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Propeller Nut Installation

Proper Inboard Motor Propeller Installation

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.


The following procedures are recommended for the proper installation of your propeller.

Handle your props carefully. The edges especially are susceptible to bending and dinging if they come into contact with other hard objects.

Smear some valve grinding compound (available at any hardware or auto parts store) around your shaft taper. Install the prop on the taper without the key and rotate 3 or 4 turns in each direction while pushing prop forward on shaft. This will remove minor metal imperfections and help assure a proper fit on the taper. Mark the shaft at the forward end of the prop hub at it's furthest point of advance with a pencil or magic marker.

Remove prop and wipe off all compound from shaft taper, prop bore, and keyway.

Fit key in shaft keyway. It should fit snugly and "bottom out" in keyway. Remove from shaft and try for same fit in prop keyway. If any areas bind or won't fit properly, dress the key lightly with sandpaper wrapped over a flat surface. If the key rocks or is loose, replace it, as looseness here can cause vibration and possible shaft damage.

Place key in shaft keyway and after aligning prop keyway, slide prop up onto shaft to the point which you previously marked. At this point the key should be completely captured by the prop and centered laterally within the prop hub. Never, ever, hammer on the propeller to make it fit on the shaft. If the prop doesn't move up to your marked point, the key is binding due to poor fit.

Take your large shaft nut and hand tighten up against hub. At this point many installers will place a block of wood between the boat bottom and end of propeller to keep the shaft from turning while they tighten the nut with a wrench. This practice should be avoided if at all possible, as it can cause blade distortion. Use a plumbers strap wrench to hold the shaft or have a friend hold the prop down close to the hub while you tighten the nut and firmly seat the prop on the taper.

If you have a single "castle" type nut, finish by installing your cotter pin at this point. If you have two nuts, remove the large shaft nut and replace with the small one. Tighten small nut using the same procedures as in step 6. Now, thread on your large shaft nut behind the small one and tighten (this is your lock nut).

Install cotter pin, and bend back prongs.