Abstract: The present invention aims to couple a propeller with a power output shaft to transmit the torsional force of the power output shaft to the propeller. The apparatus includes a first sleeve and a second sleeve that form a coupling surface between them to hold a torsional force transfer means. The torsional force transfer means is coupled with the first sleeve and the second sleeve to transmit only a selected amount of the torsional force. The first sleeve has a first outer surface and the second sleeve has a second inner surface that are engaged respectively with the propeller and the power output shaft in the rotating direction so that the power output shaft can transmit the torsional force to the propeller. In the event that the propeller is overloaded the torsional force transfer means between the first sleeve and the second sleeve slip to form a protection mechanism.