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Hello sleuths - Can you tell me why my rudder broke off at sea?

7K views 72 replies 22 participants last post by  Zanshin 
#1 ·
I finally got my 2003 43" Jeanneau hauled today to replace the rudder. The solid metal stock broke off right at the through-hull and after dropping it and doing a quick bit of cleaning this is what the remnant looked like:



There are no sharp edges, but I have been in a marina for the past 4 weeks waiting for the replacement part to arrive. I was surprised at the copper buildup and my first thoughts are that I might have an AC power issue and that the copper has been plated from the surrounding bottom ablative coat.
 
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#4 ·
Wow Zan...that is something I've never seen. I agree with the probability that it is galvanic in nature. My guess is we don't see any ypical pitting cause what you have was above the waterline. The additional time in the marina probably took care of ragged edges with the copper plating!
Sounds like you need a good marine electronics guy to check things out before you put another shaft in the water.
That picture should be in the galvanic corrosion hall of fame!
 
#5 ·
It is unfortunate that we can't see what the other piece looked like. That piece would probably give us a much better picture of what had happened.
 
#6 · (Edited)
The rest of the shaft is unmarked and smooth, with the exception of some pitting along the waterline which I noticed by dragging my fingernail across it. I couldn't find the actual breakage lines when the rudder broke off - as noticed already the whole bottom of the shaft is rather smooth, almost as if the corrosion happened over enough time to prevent sharp edges.

I am a bit surprised that the copper deposited only on the end of the shaft, as if the exposed surface was acting as a cathode.

The rudder snapped off in relatively mild conditions and with little torque applied. I couldn't salvage the rudder portion, but do recall that I saw a couple of inches of steel stock sticking up - which might indicate that the rudder fibreglass bottom had water in it, otherwise I would only have seen the bottom of the rudder above the waves.

My first thought when I saw the discoloration was that someone had (badly) welded a broken rudder back together, but that certainly didn't happen.

Addenda - Cam: The portion in the picture was under the waterline by a couple of inches, in the top section of the picture you can vaguely see the waterline. I have more pictures that I can post if people are interested enough that might help in the analysis.
 
#7 ·
Corrosion might have indeed worn the sharp edges away. How long was the boat in the water after the rudder broke??
 
#9 ·
Did it break off right at the point it exits the rudder blade?? It looks like it actually broke off inside the rudder itself, not at the point it exits the rudder, based on the lengths.
 
#11 ·
If the stock broke off below the top of the rudder, it could be crevice corrosion instead.
 
#13 ·
sailingdog said:
Did it break off right at the point it exits the rudder blade?? It looks like it actually broke off inside the rudder itself, not at the point it exits the rudder, based on the lengths.
It broke off just a couple of mm inside the through-hull, the bottom-up picture shows it but it is hard to see the depth from that angle.
 
#14 ·
I think it was oxygen starvation and crevice corrosion rather than galvanic corrosion that caused the problem.
 
#15 · (Edited)
The sacrifical zinc on the prop shaft, just about 2 feet from the rudder top, has not deteriorated too noticeably since I got the boat several months ago. I don't know if that has any bearing on possible galvanic issue but thought I'd muddy the waters a bit more.

There have been some interesting thoughts and observations posted so far, thanks for all the input! This is a caveat emptor issue and no insurance is involved, but I really would like to know the possible causes so that I (or other owners of similar boats) can avoid this type of catastrophic failure in the future. I think that, judging by the position of the fault, my surveyor didn't have a chance of visually locating this flaw.
 
#16 ·
Crevice corrosion at a localized site, with oxygen deprivation isn't going to affect any of the zincs on the boat.
 
#18 ·
brak said:
Surely most rudder shafts go through a fairly tight stuffing box and yet do not usually break off, much less in 4 years?
A logical thought ...however it has more to do with the circulation of the oxygen than the years of service. In use, the bearing surface is rotated frequently, and the area is oxygenated. In repose, the areas are starved.

Let us ignore the obvious variance of metal quality and concentrate on what we see. In the pic of the shaft within the shaft log one can see a dimple at the upper quarter. Quite possibly the initial intrusion pit. Then the smaller shallow (now rounded) pits forming about it. This would probably have occured over a period of time. Are there any indications of the amount of usage of previous owners?

Ive seen some pretty stunning stuff when it comes to stainless steel corrosion. It is not as obvious as the milder steel types , yet can be quite dramatic.

Dewey
 
#19 ·
Also, if the alloy of steel used had any inclusions in the piece... that would have made any corrosion problems worse at the point where the inclusions were located.
 
#20 · (Edited)
It looks a lot to me like there was a weld between solid shaft and tube at that point and the weld failed, induced by both electrolytic and crevise corrosion. If the welting rod had some copper content, to increase flow and penetration, that could explane the copper present. I would take it to an expert SS welder for advice. It might be a guarantee issue. (e.g. if ER630 rod were used instead of ER316)
 
#21 ·
Idiens said:
It looks a lot to me like there was a weld between solid shaft and tube at that point...
That was my first thought as well, that a weld had failed. Tnew rudder shows that the solid steel bar goes down into the fibreglass rudder stock, so it wasn't a weld point that came from the factory. And the local shop doubts that anyone would have welded a repair in that location. Plus the copper seems to be just plated on - I used a knife and scraped away a portion to expose the steel underneath.
 
#23 · (Edited)
Analysis? Perhaps Zanshin has spent enough already.

I've see this many times. An unsupported shaft flexes, fatigues, then snaps at the bearing, always at the bearing. Got a fine example in the shop and the fracture, at the bearing, in a 4" diameter shaft is very similar in appearance to the photographs.

If you wiggle it often enough, it's gonna break at point of attachment.

Hey, just a GUESS based on experience ... kinda favor my rudders with more than one point of attachment!!!!

(Bob makes a lot of boat bucks turnin' those replacement shafts!)
 
#24 ·
Bob...interesting. Analysis would be for proof for recovery from the guy that sold it to him if there was an undisclosed repair...but based on what you are saying there is not a need as this is a common failure?
What does Bob think causes it to fracture rather than bend before giving way..do the bearings score the shaft and create a weak point?
 
#25 ·
Probably started as a hair line stress crack. Then the galvanic action added to it.
On your haul outs or every other one. You may want to magnaflux test your rudder stock for hidden hairline cracks. Especially in the area where the bending forces are the greatest.
What does the manufacturer say about it?? Have it been tested by a lab?
And have you done a thorough check of your boat for other possible places of galvanic corrosion? Such as the Propeller shaft, the engine, any metal through hulls and The mast and associated fittings for the mast, etc. etc.
 
#26 ·
Another 2cents. If it were a straight fracture break in solid unwelded metal, then at least part of the area (the bit that finally failed under stress) would show the characteristic metal crystal structure. The fact that there appears to no sign of crystaline fracture suggests something else happened. Corrosion could be responsible for most of the weakening, but the last bit has to break. Or it was a weld and those bits of copper are the last remains of the "habit" that was holding it on.
If that copper were left over from a welding rod, then it would adhere poorly to SS.
 
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