deteriorated rudder post

[JD-MDR]

Hoping for some response on why my rudder post got so bad . It's all at the point where it exits the rudder and goes into the tube. I would have never saw it but I found a small crack in the fiberglass at the rudder. we saw this when we dropped the rudder.

[tkeithlu]

Oh, man, that's BAAAAAD. As in galvanizic corrosion poster child. Zincs? Bonding of zincs? Here's a short course:

https://www.intermarineboats.com/new...c-Corrosion/29

If you get serious about avoiding this sort of very expensive mishap, a silver reference electrode, for $130, is a good investment:

https://www.boatzincs.com/corrosion-...ode-specs.html

[JD-MDR]

here is another photo

[JD-MDR]

Originally Posted by tkeithlu

Oh, man, that's BAAAAAD. As in galvanizic corrosion poster child. Zincs? Bonding of zincs? Here's a short course:

https://www.intermarineboats.com/new...c-Corrosion/29

If you get serious about avoiding this sort of very expensive mishap, a silver reference electrode, for $130, is a good investment:

https://www.boatzincs.com/corrosion-...ode-specs.html

OK ,I can fit a zinc on the rudder post up above where it comes out of the tube but I thought the zinc needs to be under the water attached to the piece its protecting. But why? I don't know any one who puts zinc on their rudder.

[JD-MDR]

I think that coupling is iron. It attaches the bronze worm drive gear to the SS shaft or rudder post. Maybe it should be bronze also . Why didn't the Manufacture do that.. I'm just wondering.? How to fix this problem? Although I can just rebuild it like it was and It should last the rest of my life. Maybe that coupling is bronze. It doesn't look like it . I'll have to check it out when I go back down there

[deblen]

Originally Posted by JD-MDR

OK ,I can fit a zinc on the rudder post up above where it comes out of the tube but I thought the zinc needs to be under the water attached to the piece its protecting. But why? I don't know any one who puts zinc on their rudder.

Your rudder stock appears to be SS. What metal is the rudder tube made of?
/ Len


Edit: Can you see any corrosion on the inside of the rudder tube at bottom end?

[JD-MDR]

Originally Posted by deblen

Your rudder stock appears to be SS. What metal is the rudder tube made of?
/ Len

It's FRP tube just like the propeller shaft tube I replaced a horribly deteriorated bracket on the rudder tube with a aluminum one .. Now thats another metal type but it's not touching anything but the FRP tube

[Frankly]

A few years back I changed out my rudder shaft packing to a graphite material (using graphite on prop shaft for years). Checked a couple of years later and pitting similar to yours just a lot less. Immediately reverted to the original Teflon packing. My SS rudder shaft is connected to my bonding/ zinc system. As to why a problem on rudder and not prop no real clear explanation. The location of the rudder shaft packing is normally dry just sitting, but wet underway. Maybe a splash zone.

On the material galvanic potential charts graphite is almost off the chart.


Frankly

[deblen]

Further reading may provide clues. https://www.sailmagazine.com/diy/know-your-rudder


Len

[s/v Jedi]

That may be an electrical problem.

[JD-MDR]

Originally Posted by deblen

Further reading may provide clues. https://www.sailmagazine.com/diy/know-your-rudder


Len

Thanks Deblen. That was really informative. I think this is my problem: "Stainless steel, in particular, suffers when deprived of oxygen, which is exactly what happens up inside a rudder tube full of stagnant seawater" Here is another point: "To eliminate the need for lubrication, some rudder tubes are sleeved with low-friction plastic bearings. Unfortunately, two common bearing materials, polyamide (nylon) and polyacetal (Delrin), absorb water and swell, causing the bearings to grip rather than guide" We were thinking of using Delrin sleeves on both ends of the tube.

[s/v Jedi]

Originally Posted by JD-MDR

Thanks Deblen. That was really informative. I think this is my problem: "Stainless steel, in particular, suffers when deprived of oxygen, which is exactly what happens up inside a rudder tube full of stagnant seawater" Here is another point: "To eliminate the need for lubrication, some rudder tubes are sleeved with low-friction plastic bearings. Unfortunately, two common bearing materials, polyamide (nylon) and polyacetal (Delrin), absorb water and swell, causing the bearings to grip rather than guide" We were thinking of using Delrin sleeves on both ends of the tube.

In my experience, when stainless steel corrodes where it isn’t being exposed to oxygen, you get black pitting which isn’t the case here.

I have seen this kind of damage on stainless anchor chain which turned out to be caused by a DC current running through it when anchored.

[Compass790]

Originally Posted by JD-MDR

Thanks Deblen. That was really informative. I think this is my problem: "Stainless steel, in particular, suffers when deprived of oxygen, which is exactly what happens up inside a rudder tube full of stagnant seawater" Here is another point: "To eliminate the need for lubrication, some rudder tubes are sleeved with low-friction plastic bearings. Unfortunately, two common bearing materials, polyamide (nylon) and polyacetal (Delrin), absorb water and swell, causing the bearings to grip rather than guide" We were thinking of using Delrin sleeves on both ends of the tube.

I'd go for something called Vesconite or vesconite high lube, its made for bearings in mining & copes with slurry. I haven't actually tried it but will for a cutless type bearing eventually. Read another post where one cruiser had it for 10 years as a cutlass bearing. Just a thought about bearing material, not about your corrosion issue.