>"Is ND filter the only solution?"
Most likely, yes.
Assuming the propeller is on a plane that is outdoors, per the above rule of thumb, in bright sunlight, proper exposure will be achieved at f16 with a shutter speed that is the reciprocal of your ISO. e.g. ISO100 = 1/100 shutter. Since f8 is two stops brighter than f16 (and you can't lower ISO), your shutter will need to be two stops faster. i.e. 1/400.
Whether or not this is slow enough to show motion blur is obviously dependent on the speed of the propeller, but I'm guessing no because -- ballparking to make the math easy -- at 1200 RPM the propeller completes one full rotation every 1/20 of a second. And that shutter speed would necessitate a four stop ND, which brings us back to "most likely, yes".
However, the next issue you're likely to run into is camera shake. The rule of thumb here is that camera shake can start to impact an image when the shutter speed slows below the reciprocal of the focal length. e.g. 400mm (full frame equivalent, i.e. zoomed halfway in on your camera) = 1/400 shutter (or slower). So you'll also want to use image stabilization (and practice panning).