Prop Nuts

Has the same mechanic been installing those nuts all that time? Does he use a torque wrench, or go by feel? Does he put a wrench on the first nut while torqueing the lock nut? Thread locking compound would help, but if you ever need to remove those nuts underwater, you might curse that option.

Just wondering, are they left-hand thread? Some mechanics don't have a torque wrench for left rotation.

Blue (medium) threadlocker, not red (high-strength). When removing, you will need to spin the nuts off with a wrench, they won't spin off freely once they break loose. I'm just thinking that will be a pain underwater, and I'm pretty sure it isn't recommended for prop nuts f0r that reason. Either way, if you describe the problem, the mechanic should want to use a torque wrench. I'm not sure what torque, but you can look up the thread size and pitch, and nut and shaft material, and find the torque so the mechanic doesn't have to hunt or guess.

What is the installed order of the propeller nuts?
The thin nut, which is properly called the Jam Nut, should be forward, closest to the aft face of the propeller hub. Then the large nut goes next. Otherwise, the mass moment of inertia of the large nut can overcome the force of the incorrectly placed Jam Nut, and you have aft movement in your installation and loose nuts (sounds funny, I know!).
But it can be more complicated in the fitting of the propeller to the shaft. Dry fit it without a key and mark the forward prop hub location on the shaft. Now pull the prop, install the key, and refit it on the same shaft to make sure you are at the same location, otherwise the prop may be riding the key and you need to re-work the key, or the keyway, or check both shaft and prop tapers for correctness.
If the dry fit with the key is good and in the same location with the marked line, you can use the Large Nut to snug everything up. THEN, back the Large Nut off and remove it, install the Jam Nut, then the Large Nut, use a cheater bar and she should be good to go.
Lapping the propeller to the shaft is also a most favored process and has been covered on this forum in detail. My thought - if the yard does not properly lap the propeller to the shaft for all installations, go find a new yard. They should at least offer it as an "option", even though I would consider it "standard practice".

I agree with Pacblues comments. The props need to be lapped, and the nuts in the proper order.

I agree with comments about lapping prop to the shaft. I know the sequence of the nuts has been discussed and debated at length. The sequence can be determined by the type of nuts being used. My boat has a flanged face nut as the large nut. In this case, the large nut must go on first with the flange against the prop hub. Then the jamb nut goes on. I don't know how many boats out there use the flange face nuts, maybe not many but personally I like them. Puts a lot more surface area contact against the prop hub.

I just had a discussion on this very topic yesterday with our local machinist who has done a LOT of shafting for a bunch of builders over the last 30 years. He said the large/small nut order was a relic from the days when prop nuts were not faced and usually rough castings. Modern nuts are almost always faced and the order doesn't really make much difference. He prefers to use two of the large nuts and machine down the aft one so when the prop is properly seated, the aft face of the second nut is right at the cotter pin hole and is held from rotating by the pin.

He also said lapping more often than not reduces the contact area if not done correctly and does a demonstration every year at our community college boat building program (MARTEC) that backs up that controversial statement.

Good comments. The engineering of the shafts/props/nuts is an SAE spec so I would not believe the tale about rough casting nuts determining the order. Would like to see the lapping demo and what is the wrong way to do it, have always seen improvement in the contact area versus a reduction in my own experience.

As an alternative, I like the trick of custom machining the the nut to have the last one match up closely to the cotter pin, but would still go with the Smaller Nut first, Large Nut last sequence.

Have not seen the use of the flanged Large Nut up against the prop hub, but that Don Rosciolo was not above trying something different, maybe saw it on a European boat.

Quite honestly I like how some of the SF builders do it, with smaller diameter splined shafts instead of keyways.

We also lapped the prop onto the shaft at the Viking Service center in Riviera Beach when I worked there, but then used a propsmith to install the prop. Our "torque wrench" was a called a "slugging wrench" which was super heavy duty box end with a solid flat end for slugging with a small sledge hammer. I personally installed hundreds of props on shafts from 2 1/4 to 4 inches in my close to three years there. We never had a prop nut come loose.
My professional opinion is that the OP has his props installed improperly if the nuts are coming loose.

I saw in a rare instance where the prop shaft threads were improperly cut and that caused the nuts to always back off. I forgot who had this issue, but they determined that to be the cause once they measured the threads with a tap and micrometer.

Interesting thought on the threads improperly cut. Any lathe machinist should be good at measuring the threads with a proper thread micrometer, but they usually just spin a nut on to check the threads work. Running a die on the threads would be a good way to make sure they are properly cut, clean and undamaged. If a sharp and unworn thread die turns easily across the threads, the threads are too small or worn down. The nuts, similarly, should not jiggle on the threads.

You could buy one of these jigs to drill your prop nuts for safety wire, can even drill them without taking them off the shaft. This jig is too small, maybe they are made for bigger nuts, otherwise it doesn't look hard to make:
http://www.aircraftspruce.com/catalog/topages/nutsaftblok.php

This was a very slick way for the go fast SF pioneers to get every last ounce of performance (and reduced drag) of the propulsion packages of that era, mainly Detroits and the sprinkling of a few MAN's. It was a carry over from the Hydro-Plane engineering guys out West and was the trick set-up for the time. A couple of SF builders have applied that philosophy today, like Willis I believe, but it is still not the norm.

Now the propulsion packages have become high hp affairs, MTU or CAT, and the engineering behind the props/shafts/struts has gone up another level. Everything needs to be calculated and accounted for, including unsupported shaft lengths, strut design, including strut barrel lengths which should be full SAE spec, prop hubs should also not be cut down in length, should be the full SAE length, distances between props and struts, and now there is more attention paid to the rudders as part of the propulsion package. All the materials of the above , including today's shaft materials are under more scrutiny. I have found that a Safety Factor of at least 5.0 for shafts installed in diesels, regardless of which shaft material is selected (AQ 17/19/22/22HS) is a much safer bet for longevity then pushing the envelope to lower safety factors.

But with Cummins 450hp Diamond series engines, the OP is not yet in this high power realm. The Thread condition is an interesting point, and the proper way to check the shaft and nut threads is with a Go-No-Go Thread Gage. They are expensive, but the right tool for inspecting Threads.