Abstract: An anti-theft device for obstructing access to a nut securing a propeller on a drive shaft of an outboard engine assembly. The device comprises a two part yoke which partially encircles the housing of the engine assembly and its drive train at one end, and at its other end locates a cap over the nut threaded to the drive shaft. The two parts enable adjustment of the longitudinal dimension of the device, for accommodating outboard engine assemblies of different dimensions. In alternative embodiments, two types of locks are provided. One type is an integral key operated locking cylinder. The other type accepts a conventional padlock. In a further alternative embodiment, the cap has projections for engaging propeller blades and preventing these blades from rotating when the device is installed.