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thinwater · 14-05-2015, 18:17

It's spring any everyone is rushing around to replace tired anodes. I saw a couple boats in the yard that were clearly the other way; damage due to over protection.

Examples?

scuba0_1 · 14-05-2015, 19:12

I remembered reading this.

There is no accurate means of controlling the anode's effectiveness. Too little protection results in corrosion. Too much protection can result in caustic attack to wood, a loss of anti-fouling paint or other underwater coatings on metal fittings, caustic corrosion on aluminum boats, and the embrittlement of high strength steel.

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senormechanico · 14-05-2015, 20:16

Four years ago, after installing a big zinc on my bonding system and thinking it would last a long time.
A year later I found out differently.
The zinc was almost gone and my paint was burned around the thruhulls.
(The shaft zinc in the photo is insulated from the bonding system with a flexible shaft coupling AND an insulated Volvo transmission)
The second photo shows that burnt paint from uncontrolled zinc current in the bonding system from that big zinc.
There was similar paint damage around the other thruhulls as well.

After that, I built a controller for my bonding system with some help from other electronic guys on the 'net and installed a silver electrode consisting of a pre 1965 dime in the centerboard trunk.
I set the bonding system voltage to -0.600v with respect to the dime reference electrode.
The circuit automatically readjusts the zinc current about once a minute to hold that setting.

It's working great, the prop, thruhulls and 3 year old paint are all in good shape.
Although the shaft zinc (uncontrolled by the circuit) only lasts one year, the 3 year old bonding system zinc is still good for another year.

chris95040 · 14-05-2015, 20:37

I spent some time googling this a while back- other than wasting money and deteriorating wood somehow, it seems you really can't have 'too much zinc'. At least that was my understanding.

The voltage created by a 'battery' of dissimilar metals in an electrolyte is a function of the galvanic difference between the metals, NOT the quantity of metal. For this reason, more zinc won't lead to more voltage in whatever cuckoo circuits it's a part of, so it won't lead to more current flow, so it won't lead to more galvanic corrosion.

skipgundlach · 15-05-2015, 07:27

Following - we just put two monster groupers over the side due to unseemly consumption of our MaxProp zinc...

fstbttms · 15-05-2015, 07:37

Over-zincing a fiberglass boat is not an issue, in my experience. When anti fouling paint suffers what we call "burn-back", it typically occurs because it is in contact with improperly primed metals, like thru-hulls.

crazyoldboatguy · 15-05-2015, 07:45

Quote:

Originally Posted by fstbttms

Over-zincing a fiberglass boat is not an issue, in my experience. When anti fouling paint suffers what we call "burn-back", it typically occurs because it is in contact with improperly primed metals, like thru-hulls.

Being a fresh water sailor I am not familiar with these issues on the scale salt water sailors are. However, I plan on moving saltward in the near future so I must get up on the learning curve.

I have a wooden boat which for most purposes, could be considered cold-molded. Altho carvel planked, it has splines rather than caulking in the seams. Because of that, the hull will be sealed from outside water incursion with a full recommended coating of Interlux 2000e. I may well use marelon thru hulls to avoid any galvanic issues but am curious about your statement about "improperly primed metals, like thru-hulls". Does that mean you recommend painting metallic thru hull fittings?

fstbttms · 15-05-2015, 08:00

Quote:

Originally Posted by crazyoldboatguy

Does that mean you recommend painting metallic thru hull fittings?

I have seen many boats with unpainted thru-hulls that have no paint issues whatsoever. I am not enough of an expert to say why some boats will suffer burn-back and some won't. I suspect it has more to do with the copper content of the paint than anything else. The more copper, the more chance for a reaction with metals. That said, in general I think it's a good idea to paint thru-hulls and of course, to properly prime them before doing so.

BTW- over-zincing is absolutely a concern with wooden boats.

crazyoldboatguy · 15-05-2015, 08:33

Quote:

Originally Posted by fstbttms

I have seen many boats with unpainted thru-hulls that have no paint issues whatsoever. I am not enough of an expert to say why some boats will suffer burn-back and some won't. I suspect it has more to do with the copper content of the paint than anything else. The more copper, the more chance for a reaction with metals. That said, in general I think it's a good idea to paint thru-hulls and of course, to properly prime them before doing so.

BTW- over-zincing is absolutely a concern with wooden boats.

Thanks - that's a help.

It is a bit baffling - if the paint has a high content of copper, and the thru-hulls are copper based bronze, I would think it would be compatible. I understand there are many variables involved. I have some of the original bronze thru-hulls and some new ones that came with the boat. I may well sell those and put all marelon fittings - just to avoid the issue altogether. One zinc on the prop shaft and address the rudder fittings as well.

fstbttms · 15-05-2015, 08:40

Quote:

Originally Posted by crazyoldboatguy

It is a bit baffling - if the paint has a high content of copper, and the thru-hulls are copper based bronze, I would think it would be compatible.

While certainly not proof, I do see burn-back far more frequently with high copper hard paints than with other paints. Black Pettit Trinidad comes to mind. Can't ever remember seeing it with a white paint, for instance (which will usually have the lowest copper content in a given product line.)

thinwater · 15-05-2015, 08:46

The case that got my attention this spring was an aluminum boat with mag anodes; they have a higher potential and it had blown the paint off the back 1/3 of the boat and started some serious pitting. The boat had been in fresh water the prior year.

fstbttms · 15-05-2015, 08:48

Quote:

Originally Posted by thinwater

The case that got my attention this spring was an aluminum boat with mag anodes; they have a higher potential and it had blown the paint off the back 1/3 of the boat and started some serious pitting. The boat had been in fresh water the prior year.

What anti fouling paint was on the boat?

geoffr · 15-05-2015, 09:07

Following as we are currently replacing all our anodes... (Main, outdrive/prop, Rudders).

Would the hard anti-fouling missing around our SSB ground plate be related to anodes??

donradcliffe · 15-05-2015, 09:47

This year I did the bottom of the YC race committee boat, and there was quite a bit of 'burn-back around one thru-hull. I disconnected the bond on the thru-hull and will report on the results next time the boat is hauled.

The missing bottom paint around the SSB ground plate may also be due to connecting the SSB ground to other boat grounds.