Brass or Bronze shaft nuts?

[Roger Long]

When we hauled our boat to return home for the holidays in December, I was surprised to find one of the prop castle nuts loose. The wheel was tight on the shaft and the cotter pin was in so I figured there must have been a slip up when the drivetrains were overhauled a year earlier. I tightened it up almost a full turn and just barely got the cotter pin back in the groove.

We hauled again a couple weeks ago and were leaving the yard racing darkness and the big northeaster for our drive north . I looked at the prop nuts as I walked to the car and was surprised to find the same nut loose with the pin properly inserted. There was perhaps 1/8” in between the nut and hub. The light was bad but the nut felt and looked odd. Our zincs have been going quickly so I have some electrical things to check out but I was also wondering if this boat that was always in fresh water until our ownership might have a brass prop nut. The best thing at that point seemed to be to deal with it when we get back to VA to paint the bottom and re-launch

Anyone have thoughts on how the nut could get loose twice without getting physically smaller through corrosion? I’m sure the prop has not slid up the taper.

Now that I’m thinking about the schedule when we get back to the boat, I don’t want to be spending another day on the shaft so I’m looking around for a prop nut. I was surprised to find this:

https://www.passagemaker.com/channels/propeller-nut-myth-busting

It says:

Yes, that’s right; the alternative (and I would argue a preferred one to stainless steel propeller nuts) is brass, and any propeller nut you see that isn’t stainless steel almost certainly is, contrary to popular belief, brass rather than the erroneously assumed bronze. While I admonished readers about the use of brass in a previous column, this is an exception to that rule.
Brass, actually manganese bronze (which is a misnomer, as technically, this alloy is in the brass family), is used for a few reasons. As mentioned, it’s especially slippery, thereby protecting your valuable propeller shaft threads from galling. Brass is strong and relatively easy to machine, making it well-suited to the manufacture of nuts. Finally, many propellers are made of manganese bronze (again, that’s technically brass) and as such, a genuine bronze nut would be cathodic to the prop, making the latter more likely to corrode when the two were in contact. If you maintain your shaft anodes, propeller and nut corrosion should never be an issue. Thus, when specifying a replacement propeller shaft, or just replacement nuts, go for the brass.

Not using brass on critical items below the waterline has always been religion. The prop nuts I've always had before sure looked like bronze to me and not brass.

Poll: Should I order a brass or bronze shaft nut? Both seem to be available.

 

[ghost]

I think that article is from Tony Athens, at least that’s what my memory says as I’ve seen it before somewhere. If it’s from Athens, you can’t go wrong following his advice.

With many things brass/bronze you actually need to be more nuanced than those two labels. It’s a rather complicated rabbit hole.

 

[diver dave]

Not sure I can help with the decision, but here are some metal specs.
I'm lucky to have an XRF at work, so I can analyze most all alloys.

 

[jleonard]

I think that article is from Tony Athens, at least that’s what my memory says as I’ve seen it before somewhere. If it’s from Athens, you can’t go wrong following his advice.

With many things brass/bronze you actually need to be more nuanced than those two labels. It’s a rather complicated rabbit hole.

Not from Tony. Steve D'Antonio wrote it.

 

[sunchaser]

I think that article is from Tony Athens, at least that’s what my memory says as I’ve seen it before somewhere. If it’s from Athens, you can’t go wrong following his advice.

With many things brass/bronze you actually need to be more nuanced than those two labels. It’s a rather complicated rabbit hole.

Steve D wrote the article.

 

[Roger Long]

H & H Propeller, the go to place when I was in the boat designing business and with shops all over New England, says they never heard of using brass nuts.

 

[jleonard]

H & H Propeller, the go to place when I was in the boat designing business and with shops all over New England, says they never heard of using brass nuts.

H&H knows as much as anyone about that. Personally I would trust them more than Steve D on this subject.

 

[sunchaser]

H & H Propeller, the go to place when I was in the boat designing business and with shops all over New England, says they never heard of using brass nuts.

What nuts does H & H recommend? As previously noted, there are many different brass alloys, not all subject to de-zincification

 

[Xsbank]

Why take a chance? He says, she says...if all goes well, brass will work. If something goes wrong, not so good. Think "pink prop."

As for the nuts slipping, I suspect your wheel does not adequately match the shaft taper or it was just not installed tight enough. I would pull the wheel and check the fit. The thrust will tend to shove it further on the shaft, hence the loose nut. Make sure the taper is a good fit.

 

[caltexflanc]

There's only one nut on each shaft? Typically there is a thin "jam" nut and one thicker main nut.

Ilustrated here. "The which goes first" discussion is like anchors in many ways. http://sharpesurveying.com/propeller-nut-installation.html

 

[diver dave]

H & H Propeller, the go to place when I was in the boat designing business and with shops all over New England, says they never heard of using brass nuts.

Roger; You mentioned castle nut, and then there was a 1/8" gap forward of the nut, the cotter pin was intact and the prop did not move forward?? Castle nut prevents nut rotation.

I'm so confused.

Did the nut get thinner?

 

[Roger Long]

Did the nut get thinner?

The strut and shaft were replaced on the other side last year after hitting a deadhead. Both drive trains were overhauled by a very good and experienced yard. The prop and shaft in question have been mated for years or decades and were disassembled, inspected, overhauled, and put back together.

I've been keeping an eye on the props and struts because of the faster than normal zinc depletion. No sign of pinking or electrolysis aside from the odd appearance and feel of the nut. I’m guessing that the nut is brass, did get thinner, and that the threads are now sloppy. I wish I’d had more time and better light when I discovered it.

I’ll let you all know what I find out when we get back.

 

[Roger Long]

I just got a chance to talk to the owner of H & H, who probably knows as much about propellers and shafting as anyone on the planet. He thinks there was a mismatch between the shaft threads and the nut due to wear or out of spec manufacture. This nut may have been on since 1975 and gone through several prop removals. It would have felt tight when torqued but would not have stayed that way. Once loose, there is enough slop in the cotter that the rattling around would have worn the threads of the nut. He suggests using Locktite on the new nut. He also told me that they are having to re-tap most of the nuts they get now due to sloppy manufacture in China so, if the shaft threads are slightly worn, the new nut may fit just right.

I wish I could install a nut and jamb nut pair but there isn’t enough clearance on this particular shaft.

 

[DavidM]

With many things brass/bronze you actually need to be more nuanced than those two labels. It’s a rather complicated rabbit hole.

Absolutely. The term brass or bronze really means nothing by itself.

The brass we normally think of as bright and shiny when polished has a yellow look to it and is about 65% copper and 35% zinc. It's tensile strength is fine, but salt water leaches out the zinc, turns it pink and it looses material and strength. This is called dezincifying.

Bronze, naval bronze or sometimes called naval brass has less zinc, maybe 15% and other alloys such as manganese, nickel and aluminum. It does not dezincify and holds up much better than brass in sea water.

Interesting comment about having to retap Chinese nuts. Why can't the Chinese make something right without western management and inspection? I see the retapping problem all of the time when I try to assemble something made in China. But it is cheap!

David

 

[Lepke]

The nuts are usually the same material as the shaft so one doesn't corrode the other.

 

[Roger Long]

The nuts are usually the same material as the shaft so one doesn't corrode the other.

Maybe on your coast but "bronze" nuts are very common here. If there was going to be a corrosion issue, that big "bronze" prop you see on most stainless steel shafts would would the problem.

Stainless steel nuts on a stainless steel shaft would be a real set up for galling which is much more likely to destroy the shaft.

 

[diver dave]

I've had a few cases of galling and it really sucks sometime.

Little bit like corrosion, it can really sneak up on you. My last two galling experiences:
Dry fitting a stainless elbow on an aluminum fuel tank. That is a bad idea. I was lucky to not have to remove and reweld the dip tube union.
2nd time was a routine boat trailer hardware exchange. All brand new stainless 3/8" u bolts and self locking nuts. Assembly of 10 went fine, the next froze the nut to the ubolt, ruining it. I had used no lube. bad, bad.

If it happened on a high $ prop shaft, it would be a bad day. Conventional wisdom says that surface oxide alloys are more susceptible. Like aluminum, titanium and stainless. To this day, I have not used a lube on the prop shaft threads with SS nuts, and have gotten away with it, but I will start to.

My chart above shows red brass has about 1/3 the strength of SS, so that would be a torque consideration. Naval brass looks strong, but contains a lot of zinc. So, that's the yellow metal dilemma.

 

[AusCan]

I've always used stainless nuts on a stainless shaft. As long as the threads are in good condition, and there you use a touch of lubricant, galling will not be an issue.


Dry fitting without a lubricant, especially with new hardware or tapered threads is just asking for trouble.

 

[psneeld]

All the boats I have had or commercially worked on had brass or bronze nuts on stainless or monel shafts.

 

[sunchaser]

All the boats I have had or commercially worked on had brass or bronze nuts on stainless or monel shafts.

My experience too. Maybe something going on here that is fit and not nut metallurgy related.

 

[Xsbank]

The yard cut corners when they installed the wheel, no other reason for the nut to loosen. There is supposed to be two nuts, the thin one goes on first and the fat one last and is pinned.

 

[diver dave]

I wish I could install a nut and jamb nut pair but there isn’t enough clearance on this particular shaft.

When you view this again, see how much of the shaft taper is still visible forward of the prop hub. Maybe a chance for a shop to machine the hub to allow it to sit further forward, thereby making room for a proper double nut job.

If there is no extra taper to work with, then maybe machine off some hub on the aft end. I'm sure a prop shop would have good suggestions.

And, of course, make REAL sure the prop is not sitting on the key. I use prussian blue for the test.

 

[Benthic2]

If you are going to get a new nut, could you order a slightly thinner one to make room for a jam nut ? In other words, if your one nut now is an inch long, order a 1/8 inch jam nut and a 7/8 inch main nut.

 

[Bacchus]

I would also suspect the fit of key. Even if the prop & shaft are mated properly a key can be problemental.
I would get some blue and ensure fit w/o the key and mark the hub location.
Confirm seating.

I find it hard to believe that erosion / corrosion would not be obvious???

 

[Comodave]

Agree that it is likely the key was not seated properly when the nut was tightened and over time the key moved a bit allowing the prop to seat further.

 

[C lectric]

Don't assume the prop has not slid further up the shaft. The load from driving may have pushed it up more if the prop hub was riding the key and/or not well fitted to the shaft.

The fact that you have a castellated nut with a cotter pin in it tells me the nut will not have backed off. Most likely the prop does not properly fit the shaft.

Go to Seaboard Marine and look in TONY'S TIPS and look for his article:
Propeller installation / Big Nut vs. Little Nut

Found under the sub heading: BOATS & REPOWERS

THe article cannot be copied to add here.

Pay attention to his description of how to 'Fit a prop to a shaft' with a marker pen and some valve grinding compound, often known as Clover compound.

I had to do that 10-12 years ago. I found my prop was riding the key. After dealing with that and then using the CLover the prop slip another 3/8" further up the shaft. THe cause of the key riding in my case was an improperly cut keyway in the prop hub. It actually had a bit of rocker in it, ie. the keyway bottom was not flat or straight.

Check it out as eventually it could cause some damage.

 

[HopCar]

The original post refers specifically to Manganese Bronze which is a specific alloy. It is to me a low zinc brass at about 22% zinc. It is somewhat subject to dezincification. I think you would be better off using a lower zinc alloy.

I consider Maganese Bronze to be brass but Red Brass is really Bronze. Red Brass only has about 5% zinc and is very resistant to dezincification.

Manganese Bronze is used to make propellers but I think it’s a trade off. They use it despite its high zinc content because it machines very well. It’s certainly much better than common brass which has a zinc content of 35% or so.