Propshaft corrosion

davidwf

davidwf

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Just discovered that my propshaft on a 5 month old boat has badly corroded under the rope stripper see picture below. Prop is OK. The Stripper is unmarked its just the shaft, I suspect I need to replace the shaft, any ideas what would have caused this? If it was electrolytic corrosion I would have thought the brass prop would have corroded first. I did note that there was no shaft anode on the shaft, although I was told one had been fitted so that might well have coroded and dropped off. Boat has a galvanic isolator and has only been on shore power for about a month.

propsmall.jpg
 
a friend has a stripper (the rope type) ,he found that after a year in the water the bolts supplied with it had corroded away almost to nothing and it had eaten the shaft slightly but not as much as yours. suppliers said that the securing bolts were wrong but also the shaft was made of incorrect material as well.
 
That could be electrolysis but is more likely to be crevice corrosion. When bolting anything around a propshaft it is always worth using a seating of some kind of sealant. If moisture can get in the gap but there is no free oxygen circulation you will get this (Google crevice corrosion for a fuller explanation)
 
As Boatmike says almost certainly crevice corrosion. Stainless steel needs to be in good oxygenated water if it is not going to corrode. When you get stagnant water trapped in a crevice its oxygen concentration becomes depleted it looses its protective oxide film and an electrolytic type of corrosion mechanism occurs between the oxygen rich and the oxygen depleted areas. the prop may not have helped matters.

Brass and stainless complete with its oxide film are in a similar position in the galvanic series and as a result they are normally OK together. The prop could still dezincify if it is brass though.

Maybe a shaft anode would solve both problems. But that must be fitted so that it makes good contact with the shaft and not insulated from it by any sealant.
 
The rope cutter is a PropProtector, I was told that an anode had been fitted at launch.

Prop still has a nice ring to it, its a varifold, I said it was brass in origional post just looked it up and its nickel-aluminium-bronze alloy, and I've cleaned it up and there is a little bit of pitting but nothing more than I've seen on previous boats. No damage to it at all on the leading or trailing edges where it is thinest so thats good.

I have passed pictures back to dealer so he can discuss with the boat builders after sales guy who is on the boat show stand with him so thats handy.
 
This looks like the cutter and the shaft are not the same grade of stainless. They should both be 316 and if they are you will have no problems. The anode is irrelevant for the shaft/cutter if they are the same grade. It is there to protect the prop.
 
[ QUOTE ]
its nickel-aluminium-bronze alloy

[/ QUOTE ] I don't think you can do much better than that. If I was a Ni/Al bronze prop and someone called me brass I'd unbolt myself and go find a new owner. /forums/images/graemlins/grin.gif
[ QUOTE ]
This looks like the cutter and the shaft are not the same grade of stainless. They should both be 316 and if they are you will have no problems

[/ QUOTE ] I was under the impression that 316 is often not used for prop shafts, but sorry I cannot elaborate on that!
 
Salt water and no oxygen (trapped static water) where ss is clamped or in bolt holes will result in corrossion. Any Stainless items that will get wet either need to be open to the water flow or sealed. Clamp on items like disc cutters will leave a film of water under neath them that is trapped. Bed it on epoxy, as you should with all emersed ss bolts.
 
I was under the impression that 316 is often not used for prop shafts, but sorry I cannot elaborate on that!
_______________________________________________________________

It is actually quite commonly used Vic and most times OK if there is a free flow of water around it. The alternatives (that is those with any reasonable tensile strength) are a Duplex grade or if you have loadsa money, Monel. There are also other grades often grouped under the generic term "underwater stainless" that are marginally better that 316 but IMHO not worth paying more for. In a practical sense Duplex is great but not often specified due to expense. As far as boatbuilders are concerned it is the least worst that they can get away with I am afraid......
 
I get the principle of preventing corrosion by stopping the trapping of water, but does the prop not end up trapping some water or is the taper so well matched to the prop that it excludes water?

I do thank you all for the information it's confirmed my thoughts on the subject. I won't be refitting the cutter without epoxy. Now need to decide whether or not I need to change the shaft.
 
I do not feel qualified to take about metals, but I remember seeing a friend's shaft very similar to your own.

It turned out that his shaft was not 316 stainless - It was an honest mistake and was replaced free of charge by the engineer.

Might be worth checking.
 
From just the photo it's quite difficult to decide between crevice and galvanic corrosion here. It might be best to take precautions against both.

As has been said, bedding the cutter on a mastic to seal the crevice will exclude seawater, and therefore that type of corrosion. Sikaflex 291 should be perfectly good as it does exactly that for deck fittings.

For the problem to be galvanic corrosion the metal from which the rope cutter is made must be more noble than the shaft. The Prop Protector website says its product is made from 'high grade 316 stainless steel' but it does not give a specification. Assuming the shaft to be a standard 316 stainless, it is unlikely that any high grade version of the same alloy would react galvanically with it. It seems possible that your shaft may be a lower grade, maybe a 304 or something else. Again, isolating the cutter from the shaft will overcome the problem but this is not so straightforward. Sikaflex will probably not do it but a layer of thin plastic should be OK. In this case, use the Sikaflex as well to exclude the crevice possibility.
 
Yes indeed Vic is correct, that the prop should be a "ringing fit" on the shaft. You should not use any form of sealant at this joint because you need the anode to be electrically connected to the prop. The shaft cutter should however be bedded on any good waterproof sealant. Silicone is usually a good choice as it will break easily away when you want to remove the part. I have also used the setting variety of Hermetite before too but personally would not use Epoxy or even Sikaflex as you may want to remove the part in future. As far as replacing the shaft is concerned it does not look as if you have severe enough pitting to weaken the shaft but I would polish it up with emery before refitting the shaft cutter and then re-examine to see if there are any deep pits left. As this is not a bearing surface I would not be unduly worried about it. Also, if you don't have corrosion elsewhere (possibly worth dropping the shaft back and examining bearing surfaces) I would think the question of wrong material being used is a red herring.
 
My comment on the materials was simply an attempt to explain the possibility of galvanic corrosion. Not knowing exactly the composition of the cutter makes this a problem. Although I fully agree that crevice corrosion is a possibility, it is certainly not the only one. IMHO one photograph is insufficent information to make a definitive diagnosis.

I don't think Sikaflex would prevent future removal of a device such as this, which can very easily be levered without any risk of damage.
 
Just thinking about this question while watching the snooker. What's the condition of the bore of the cutter? If it's similar to the shaft it would suggest crevice corrosion. If unmarked it would suggest galvanic. Still not absolutely definite but maybe a good indication.
 
We know that the PropProtector is made of 316 so it does seem very likely that the shaft is not.
Whether it is suffering from galvanic corrosion or is an alloy more susceptible to crevice corrosion I would not like to say, but I would not have thought that two stainless steels were far enough apart in the galvanic series for one to suffer galvanic corrosion as such.

All probably rather academic as crevice corrosion itself proceeds by a "galvanic" mechanism anyway. The areas in oxygen depleted water water being anodic to the area in oxygen rich water.

Various stainless steels other than 316 are used for prop shafts.
 
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