What Do You Zinc ?

[Mark Johnson]

Originally Posted by GordMay

Wrong.
It has everything to do with the electro-chemical issues related to surface area & mass (both related to volume), as well as the peripheral mechanical issues you mention.

I hate to disagree with you Gord, I very seldom do, but in this case... My Zinc would protect my shaft and strut equally well at half it's original size, VS new. I only change them out at "half way gone" for the reasons I listed. (Less contact, less strength, and high likelihood of failure).

I would readily agree that one can be "over Zinced", or "under Zinced" in the first place, however, if one has just the right amount...

In my case it was determined from over 15 years experimentation, with several Zinc volume changes, as well as testing with galvanic testing equipment, (which I did as an ABYC member according to their procedures). If you have the right amount originally, then when your shaft Zinc has lost half of it's mass, it will be going away "faster", but still protect the metal if it is solidly in contact. It is "close enough" in volume.

The primary issues remain... At this point it's ready to slip, fly off, or has lost partial contact, in which case it is NOT protecting as well, for that reason. Regardless, this is why conventional wisdom, and my personal experience as well, is to change shaft Zincs at about half way consumed. If it is winter, and you are not using the boat, it is usually safe to go longer, just start new in the spring.
M.

[Wand]

Originally Posted by doug86

First you said that you always waited until it was 90% gone, now you admit you never had one to begin with. How can your 30 years of not using a shaft anode possibly have any bearing whatever on the issue of shaft anode erosion?

If you are going to use quotes, at least be accurate. I never used the words mere and unreliable

It's true that you did not used those exact words, which were that the recommended zinc changeover time "is based on chemistry and scientific facts, not anecdotal beliefs". If you don't think my paraphrasing as a true and accurate reflection of your views on anecdotal evidence, maybe you could tell us then just how you do regard it?

The 90% gone policy I was talking about referred to the anode protecting the thru-hull area. I intend soon to replace it with a non-metallic fitting, isolate the engine and shaft and all related gear from the hull, and ensure that that my electrics are properly grounded to the negative pole, and will then dispense with anodes altogether. Go on, say I'm nuts

[Wand]

Originally Posted by Mark Johnson

I hate to disagree with you Gord, I very seldom do, but in this case... My Zinc would protect my shaft and strut equally well at half it's original size, VS new. I only change them out at "half way gone" for the reasons I listed. (Less contact, less strength, and high likelihood of failure).

I would readily agree that one can be "over Zinced", or "under Zinced" in the first place, however, if one has just the right amount...

In my case it was determined from over 15 years experimentation, with several Zinc volume changes, as well as testing with galvanic testing equipment, (which I did as an ABYC member according to their procedures). If you have the right amount originally, then when your shaft Zinc has lost half of it's mass, it will be going away "faster", but still protect the metal if it is solidly in contact. It is "close enough" in volume.

The primary issues remain... At this point it's ready to slip, fly off, or has lost partial contact, in which case it is NOT protecting as well, for that reason. Regardless, this is why conventional wisdom, and my personal experience as well, is to change shaft Zincs at about half way consumed. If it is winter, and you are not using the boat, it is usually safe to go longer, just start new in the spring.
M.

Thanks for this post Mark and the earlier one. You've clearly explained the reasoning supporting your views on zinc changeover and shown how your experience over the years has informed those views. It's this sort of information drawn from careful observation that makes this board so useful.

Obviously my views are different regarding the changeover times but I absolutely agree that the electrical connection issue is the key. I do clean and tighten the zincs with every haulout (so they don't fall off!) but will from now on take a closer interest in the actual connection points.

BTW, for the record, the 90% erosion point I mentioned was using a bit of poetic licence for purposes of illustration of the point. While I have gone that far without problems, they're usually about 3/4 gone when I replace.

[Wand]

Originally Posted by lancelot9898

A surveyor once told me that you could over zinc a boat. I had trouble following that claim thinking that the more zinc you used the better the protection, but he explained it by saying that the larger quantity of material would cover a larger area of water and thus more ions flowing. The protection would still be there, but why put more than you need. No explaination as to how you determine the optimum amount of zinc to use.

This query seems to have been overlooked in the flurry that followed. I guess the answer is, 'it depends'.

Because that's just the nature of investigations in boating, as it is in most areas of genuine enquiry (at least for those who resist the 'right and wrong' approach to their investigations).

But one rule of thumb is a pound of zinc for every foot of waterline. Another rule of thumb is to measure the square metres of underwater hull area and divide by 3.43, with the answer being the kilos of zinc required. If you want a more technical approach, you'll need a little bit of pure silver and a multimetre to measure voltage between the different underwater components.

The rule of thumbs are adequate for me - I don't think things are that critical. But the voltage testing is fun and aids the undersanding of what is really going on.

My two cents...

[SPCarroll]

My question is: What do you need to Zinc on a fiberglass boat that's going to be in fresh water? I have an inboard engine, and it looks like there are Zincs on the rudder, and one for the prop shaft.
Also I have what looks like a grounding plate for lightning. I am putting on a different mast on the boat, how would I go about hooking up the grounding plate?

[Lancerbye]

Originally Posted by SPCarroll

My question is: What do you need to Zinc on a fiberglass boat that's going to be in fresh water? I have an inboard engine, and it looks like there are Zincs on the rudder, and one for the prop shaft.
Also I have what looks like a grounding plate for lightning. I am putting on a different mast on the boat, how would I go about hooking up the grounding plate?

You don't need zincs on a fresh water boat. It is the conductivity of salt water that allows electrolosis. As for your grounding plate. Are you sure its for a lightning dispersal system. Could have been (most likely) for an SSB radio. Your boat must have been a salt water cruiser at one time.

[Sandero]

Seems like the corrosion of a zinc is directly related to its surface area and the electrical environment which can change depending on where the boat is. I had a zinc on for exactly 10 months and it came off about half the weight... I am going to weigh it and a new zinc and report the amount of loss. The boat was 5 months in a mooring field when not sailed and a 5 months in a marina in water storage in salt water where the shore power was connected and there are live aboards withing 50 feet. The zinc was a 30mm shaft donut supplied from Beneteau. The boat is 26 years old and the bronze thru hull show no pink and the ss shaft shows no signs of corrosion. '

Whether or not my metal is being protected by the zinc remains to be seen... but it is disappearing bit by bit... month by month. I'll add some photos and data later.

[gbendaly]

hi everyone
aside from a 2 part shaft zinc connected to absolutely nothing the underwater profile of Kavala is free from any metallic protrusions, yes I have an islolator on board and I am about to change the zinc in june after two years of loyal services.can you beleive it ?
George

[Pete7]

Originally Posted by Lancerbye

You don't need zincs on a fresh water boat.

True, but you should fit magnesium instead for fresh water

Cathodic Protection - Marine Cathodic Protection - MGDUFF

Pete

[Sandero]

Here is the result:
donut zinc - 30 mm shaft

10 months in Long Island Sound - July1 - May 1
On Mooring from July 1 - Nov 1
In marina from Nov 1 - May 1

weight of new zinc = 13.25 oz.
weight of eroded zinc = 7.875 oz.

.54 oz / mo. of loss

good, bad or what?

[GordMay]

Originally Posted by defjef

... good, bad or what?

Roughly 1/2 eroded per year, which is "about" right.

[aboutgone]

And zinc's do nothing on the shaft in the engine room!!!!!

[Sandero]

duhhhhhhhhhhhhhhhhhhhh

[aboutgone]

Was just making reference to the pic in one of the first post......Collar zinc on the shaft next to the stuffing box.....That's all

[capnorv]

Originally Posted by aboutgone

And zinc's do nothing on the shaft in the engine room!!!!!

True, but I keep one there to keep the shaft in the hole just in case the coupling should let go. $10 seems cheap for insurance. Granted the key might do the same, or ruin the log. Also a good place to keep a spare or two.