Zinc Life

[Tom Potter]

We've owned our 28 since August. We paid a diver to clean the bottom last weekend. During his bottom cleaning he informed me that the 2 zincs on the shaft needed replacing. (They were new 4 months ago) He brought up the old ones and sure enough they were in desperate need of changing. One of the two bolts on each zinc was compromised. So my question is what kind of life do you get out of your zinc's?

 

[RAD]

I get a season April to Nov out of one zinc on the shaft....Just checked today cause the boat got hauled and there seems to be a little life left in it, this is the first season that I had one as previous years I had two and it seemed that they both would wear evenly and have a lot of life in them but I wouldn't leave them on in fear they would wear out before the following season ended
Now if your saying you went through two zincs since August then I would say get a good boat electrician and check for electric leakage either from your boat or your neighbors,another just in case would be an extra zinc hung over the side and connected to your boat grounding system if the problem is from the marina

 

[Tom Potter]

I pretty much think I have a problem somewhere. I would guess that 50% of both zincs were gone. I'm plugged in 24/7 at my dock no other boats around. I run my newly installed Xantrex Truecharger 2 and just recently a west marine air dryer.

The boat is new to me, looks as if I need to start looking around and see if I can find the problem.

 

[Tom Potter]

So after making my last post I went down to the boat and poked around a little. I think I found the problem. The bonding wire that ties all the thru-hulls together has a wire that also goes to the engine block. The connection that mounts to the engine block is broken and not making contact. I hope that by making this repair it will solve the issue. I will find out in March, that's when the diver is scheduled to return.

 

[jibes138]

Galvanic Isolator?

You may want to consider installing a galvanic isolator. I just dove on my boat and after two years I have averaged one zinc on the shaft per year. The ground wire shouldn't be your root cause. I disconnect my boat completely from the marine power system when I am not using the boat. I think leaving it plugged in all the time may be your problem. Do you live near Belhaven and can check on it easily? I'm about 80 miles from Beaufort so I have more piece of mind with the power off while I'm not around. I don't have an isolator installed but have one in a box ready to go.

 

[Tom Potter]

Thanks Dave,
I read up on the galvanic isolators, and have it on my list as a back up if the broken bonding wire fix don't correct the issue.

There is somewhat better news (I think) than I first thought. I discovered the zincs were not 4 months old, but one was 10, months the other 7 months. Although very worn they both were still connected to the shaft. This being my first boat with zinc's I'm unfamiliar as to the wear times.

My thoughts on the broken bonding wire to the engine is this. My understanding is when you have two metals in the water, that it creates current between them. The zinc being the sacrificial metal gives itself up to be eaten so that your shaft/prop and thruhulls don't suffer. I was thinking that since the bonding wire that connects all the bronze thruhulls together with the engine block was broken that I essentially had two separate metals in the water, the bronze thruhulls bonded together and then the engine shaft. My thoughts were that this could be what was causing the zincs to be eating up faster than normal (keeping in mind I'm not sure what normal is). I figured that when I fixed the broken bonding wire it would tie everything together thus giving me essentially only one metal in the water and would slow the wear of the zincs.

Is my thinking way off with this??

Tom

 

[jibes138]

Galvanic corrosion

The thru hulls and the zinc are separated by a very large distance. What you are somewhat describing is an anodic plating process where and anode sacrifices atoms under an electric current to the cathode. Whil this may in fact be happening it would be at an extremely low rate due to the very small current,voltabe potential and enormous distance. This is a much bigger concern when two dissimilar metals are installed and actually touching each other like a steel screw in an aluminum hole, where the galvanic current between the two will cause galvanic corrosion. I agree you should fix the ground wire although not all will agree. My boat is also bonded to all thru hulls, keel bolts, etc. I think it is good to have lots of surface area in the water for possible lightning strikes. having the prop, shaft, and all thru hulls and lead keel as a conductor makes me feel a little better when faced with a storm a hundred miles out.
After 12 months my zinc was pretty eaten up, one of the screws was loose enough that the hex socket for the nut would not hold it tso I had to jam a screwdriver in to get the allen wrench to take it off. But it was still holding solid on the shaft and probably 1/3 to 1/2 of the zing was left. I made a note to change it more often just for ease of changing it without a haulout. Up on the hard it is a piece of cake of course. Under water you have to have all your tools, can't see, numb fingers in cold water, etc.
I've been amazed at the growth on underwater surfaces exposed to light in the beaufort area. A lot of current and warm water it seems to be a barnacle farm. So my other learning is frequent dives for cleaning. The rudder and prop are the two main culprits but the raw water intake was pretty clogged up. i am up on the hard at Bock Marine at the moment. Not just barnacles either, but oysters seem to like whateve bottom paint was applied, I'll be busy this weekend winterizing, sanding and scraping if the weather holds up. Really great people at Bock, hauled me out in the pouring rain day before thanksgiving.