Nuts ?

[Time To Relax]

Is this the correct order?

Prop
Small nut
Big. Nut
Cotter pin

Or

Prop
Big Nut
Small nut
Cotter pin

 

[Air O'Nautical]

Found this to explain.

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.

Still,
i walked around a large marina and looked at the boats stored for the winter, about half have the large nut first....
None seemed to have fallen off.
I use the procedure above with no problems.

Cheers,
Mark.

 

[PIBYC]

Great explanation. Thank you.

 

[OldSkool]

Since the thread area is shorter on the thin nut, the pretension can be a lot greater than on the larger nut using the same torgue spec for both. The larger nut spreads it's pretension over a larger area so it's effect is to jam the smaller nut. The thinner nut holds the pretension on the bearing, prop, bushing, or what ever.

 

[Ka1oxd]

OK I am going to mix it up. I have Prop, washer, 2 nuts the same size, then pin. I would hate to have one smaller than the other.

 

[carterchapman]

Mark - My Sea Ray Sport Yacht Manual for our 410 has different instructions...Note Steps 4 and 5:

 

[Air O'Nautical]

Carter,
I think both ways will work because, I haven't heard of anyone loosing a prop.
Here is another Internet find on the subject..


Thursday, 09 February 2012 09:24
Propeller Nut Myth Busting
Written by Steve D'Antonio


How can you go wrong with something as simple as a propeller nut? The answer might surprise you. While few would deny the important role a propeller nut plays, many folks, professionals and boat owners alike, underestimate the variety of ways they can go awry.
WHICH NUT FIRST?
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.
I know for all you gearheads out there this appears to be heresy or worse, and you are wondering if I’ve lost my nut-and-bolt mind. When I first began proliferating this information several years ago, it was met with the hue and cry of many in the boat-owning and marine repair and boatbuilding communities, proclaiming it sacrilege, barbaric, and just plain stupid. Some even claimed that I had intentionally misled folks to drum up business for myself and my colleagues in the industry by propagating a spate of lost propellers. Even today, whenever I share this approach, the skeptics come out of the woodwork.
Rest assured, not only is this approach one I wholeheartedly endorse, but it’s also backed up with good engineering and it’s endorsed by propeller manufacturers, the Society of Automotive Engineers, the American Boat and Yacht Council, the U.S. Coast Guard, and the U.S. Navy. The latter spells this procedure out in excruciating detail in chapter 75 of their sublime (if you like this stuff) NavShipsTechManual (Chapter 075-5.3.4, Fasteners). It can be downloaded as a pdf, courtesy of Uncle Sam; I keep a copy on my iPad for “light” in-flight reading.
WHICH ALLOY?
A few months ago, I was standing beneath a client’s vessel, watching as a mechanic removed the propellers. After using a wrench to break the full-height stainless steel nut free, he tried to remove it by hand. Normally that’s easy to do even on a large, 3-inch diameter shaft such as this one. Yet, this one remained extremely difficult to remove and required the use of a wrench for nearly the entire unwinding process. That caught my attention and as I looked more closely, after both the full and half-height nuts were removed, I quickly found the culprit for the resistance. The nuts and the shaft had entered an unholy alliance that led to a phenomenon known in the world of metallurgy as “galling.” In plain terms, galling is a transfer of metal from one surface to another by friction-induced heating, thereby deforming the threads on the propeller shaft, which in turn made the nut’s removal more difficult. In essence, it’s micro-welding, and stainless steel, as it happens, is especially susceptible to this peculiarity. On small fasteners, a single galling episode can spell their ruination. For larger threads, as in this case, the shaft’s threads were salvageable while the nuts’ weren’t.
As is often the case, the best solution to this problem is prevention. The easiest and least expensive preventive approach involves lubricating the threads, something you should consider for any highly loaded stainless steel fastener. The problem with lubricating propeller nuts is the lubrication can lead to highly undesirable loosening. An alternative, albeit a less than perfect one, involves utilization of a thread locking compound that will temporarily lubricate the threads until it sets.
However, an even better approach, and one that doesn’t require any additive or compound, involves the use of a nut whose alloy is self-lubricating and is therefore immune to galling: brass. Yes, that’s right; the alternative (and I would argue a preferred one to stainless steel propeller nuts) is brass, and any propeller nut you see that isn’t stainless steel almost certainly is, contrary to popular belief, brass rather than the erroneously assumed bronze. While I admonished readers about the use of brass in a previous column, this is an exception to that rule.
Brass, actually manganese bronze (which is a misnomer, as technically, this alloy is in the brass family), is used for a few reasons. As mentioned, it’s especially slippery, thereby protecting your valuable propeller shaft threads from galling. Brass is strong and relatively easy to machine, making it well-suited to the manufacture of nuts. Finally, many propellers are made of manganese bronze (again, that’s technically brass) and as such, a genuine bronze nut would be cathodic to the prop, making the latter more likely to corrode when the two were in contact. If you maintain your shaft anodes, propeller and nut corrosion should never be an issue. Thus, when specifying a replacement propeller shaft, or just replacement nuts, go for the brass.
Last modified on Tuesday, 14 February 2012 14:35
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Published in Channels Feb Vol 1
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[OldSkool]

In layman’s terms, the half nut though having a higher torque in some cases can be loosened with a quick snap of pressure from the prop. Since the threads are few it can only handle a large torque load for a millisecond then snaps loose. The nut can be virtually turned off by hand. The larger, longer thread nut will better hold the torque pretension even under lighter torque spec tightening. The nut is not prone to the snap load loosening like the half nut.
The large nut becomes a better lock nut for this reason.
Another reason the lock nut has a lower torque is because you don’t want the jam nut the turn any tighter once it’s set. Therefore the larger nut must be used as the lock nut against the half nut.
Set the prop. Install the half nut to high torque. Hold the half nut to prevent additional torque loading then install the large nut against the half nut torqueing it to final spec.