CD 36 Rudder post pitting

[Jbodkin]

Greetings, My 1982 CD 36 has recently developed 2 small pits evident on the rudder post between the hull and rudder. Pit dimensions are about 2 to 2.5 mm in depth (maximum) and about 3-4 mm in height (see photo). Does anyone have experience with this type of corrosion on their rudder post and any solutions? I understand that prior painting of the post with anti fouling paint may be the cause, but am unsure. This corrosion has occurred over the past 3 years since the last haul out. The prop shaft has no evidence of corrosion. If the post/rudder requires replacement, has anyone either rebuilt/replaced the post and rudder or purchased a new one. I am planning on hauling out next year for repair, rebuild or replacement. Thnaks for any thoughts and suggestions.

Jim

[Frenchy]

Perhaps the anti-fouling paint applied on the shaft is the culprit. I don't paint the shaft or other underwater
metal with anti-fouling. I understand it can be done with several coats of primer. I tried that on a propeller
one year, top coating with the recommended hard bottom paint. I got the same barnacles I always had so I
gave up on that.
If I had the same problem as you have, I think I would grind and clean out the pits, fill with thickened
epoxy, smooth over and leave bare. Hopefully the rest of shaft farther up has not been affected. Probably not.
A boat yard manager would give you better advice, I'm sure - or a surveyor friend if you have one.
- Jean

[John Stone]

I had a little pitting on mine. Does not seem to be a consensus about it being a problem. I am cynical enough to think a boatyard would be happy to tell you it's a problem then charge thousands to "repair" or better yet build you a new rudder. Gather info, talk to some folks you trust and make the best decision you can.

Here is our rudder project with pictures of the corrosion. Scroll down about 1/2 way for the work on the rudder. Below I excerpted the discussion on the corrosion.

http://www.farreachvoyages.com/blank-40/

"Before I started this last phase of the rudder modification, I hauled the rudder up to Bircher's Machine Shop in Morehead City to have Jim Bircher look at the immersion corrosion on the back side of the post about three inches above the top of the rudder. It's a small area about 1/2" wide and 2" long vertically. He thought it was not too big a deal. His comment was that the rudder post had lasted for 20 years and the corrosion was pretty small. He told me I could have a new post machined, build a new rudder, and it could look the same in two years. He thought the rudder post would easily last another 10-12 years. This was all just an educated guess but it worked for me. I decided to keep pushing forward and use the rudder I have. I'll be able to keep an eye on it because I can see this spot with the rudder mounted. He did say one should avoid putting any anti-fouling paint on SS. Once deprived of the protective oxygen it can quickly corrode, which is probably what happened."

[Matt Cawthorne]

I have hull 79 also from 1982. About 6 years ago I noticed pitting in my rudder shaft. I used a dremel to trace the pits to their end cavity. I tried to passiviate the shaft, but it was really too cold to achieve that. I worried that the area under the fiberglass could be worse so I made a new rudder around an inconel 625 shaft and replaced the original the following season. I have studied corrosion and crevice corrosion can be aggressive. BTW on my rudder, the area under the fiberglass was not experiencing corrosion, but by the time I convinced myself of that, it was not worth saving.
Matt

[JD-MDR]

My rudder looked like that. I found I needed to replace the post. Here are photos. You can find more on my posts. I wish i would have made my post more organized so its easier to search.

[John Stone]

JD I remember that post from before. That's a lot of corrosion. Once it gets like that I think it accelerates. Good call. My rudder post seems to be about the same as before last time I checked.

I have to wonder how bronze would do over the long haul. You almost never see bronze prop shafts or rudder post any more.

John

[Jim Walsh]

JD’s rudder post still sends shivers down my spine.
Orion’s original bronze prop shaft was bent when the previous owner got into an altercation with a mooring pick-up buoy in Newport Harbor. It was replaced with a monel shaft…which I subsequently shortened by a couple inches to accommodate my feathering prop. I delivered my new prop to the shaft shop so the prop and shaft were lapped for a perfect fit. My shaft coupling also went along so the coupling could be faced for a proper fit.

[JD-MDR]

I don’t think my Rudder post looked as deteriorated as OP’s. I decided to drop it and check it out because when I wiggled it moisture would seep out. Im lucky I found out while in the yard