My Zinc was eatten

[Bad Obsession]

I dove on my boat for the first time this season. I left it in the water over the winter and I am going to haul out in Late June to do the bottom. When I was under I checked my Zinc. It was almost gone. It was well over 1/2 the way gone!! The boat has only been in the water for a year and I am surprised how fast the Zinc went. I was able to put a new zinc on while I was cleaning the boat bottom (it is really fun to do without scuba).I am wondering how my zinc dissolved that fast. My Uncle's boat has been in the water the same amount of time that mine has been in the water but he is on a different dock (his zinc was fine). The only thing that I can think of is that the people on my dock let their electric cords dangle in the water and I am constantly picking them up only to find them back in the water later on. Does anyone else have an idea why my Zinc may have dissolved so fast?

 

[Bill]

normal

That's actually pretty normal- in fact, not too bad. Stray ac current is generally not the sole culprit, it's usually poorly grounded DC (Nigel Calder has a great explination of it). The power cords in the water are not the issue.You can do a couple of things- Get one of the fish zincs that hooks to the rigging and the fish goes in the water or double zinc your prop shaft; get a galvanic isolator; have a thorough check or your boat's electrical system (you are as likely a culprit as anyone else).On a safety note, only dive the boat at anchor- Diving in a marina is asking to get electricuted.Seriously, though- if you only have one zinc on your prop shaft, and ANY of it is still there after a year- it's a pretty good situation overall.B

 

[Don S/V ILLusion]

wish mine lasted that long

I winter in the water biannually and when we short haul, there is nothing left of either zinc so you're not doing too badly. If you plug into shore power continuously, it's likely the AC circuit which can be rectified by adding a galvanic isolator.

 

[Warren Milberg]

Don Casey's reply...

.. to my question about this same problem:"Current flowing to your boat from an outside source would indeed eat away your anodes, but this requires a complete circuit, meaning your boat would need to be plugged in to the shore circuit. A more likely scenario is that your shaft is connected to the ground side of your electrical system through the ground lug on your engine, and somewhere in your electrical system is a small ground-fault that is energizing the shaft. This assumes that you feel confident that the anodes are not sacrificing themselves simply due to dissimilar metals in the water, given that your shaft is bronze rather than a steel alloy. There is also the issue of what alloy these anodes are compared to the one you had installed last year. Not all anodes are the same on the galvanic scale, which will affect how long they last.This seems like a case where you should check all of your onboard circuits for leaking current. You can find instructions for doing that in Sailboat Electrics Simplified. If this effort does not reveal a current leak, then the recommended treatment is what doctors now call watchful waiting. Put on fresh zincs and keep an eye on them."

 

[Fred]

While it's true that usually a ground fault

is why your zincs go away, and a good marine type battery charger or inverter and clean grounding should make zincs last a while,There is such a thing as a hot dock, even when things are set up right.when I was with the Port of Port Townsend, Wa. we had one dock where zincs and props got eaten too fast. Owners would swap slips to one of the other docks and not have a problem.After a couple of years of consultation with some pretty smart folks, who were mostly baffled, One guy figured it out.Each dock has its own transformer, where 440 volts is stepped down to 220 volts, which is sent down the dock and split in boxes to provide 110 outlets.Since the current cycles at 60 cycles a second, if one the 440 from transformers is not synchronised, the cycles can be opposite on different docks. Our hot dock was on the opposite cyle from the rest of the docks.When we had Puget Power check and swap the offending outputs, the problem went away.

 

[Victor]

Permanent Zinc

A zinc lasting a long time is not necessarily a good thing. Several years ago, my neighbor at the marina was in the water replacing his zinc. He offered to do mine too and I accepted. That zinc is still there in perfect condition years later after several other zincs have come and gone. I believe he did not clean the shaft adequately, the zinc he installed is not electrically connected to anything, and will remain as a permanent decoration. If it sounds too good to be true, maybe it is.

 

[Jim Seamans]

I was out in San Fran last week and heard this

Two power boats, one was a friend was having electrical problems where thru hulls were getting pitted. The marina said they were over zinced(sp). I have never heard of having to many zincs. I felt the marina was giving them a load of you know what.Has anyone else?JimS/V Java

 

[Dan Johnson]

Jim...

I've never heard of being "over zinc'd" and still have pitting of metal thru-hulls. Zinc is used because it is a metal of low "nobility" compared to the expensive stuff (bronze, stainless, aluminum outdrives, etc.). The more exposed zinc surface would spread the electrolisis over a greater area compared to just a single point such as on the prop shaft. If it's being eaten at a "fast" rate, add a second one to the prop shaft. As a matter of policy, I unplug the shore power when leaving the boat each weekend. Two years seems to be the average life of my shaft zinc (1/4 to 1/3 eaten) in our marina.

 

[Fred]

I don't know of a problem with too much

zinc on a glass boat, but wood boats can get "galvanic corrosion" from too much zinc, especially in Oak. I've seen it a couple of times on older Engiish boats, once in the stem, and once in butt blocks around a through hull where the problem had spread to planks and frames. In both cases there was a white powder on the surface of the wood like you see on battery terminals sometimes. The wood looked sort of petrified. It was hard and very brittle. No strength left in it.On a glass boat, problems would be with "everything else", not the glass.I sure like my plastic through hulls all above the water line and outboard that lifts clear of the water when the subject of electrolysis come up. Oops, forgot about the rudder, keel pin, ballast tank plug, trsnsducer........