Prop Savers help prevent propeller and motor shaft damage in the event of a prop strike.
This Prop Saver has long screws which allow two rubber 'O' rings to be used for high power applications, although one 'O' ring is usually sufficient.
Most GWS 2 blade slow fly props and APC slow fly props fit this prop saver. Be sure to use the correct prop insert provided with the APC props.
If your motor has a long shaft, I recommend that you mount the Prop Saver close to the motor and cut the motor shaft even with the Prop Saver. This allows for maximum flexibility for the prop in case of a prop strike, and helps prevent bending the motor shaft.
If you allow the prop shaft to extend through the prop hub, most any prop with a 3mm hole can be mounted, but the Prop Saver does not protect the prop very much, as the prop cannot flex in all directions.
The picture below is for reference only - no motor or propeller included.