Blocking the prop

[Nonchalant1]

Some recent threads have indicated that installing or removing a prop by using a wood block to keep it from turning while you tighten or loosen a prop nut is a bad idea. However, this month's Passage Maker magazine article by the PMM's technical editor and VP of operations for Zimmerman Marine, indicates differently. He says:

"Once you've spun the large nut on, place a piece of soft wood between the edge of one of the propeller's blades so the propeller is on the upswing, travellling towards the hull. This will hold the prop stationary as you torque the nuts."

'nuff said,

Doug Shuman

 

[Bob Bradley]

Does he give any indication as to how many foot pounds to torque the nuts?

 

[Nonchalant1]

Does he give any indication as to how many foot pounds to torque the nuts?

Bob,

Yes. He says crank it as tight as you can by hand without any help from pneumatic devices, etc. That's the same advice given by Karl (Genesis), when I asked him, although Karl's was more colorful prose.

I use an 18" long 1/2" drive wrench handle with a big socket (then same size that fits your various tank senders), and just crank it real tight by hand. He reiterates to tighten it with the big nut first, then take the big nut off and tighten the thin nut against the prop, and then put the big nut on last and tighten it against the thin nut. This unloads the thin nut and leaves the pressure on the big nut which has more threads. The thin nut acts as a locking nut even though it's up front.

Also, always use stainless steel cotter pins only. The largest diameter that'll fit your shaft holes.

Doug

 

[Bob Bradley]

Do you use 2 wrenches, to lock the nuts against one another, or just tighten the large against the small?

 

[Nonchalant1]

Just tighten the large against the thin. After torquing the large nut, take it off and put on the thin nut. Give the thin nut a good crank, but not as hard as you'll torque the large nut. Then put the large nut on again and crank it down hard.

Doug

 

[Trojan]

The book from Hatteras for My 1973/43 with a 2in. shaft and a 1in. thread. Reads, tighten the small nut to 100 pounds of torque. Tighten the thick nut to 100 pounds of torque. It does not say how to block the prop. I use a 1.5 in dia. thick wall pipe 5ft. long stuck in the root of the prop againest the strut and a big socket on My torque wrench. Bill

 

[MikeP]

I don't understand the logic in this particular mechanical fastening arrangement.

I DO understand double-nutting to keep nuts from loosening. What I don't understand is installing the thin nut under the thick nut. In any other mechanical application I have seen, the thin nut would be installed on top of the thick nut to provide the lock. Additionally, if you don't lock them together by putting wrenches on both to ensure they are tightly locked, it isn't reliable at all. Everything from mechanical rocker arm adjusters to connecting rod bolts with lockers. You must tighten the lock nut against the main nut or it won't lock.

If the purpose of the two nuts is truly to prevent the prop nut from unscrewing, then a far better way, as Genesis suggested is a castellated (sp?) nut and cotter pin or a hole drillled through the edges of the nut which can then be safety wired. If I found a shaft and a prop laying on the ground, never having seen such a thing before, and somone asked me how too attach them, I'd say use an appropriate flat washer, then a single nut torqued to the appropriate specs for the nut/shaft thread material/size. And, even though it is not possible for any "unscrewing" force to be applied to the nut, since the prop can't turn on the shaft due to the key, it would seem prudent to ensure that should the key break, the prop couldn't spin the nut off the shaft. Therefore, I would probably use a castellated nut/cotter pin.

Just my thoughts re the logic - I'm not suggesting anyone change anything that's been working just fine!

 

[spartonboat1]

The torque spec callout from the owners manual post of !00#'s is essentially the same as an automobile lug nut, which is a smaller dia. So how the 100# figure was chosen is puzzling. I know there are sites on the web that will provide specs for given nuts and shafts of a given material.

Also, I not seen a mention of Loctite type of anerobic fluids, which come in various grades in terms of how well the threaded fastener should be retained. Some claim that the nuts or shaft would need to be broken in order to break loose the bond.

Any thoughts on the torque or Loctite?

Finally, the cotter pin basically acts to prevent the outer nut from being lost, since the pin hole sits too far back from the nut for a castellated arrangement to be functional.

 

[spartonboat1]

Post, post...re: Loctite.
Loctite comes in various grades, some of which are very strong. Ergo, my remarks that some [grades of Loctite] require damaging or breaking the fastener to loosen it...almost epoxy like...

 

[MikeP]

Again - these shaft/prop fastenings seems a strange concept. The cotter pin is there but, since it is so far aft of the nut, it won't stop the nut from unscrewing; so we add a thin "lock" nut, and put it on the wrong side of the assembly.

Even though this seems standard, it almost sounds to me like a design screw-up back in the beginning of time that was corrected by jury-rigging. I can see it now...


Machinery Operator: "Oops, I drilled the shaft cotterpin hole too far from the propellor, I need another shaft!"

Foreman: "No, that costs too much. Just add a second nut to lock it and stick the cotter pin in the hole behind the two nuts anyway."

Machinery Operator: "OK, I need another one of these nuts."

Foreman: "No we have some real thin bolts that cost a lot less, use them instead."

Machinery Operator: "But now the cotterpin is way behind the nuts, It looks ridiculous!"

Foreman: "We'll have the marketing department tell everyone that we designed it to be that way. In a few years, everyone will accept is as normal!"

 

[Trojan]

If the taper on the shaft is properly matched to the the prop in theory you don't need the nuts at all. Just to seat it. Thats why there so damn hard to get off. But due to flexing of the prop hub the bond can be broken so enter the nuts. Torque is determined by the yield point of the shaft. There are set standards for bolt sizes, grades and type of joint. Wheel Lug bolt are a grade 9 or better which is quite hard. Shafting is near 8. Standard bolts are a grade 5. I can't remember the exact grades without digging in my books. We used to tighten rod bolts on our race car by measuring the stretch of the bolt. The small nut will deform the threads at 100ft.just before yield locking it on the shaft. The large nut gives the joint strength and will not deform.Threadlocking the nuts togeather. The pin just keeps it there if it comes loose. Bill

 

[MikeP]

You would need something to hold the prop on when in reverse because the prop would pull itself off the key/taper if something didn't prevent it from doing so. But I maintain that the method is mechanically inefficient and decidedly inelegant though it apparently works fine. If it was an airplane (or an old British motorcycle), it would have one flat washer, a nut and safety wire.

Note: The washer, nut and safety wire would not have been OEM on the British motorcycle. It would have been added later by the owner in hopes of keeping any remaining parts from falling off.

 

[Avenger]

Which reminds me of a joke:

Do you know why the British drink warm beer?

Because Lucas makes refrigerators too.

 

[spartonboat1]

C'mon, there has to be a registered PE mech engr who reads this site that can give the real answer (or what maybe it should be, and not the accepted practice).

E.g., please note that the reason [a contributing factor] that the foam has fallen off the Space Shuttle launch vehicle, is that several years ago the environmental types requested a change to a bonding agent that did not outgas naughty environmental products, but in the process a product was elected that was less effective, with weaker bonding. Some engineering team capitulated [lets blame Clinton's watch]...anyone knows for sure if that's not true, state it here. I know the weaker glue aspect element is accurate. I am curious if they ever tried 3M 5200!

 

[captddis]

I will add my two cents. I understand the logic about using the small nut first. But..... I have always used the big nut first and see no reason to change. If the nuts are tight they work regardless of orientation. I remember working for a Hatt. dealer and I remember using a porta power hydraulic puller and standing on the pump handle , then having to use heat also. This is common , the prop would not come loose even if the nuts were gone.
Tighten the nuts and let the engineers argue about the orientation. Me I will be out on the boat.

 

[MikeP]

You'll be out on the boat? What the heck kind of comment is that? I thought the only reason to own a boat was to work on it!

 

[captddis]

I guess I need to clarify. I will be out on the boat one weekend a month after working the previous three. Although I have two boats so every other month might work. Dave

 

[Norm Mayer]

No one has mentioned a trick I saw used on my boat at Dog River Marina Mobile AL a few weeks ago after having my props reworked. They installed the props first, without keys, with valve grinding compound between the shafts and the props. Then, with the nut installed and only lightly tightened, they spun the props, seating the props to the shafts. They then disassembled the props, cleaned the grinding compound off and then installed the props and as mentioned previously, torqued them. Sorry, don't know what spec they used. They impressed me greatly at Dog River, and I am a master automotive technician. Hope this helps!

 

[Nonchalant1]

Here's how I understand why you gott'a have big nuts to operate a Hatteras:

Whichever nut is the last one put on unloads torque from the first one. The big nut has more threads and won't gall the shaft, so it's the last one.

The thin nut was cranked against the prop. The PROP holds it enough to create the binding necessary between two nuts on the same threaded shaft to lock them in place.

I note how this has held my props on OK over the years. It's good enough for me.

Doug

 

[Trojan]

You are supposed to lap the prop to the shaft every time you install a prop. BILL