Advice on Anodes? Steel Rudder Stock And Bronze Fittings?

[andyskip]

QUESTION: Is is sensible to wire a stainless rudder stock and bronze fittings to the same anode?

POLL: see above poll add your vote!

I'd really appreciate some expert advice. I'm restoring an old contessa and thinking about anodes / galvanic protection.

I have one large Anode on the hull, and then separately a small ring anode on the prop shaft.

At the moment the engine block and some (not all) of the bronze through hulls are wired together and onto the main anode.

The stainless rudder stock used to be wired into the same circuit but has lost that connection. Before I connect it back in again I was wondering....

Assuming the anode is present and all of the connections are good, then no problem... the anode will prevent corrosion on both the steel and the through hulls.

But what if the anode connection fails or the anode disintegrates before I notice? This is the point of my question....

In this case it seems to me that I have then electrically bonded bronze fittings straight to a steel rudder stock, both in the water. Given that steel is less noble than bronze, will it not actively start corroding in that electrical circuit?

I am wondering whether it's better to:

- Leave the rudder stock disconnected. It's well insulated from other fittings
- Add a dedicated anode for the rudder stock
- wire everything together like it was before and check the anode regularly

Please let me know what you think.....

[s/v Jedi]

My recommendation is not in the options so I put it here:

- remove all bonding wires and grounding plates, then only zincs on the prop/shaft.

[andyskip]

Originally Posted by s/v Jedi

My recommendation is not in the options so I put it here:

- remove all bonding wires and grounding plates, then only zincs on the prop/shaft.

Really interested in this reply..... can you expand on this? Obviously the contessa has a GRP hull. So individual fittings are insulated from each other and are unlikely to pick up any voltage from electrical circuits.

And many of the though hulls aren't connected at all. Looks like they never have been. Only the raw water intake.

But the main anode came with the boat. I'm thinking that the engine block should be connected to that at the very least given the various electrical connections into it?

Thanks! Very interested to hear your point of view on this.

[s/v Jedi]

Originally Posted by andyskip

Really interested in this reply..... can you expand on this? Obviously the contessa has a GRP hull. So individual fittings are insulated from each other and are unlikely to pick up any voltage from electrical circuits.

And many of the though hulls aren't connected at all. Looks like they never have been. Only the raw water intake.

But the main anode came with the boat. I'm thinking that the engine block should be connected to that at the very least given the various electrical connections into it?

Thanks! Very interested to hear your point of view on this.

The zincs are only to prevent galvanic corrosion. If the engine is at risk, it should have a zinc installed in it’s heat exchanger. Check the engine installation manual to be sure, but I never heard of them asking for a cable to a zinc.

I have a zinc on my prop and a zinc around the propshaft. Use a silver reference cell to check if you have the correct amount of zincs and you’re done.

For safety with shore power, just use an isolation transformer.

[andyskip]

Thanks s/v Jedi. Really appreciate your thoughts on this.

The engine is a Yanmar 2GM20 (retrofit not original) and that does have a small pencil anode in the head somewhere in the raw water cooling circuit.

I'm going to call Jeremy Rogers and ask how they wire things as well...... the boat does come with a large anode plate as standard so I'd like to understand their reasoning.

I'll post another update here FYI.

[deblen]

Originally Posted by s/v Jedi

My recommendation is not in the options so I put it here:

- remove all bonding wires and grounding plates, then only zincs on the prop/shaft.

Agreed.
Plus some pics of a mild steel rudder,SS stock,Bronze prop & bronze stuffing box, common on local fishing boats. The zinc is mounted to the object to be protected. Bronze thru hulls do not need protection.

[andyskip]

OK I just spoke to Jeremy Rogers (who designed and built / service the Contessa fleet) and they largely agree with the comments here but in any case I found it quite interesting.

JR normally bond the main hull anode to a bolt on the gearbox housing. This is to give the prop shaft and prop a level of protection, to the extent that connects through the mechanics and bearings... not a perfect connection but something.

They do recommend a prop shaft anode if at all possible (in my case this is difficult as there is an aquadrive on the prop shaft meaning there is almost no exposed shaft inside or out. And no room behind the prop).

In summary, and based on what JR have seen when servicing old Contessas their thoughts were:

- DO bond the bronze raw water intake / strainer as that is more delicate
- DO bond the engine near the prop shaft
- DO bond the B&G logs (they have a ferrous wheel)

- Don't bond the stainless rudder stock - "it's well away from anything and we don't tend to see significant corrosion there just a bit of crevassing after 40 years."

- Don't bond the other seacocks - "those were very well made with a decent bronze mix and seem to be pretty bulletproof."

So that agrees with the above comments - my takeout is...

1. Anode on the rudder is the main thing and as close as possible to the steel / bronze join).

2. Bonding the engine gearbox doesn't really protect the engine as such - it has it's own anode immersed in cooling water). But it may help the prop to an extent.

3. Don't worry about much else (apart from the logs in my case which do have a bonding point).

[CharlieJ]

Since the vessel does not have enough exposed propulsion shaft to install a conventional shaft anode, I recommend that you investigate installing a shaft brush. The AWG 8 green conductor from the hull anode is connected to the shaft brush to provide cathodic protection to the stainless shaft/bronze propeller system.

Even a simple shaft brush will provide better electrical connection between the anode and the shaft/prop than depending on an electrical path through bearings, oil, seals, etc. These are available at most good chandeliers. Here is an example: https://www.boatcorrosion.com/produc...ssemblies.html

The ultimate shaft brush is manufactured by ElectroGuard and is far more efficient in transferring cathodic protection current from the hull anode to the shaft/prop: https://www.boatcorrosion.com/product-slip-rings.html

[andyskip]

Originally Posted by CharlieJ

Since the vessel does not have enough exposed propulsion shaft to install a conventional shaft anode, I recommend that you investigate installing a shaft brush. The AWG 8 green conductor from the hull anode is connected to the shaft brush to provide cathodic protection to the stainless shaft/bronze propeller system.

Even a simple shaft brush will provide better electrical connection between the anode and the shaft/prop than depending on an electrical path through bearings, oil, seals, etc. These are available at most good chandeliers. Here is an example: https://www.boatcorrosion.com/produc...ssemblies.html

The ultimate shaft brush is manufactured by ElectroGuard and is far more efficient in transferring cathodic protection current from the hull anode to the shaft/prop: https://www.boatcorrosion.com/product-slip-rings.html

Thanks Charlie I agree with you... good advice however in this case we have less than 20mm bare shaft inside the boat.

We have an aquadrive between the engine and the stern tube which takes up all available space which makes this one a total pain!

[CharlieJ]

@andyskip:
That is difficult!

To make the most of the cathodic protection system, when waterborne have a "hull potential" reading taken between your propulsion shaft and a silver/silver chloride reference cell. If the reading falls between -550mVDC and -1100mVDC then the shaft/prop system is being adequately protected. If the reading is not in that range, change the hull anode to aluminum and retake the "hull potential" reading. If still not in range, you may have to increase the surface area of the aluminum hull anode.

Recommend engaging an experienced marine corrosion tech to help sort through this.

[andyskip]

Thanks Charlie I will do.

And in addition (in case interesting to others) I am going to tap and bolt a bonding wire onto the aquadrive's thrust bearing mounting plate as well i.e. just in front of the prop shaft clamp.

It won't be as good an electrical connection as a carbon brush on the prop shaft but I think it will help.

I'll upload a pic when it's done.

In the meantime and more generally... I am waiting in hope for a boat job that ends up simpler than expected...