Question

What makes the inner prop nut and the outer come loose? How do I reset them to be sure this does not happen again

There has been quite the discussion about this in the recent past regarding the proper placement of the main nut and the Jam nut. As I recall the Jam nut actually goes between the prop and the large nut which is counter intuitive. Some one posted a white paper on the subject that was fascinating. Sorry for not linking it for you but should be easy enough to find.
CD

dwna1a said:

What makes the inner prop nut and the outer come loose? How do I reset them to be sure this does not happen again

Click to expand...

https://www.seattleyachts.com/news/clearing-up-the-confusion-of-prop-nuts
https://www.sbmar.com/articles/propeller-installation-big-nut-vs-little-nut/
https://www.passagemaker.com/channels/propeller-nut-myth-busting
And the list goes on.

Tech, that’s how it is set up, halfnut first. Two wrench’s to set them.
But what makes them come loose? Vibrations?

dwna1a said:

Tech, that’s how it is set up, halfnut first. Two wrench’s to set them.
But what makes them come loose? Vibrations?

Click to expand...

I have always tightened them tight, one turn or ten. I wouldn't stop at two.

To install props correctly put them on the shaft without the key, make a pencil mark install the prop with the key and make sure it hits that mark.

In most cases the prop rides up on the key slightly and when shifting, that action loosens the nuts because the prop is not seated on the shaft correctly. To help stop that, don't put the key all the way in with the prop. Tightening the half nut will push the key in the rest of the way and the prop should be right at your mark. Then tighten the larger nut and your good to go. Also make sure the key is the correct length and not approaching the key way curve.

Improper installation makes them come loose. What's proper installation? Eh, I snug them up with some 5200 on the thread and between the nuts and they never come loose. Cotter pin prevents anything really bad from happening anyway, loose or not.

Ok today I snugged the inner nut to the prop. Then the outer nut. New pin. The prop never moved off the key way or key. Just the two nuts

Shaft nuts coming loose can be caused by a host of issues which need to be identified and corrected.
I posted this in another thread -

In theory there should be no nuts required if everything is fitting correctly. If the nuts have loosened more than once you have other issues that need to be corrected. If this is the case then:

• Disassemble and remove the prop from the shaft and inspect everything. Especially look for cracks and deformations.
• Make sure there is no rotational play between the prop and shaft - the key and keyways are tight.
• Cleanup the prop bore and shaft and using machinist dye check the fit between the prop and shaft.
• Lap the prop to the shaft as required and re-check with the dye. After cleaning up the lapping compound/dye then re-fit the prop and scribe a prop location (index) mark on the shaft without the key installed.
• Reassemble dry (no lube at all) and clean with the key installed and make sure the prop installs to the index mark.
• Get new nuts and install the thin nut first (no lube here either). I use a 3 foot breaker bar and put all of my 172 pounds of weight against it. That's right at 500 LbFt for my 2.5 inch shafts.
• Check that index line to make sure it remains as was after lapping. If it is not in the same location the prop is cracked and will require replacement.
• Put the thick nut on and torque the same then install the cotter pin.

ttmott said:

Shaft nuts coming loose can be caused by a host of issues which need to be identified and corrected.
I posted this in another thread -

In theory there should be no nuts required if everything is fitting correctly. If the nuts have loosened more than once you have other issues that need to be corrected. If this is the case then:

• Disassemble and remove the prop from the shaft and inspect everything. Especially look for cracks and deformations.
• Make sure there is no rotational play between the prop and shaft - the key and keyways are tight.
• Cleanup the prop bore and shaft and using machinist dye check the fit between the prop and shaft.
• Lap the prop to the shaft as required and re-check with the dye. After cleaning up the lapping compound/dye then re-fit the prop and scribe a prop location (index) mark on the shaft without the key installed.
• Reassemble dry (no lube at all) and clean with the key installed and make sure the prop installs to the index mark.
• Get new nuts and install the thin nut first (no lube here either). I use a 3 foot breaker bar and put all of my 172 pounds of weight against it. That's right at 500 LbFt for my 2.5 inch shafts.
• Check that index line to make sure it remains as was after lapping. If it is not in the same location the prop is cracked and will require replacement.
• Put the thick nut on and torque the same then install the cotter pin.

Click to expand...

That's the post I was looking for when I posted the above thread.

@ttmott "Whenever work on a propeller is done the prop should be fit on the shaft with some machinist dye and then removed and the dye pattern inspected to ensure all of the dye is disturbed (the verifies there is full contact between the propeller bore and the shaft taper); this is an indicator that the taper on the shaft and taper in the prop match and there are no dings or bends that will prevent a proper fit. If it does not properly fit then it must be lapped in; have the propeller shop or your mechanic do this. Once that is done then the prop is fitted on the shaft without the key and snugged up with one of the nuts then scribe a line with a pencil or marker where the prop is on the shaft; this is done to ensure the key does not slide up and prevent the prop from fitting to the proper location on the shaft. A proper fit between the shaft and propeller does not require the nuts to be extensively tightened. You can place a soft wooden block between the hull and one of the blades and then tighten the thin nut to 300 - 400 lb ft for nuts that size. If you have a 24 inch long handle on the wrench and apply 150 - 200 pounds force on it then you are good. Many say not to block the propeller but when you look at the force placed on the end of a blade to tighten the nut it really is not that extensive. Put the thicker nut on, torque it the same as the thin nut then install the cotter pin and you are done. There is no need for any locking compound on the threads (I have never seen this done); in fact it is actually best to apply oil on the threads during the nut tightening. The simple fact that there are two nuts and the reason for two nuts is to provide a locking effect there is no need for any additional locking features."

My only contribution is that, it's all about perfect fit and proper clamping force.

T, we put the thin nut on yesterday and put a bar on it, the second we put one with another wrench and slugged the two tight. There is no play before or after, the key is new. The prop never got loose on the shaft or key. I just found it odd.