How long should a zinc anode last?

[windsmaker]

Replaced the zinc anode of the stainless shaft on my Beneteau which I keep in water year round in Louisiana It lasted only 4 months !! it was gone , came off in my hand.
I think a anode should last a year
Do I need a galvanic isolator to protect me from shore power.
How can I measure the problem?

 

[Don S/V ILLusion]

If you are usually connected to shore power, then probably yes, a galvanic isolator might help prolong the life of the anode (although not definitely). Not sure there is one, single definitive answer to the question of normally expected lifetime of an anode other than to say 4 months, as I infer you already know, is too short.
Determining the existence, amount and source(s) of stray current corrosion is not as simple as connecting a digital voltmeter and measuring potential differences and if I took up the bandwith describing what you'd need and how best to measure, it would be a waste of time frankly as it's unlikely you would have the equipment necessary, i.e., silver chloride anode, etc....

Better you pick up any of a number of marine books on boat electrical management, any of which will describe the causes, means of identification and solutions and provide you with other info you might someday need.

 

[Joseph Shirley]

4 months is a little too short but it may be normal where you are. A galvanic isolator is something you should have anyway to protect you from stray currents in the neutral wire of your shore power connection, but it may not help where you are. I once moored near a marina/boatyard where they normally grounded their welders by dropping the ground in the water. It made the water at the marina pretty hot and people went through zincs very quickly. The club where I moor now periodically checks the water electrical potential but I'm not sure how they go about it.

Joe S

 

[BobM]

How about a zinc fish? I am not at a dock, but I think one would be cheap insurance.

 

[bobtigar]

@ things to extend the life of a shaft zinc.
1. Some one close to you i runing their air conditioner, battery charger ,etc. When you dont need 110 on your boat disconnect the plug to reduce the chances of stray current.
2. like someone else said -use a fish zinc.
My prob were like yous 3-4 mn andf the zinc was gone. With the fish-2 years+ Bob

 

[Warren Milberg]

I recently submitted a question like this to BoatUS expert Don Casey: Do-It-Yourself Guru.Here is my original question and his reply, fyi:

Original question:
Last year the zinc anode on my bronze shaft of my sailboat was about 75 per cent dissipated at the end of the season. I had connected to shore power only for about 10 hrs the whole season. Thinking I would gain protection this season, I put on two zinc anodes. When hauled a few weeks ago, both anodes were equally dissipated also to about 75 per cent gone. Only this season I never connected to shore power at all. Does it make sense to double the number of anodes? Would agree that the problem would seem to be ambient electricity in the water at my marina? What else could cause this problem? Thanks.

Casey's answer:
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.