.  The texture I use includes a general blurred effect and a stronger, still blurred,  impression of the blade.  For animations I add some rotation to the disc so that the stronger impression of the blade isn't in the same place in each frame.

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