How do I zinc properly?

[SFS]

I've had my 1983 H31 on the hard for two weeks, getting a bottom job and a strut/shaft replacement. The strut is being fabricated in stainless by a local prop shop (a well known, well respected shop). The strut that failed was the victim of extreme electrolytic corrosion, and the shaft is well pitted too. (Got an error message trying to post pics, but I'll close the browser and try again.)

PO has said that zincs were being eaten quickly, and other nearby boats had similar problems. The problem may or may not have been a particular nearby boat that has not been attended to in some time. That doesn't matter now, as I've moved the boat to another marina, where my dock neighbors say stray current is not an issue.

I've paid the yard to check the bonding on the boat, and they tell me that everything seems to be well bonded. However, they also said that the broken strut they took out was bonded, and it seems as though that didn't help. I'll have them check the cable before using it to bond the new strut.

So, with my newly barrier-coated and painted iron keel, my new stainless shaft, and my new stainless strut, where should I be placing zincs? I've read the threads here, and am sort of confused. Part of me wants a zinc on the keel, one on the shaft, and one on the strut. Would that mean the boat would be "overzinced". Is there a consensus on whether "overzinced" is really possible?

 

[fstbttms]

It is, in theory, possible to be "overzinc'd" but the arrangement you have in mind is not nearly enough to cause it. I would recommend one or two anodes on the shaft, one on the strut (but only because the boat has a history of corrosion there, otherwise I'd say "If it ain't broke, don't fix it.") As for the keel, don't waste your time or money, IMHO. You would have to mount a very large anode on the keel to protect that mass of metal from corrosion and typically, rust is the main concern there anyway. Zinc anodes will not prevent rust.

 

[Charles Erwin]

Overzincing is not a problem in your case . Your hull is not wood, aluminum, or steel. In those cases an active galvanic impressed or monitored current system is the solution but such measures are quite unnecessary in your case.

As to zincs - see to it the rudder shaft, strut, and so forth are connected to a zinc. Bonding itself is not going to help in your case unless the bonding circuit ends in an active and clean zinc. Obviously, the propeller shaft/propeller assembly needs to have zinc protection same as your other underwater metals. No reason why the keel should not have its own zincs either.

As to bonding - one should never feel comfortable when somebody says 'It is OK.' This might work assuming they are willing to tell you what things they considered in order to conclude the bonding system is 'OK.' But absent knowing what exactly was examined it is impossible to know whether the bonding system is actually 'OK.'

This is a guess but based on what you say it is more likely something is wrong on your boat - and not as likely there is some external gremlin at work. However if you do not have galvanic isolation then either get it or disconnect the shore cord altogether.

To the point - if any part of the bonding circuit could conduct any DC- current it is NOT OK. More directly to the point - unless there was a hull potential assessment done - which requires the boat is in the water - together with a logical process of things turned off and on - the conclusion "It is OK' is completely unreliable.

Get someone who knows the score to make an independent analysis of every underwater fitting, including struts, shafts, posts, and the bonding system, with shore power engaged and disengaged before you rely on any conclusion.

ABYC corrosion trained people should be easy to find where your boat is. Use them.

Charles

 

[Ted]

How do I zinc properly

A bit off topic but a friend of mine had the strut on his Hunter 31 break too. This was not caused by a wrapped line or such. I wonder if this could have been a design or manufacturing defect. His strut did look a bit fragile compared to similarly sized boats in the yard.

 

[SFS]

Well Charles, your advice is well founded. Further questioning in the yard today (as we get closer to splashing the boat) revealed that the project foreman was just using "the bonding is ok" to put me off, as it had not yet been checked. (Why not just say so?) When the tech actually started looking around yesterday, he found that there is virtually no bonding in place. I elected to not put any more money into this yard's pocket, and will follow up with someone else, an AYBC accredited corrosion specialist (this yard is supposedly an AYBC yard).

Does anyone have anyone they would recommend in the Tampa area?

 

[JamesG161]

Charles...
I am new to the forum, but you are the first I have read who KNOWS about galvanic corrosion.

Simply put...
If a Zinc anode is to "sacrifice itself" it must be in "electrical connection/galvanic current must flow" with the metal to protect.

Bonding is not intended for galvanic corrosion protection, but to assure all electrical gear is at same ground potential.

Although Stainless Steel sounds good, it is not always the best for submerged chloride service. The grade of Stainless is key.
Jim...

 

[Charles Erwin]

Right. However, the least noble member of the bonded string will sacrifice to protect the rest just the same as a zinc anode bolted (bonded) tight to the shaft sacrifices to protect the shaft/propeller . But if you end the bonded group with a zinc (the zinc is automatically the least noble) it sacrifices to protect all the other metal items in the string.

An added benefit with well maintained bonding (even if the string does not end with a zinc) is that bonding will trap stray current, For example, a wire exposed to bilge water that allows DC current to pass to a sea-cock and return via the shaft is a very not good condition. Bonding the sea-cock prevents stray current damage in that case.

There are also plenty of reasons to discard/remove bonding cables all together. One should probably never include a sail drive in a bonded group. And there are other exceptions and conditions that need to be considered because every boat is different. .

So there is every reason to consult with a person who has gone to the trouble of getting the training to understand this stuff.

Charles

 

[MitchM]

your local US power Squadron teaches a course, ME 101 Marine Electrics which has an entire chapter on bonding and galvanic corrosion. the course manual is a great resource on how to install or maintain bonding systems and zincs.

 

[Maine Sail]

I elected to not put any more money into this yard's pocket, and will follow up with someone else, an AYBC accredited corrosion specialist (this yard is supposedly an AYBC yard).

Does anyone have anyone they would recommend in the Tampa area?

A yard can't be an ABYC certified technician. A yard can be an ABYC member but the individuals who work there are the ones who carry any ABYC certifications and who have actually taken the courses and passed the exam. Ask if thay have any marine technician employed that is ABYC Electrical or ABYC Corrosion Certified. Often times only a few workers will be certified in "general systems" but you are best to find a specialist who does only electrical etc... An ABYC corrosion tech will have been trained in ABYC standards E-2, A-28, A-31, E-10, E-11, T-1, TA-27 and TE-30. There are many good electricians out there who are also good at corrosion but the best way to be sure is to ask for an ABYC electrical or corrosion certification.

 

[Maine Sail]

An added benefit with well maintained bonding (even if the string does not end with a zinc) is that bonding will trap stray current, For example, a wire exposed to bilge water that allows DC current to pass to a sea-cock and return via the shaft is a very not good condition. Bonding the sea-cock prevents stray current damage in that case.

Bonding with a DC leak only serves to give you more metal mass and spread the load, if you will. This serves to slow the corrosion process but does not protect against a DC leak. DC leaks are very, very dangerous regardless of bonding or no bonding.

A bonded boat can survive a DC leak for a bit longer but bonding also has drawbacks... Stan Honey has written extensively on this. Stan is both a Yale and Stanford educated EE and the the guy who put first down lines on the football field (Sportvision) and into TV sports including the Americas Cup & NASCAR and is regarded as perhaps the worlds best ocean sailboat racing navigator. He's no slouch and anyone keen on corrosion would benefit from reading his thoughts on bonding. I speak with Stan a few times per year and worked with him to design the LiFePO4 bank on his personal boat, a Cal 40, and also discussed the LiFePO4 system on Comanche with him when he was working with Hodgdon Yachts here in Maine on the construction of her. I have not bonded my own boats, since before knowing Stan, but he really pushed the envelope of thought on bonding..

As you and I have said many times before any decisions to change your vessel with regard to corrosion should be done with proper testing not just a shoot from the hip approach..