Steel centerboard zinc - SailNet Community
 
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post #1 of 9 Old 01-09-2007 Thread Starter
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Steel centerboard zinc

I bought a 1976 Helms 25 this past fall and it was used in fresh water all its life. It will now be in salt water and I noticed its 300 pound cast iron or steel centerboard does not have a zinc or a hole for a zinc. Steel and iron are low on the galvanic scale and I would think they should be protected by a zinc, especially in salt water. For all those iron centerboard boat owners out there in salt water, do you install zincs? Where on the centerboard and how big? My centerboard is 6' long and about 14' wide and fortunitally in good shape right now. the previous owner said he had it sand blasted and immediately put Interlux barrier coat on it and bottom paint. This was in the late 90's.

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Wayne
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post #2 of 9 Old 01-11-2007
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I'm no authority, but I'd put a good size one at the trailing edge. That's where all of the turbulence is going to be and hence the corrosion.
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post #3 of 9 Old 01-11-2007
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Quote:
Originally Posted by sailaway21
I'm no authority, but I'd put a good size one at the trailing edge. That's where all of the turbulence is going to be and hence the corrosion.
Ummm... not sure what kind of corrosion is caused by turbulence, but whatever it is a sacrificial zinc anode is not going to prevent or remedy it.
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post #4 of 9 Old 01-11-2007 Thread Starter
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Fstbttms:
I'm glad you replyed. Hopefully you have cleaned a number of hulls with iron cenrtboards like mine. Do you recall if most of them had zincs bolted to them.

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Wayne
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post #5 of 9 Old 01-12-2007
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Quote:
Originally Posted by Wayne25
Fstbttms:
I'm glad you replyed. Hopefully you have cleaned a number of hulls with iron cenrtboards like mine. Do you recall if most of them had zincs bolted to them.
I have seen a few swing-keel boats use zincs on the keel, don't believe I've I seen any centerboarders that do. I wouldn't recommend it in any case, for two reasons:

1.- The centerboard is likely isolated from the onboard electrical system and therefore not at much risk of electrolytic corrosion. The same applies to any risk of galvanic corrosion. The centerboard has little (if any) connection to a dissimilar metal.

2.- Adequate protection of the relatively large amount of metal in your centerboard would require a whole bunch of zinc, more than you could conveniently attach. The zinc anodes commonly used to protect props, shafts and other running gear are probably inadequate for the job.

My recommendation would be to not lose any sleep over the centerboard but when you have the boat cleaned or hauled, inspect it. Attach anodes only if there is an obvious problem with corrosion. If it ain't broke, don't fix it.
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post #6 of 9 Old 01-13-2007
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Turbulence brings oxygen into the water and it is the oxygen that causes corrosion. The forward edge of a keel, or bow for that matter, seperates the water but causes little or no turbulence. As the water flows along the surface, turbulence is induced by small imperfections in the surface, and the further aft it flows these eddys interact with one another creating a greater effect as you go. At the trailing edge, seperate flow paths meet and this is where the greatest amount of turbulence is generated. The oxygen molecule is unstable and extremely desirous of bonding with something. The sacrificial anode made of zinc in this case bonds more readily than the steel of the keel or centerboard.
Zincs are always placed where turbulence is greatest. Hence they are found on rudder posts, the rudder blade, and in the area of the screw. On a steel hulled vessel they are found virtually nowhere else other than at or near the stern.
It sounds like your centerboard is pretty well protected in terms of coating. You should bear in mind though that what appears well coated to the human eye may not be well coated to the indefatigable corrosion process. Your first clue may be bubbling of the coating or it may just work away beneath with little outward signs. The easiest way to determine if these factors are present is to mount a small zinc and see how long it lasts. Further action, or inaction, can be based on your results.
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post #7 of 9 Old 01-13-2007 Thread Starter
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Sailaway & Fstbttms- Thanks for your input. I was more concerned about galvanic corrosion between the copper bottom paint and the 300 pound iron centerboard.
Sailaway21- I've seen that type of corrosion on props. It usually is in the area of the prop that has a lot of turbulence and cavatation. I don't think I would have enough turbulence and more importantly, time in turbulence to take a toll on the centerboard.
Fastbttms - I'll take your advise and just look for signs of pitting from galvanic corrosion before I get too concerned.

Wayne
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post #8 of 9 Old 01-15-2007
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Quote:
Originally Posted by sailaway21
Zincs are always placed where turbulence is greatest. Hence they are found on rudder posts, the rudder blade, and in the area of the screw.
If you're suggesting that the reason we use sacrificial zinc anodes on pleasure craft is to prevent turbulence-induced corrosion, I'm gonna have to disagree. We put zincs on the above-named items because these are generally the only large metal parts on a fiberglass or wooden boat that are exposed to saltwater and the two main kinds of corrosion associated with it, galvanic and electrolytic corrosion.

Quote:
Originally Posted by sailaway21
On a steel hulled vessel they are found virtually nowhere else other than at or near the stern.
This statement is simply not true. Large steel-hulled vessels will typically have anodes welded or bolted at intervals along the hull and keel. The larger the vessel, the more anodes it will use. Many fiberglass sailboats have zincs attached in recesses along the hull as well. Again, not to protect anything from turbulence, but from corrosion associated with electrical current.

Last edited by Fstbttms; 01-15-2007 at 03:17 AM.
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post #9 of 9 Old 01-15-2007
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Quote:
Originally Posted by Wayne25
Sailaway & Fstbttms- Thanks for your input. I was more concerned about galvanic corrosion between the copper bottom paint and the 300 pound iron centerboard.
You are far more likely to experience rust on your iron centerboard than any corrosion caused by the anti fouling.
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