Zinc in fresh water ?

[Mo]

What Hanley said, especially if you are on a marina that has power. You never know what you are docked next to. We have an area of our marina that extends toward the Navy Jetty...the boats closer to that have their zincs eaten more the closer they are docked in relation to that Jetty. It has power rigged out there everywhere for any conceivable requirement.

[Ball Racing]

In salt water you use "zincs", fresh and brackish you use aluminum, and magnesium.
Right????

[hanleyclifford]

In salt water you use "zincs", fresh and brackish you use aluminum, and magnesium.
Right????

Interesting notion and I have read that somewhere else, too. What intrigues me is that zinc lies between magnesium and aluminum near the bottom (anodic) end of the series chart; so how come they are both better than zinc in fresh water?

[Ball Racing]

I read that zinc in fresh water forms a "skin" over it as a first reaction,
therefore it can no longer erode away. Making it no longer the sacrificing metal.
The others types seem to not form a protective skin over themselves.
But I have no hands on data to show this as proof positive.

Daniel

[Mo]

hmmm

Interesting. Can anyone explain to me why the zincs near (what I think is electricity) don't last as long...I just heard it happens but not sure why.

[roadnsky]

I'm in fresh water.
My zinc is 6 years old and looks almost new.
I'm also on Shore Power.

Just lucky?

[edwardc]

Interesting. Can anyone explain to me why the zincs near (what I think is electricity) don't last as long...I just heard it happens but not sure why.

Mo,

One cause of stray currents is from other boats nearby that are not properly protected. This is an excerpt from my Galvanic Isolator project on my website (http://www.chessie.com/boat/projects.shtml) that explains it and what to do about it:

============

...Soner or later, every boat owner gets introduced to the galvanic corrosion that happens when you put two dissimiliar metals that are electrically connected into saltwater. You quickly learn to install and maintain an appropriate number of zincs to protect your valuable underwater hardware, such as through-hulls and prop shafts.

What some don't know, however, is that it's possible to do everything right and still have galvanic corrosion! The problem happens when you're connected to shore power. Even if all your A/C devices, like your battery charger, are transformer isolated, there's still the safety ground. This is connected directly to your boat's system ground, to which all your underwater hardware is bonded. As a result your boat's ground, through the marina's wiring, is now connected to every other boats grounds and hardware! And if any of them aren't properly protected, they'll cause a galvanic current that'll start eating your zincs! This uses them up too quickly, and then least noble of your hardware starts going.

The obvious solution is to disconnect the safety ground. But this is a serious safety hazard unless you are absolutly sure that everything is transformer isolated, and remains that way. So, what we really want, is something in series with the safety ground that doesn't conduct most of the time, but will conduct the full service current in case of a short.

It turns out that there is such a device. It depends on the fact that virtually all galvanic circuits are driven by only a few tenths of a volt. A silicon diode drops about 0.6 volt. Below this, it doesnt conduct at all. Above this, it conducts heavily. Since 0.6 volt isn't really enough, we put two in series for 1.2 volt. And since diodes only conduct in one direction, we put another pair in parallel in the opposite direction.

This is basis of a galvanic isolator. Real-world devices are more complicated, including things like fail-safe, A/C leakage bypass, and monitoring. But at their heart, there are four diodes. ...

[Mo]

Excellent explanation

Thanks Edward,

I got it...very well presented. I looked at your site....nice job!!

[Laker]

Thanks , gang. My shaft is SS . I have very little exposed shaft. (Raised Catholic , very modest.) I am in a marina. I generally do not keep the shore power rigged unless there is a specific need ; power boats either side of me are on all the time. Prior to my 7-8 year renovation layup which ended last year I know I had wiring/grounding problems for a variety of reasons , including the unpleasant experience of nearly being put on my ass when brushing my arm against the centerboard pennant! That has not happened since re-wiring. As previously stated , my old prop is in good shape , as is the bronze intake strainer which I removed , sand blasted (walnut shells , not glass beads) and re-installed recently.

Soon the yard will fill with laid up boats up here at 43 dg north , and will take a tour to see which sailboats have zincs and where they are located.

Parting question : with no space available on the prop shaft , what is a good location for zincs on a sailing hull.

All in the interest of preventing electrogalvonics...

[hanleyclifford]

This is the best zinc solution I have found.

[edwardc]

... My shaft is SS . I have very little exposed shaft. (Raised Catholic , very modest.) ...

Almost choked, I laughed so hard!

[sastanley]

Hanley, is that the highly prized "Dave Neptune prop"?

I can't imagine 2,500 RPM in gear with an A4!

[hanleyclifford]

That is indeed the Dave Neptune prop. 2500 rpm was easy so I know I have to get it repitched.

[Mo]

Hanley, while I was fixing the steering on my boat a couple of weeks ago I found another 3 bladed prop under my gas tank. Took it home to my garage and put the grinding wheel to it....can't see markings...any ideas where they should be except on the blade like my last one.

[sastanley]

Mo, I found mine on the hub face..like where the prop nut would contact it.