Abstract: An anti-theft device, for preventing unauthorized access to the propeller retaining nut, includes two sections that fit together to form an access-preventing cap. The cap fits around an expanded rim normally found on propeller center-bodies, and the cap cannot be pulled off the propeller center-body without disengaging the two sections from one another. A tongue extending from one section is engaged by a grooved dowel going through the base portion of the second section to hold the two sections together. A key operated locking flap engages the groove in the dowel to prevent removal of the dowel without possession of the appropriate key. In a second embodiment, an adapter is placed on the propeller shaft, between the propeller and the propeller retaining nut. The access-preventing cap is of smaller dimensions than in the first embodiment and is shaped to engage the adapter.