WHY DID MY PROP COME OFF?
The three most common prop-locking mechanisms are:
Looking like the ramparts of a castle, these nuts have slots for a cotter pin to be pushed through a hole in the prop shaft. The pin is then bent to prevent it falling out, and it prevents the nut from coming loose. Although this is by far the most popular prop securing method, it’s not particularly reliable as only a cotter pin prevents the nut from coming loose.
This arrangement uses two nuts of identical thread size, but one twice as thick as the other. The thinner nut is the ‘jamming’ nut and always screwed on first, tightened to the specified torque. Then the thicker nut is fitted and tightened with considerably more force up against the thinner one.
This causes the nuts to lock tight against each other with the bigger, outside nut taking most of the load. Usually, an additional cotter pin is installed in the driveshaft outside the second nut. This is arguably the most secure locking method as two separate mechanisms prevent loss.
TABBED WASHER AND SINGLE NUT:
Either a nyloc or castellated nut, this is the most common mechanism used on outboards, where a tabbed washer is fitted between the prop and the nut.
Various types exist, but they all have some way to stop the washer turning on the shaft – either an internal protrusion that fits into the keyway on the shaft, or cut-outs to match the splines on the shaft.
The nut is tightened hard up against the washer, after which the tabs on the outside of the washer are bent, either tight against the nut sides or against the castellated part of the nut.
Since the washer cannot rotate due to the splines, and the bent tabs prevent the nut from rotating, the prop cannot come off. If a nyloc nut is used then there are two mechanisms to stop the nut unscrewing, but if a castellated nut is used then an additional cotter pin is recommended.
Both the cotter pins and tabbed washers are designed to be used once – when removing and replacing the prop for maintenance, replace these with a new one to prevent them failing due to metal fatigue.