Propeller shaft prep for prop

On inboard boats I usually put the prop on after cleaning the ID of the prop and the taper of the shaft w acetone. Some say to use never-seize. I did that once and the prop DID come off quite easily. I worry though if the prop, through torsional vibration work (move back and forth) a tiny bit and wear the propeller bore. What do the rest of you do?

The last time we had a prop at a*shop in*Seattle, the prop guy said it was OK to use Never-seize but he said to use a real light application.* When we hauled-out 2 plus years later the prop nuts were as tight as when I installed them.

Larry/Lena
Hobo KK42
Zihuatanejo, MX

One yard used some kind of cutting oil on my previous boat, a 45' Californian MY with 485 hp DD's. The guy sat on a box turning the prop on the shaft for about 5 minutes before he installed the nuts.

The way i do it is to use valve grinding compound and put it on the shaft and slide the prop on and turn the prop for a while* without the key to get rid of any high spots,then push hard on the prop to make sure that is all the way up without the key,and then mark where it is on the shaft, take the prop off and put it on with the key in it , and make sure it goes all the way up to the mark,if it dont then you have to file the key i little till it goes all the way.if you dont it might become key bound, and you could get a vibration,because the prop is riding on the key instead of the taper

capt jerry wrote:

The way i do it is to use valve grinding compound and put it on the shaft and slide the prop on and turn the prop for a while* without the key to get rid of any high spots,then push hard on the prop to make sure that is all the way up without the key,and then mark where it is on the shaft, take the prop off and put it on with the key in it , and make sure it goes all the way up to the mark,if it dont then you have to file the key i little till it goes all the way.if you dont it might become key bound, and you could get a vibration,because the prop is riding on the key instead of the taper

Click to expand...

You clean the valve grinding compound off before the final installation, right?

*

Valve grinding compound. That's what I meant.

capt jerry,That's the way I've done it except once. I think I was "key bound" a bit. Must have had a lapse or was in a hurry. Some vibration resulted.*I'd always put them on dry but the local mechanic said use Never-Seize. So I tried it but it wasn't long before I needed to pull it off and it did come off easily but that may have been related to the key bound situation. The only way I can imagine the Never-Seize being a problem is if the shaft and prop were to move back and forth causing wear but Larry's experience suggests that dosn't happen. If so I think the boat mechanic here was right.
Use Never-Seize.

After cleaning off the lapping compound, coat the taper with machinist's bluing then trial fit the prop. The bluing will show the area of contact. Typically, 85 percent contact or better is considered a good fit.

Great now that it fits , will we resume the big nut inside vs the small nut inside war?

MY way,
Big nut , big wrench to seat the prop, then take OFF the big nut , install the thin nut reasonably tight , then the big nut really tight up against the small nut.

Other religions?

Is the smaller nut not a lock nut of sorts? The bottom line here is to hold the prop on so why on earth put the small nut on first? I've heard it's the propper way to do it but it seems so bass akwards I still don't put the small one on first because I can never remember the why. OK*** ...I'll bite FF***** .....why?

FF wrote:

Great now that it fits , will we resume the big nut inside vs the small nut inside war?

MY way,
Big nut , big wrench to seat the prop, then take OFF the big nut , install the thin nut reasonably tight , then the big nut really tight up against the small nut.

Other religions?

Click to expand...

What no cotter pin?*

*

Depends on size of prop & shaft but for mine there is no lock nut, big boss nut with a 1/4" brass rod thru nut and shaft bent over each end.
Here we go a little photo for description.
Benn

-- Edited by Tidahapah on Saturday 25th of December 2010 09:18:11 PM

For yrs and yrs all the older Bertrams from the factory* had the small nut on first

Why

Eric,
Yes Why.
A nut requires its dia in depth for any holding power.
A 5/8 nut must be 5/8" deep
A lock nut is usually a 1/2 nut tightened up on top of the full nut.
Go figure!!!
Benn

The thin nut is a jam nut. It purpose in life is to prevent the larger load bearing nut from backing off. It goes on first.

The way it works with props is the large nut is used to push the prop tightly on the taper. It is then removed and the thin jam nut is torqued to about half of its rated torque. This is just to ensure that the nut is hard against the prop hub and its threads are loaded. The jam nut is held with a wrench and the large nut is then tightened against it.

What happens is the threads on the jam nut are at first in contact with the loaded or working side of the threads, picture the nut pulling itself down the shaft by gripping one side of the threads. A nut works because torque on the threads creates a tensile load on the shaft or bolt, it stretches it. The parts are held together because the shaft or bolt wants to spring back and whatever is between the nut and the shaft gets squeezed and held by that tension. The torque figure you use or how much umph you twist the nut or bolt depends on how much load the fastener can take before it breaks in tension, or breaks across the threads because of torsional failure or thread stripping. The figure for tension or elongation is calculated by the loads imposed on the shaft or bolt by the machinery it is holding together.

Head bolts must withstand the forces of compression and combustion that want to separate the head from the block. The bolts are tightened so that the force holding the head on is several times higher than those trying to lift it off. If the tension is not high enough, the forces will stretch the bolt each time the cylinder fires and then when that pressure is relieved, the bolt is actually loose for a moment and vibration will loosen the bolt. The best way to prevent firing pressure from stretching the bolt is to pre-stretch it more than the engine can.

Anyway, back to the jam nut. When the jam nut is in place it is applying a force toward the propeller. That force comes from the contact of its "forward" thread surface on the "aft" thread surface (we can use those terms because the shaft is horizontal and we are moving the prop forward) of the shaft. Most of us know that there just aren't enough threads on the jam nut to hold the prop on without strippiing and this is where the controversy comes from. The job of the jam nut is not to hold the propeller on, it is to prevent the prop nut from loosening.

The prop is held securely because when the prop nut is torqued fully it pushes the thread contact area of the jam nut off the "aft" threads of the shaft and unloads the jam nut threads completely. Torgue applied to the prop nut stretches the shaft by pushing against the jam nut which in turn pushes against the prop hub until the jam nut threads contact the "forward" thread face of the shaft. This jams the two nuts together preventing any movement between them.

It's not religion, it's mechanics.

Rick,
You're the head gear head from now on. That's the best explanation I've ever heard. So the jam nut just sit's there jamming the two nuts and keeping the load on the big nut that is capable of holding the prop on YES! If the jam nut is installed last the big nut transfers part of it's load to the jam nut but if the big nut goes on last the jam nut transfers it's load to the big nut so the big nut holds the prop on AND carries the jam nut load as well.
Did I get it?
The part that most frustrates me most about propeller installation is the cotter pin, the hole in the shaft and the hole in the big nut. It seems most of the time the when the nut is torqued the right amount the holes are as far out of alignment as possible! Then I over tighten the nut, put the cotter pin in the holes and back off the nut until the holes prevent movement of the cotter pin. I've often thought of drilling more holes in the nut but never have. Is the properly installed jam nut without cotter pins as secure as with the cotter pins? I think there are shim washers to solve the hole alignment problem.
Never seen'em though. Is this the answer or what???


-- Edited by nomadwilly on Sunday 26th of December 2010 11:07:45 AM

i had to use shim washers on a boat that i had,because i put on* prop of differnt make, as the hub was shorter. igot it from a prop shop that had diff. sizes

capt jerry

Please see the OFDE post on capitalization.

nomadwilly wrote:

So the jam nut just sit's there jamming the two nuts and keeping the load on the big nut that is capable of holding the prop on YES! If the jam nut is installed last the big nut transfers part of it's load to the jam nut but if the big nut goes on last the jam nut transfers it's load to the big nut so the big nut holds the prop on AND carries the jam nut load as well.
Did I get it?

Click to expand...

Sort of ... think of the jam nut as a spacer that transfers the force created by the prop nut onto the prop hub. It is sort of a a washer that pushes back at the nut at the same time.

Because the jam nut doesn't have enough thread to push the prop nut forward without stripping, all you have done is tighten a jam nut against the prop nut with a few threads, not enough to stretch the shaft between them and produce a force that will hold the two of them together.

*


-- Edited by RickB on Sunday 26th of December 2010 03:45:14 PM

Rick--- I understand (I think) your explanation of why the thin nut goes on next to the prop and the heavy nut after that.

But I'm curious--- if the purpose of the thin nut is to jam the large nut in place and keep it from backing off, thus holding the prop in position, why wouldn't this work if the jam nut was put on behind the main nut?

Is it because tightening the jam nut would "unseat" the main nut from the forward face of the shaft threads and so allow it to succumb to vibration and eventually loosen everything?

If the jam nut was capable of pushing the prop nut forward enough to shift the thread contact to the aft face of the shaft threads then it would work. But since the thin jam nut has only a few threads it will (probably) strip before it can produce enough force.

If you have enough threads then a second prop nut would jam the first one but then you risk exerting more torsion or elongation than the shaft could withstand in way of the thread roots..

It's easy to demonstrate the effect if you take a bolt in a vice and put two identical nuts on the threaded portion. Tighten them together as hard as you can then put a wrench on the inside nut. You will easily unscrew both nuts togther just by turning the inside nut.

That happens because the bolt is not stretched to produce tension to hold the nuts together.

Thanks for the explanation. I can understand the logic.

RickB* Your explanation is good, but the part that has always flustered me in boating, not other applications where a cotter pin is not used, is the one Eric raises. Why after tightening properly,*then turn the nut to fit the cotter key/pin?

sunchaser wrote:

RickB* Your explanation is good, but the part that has always flustered me in boating, not other applications where a cotter pin is not used, is the one Eric raises. Why after tightening properly,*then turn the nut to fit the cotter key/pin?

Is not the cotter pin solely to ensure that a nut cannot back completely off the shaft? It seems to me that the pin has nothing to do with the nuts and their order of placement and torquing?

Click to expand...

*

sunchaser wrote:Why after tightening properly,*then turn the nut to fit the cotter key/pin?

Click to expand...

In an ideal world, the shaft is drilled to match the notch (crenula?) in the castle nut. If torquing a bit more won't line up a notch with a hole then a shim or washer is called for to move them in line. Backing off unloads the whole assembly.

Considering how infrequently props fall off or shear the key, it shows how well a tapered fit holds the prop on the shaft since a lot of other people probably also back off the nut just a hair to get the pin to fit.

Mike wrote:I have never owned, or even seen, a prop nut that has a hole drilled thru it for a cotter key.

Click to expand...

There's all sorts out there. Castellated nuts are very common, as are plain nuts like the one in your picture. I have seen plain nuts with a hole drilled through them and even nylock or fiberlock nuts on smaller shafts.

Not all shafts are machined on the end to provide a section of reduced diameter that provides a place to drill a hole for a pin. When I mentioned drilling the hole, it was in reference to a new undrilled shaft with a castle nut tightened in place. The hole is drilled through the shaft in a notch so the castle nut does what castle nuts do.

If you can imagine a way to secure a prop to a shaft, it is being done that way somewhere.

Let me know if you can figure how this one is attached ...*

The prop nut has holes around the outer circumference that are spaced a little wider (or narrower) than the holes on the prop face. This gives you a chance of alignment between the holes in the nut and prop ( one of the two will be threaded to hold a lock pin) at almost any position that you determine is the appropriate torque ?
A guess.

Forkliftt wrote:The prop nut has holes around the outer circumference that are spaced a little wider (or narrower) than the holes on the prop face.*

Click to expand...

Nope, good guess though ... here's a view from another angle.



*


-- Edited by RickB on Monday 27th of December 2010 10:05:15 PM