nmanno

Carbon fiber hull - corrosion

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Hi guys,

I've got a Carbon hull race/cruiser and live in warm Asia. 

I have an aluminium sail drive and aluminium rudder stock and they are getting eating like soft cheese by what I assume is galvanic corrosion.

The sail drive has the usual anode and I've even put an anode connected to the rudder stock when the boat is on her mooring which still doesn't seem to help.

The antifouling used is VC Offshore which is a copper based hard racing antifouling but the yard here paints 1m2 around the sail drive with a non-copper based antifouling. 

In my marina, there were heavy tunnel works over the last couple of years and I wonder if those works were a reason for the strong corrosion (i.e. potentially some strong electrical currents in the water).

Anyhow, what should I do as it's not viable to change the entire saildrive and rudder every 4-5 years! How best to test for stray currents etc...

Any views appreciated.

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From what you're describing its going to be very hard to stop.  Carbon fibre + Aluminium + salt water = battery you need to make sure the aluminium components are isolated from the carbon structure.  If the boat has carbon engine beds this won't be easy or cheap.  Some light reading here to help you diagnose the issue.  Or call this bloke and offer him a trip to Hong kong.  He spends his life in planes anyway.

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You can remove the aluminum bits and powder coat them, which will help a good deal. There are other ways to isolate the carbon from the aluminum, one trick is to apply a layer of fiberglass to the carbon in the area of contact but what may get you are any fasteners still provide an electrical path. They can be painted which will help, or at least the liberal use of TefGel will do wonders.

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If the Carbon is coated properly it shouldn’t give any galvanic problems. As long as the sea water does not touch the carbon (the barrier coat should take care of this) there won’t be a galvanic chain. I suspect the copper will be a bigger problem, lower on the galvanic chain than carbon but a big area and a small area of Aluminium (the bigger the ratio the bigger the corrosion)

 Big steel structures could be a problem depending on the shore power connection you are using, earthing, galvanic transformer, etc...

the picture below is for copper and stainless, but you can change those to carbon and Aluminium, or copper and Aluminium. The picture gives an indication of the different ways you can try to avoid galvanic corrosion.

ways-to-prevent-galvanic-corrosion.png

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I don't know if CF hulls need to be treated differently but I would think not.

The basic test for a 'hot' marina involves measuring the current in the main ground wire. 

You could ask fellow boaters if their zincs are requiring frequent replacement. Also if there are any ali bottomed RIBS ask the owners if they are having issues with corrosion. I have seen 2 where the bottom was eaten away in a year. A hot marina was the suspect. 

If you are plugged into shore power you want to look into using a galvanic isolator.

 

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Are you sure you're using the correct anode?  And enough of the correct anode?

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19 minutes ago, voodoochile said:

Are you sure you're using the correct anode?  And enough of the correct anode?

and not too much of the right anode..

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Just call Brian Gatt, linked in my first post,  a return flight from Sydney to Hong Kong is nothing compared the the boat bucks you're already spending.

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I am not sure what your complete solution is. But if you are at RHKYC that place is hot as aych eee double chopsticks. Crimeny, you've got 220v power supply to a couple hundred mooring balls that are like 30+ years old. 

Hopefully that may help someone t/s the problem. 

 That marina eats metal for fun I think. Also very warm and very polluted.

 

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Some sail drives are meant to be electrically isolated from the engine and the rest of the grounding system. Often in installation or subsequently this isolation Is breached. One thing I'd do is check with an ohmmeter and make sure they are still isolated.

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On 12/4/2018 at 4:40 AM, SCARECROW said:

Just call Brian Gatt, linked in my first post,  a return flight from Sydney to Hong Kong is nothing compared the the boat bucks you're already spending.

Absolutely, I will go down that route if needed, just want to try a couple of ideas first.

 

On 12/4/2018 at 6:58 AM, DDW said:

Some sail drives are meant to be electrically isolated from the engine and the rest of the grounding system. Often in installation or subsequently this isolation Is breached. One thing I'd do is check with an ohmmeter and make sure they are still isolated.

You see, that's an interest angle here, as I went around the boat with the ohmmeter, I got a reading between the bare carbon hull and the drive, the engine, the rudder stock etc... it seems like my entire boat is a huge battery with both the sail drive and rudder stock being the 2 electrodes!

 

On 12/4/2018 at 4:50 AM, Baldur said:

I am not sure what your complete solution is. But if you are at RHKYC that place is hot as aych eee double chopsticks. Crimeny, you've got 220v power supply to a couple hundred mooring balls that are like 30+ years old. 

Hopefully that may help someone t/s the problem. 

 That marina eats metal for fun I think. Also very warm and very polluted.

 

Yes, RHKYC marina in causeway bay, indeed, very hot, polluted and with so much works and power running in the water, I wonder if the entire place is not a power plant! 

And, yes I use the right anode and change it regularly but I don't see the anode being "eaten away" faster than the sail drive.

On 12/4/2018 at 12:25 AM, TQA said:

I don't know if CF hulls need to be treated differently but I would think not.

The basic test for a 'hot' marina involves measuring the current in the main ground wire. 

You could ask fellow boaters if their zincs are requiring frequent replacement. Also if there are any ali bottomed RIBS ask the owners if they are having issues with corrosion. I have seen 2 where the bottom was eaten away in a year. A hot marina was the suspect. 

If you are plugged into shore power you want to look into using a galvanic isolator.

 

I am not connected to shore power, just a mooring in the middle of a marina.

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On 12/3/2018 at 7:52 PM, nmanno said:

Hi guys,

I've got a Carbon hull race/cruiser and live in warm Asia. 

I have an aluminium sail drive and aluminium rudder stock and they are getting eating like soft cheese by what I assume is galvanic corrosion.

The sail drive has the usual anode and I've even put an anode connected to the rudder stock when the boat is on her mooring which still doesn't seem to help.

The antifouling used is VC Offshore which is a copper based hard racing antifouling but the yard here paints 1m2 around the sail drive with a non-copper based antifouling. 

In my marina, there were heavy tunnel works over the last couple of years and I wonder if those works were a reason for the strong corrosion (i.e. potentially some strong electrical currents in the water).

Anyhow, what should I do as it's not viable to change the entire saildrive and rudder every 4-5 years! How best to test for stray currents etc...

Any views appreciated.

Hi nmanno, FWIW I will offer you this free advise, can pm me if you require consulting advice. I have been a Corrosion Engineer and run a sacrificial anode manufacturing plant for 35 years. The fact that you have a carbon fibre hull is likely to play no role in the corrosion problem. Your carbon is fully encapsulated in epoxy and presumably barrier coated with epoxy paint under the antifouling.

The use of  Trilux antifouling on the Saildrive leg and surrounding area will make no difference to the corrosion of your Saildrive leg/ rudder stock. I recommend and on my own boat use the same antifouling on the drive leg as bottom. (International  Ultra). My own research and other studies have shown that, similar to the carbon, all of the copper is electrically isolated from any aluminium by the paint  binder. I get a longer life between slipping as the Trilux always appears to foul up far more readily than regular antifouling. 

Propspeed provides a useful barrier coating to bronze running gear and greatly reduces anode wear. 

For the Saildrive leg I have developed a kit which is used to “boost” or increase the life of the ring anode and allows me to monitor the corrosion potential of the Saildrive leg. I have electrically bonded the aluminium rudder bearing housing to the Saildrive leg in my boat to remove a common corrosion issue which occurs on these housings.

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I had a similar problem with my carbon fibre race boat with my Yanmar SD20 sail drive. There are four preventitive actions I did to remove the problem which I believe eliminated my sail drive being eaton up :

1. I found that the carbon fibre plate installed around my sail drive was making contact with the sail drive and causing electrolysis through to the sail drive as this is the area being eaton up and disappearing. I removed the carbon plate and replaced with fibre glass plate to provide an insulator to the carbon fibre haul to the sail drive

2. Installed a 0VDC isolator from all my batteries to avoid any stray currents from the batteries to the carbon haul.

3. Installed anodes on each side of the boat connected to the sail drive via a cable to one of the bolts. This was only when moored.

4. Use probspeed on the prob which is a 3 blade Gori racing version

the problem has all gone away as a result of these changes with minimal loss of anodes on my sail drive. The carbon fibre boat next to me had a similar issue and the owner only installed 0VDC isolators on his batteries and I believe his issues also went away.

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