'Zinc' Analysis II

[Knot Salted]

Greetings.

I am diverging too far from the original questions here, so starting a new (old subject I am sure..) page.

When we bought her, Knot Salted had only one large zinc plate anode on the transom to which all water contacting metal on the boat including seacocks, through hulls, Struts, rudders, engines and trannys were bonded. Not much erosion of the plate over 6 or more seasons including some with the PO, and no obvious erosion of any components visible. She has always been in fresh water.

We have had a few new neighbors at the dock, but no changes over the last two seasons. Last year I went to magnesium anodes, replacing the transom zinc, and adding mags to trim tabs and prop shafts. Rudders are bonded to the transom anode, and no mags were added to them directly.

My reasoning for the swap and additions was reading about zinc being of little real protection in fresh water, magnesium being a better choice, and wanting more coverage - just in-case of a change at the marina.

After one season, all the mag anodes are really chewed up. -I mean they will bite you if you grab them. The bulk of each of them is still present. At the end of this season, I will need to replace all for sure. Of course Mag is much more reactive, but it is still very surprising.

Have I created electrolysis? All mag anodes are mounted on directly bonded parts, except for the new prop shafts mags.

All anodes are Magnesium. No Zinc. Maybe this is my new normal?

 

[Comodave]

Yes, you are now actually protecting your underwater metal. Zinc tends to get a coating in fresh water that will stop it from eroding. That is probably what you were seeing with no wastage of the zinc anode. Now with magnesium you are seeing normal erosion of the anode. We are in fresh water also and use aluminum anodes since I cannot get my hull anodes in magnesium. We see about 40 to 50% wastage in a season here. My boat is docked behind our house with no other boats within 200’. So we don’t have other boats causing our anode erosion.

 

[Knot Salted]

Good to know. I am glad we do not seem to have damage for such long exposure...

I was wondering about going to aluminum. Still protective in a hostile environment, but not going for nothing like these mags seem to be doing.

Thoughts?

...and I do want to get over there to see that President!

 

[Comodave]

I would have gone with magnesium if there was one that would fit on my hull mounts. I didn’t want to drill new holes to fit what was available in magnesium. I do use magnesium in my heat exchangers since I could not find aluminum anodes in that size. It is ok to use different type of anodes in the heat exchangers and on the hull since they are considered different bodies of water. Aluminum anodes can also be used in salt water but not magnesium.

Anytime you are this way you are more than welcome to stop and see my baby. I am always ready to talk boats. While we don’t get over to the sunset side often, I would also like to see your baby if we do get over there.

 

[Knot Salted]

Sounds good Dave,

I did recall reading on the zinc for salt, and Al and Mg for fresh, but had forgotten that zinc becomes somewhat sealed in fresh water if not in an 'electrically' active environment.

Thanks for that!

 

[Flybull]

Have there been any changes at all made to the electrical/electronics in the boat coincidental of the new anodes...?

 

[Knot Salted]

No, nothing new.
Good question though.