This is a Reverse Rotation or Pusher prop. Pusher props are great for setting up twin engine planes with counter-rotating motors to eliminate the torque problems which can occur when both motor turn the same direction.
The GWS 3-Blade propellers can be used on models that need a bit more ground clearance, or for those that want a more scale look for their aircraft.
The GWS 8x4x3RR prop has a 1/4"center hole pre-molded, and the hub measures 18.5 mm in diameter and 11.6 mm thick. The hub is designed to use the included spacers to adapt the bore size to various prop adapters or shaft sizes. The included adapters allow you to adapt the center hole to fit 3 mm, 4 mm, 5 mm, 6 mm, 8 mm, or 1/4 inch prop shafts.
According to the widely accepted Maximum RPM formula of 100,000/D for GWS DD series props, the 9x5 DD props should not be used over 10,000 RPM.