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Old 22-02-2017, 11:52   #1
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How to attach a zinc to a cast iron keel?

Hello, I am a new member and am hoping to get some guidance. I have a Catalina 22 with a well maintained painted cast iron swing keel and am looking for a good way to attach a zinc. The keel pivots on a bronze pin and hanger castings which are bolted to the hull with stainless bolts and weldments. The lifting cable and attachment eye bolt are also stainless steel. The question is how to provide a good electrical connection between the zinc and keel while still preventing the formation of rust. I have not been able to find much info. on attaching a zinc to cast iron anywhere on line. I thought of welding studs on the keel but would like to avoid the cost of welding and re-painting the keel. My best idea so far is to drill and tap the keel to thread in (with waterproof grease) a stainless stud to bolt on the zinc. My questions with this approach are; 1) would the contact between the nut (on the stud) and the zinc be sufficient, and (2 would this approach also protect the bronze hangers and stainless steel parts? I would greatly appreciate any comments or other ideas. Thank you!
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Old 22-02-2017, 12:21   #2
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Re: How to attach a zinc to a cast iron keel?

On my Tanzer 22 I have done as you suggested and drilled and tapped two 1/4-20 holes in the cast iron keel, one to port and one to starboard. I took a small rudder zinc and bolted one half on each side of the keel with stainless socket head cap screws. I had to do a little work on one of the two zinc halves to fit the cap screw. The zinc corrodes away, and rust is slower to appear in the nicks in the paint at the leading lower edge of the keel. Now that the boat is always in fresh water, I have replaced the zinc with magnesium.
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Old 22-02-2017, 12:31   #3
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Re: How to attach a zinc to a cast iron keel?

Stainless steel does not like to be submerged, especially in salt water ! L understand that your cable is stainless, etc. But it would be out of the water when raised ! It would slow down degradation of the more noble metals !
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Old 22-02-2017, 12:58   #4
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Re: How to attach a zinc to a cast iron keel?

Thanks, unfortunately the swaged end of the lifting cable and eye bolt as well as the keel hanger bolts are all submerged all the time. That is why I am especially concerned about the quality of the electrical connection between zinc and keel.
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Old 22-02-2017, 19:06   #5
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Re: How to attach a zinc to a cast iron keel?

1.- Anode protection is a matter of surface area relative to the part being protected. A piece of iron the size of your keel would require multiple large anodes to adequately protect it from galvanic or electrolytic corrosion.

2.- Your keel is unlikely to suffer from galvanic or electrolytic corrosion.

3.- To get a good electrical connection between your keel and an anode requires metal-to-metal contact. This means exposing bare iron to saltwater. Rust will occur.

4.- Anodes do not protect metals from rust.
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Old 22-02-2017, 19:25   #6
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Re: How to attach a zinc to a cast iron keel?

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Originally Posted by fstbttms View Post
1.- Anode protection is a matter of surface area relative to the part being protected. A piece of iron the size of your keel would require multiple large anodes to adequately protect it from galvanic or electrolytic corrosion.

2.- Your keel is unlikely to suffer from galvanic or electrolytic corrosion.

3.- To get a good electrical connection between your keel and an anode requires metal-to-metal contact. This means exposing bare iron to saltwater. Rust will occur.

4.- Anodes do not protect metals from rust.
Thanks for your reply. The boat is in the water for only 5 weeks a season so would two 3" diameter zincs be any help or would more zincs be needed no matter the time period? I was thinking that the greased threads on the stud and keel would keep rust from forming.
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Old 22-02-2017, 19:36   #7
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Re: How to attach a zinc to a cast iron keel?

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Thanks for your reply. The boat is in the water for only 5 weeks a season so would two 3" diameter zincs be any help or would more zincs be needed no matter the time period? I was thinking that the greased threads on the stud and keel would keep rust from forming.
I think that you are over thinking this issue! I kept a CAt-22 in the water year around in the SF bay for 7 years of ownership. I used NO anodes at all during that time, and had no problems with keel corrosion, pennant degradation or pin issues. I was not "plugged in" to mains power during that time, and had no shore power system on the boat at all for that matter!

Rust on the cast iron from breakdown of the (rather poor) factory painting caused a few lesions to appear each year, but they were addressed at anti-fouling time and were not much of a worry.

Relax and enjoy your oh-so-short sailing season.

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Old 22-02-2017, 19:40   #8
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Re: How to attach a zinc to a cast iron keel?

I have a 26 YO IP keel centerboard yacht. My CB is glass so this is not a galvanic issue. My lift system consists of a SS cable connected to a SS bale built into the CB. The cable passes through a SS tube and over a SS sheave assy as it attaches to the bale. Although SS is not an ideal choice for continuous SW submergence, I have found it satisfactory if I keep the zincs up to snuff. All components except the lift cable are original and in good shape. A few years back I added an extra zinc on the side of the keel to extend the time between zincs. All the zincs are connected electrically to the cable tube and I depend an mechanical contact of the lift cable to tie the remaining components to the bonding system.

In your case I wouldn't worry about the iron CB but concentrate on the other components. Just try to keep epoxy and bottom paint on the iron and get some zincs connected to the remaining components and be religious about keeping an eye on them.

I get 10 years out of a lift cable and my yacht sits in salt water 365. Only cable issues are a few broken strands where the cable bends over the sheave.



Use 316 SS for the wire and a copper Nicopress sleeve for the wire connection. Watch out for aluminum sleeves they won't last long.
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Old 22-02-2017, 19:46   #9
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Re: How to attach a zinc to a cast iron keel?

Quote:
Originally Posted by RNWilson View Post
Thanks for your reply. The boat is in the water for only 5 weeks a season so would two 3" diameter zincs be any help or would more zincs be needed no matter the time period? I was thinking that the greased threads on the stud and keel would keep rust from forming.
You're missing the point. You don't need anodes on your keel.
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Old 22-02-2017, 20:19   #10
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Re: How to attach a zinc to a cast iron keel?

Thanks everyone for the advice! I guess I will skip the zinc and just keep an eye on everything since it is only in the water 5 weeks a year anyway. I am happy to have one less job on the list! Thanks!
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Old 22-02-2017, 21:01   #11
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Re: How to attach a zinc to a cast iron keel?

FWIW: If y ou should need to replace the keel lift wire, give some thought to using dyneema instead. NO corrosion issues, very good chafe resistance, easy to splice (soeasy to attach to the keel) and not expensive.

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Old 23-02-2017, 09:06   #12
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Re: How to attach a zinc to a cast iron keel?

This is a useful thread to us as well. Our 2011 Beneteau Oceanis that we bought last fall showed some minor rust pitting on her cast iron keel. The surveyor recommended adding a zinc anode but I haven't found anyone else that seems to agree with that recommendation. We are doing bottom paint and the paint shop recommended blasting the keel and re-coating with epoxy prior to bottom paint. I have tentatively agreed with this. Painting will not happen until early May so I still have time to change my mind.
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Old 23-02-2017, 09:59   #13
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Re: How to attach a zinc to a cast iron keel?

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This is a useful thread to us as well. Our 2011 Beneteau Oceanis that we bought last fall showed some minor rust pitting on her cast iron keel. The surveyor recommended adding a zinc anode but I haven't found anyone else that seems to agree with that recommendation. We are doing bottom paint and the paint shop recommended blasting the keel and re-coating with epoxy prior to bottom paint. I have tentatively agreed with this. Painting will not happen until early May so I still have time to change my mind.
With my admittedly less than complete understanding of galvanic corrosion I would think that if the keel was completely painted it would not be involved in any electrolytic action since it would be insulated by the paint. That is assuming there is no other submerged metal attached to the keel. In my situation I was concerned about the submerged stainless and bronze which I could find no way to protect except by attaching a zinc to the keel which is electrically connected by the pivot pin. Truth be told, although I have generally been advised here to not worry about it, I don't understand why a stainless shaft with a bronze prop would need a zinc and my bronze keel hangers with stainless bolts would not.
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Old 23-02-2017, 10:03   #14
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Re: How to attach a zinc to a cast iron keel?

Some time back, I have filled the anode holes with foam and overpainted the whole slab with thick epoxy.

Ever since then, there is way less rust on the keel when we pull her up.

Our keel is not used for electric grounding of any sort, so our slab does not need any anodes.

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Old 23-02-2017, 10:11   #15
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Re: How to attach a zinc to a cast iron keel?

Quote:
Originally Posted by canyonbat View Post
This is a useful thread to us as well. Our 2011 Beneteau Oceanis that we bought last fall showed some minor rust pitting on her cast iron keel. The surveyor recommended adding a zinc anode but I haven't found anyone else that seems to agree with that recommendation. We are doing bottom paint and the paint shop recommended blasting the keel and re-coating with epoxy prior to bottom paint. I have tentatively agreed with this. Painting will not happen until early May so I still have time to change my mind.
Rust is caused by oxidation of the metal in question. Anodes do not prevent this. Anodes protect from other types of corrosion, primarily electrolytic (stray current) corrosion and galvanic corrosion, which is occurs when two dissimilar metals are in contact with each other while submerged in an electrolyte, like seawater.

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With my admittedly less than complete understanding of galvanic corrosion I would think that if the keel was completely painted it would not be involved in any electrolytic action since it would be insulated by the paint.
Your keel hangers are electrically isolated and unlikely to suffer corrosion, especially since your boat will spend limited time in the water. Regarding your keel, your anti fouling paint is unlikely to protect the metal from corrosion (rust or otherwise), considering it is not a waterproof coating and is, in fact, loaded with copper, an excellent conductor of electricity.
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