Plastic Classic Forum

When putting the two nuts on the end of a prop shaft to hold the propeller on, which nut goes on first, the thick or thin one?

The best information I have is that the little one should go on first, but I would like to confirm this.

I am thinking from a functional standpoint it doesn't make a difference. Both nuts will be carrying the load. Given the option I might put the big one on first theorizing that the little one is just there to 'lock' the big nut in place. I have been known to think too much about stuff like this though.

-Britton

Funnily enough, it's exactly the opposite of logic.

The small nut goes on first, and is locked by the big nut.

I forget the functional reasons for this, and no longer care, but this is the correct way to install a propeller.

Lots of props are installed incorrectly, however, and the world has yet to end. So after all that, I suppose it doesn't really matter that much.

If memory serves, the functional reason is something along the lines of "The second nut actually is the one to take the load once it's tightened against the first."

I infer that the purpose of this arrangement is to use the two nuts locked-tgether to hold the prop on the shaft, rather than using the pressure of a single nut, which either A) loosens as the prop works itself higher up onto the taper of the shaft, or B) forces the prop up onto the taper of the shaft so hard that removal requires a team of oxen.

Yes, of course I put mine on backward at least half the time.

Figment wrote:If memory serves, the functional reason is something along the lines of "The second nut actually is the one to take the load once it's tightened against the first."

I infer that the purpose of this arrangement is to use the two nuts locked-tgether to hold the prop on the shaft, rather than using the pressure of a single nut, which either A) loosens as the prop works itself higher up onto the taper of the shaft, or B) forces the prop up onto the taper of the shaft so hard that removal requires a team of oxen.

That sounds about right.

My new shaft arrived yesterday.



Time to enable that :rolleyes: smiley, Tim.

Well! I'll be! I guess I'll have to look at Quetzal. Dad used the cylindrical nut out of the folding prop when he fitted a fixed blade prop on the Morgan 27; the nut was wired on. Here'e an article I found on the web; another site (the plastic propeller folks) said you had to keep the inner, thinner nut from turning (possibly to avoid deforming the plastic):

PROPER PROP NUTS:

I'll start with a minor and somewhat amusing controversy. There are two nuts holding the propeller on the shaft: a full-height nut and half-height locknut. Which nut goes directly against the propeller hub? Actually, most boats have it wrong, with the full-height nut against the prop.

It seems intuitive that the larger nut against the prop would be doing most of the work and that the smaller nut should go on second (see picture). In fact, the smaller nut should always go against the load. This is because when the second, outer nut is tightened down, it compresses and deforms the lower nut
a tiny bit, and rotates the lower nut a fraction of a turn. This effectively unloads the threads of the lower nut and engages the threads of the upper nut. Thus the top or outer nut actually takes all of the load. No matter how many times I explain this, some folks still seem dubious. You can also see an illustration from Engineering Drawing and Design (by Jensen/Helsel, McGraw Hill, 1985) that shows how this is standard practice for all lock nuts; and an illustration from SAE standard J756 and J755, for propeller shafts, which clearly shows the half-height nut against the hub.

If you go to any marina with the boats hauled for winter lay-up. Over 90% of the prop nuts will be on the wrong way. I wouldn't worry too much about the nuts being on backwards. It has proven sufficient, but you might as well put them on correctly next time you install a prop.

by Dave Gerr, NA.
This article reproduced with permission

While you are at it you 'might as well' change the cutlass bearing.

While you are at it you 'might as well' change the packing gland.

While you are at it you 'might as well' buy that new propeller.

While you are at it you 'might as well' install that anti-pot-warp cage

While you are at it you 'might as well'...

Congrats on the new shaft. It looks pretty and will most likely run very smoothly.

-Britton

Cutless bearing. Check.

Packing gland? Why? I'll change the packing of course, but what on earth could possibly wear out about the gland fitting?

Propellor purchased last year (or was it the year before?) This new shaft is a deferred purchase, finally going to see if Mr. Stevens' claims of self-antifouling hold water.

Anti blahblahblah. Yeah I'll get right on that!
Or maybe I'll just brush up my Radio Voice for all those calls to SeaTow instead.

but what on earth could possibly wear out about the gland fitting?

Nothing of course but the old one is going to look bad on the shiny new shaft.

Self anti-fouling? What is that about? Do I need a new shaft too?...

Email Tom and find out. I seem to recall, however, that your prop was cast before he started using the new alloy that reacts with stainless steel in just such a magical way...

"No Barnacles" if installed on a stainless shaft with no zinc.

We shall see.

Interesting. I am not going to replace my new bronze shaft with stainless quite yet though.

-Britton

Tim wrote:Funnily enough, it's exactly the opposite of logic.

The small nut goes on first, and is locked by the big nut.

I forget the functional reasons for this, and no longer care, but this is the correct way to install a propeller.

Lots of props are installed incorrectly, however, and the world has yet to end. So after all that, I suppose it doesn't really matter that much.

The narrower nut is called a "jam nut" and always goes on first. It is intended to be jamed by a thicker nut behind it.