The three most com­mon prop-lock­ing mech­a­nisms are:

Boating NZ - - Feature -


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.


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.

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