Propeller Nuts

Although it’s not a hard and fast rule, propeller nuts are frequently available in two thicknesses or heights: full height and half height. The American Boat and Yacht Council, using Society of Automotive Engineers’ references, refers to these components as the “jam nut (thin)” and “plain nut (thick).”

The reasoning behind this differing height nut approach is straightforward enough. The full-height nut, because it has more thread engagement, is designed to carry the lion’s share of the load, while the half-height nut is designed to act as a locking mechanism. That’s the aspect of propeller nuts most folks understand and with which they agree. The tricky part, and where the controversy comes in, involves their order of installation. Contrary to commonly held beliefs, when assembly is complete, the half-height nut should be against the propeller hub, having been installed first, while the full-height nut is farthest from the propeller hub, having been installed last. The logic for this seemingly counterintuitive approach goes like this: when the first nut is installed and torqued down, it carries all the load or tension. When the second nut is installed against the first nut and it is torqued, much of that load or tension is transferred to the second nut. Thus, it only stands to reason that the second nut (the one that carries the majority of the load), should engage more of the propeller shaft with more threads.

There is a misconception by many people on the proper way to install a propeller and specifically the retaining nuts for the propeller.  We often see the two nuts installed in the wrong sequence.  In accordance with ABYC standard P-6 and SAE standard J755 the thinner nut should be installed against the propeller and the thicker nut should be installed against the thinner nut with the cotter pin installed last.  The reason for this sequence is clear when you think about what happens when you tighten up the outer thicker nut.  As the outer thicker nut is tightened to lock the two nuts in place it partly releases the pressure on the threads of the inner thinner nut.  Clearly we want the thicker nut, which has more thread surface, to be the nut carrying the load.