Trojan Boat Forum

specifically prop shaft

Isn't the standard practice to have the small nut first and use the big nut (last) as the lock nut? I know the steps to install, remove nut etc., my question is about the order. Lots of variations in the yard and this came up as I was helping a guy who has a Trojan 370. He has it opposite of me and his manual states the same (small nut last as lock nut).

Bob

I'm in the small nut last group. Here's why?
The first nut is to keep the prop on the shaft so I would rather have more threads absorbing any load hence I use the thicker/larger nut closer to the prop. The second/locknut being thinner stretches a little to prevent it unwinding while working as a locknut.

Let the debate begin

Interesting reading that suggests there are other areas to pay more attention to, the taper contact, the key fit, etc....
http://www.sbmar.com/articles/propeller ... ittle_nut/

And this guy says my theory get's reversed when tightening them up, so do I need to rethink even though the first guy has my nut order?
http://www.passagemaker.com/channels/pr ... h-busting/

On further research I like Tony Athens reasoning as the practical way to do things in message #60 here:
http://www.samsmarine.com/forums/showth ... Nuts/page6

reading from page 1 is also worth it

Interesting. The Propscan place where I bring my props tells me the nuts are only there as a backup. The taper actually holds the props on the shaft and if it's seated correctly, you could run all year with them removed.

Per Steve D'Antonio writing for Passagemaker Magazine, the small or half height nut goes on first and the full height nut goes on second. The half height first pulls the tension and when the full height nut goes on much of the tension then transfers to the full height nut. This is endorsed by the SAE, ABYC, USCG and the US Navy. He has an article on PMM's website on propeller nut myth busting.

BobCT wrote:Interesting. The Propscan place where I bring my props tells me the nuts are only there as a backup. The taper actually holds the props on the shaft and if it's seated correctly, you could run all year with them removed.

+1
covered in one of my links... so do we need more than 1?

And then you have the 4th layer, a cotter pin. Me thinks it should not come off no matter which way nuts go on, but it does and has happened.

Small nut first:

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

Those of us who have an engineering background along with boating experience would say big nut first, small nut last. The big one is called the full nut, and it is the one that is doing the 'work'. The little one is called the jam nut, and its purpose is to apply force on the surfaces where the threads of the shaft meet the threads of the full nut to prevent is from turning in a variety of load situations, hence the name jam nut, it jams the threads and keeps them from turning.

I agree that the taper should be doing the job pretty well by itself, but the redundancy is necessary even so for safety reasons.

I know this old chestnut has been kicked around over and over ad nauseum and that folks do feel strongly one way or the other depending on what they have been told. I'll never change my mind on the topic either, but thats because I gave a hundred grand to Clarkson University, not because I read something on the internet or was told something by somebody in a boat yard. YMMV.

P-Dogg wrote:Small nut first:

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

Everything I learned in mechanical engineering says this is completely wrong. Big nut first, full torque, even if in some applications you have to step up to full torque for various reasons such as seating. 1/2 to 3/4 torque on the jam nut to prevent it from spinning. Full torque on the outside nut and 1/2 torque on the inside nut means the nuts are torqued together, not the item they are intended to seat, which would be under whatever torque you set them to. You cannot transfer torque through one nut to the other and then onto whatever assembly they are holding together. The principal of using a jam nut in the first place prevents this from even happening. If you half torque the first nut (no matter what its size) and full torque the second nut, all you have done is to effectively lock the assembly into a half torque situation.

Another reason to use the full nut closest to the assembly is because that is where you need to get the torque value right and in high torque situations, the larger nut allows you to get a better grip and reduce the possibility of damaging the faces of the nut in the process. Additionally, by putting the small nut on first and then torquing the crap out of the full nut over it you run the risk of cracking the small nut or otherwise damaging it. This will not happen to the full nut on first because the jam nut would fail before damaging the full nut in a catistrophic way.

Some rare exceptions are the use of castelated nuts, which in marine inboard applications are seen, but its an old practice which in recent times has been eschewed in favor of the more traditional full nut/jam nut arrangement. Unless its a sterndrive or outboard, I see most boat owners with castellated nuts on their inboard shafts tossing them and replacing them with the full/jam arrangement when they haul them.

Small nut first . This should help explain the why .

http://www.sharpesurveying.com/propelle ... lation.htm

If you understand engineering and boats your should know this

With that said close to half the boats you will find are backwards . What ones come lose the most ? I would say its a draw .

After lapping in the prop , I install the prop using the big nut and tighten . I then remove it install the small nut retighten and install the large nut .

I find the same fault with the information in Pauls link. The jam nut does not releive torque on the threads, it ADDS it in such a way that force is applied to the faces of the threads of BOTH nuts, albeit in opposite directions. This is what makes it work.

Shingley's Mechanical Engineering Design says when in doubt, use two of the same thickness.

Commissionpoint wrote:I find the same fault with the information in Pauls link. The jam nut does not releive torque on the threads, it ADDS it in such a way that force is applied to the faces of the threads of BOTH nuts, albeit in opposite directions. This is what makes it work.

Shingley's Mechanical Engineering Design says when in doubt, use two of the same thickness.

Bob you should add a poll , I bet we would be close to a draw .

So I would guess what you guys are saying is, if not sure it really does not matter will do the job either way.

Craig

From NAVSEA:http://www.fas.org/man/dod-101/sys/ship/nstm/ch075.pdf

Scroll down to page 75-96 for the Navy's take on big nut/small nut first.