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

[Zanshin]

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.

[Giulietta]

Looks like galvanic corrosion did it first, as it was at the thru hull. Then as you sail, and turn, torsion forces weakened it.

If it was impact it woud "tear" towards on side

The copper was deposited after.

[donrr1]

It looks too smooth to be from an impact as Giu says, but it also looks too precise to be galvanic corrosion. The rest of the shaft doesn't appear to be pitted at all. I don't know...

Don

[camaraderie]

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!

[sailingdog]

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.

[Zanshin]

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.

[sailingdog]

Corrosion might have indeed worn the sharp edges away. How long was the boat in the water after the rudder broke??

[Zanshin]

Here is picture taken immediately after the haulout from directly below the rudder through-hull


A picture comparing old vs. new


This is a full frontal picture prior to any degreaser and acetone

[sailingdog]

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.

[Giulietta]

Galvanic.....