Thin and thick nuts on propshaft: sequence??

[theoleduc]

We "sail" a KROGEN Express 49 "Ritser"...
At at the tail end of my prop shaft are two nuts: a thin one and a thick one. Is there a recommended sequence? I read somewhere that the thin one should be against the propeller, and the thick one should be abaft the thin one. If they are reversed (thick one against the prop), then the installation is weaker.
1) True?
2) What is the explanation?

Additionally I insert abaft the second nut a 4 mm split pen (not through the nut but through and folded around the shaft).

Is additional "fixation" of the nuts by means of a kind of "waterproof" Loctite meaningful?
Thank you in advance for comments - Watersportgreetings / Ritser / Theo

[Allan S]

The thick nut goes against the prop, the thin one,known as the lock or locking nut goes on the outside.

[slug]

Originally Posted by theoleduc

We "sail" a KROGEN Express 49 "Ritser"...
At at the tail end of my prop shaft are two nuts: a thin one and a thick one. Is there a recommended sequence? I read somewhere that the thin one should be against the propeller, and the thick one should be abaft the thin one. If they are reversed (thick one against the prop), then the installation is weaker.
1) True?
2) What is the explanation?

Additionally I insert abaft the second nut a 4 mm split pen (not through the nut but through and folded around the shaft).

Is additional "fixation" of the nuts by means of a kind of "waterproof" Loctite meaningful?
Thank you in advance for comments - Watersportgreetings / Ritser / Theo

Bolt science

https://www.boltscience.com/pages/twonuts.htm

[GordMay]

The thin nut should be placed on the bolt (shaft) first (against the prop’ hub). This nut is typically tightened to between 25% to 50% of the overall tightening torque. The second (thick) nut is then placed on the bolt, and the thin nut held to prevent rotation by a spanner (wrench), whilst the thick nut is tightened to the full torque value.
More ➥ https://www.boltscience.com/pages/twonuts.htm

See also ABYC standard P-6 (Figure 1, page 17), and SAE standard J755
P-6 ➥ https://docplayer.net/30165451-P-6-p...g-systems.html


This should be fun.

[theoleduc]

Thank you. Crystal clear. Theo

[a64pilot]

Originally Posted by Allan S

The thick nut goes against the prop, the thin one,known as the lock or locking nut goes on the outside.

That would seem to be logical, but it’s incorrect, I cannot explain why.

Only thing I would change from the accepted procedure would be to put the thick nut on first and torque to full torque in order to press the prop onto the shaft, it is a press fit and metal is elastic and the more torque the further on the shaft it will go.
Then remove the thick nut and install the thin one IAW the accepted procedure.

In my opinion based on other mechanical “things” it would seem to be logical that the thick nut should go on first and provide the actual tightening force and the thin nut just “locks” the thick one, it doesn’t provide any actual tightening force, just a locking agent and therefore doesn’t need to be as thick.

By placing the thin nut on first and then the thick one and tightening the thicker one to twice the torque it would seem the force is going to distort the threads and not provide a tightening force on the prop.
Distorting threads isn’t something that I would think you would want to do. It leads to stripped threads after all.

Not saying ABYC is wrong, just can’t understand the reasoning is all.

[GordMay]

Originally Posted by a64pilot

... Not saying ABYC is wrong, just can’t understand the reasoning is all.

Read the explanation ➥ https://www.boltscience.com/pages/twonuts.htm

[WSMFP]

I just replaced my shaft and the shop (who does OEM work for catalina and a large number of other builders) says the larger nut goes against the prop.

[tkeithlu]

If common sense beat logic and then empirical evidence testing logic, we'd have no justification for any of the sciences.

Common sense tells me that those stars are all on a black dome up there and the earth has always been just like it is now.

[GordMay]

What tkeithlu said (I think).

According to Dave Gerr* in "The Nature of Boats", page 260:
"Install the small, half-height jam nut first—against the hub. Tighten it up as hard as you comfortably can by hand with a standard wrench. Then, screw on the full-size nut and tighten that down independently—again, as hard as you comfortably can, by hand with a standard-size wrench.”

*Dave Gerr, author of “Propeller Handbook” and others ➥ https://www.gerrmarine.com/Books.html


SAE Specification J755 ➥ Marine Propeller Shaft Dimensions: SAE Specification J755

[psneeld]

The US Navy agrees with small nut on first against prop, then the larger one as it somewhat unloads the smaller one and has more threads to handle the load, the smaller remains the locker.

[skipmac]

Every technical article I've read and the opinions from the experts that I trust the most like Rod Collins aka Mainesail of Compass marine and member of ABYC and Nigel Calder who literally wrote the book all agree thin nut first.

Who am I to argue?

[a64pilot]

Originally Posted by GordMay

Read the explanation ➥ https://www.boltscience.com/pages/twonuts.htm

I read the explanation, specifically the part warning of stripping the threads.

However the other part that is non sensical is not using a castellated nut for the cotter pin or lock, in any other more critical areas like aerospace for instance that is done.
My take is this stuff is “old” way of doing things, where better ways exist, but for whatever reason the old way is still used.
My take is that most likely it’s an over sized, not highly stressed fastener is why, were it more highly stressed etc. a castellated nut and cotter pin would be used.

Often we are putting bronze nuts on a stainless shaft shaft anyway, so assumption is the nut would strip first anyway, so why not?

[GordMay]

Originally Posted by a64pilot

... However the other part that is non sensical is not using a castellated nut for the cotter pin or lock, in any other more critical areas like aerospace for instance that is done...

Indeed.
I think fstbttms got it mostly right, here (#5):
➥ http://www.cruisersforum.com/forums/...ml#post2384793

[a64pilot]

Originally Posted by GordMay

Indeed.
I think fstbttms got it mostly right, here (#5):
➥ http://www.cruisersforum.com/forums/...ml#post2384793

You like that cause it states the accepted way, but it’s not “best”
The cotter pin essentially does pretty much nothing in that installation, it may keep the nuts from falling off but once they loosen the prop will wallow out on the shaft ruining it.
The castellated nut like on the prop anode is “better”, the cotter pin is an actual locking mechanism.

The quote you made where the Professional advice was tighten it as much as you comfortably can with a standard wrench, with no regard to thread size etc was interesting to say the least, especially where the torque for both thin and thick nuts were equal.

Pretty much says this whole thing is pretty loosey goosey, meaning that good enough is all that you need, it’s not a critical application, good ole good n tite is good enough.