Crevice Corrosion - Is My Rudder Toast ?

[Matt Johnson]

Don Casey's book "Inspecting the Ageing Sailboat" showed a few things to look for with the rudder when we were buying our boat. Our surveyor checked the entire rudder for water penetration and play (was perfect)... I thought we were all set! Ignorance is bliss.

Today we had time to drop the rudder and inspect for corrosion of the rudder shaft. Well, while we do not have the corrosion where the rudder shaft enters the hull of the boat, I do have pitting between the top of the rudder and the hull. Looking at the photos from before we bought the boat, I can see this issue isn't new electrolysis, but from before we owned it.

The pits aren't huge, but I'm not sure if this rudder is a write off or something that can be ground down and corrected.

Here are a few photos... Please let me know what the forum thinks.

[barnakiel]

My two guesses (pretty wild ones, I guess):

- scrub the part, grind it very, very lightly - see what is under,
- X-ray,

Looks more like some sort of a sediment than corrosion. If it is corrosion though then it looks pretty ugly. Ugly=bad? No idea.

How old is the boat?

Surveyor said what?

b.

[Matt Johnson]

The surveyor never mentioned it in his 12 page report... He looked at everything else, but this didn't seem to interest him.

I should have seen it, but didn't know what I was looking at. The boat was purchased in December of 2009 and is a 1989 Sabre 34'.

[Cheechako]

hmmm..... you would think any oxygen depletion corrosion would be inside the bearing... and that oooks good. I would grind it some and see how deep it looks. Almost looks like cast material and the rudder was machined above that point... but that's unlikely I imagine. Is it pipe or solid rod?

[barnakiel]

It must be still mighty strong since it has not snapped of by now. I would very gently grind a small area and see what the matter is all about. Looks pretty regular and pretty granular.

We had crevice corrosion on chainplates and it looked different.

b.

[delmarrey]

What material is the lower bearing made from. Not bronze, I hope!

But yeah, I'd file off a bit of the surface to see how it looks, and go from there.
Is a Sabre 34 a coastal or off shore boat?

[foggysail]

Don't overlook cavitation. Pock marks caused by cavitation on power boat rudders are common. But they would only be in areas where the prop wash has access.

Foggy

[Matt Johnson]

It's a 2.86" stainless post. Not sure on the thickness of the pipe, but if I had to guess... I would say about 1/4".

I was looking for corrosion in the bearing area because that's where I read oxygen depletion usually occurs. From what I'm reading online, I think it was from stray current in the POs marina. All the seacocks/thruhulls are bronze and showing no issues, but they are bonded. The rudder shaft is the only thing not bonded on the boat. Does this sound right?

[delmarrey]

Originally Posted by funjohnson

It's a 2.86" stainless post. Not sure on the thickness of the pipe, but if I had to guess... I would say about 1/4".

I was looking for corrosion in the bearing area because that's where I read oxygen depletion usually occurs. From what I'm reading online, I think it was from stray current in the POs marina. All the seacocks/thruhulls are bronze and showing no issues, but they are bonded. The rudder shaft is the only thing not bonded on the boat. Does this sound right?

That would be a 2-1/2, sch. 80, SS pipe.

Does the rudder post sit in the water at the dock? Most rudders are designed to have the post slightly out of the water. This could be the case.

[Gaucho Marino]

Funjohnson:
From the pictures I would venture to say it is from stray current as you mention. I would grind it and have a SS welder refill the voids and then reshape it to its original thikness or if possible make it a but thicker at that point. You do not want that particular part of the rudder post to be weakened as it is the part that works the hardest. Then make shure you ground it.

[Matt Johnson]

Originally Posted by delmarrey

That would be a 2-1/2, sch. 80, SS pipe.

Does the rudder post sit in the water at the dock? Most rudders are designed to have the post slightly out of the water. This could be the case.

It sits about a foot below the waterline.

[afmstm]

If you have stray current in the marina, the worst thing you can do is bond all the immersed metal bits. What happens is the stray current flows into one of the metal bits, through the bonding and exits another metal bit. The metal bit where the current flows out will be eaten by electrolysis. See here for a good, intro article: Grounding.
You should consider de-bonding any metal through hulls that do not need to be electrically connected to anything else. Since your rudder post was not bonded, that implies stray current corrosion was not the culprit. Could it have been bonded in the past?

Stainless steel is unfortunately self-corroding in anaerobic sea water. You should have a dedicated zinc for the rudder, either on the shaft, or failing that in a nearby zinc mounted in the hull and connected to the shaft by a tinned braid or shaft brush. This zinc should not be connected to anything else so you reduce the chances of stray current corrosion....

Regarding crevice corrosion. Your photo showing the band of pitting is pretty scary. I would do more than take a file to it - take a dremel tool with a pointed grinder, and just like a dentist going after a cavity, dig into some of the holes. You have to determine how deep they go. Just like a dentist, your goal is to get to the bottom of the cavity and clean it out. It's OK to remove material -after all, if it's pitted it's not adding much strength, and also, it's unlikely you are removing a large percentage using a dremel tool. I have done this on chainplates and found some holes penetrate several mm.

If the band of pitting does indeed extend deep into the metal, you can calculate how much strength you have lost. Basically, you need to measure the depth of the pitting, and use that to calculate the % loss in cross sectional area. Strength varies more or less with area (strictly speaking, it's also related to the distance from the center of rotation, but just ignore that for now).

Getting a welder to look at it seems like a good idea, although of course welded metal is often more corrosion prone.

The concern I would have on my own rudder shaft is that crevice corrosion has eaten away the metal inside the rudder where you can't see it....

Good luck....

[Gaucho Marino]

Funjohnson:
Another thing you can do if the rudder stock is a pipe (not solid) is to introduce an inner pipe or solid bar, whose OD (outside diameter) is as tight as possible to the rudder stock ID, this will ensure the post is strong and will not fail at sea due to the corrosion

[Matt Johnson]

That is a good idea too. Sabre welded the top of the pipe for the attachment of the emergency tiller, but I'm sure on of the marina service guys could figure out how to get to it.

[CarlF]

You are in the lucky situation of owning a well built boat and the builder is still in business. Call Sabre -

From their web site:


Contacts in Sabre's After Sales Service Department are divided into sail and power. Motor yacht representative Tucker Thompson and sailboat contact Glen Chaplin may be reached by calling 207-655-3831 or by filling in the contact form.