Zincs going going gone....

[catwithhat]

The prop zinc (standard 3-blade prop) on my Benetau 343 seems to be going rather fast. I get about 3-months before its half gone. I tried adding a doughnut zinc on the shaft but now it seems like 2 zincs are going at about the same rate. I decided to invest in a silver-chloride electrode and measure the potential difference between it and the boat ground. I get about -400-500 mv which appears a little low (but my zincs are probably part gone already). Hanging an additional zinc off the stern seems to get me to the -700 mv where I need to be to protect prop and shaft (stainless)... These measuements were made with the shore-power cable disconnected. When I plug it in, I get -800 mv. Now I'm confused? What does this tell me? Am I getting the benefit of all the zincs from other boats that are connected to the shorepower ground? Is the difference between my "boat ground" and the "Shorepower" ground an indication of "stray currents" Thanks much for any advice. A 3-months dive interval is getting a bit pricey! Cat

 

[Warren Milberg]

I sent Don Casey

a similar question. Here is copy of the question and his answer, fyi:My 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.Our expert'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.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.

 

[Mike]

Check your neighbor's

Check to see if any of the boats in the surrounding slips have their power cords set so they're submerged, either full time or at high tide. If so it's enough to create a small amount of current flow which will eat your zincs prematurely. This goes for power lines running under the docks as well.

 

[Richard D.Bantsari]

Prolonging Zincs

I am having the same problem with our Beneteau 373. I have added a galvanic isolator to the shore power system and have added a large "guppy Zinc" over the transom . This "Guppy" is connected at the moment to the engine block but I may add a shaft brush and connect the guppy to the shaft brush.