Graham_Wright
Well-Known Member
Apart from crevice corrosion and galling (as if they weren't enough) why not?
I can make one of those M24 X 2 in 1.4404/316.
I can make one of those M24 X 2 in 1.4404/316.
Prop nut - stainless?
- Thread starter Graham_Wright
- Start date
Tranona
Well-Known Member
Why not? For exactly the reasons you state. Use the proper bronze.
vyv_cox
Well-Known Member
Galling is a serious reason why not. Galling is a high temperature mechanism that is effectively welding and parts galled together are immovable. As if getting a prop off is not difficult enough in some circumstances, having to cut the nut off without damaging the shaft or the prop would be a step too far. Why not make it in brass? Most are nowadays and they last at least ten years.
Graham_Wright
Well-Known Member
I have experience of galling both in parts manufacture and on the boat. I would not restrict it to high temperature though. I had to grind off the chain plate nuts! The trick explained to me by one of my (very experienced) staff, is to tap around the periphery of the nut (in our case ring) while attempting to rotate it.
Brass?! I have loads of CZ121 but what about de-zincification? My stern tube (produced by Norris Engineering) turned out to be simple brass which they reluctantly accepted back.
I thought it would be easy to buy a nut! M24 X 2. M G Duff were a possibility which faded on examination.
Tranona
Well-Known Member
www.lakesterngear.co.uk will supply a nut - as will any prop or stern gear specialist. £34+VAT in their catalogue.
vyv_cox
Well-Known Member
It is a high temperature mechanism but that does not mean it only occurs when it is hot. Localised welding takes place between the microscopic high points on the two surfaces, due to the load, not the temperature.
The nice example of a dezincified prop nut on my website (brass and bronze) is mine. It was around 15 years old when I took it off.
Graham_Wright
Well-Known Member
We had trouble with two mated threaded 304 components. The end of one had to be welded whilst the pair were mated. There was an unacceptable frequency of galling. Changing the male component to 316 seems to have cured the problem.
DannyB
Well-Known Member
I once screwed a very expensive stainless steel level sensor into a stainless steel tank, it was just for checking the fit and I screwed it in by hand. It stopped short of the sealing washer, and I assumed there was some dirt in the threads, tried to unscrew it, and it wouldn't budge, went and got a spanner to fit on the hex end, still wouldn't budge. It took two of us, a three foot Stinson and a scaffolding pole to remove it, of course by then it was no longer a functioning piece of electronics.
I was told later that if screwing stainless parts together to use milk of magnesia as a lubricant, stops it galling apparently.
I was told later that if screwing stainless parts together to use milk of magnesia as a lubricant, stops it galling apparently.
Topcat47
Well-Known Member
My prop nut is definitely NOT brass. Originally castellated to allow the use of a split pin, sadly the holes do not match the castellated portion any more. It's almost as if the prop has walked back up the shaft by about 3mm. The nut has never shown any sign of rust and hasn't stuck to the SS shaft so whatever material it's made of must be pretty good. I always put a spot of PTFE grease no the shaft, these days but when I first got the boat, I used Vaseline. Enough to prevent Galling? I wouldn't have thought so.
Graham_Wright
Well-Known Member
Thanks for the steer. However, they have this morning told me that their nuts are made of Navel (sic!) brass which has an even higher zinc content than other high zinc brasses.
Am I missing a trick? How can it claim to be corrosion resistant?
Tranona
Well-Known Member
See post#3. Those nuts seem to be satisfactory and have been in use for many, many years. Pretty sure that if they were not then they would have been discontinued a long time ago.
Graham_Wright
Well-Known Member
I guess the word naval (not navel!) should mean something but with around 40% zinc, why doesn't it leach out?
I'll continue looking for phosphor bronze or make one myself.
vyv_cox
Well-Known Member
Naval brass is 60/40 copper zinc with about 1% of tin added. Slightly better corrosion resistance, but as I said above, ordinary 60/40 brass will give you 10 years so for a low cost item is perfectly OK.
Graham_Wright
Well-Known Member
I've looked at the horror pictures on your website. I don't think low cost is an overriding factor!
If the prop falls off in difficult circumstances, it could be life-threatening.
Tranona
Well-Known Member
Not sure that is a realistic scenario. Not many even stories never mind evidenced fact showing systemic failure of these nuts leading to lost props.
Plenty of other things to worry about!
Graham_Wright
Well-Known Member
After all these comments, it seems we should be happy using brass fittings on board.
We have all seen corroded hull fittings, I have experienced brass screws turned to copper (and that was out of the water).
Despite this we are urged to use anything but brass under water (except for prop nuts!).
Baffled!
We have all seen corroded hull fittings, I have experienced brass screws turned to copper (and that was out of the water).
Despite this we are urged to use anything but brass under water (except for prop nuts!).
Baffled!
Graham_Wright
Well-Known Member
……like brass hull valves?
Tranona
Well-Known Member
Again, problem is vastly overstated. Tens of thousands of brass valves in use in thousands of boats with no evidence of systemic failures. However, alternatives have been developed - but not for brass prop nuts. This suggests that prop nuts are even less of a problem. Nothing baffling about that.
Evidence based decisions are usually superior to relying on theoretical possibilities that are not supported by the evidence. It is where there is a lack of empirical evidence that the theoretical approach may be appropriate. So in the case of prop nuts there is ample evidence that they are satisfactory.
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Graham_Wright
Well-Known Member
Ok I'm a dog with a bone.
My fixings and fasteners company has quoted me £3 for M24 coarse pitch nut in CZ121. This has the same zinc content as CZ114 but no tin or aluminium. M24 X 2 they guessed as ~£9.
They are currently seeking a price for M24 X 2 in Pb something.
I'll be back.
My fixings and fasteners company has quoted me £3 for M24 coarse pitch nut in CZ121. This has the same zinc content as CZ114 but no tin or aluminium. M24 X 2 they guessed as ~£9.
They are currently seeking a price for M24 X 2 in Pb something.
I'll be back.
vyv_cox
Well-Known Member
After all these comments, it seems we should be happy using brass fittings on board.
We have all seen corroded hull fittings, I have experienced brass screws turned to copper (and that was out of the water).
Despite this we are urged to use anything but brass under water (except for prop nuts!).
Baffled!
Don't forget that the vast majority of yacht propellers and P-brackets are made of manganese bronze which, despite its name, is a 60/40 brass with minor additions. The thinner sections of them suffer dezincification after many years but those with an anode on either the prop itself or the shaft derive a certain amount of dezincification protection from them. My P-bracket was 20 years old, made of manganese bronze, and when removed from the boat due to other problems showed very little evidence of dezincification despite never having had an anode.
I'm not suggesting that a brass propeller nut is going to last for ever but I understood that you had some material and wanted to make your own. I feel certain that you would inspect it annually and can make a decision as to its suitability for continued use.