Are galvanic isolators worth installing?

Frank Langer

1984 Ericson 30+, Nanaimo, BC
The zinc anodes on my prop shaft need replacing about every 5 months. A mechanic told me a galvanic isolator would significantly extend that time interval, but they cost about $400. Has anyone installed one, and noticed a significant decrease in erosion of the zincs? Is this a good value for dollar, or merely another of the many ways to spend our boat bucks?
Thanks for any advice or suggestions.
Frank
 

eknebel

Member III
Wow, you are going thru zincs! If you leave the boat plugged in most of the time, the isolator should help, especially if a new slip holder has a electrical neutral/ground problem. But you can buy quite a few zincs for $400 :(
 

Loren Beach

O34 - Portland, OR
Senior Moderator
Blogs Author
The zinc anodes on my prop shaft need replacing about every 5 months. A mechanic told me a galvanic isolator would significantly extend that time interval, but they cost about $400. Has anyone installed one, and noticed a significant decrease in erosion of the zincs? Is this a good value for dollar, or merely another of the many ways to spend our boat bucks?
Thanks for any advice or suggestions.
Frank
Five Months? Yikes.
I am not an electrician, but know several. Strictly IMHO, you are moored in a Very Damned Hot marina! :eek:
About a decade ago I installed a galvanic isolator for about $100. Strictly as a precaution. What I would prefer, but have not done, is the better solution of an isolation transformer. These have always been quite heavy and expensive. Still think about it occasionally....

OTOH, I replace our anode (aluminum these days for fresh water) every haul out, at about three years. They are usually over half OK when replaced.

I should note that our YC has not only replaced all of the wood headwalks with concrete in the last 20 years, but more recently upgraded all of the power post 30 Amp 'shore power' outlets to GFCI type. We do not have the fancy outlet box pictured in the article, but same standard.

BTW, one factor that pushed our little club from considering this a "nice to have" to a "requirement" was that we found that once the Board was on record (several times...) discussing the risk inherent our older non-code-compliant outlets, those Minutes could be subpoenaed in any future possible lawsuit over any electric shock 'drowning', and we would all be financially as well as morally involved.

I would guess that either you dive or hire a diver to replace your zinc's. Brave Divers!
Admittedly there in less risk to swimmers in salt water; less, but not zero.

Take care.
 

Parrothead

Member II
Before you invest in a galvanic isolator on somebody else's say-so, remove the shore power cord to see if it makes a difference on zinc life. If it doesn't, the isolator won't help.
 

Frank Langer

1984 Ericson 30+, Nanaimo, BC
Before you invest in a galvanic isolator on somebody else's say-so, remove the shore power cord to see if it makes a difference on zinc life. If it doesn't, the isolator won't help.
Thanks for all your replies so far! Our marina dock shore power does have GFCI which should help avoid electricity causing drowning for divers and others. I do all the diving myself on our boat to change the zincs (and yes, it is cold even with a wet suit in the winter!), clean the prop and the hull underwater, as it is just too expensive to hire a diver at $100 or more per dive. I have tried to remove the shore power more often in the summer months, and have not noticed a significant difference. I have also tried hanging another zinc over the side, connected to a negative battery terminal on the boat, but that also didn't make much difference and that zinc seemed to barely corrode at all, as if it wasn't really working. I do want to continue to connect to shore power during winter, as I keep two small heater/dehumidifier fans running to avoid moisture/mould. I will continue to think about whether it's worth investing in a galvanic isolator--$400 is a lot of money, but at $25 for two zincs twice per year that adds up too, and it might help me avoid that mid-winter dive to replace the zincs. I'm not getting any younger! :)
Frank
 

Pat C.

Member III
You may want to check that all your negative DC wires are connected to the batteries negative terminal, not just to the engine. You may need to add negative wires back to the battery for the alternator and the starter, these normally ground through the engine which can lead to anode consumption.

Bilge pump wiring is notorious source of this I have heard. Make sure everything is heat shrunk and leads back to the battery with good connections.

A galvanic isolator is a good idea to protect your anodes from other boats when you're plugged in, since you are then sharing a common ground. Just learn how to check it occasionally to make sure the diodes are good.
 
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Kenneth K

Sustaining Member
Blogs Author
I'm curious whether you have a direct-coupled propshaft or a drive-saver coupling?

20171007_134606.jpg Drive-saver

And, if you have a drive-saver, do you have a bonding strap connecting the engine and prop sides of the shaft?

I've posed this quandary before but no one seems to step up to answer it: I believe the bonding-strap is "required" with a drive-saver to complete the ground for the AC electrical system. However, it seems, many don't bond their shafts, either because they're not aware of the need to do so, or because they are aware and fear that it worsens the problem of engine & shaft galvanic corrosion.
 

HerbertFriedman

Member III
A galvanic isolator is always a good idea, but they are very simple to make. Four diodes and a hunk of aluminum (for a heat sink), about $10 in parts, circuit diagram attached.
 

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Frank Langer

1984 Ericson 30+, Nanaimo, BC
Hmmm, lots to think about now! Pat, most of the negative grounds go to a stud on the engine, and from that same stud, to the negative battery, terminal but I'll check if there might be one from the starter or alternator that just goes to the engine.
Kenneth, I don't have a drive saver, so the prop shaft attached directly to the tranny coupler, tranny and engine. I've read about bonding but there seems to be alot of controversy about it so I've not bonded all the metals on the boat.
The idea of constructing a galvanic isolator is interesting, so I'll think about that. Not sure where I would mount the required size aluminum plate.
Thanks, all!
Frank
 

HerbertFriedman

Member III
I have the full set of instructions and appropriate diode part numbers, e-mail me and I can send them out you. As far as location, you want to mount any galvanic isolator right where the shore power cord comes into the boat. this isolator goes in the ground leg of the shore power.
 

supersailor

Sustaining Member
Having a prop and shaft eaten by electrolysis, I added a galvanic isolator, a drive saver and a huge (50lb) zinc. I used to go under five months with having the two shaft zincs totally destroyed to having them never seeming to need replacement. I did too many things at once so I can't point out which one stopped the rot but I'm not complaining.

Maybe I should think about a folding prop again!
 

Parrothead

Member II
Do not be surprised if the zinc life with an isolator installed is the same as when you had the shore power disconnected (no significant difference with the shore power connected or disconnected per your assessment).
 

HerbertFriedman

Member III
Bob, there is such a thing as too much zinc, it distorts the potential of the boat with respect to the salt water. I hang a small zinc over the side just in case my prop shaft fall off but it is not a large one. There are lots of papers on line showing how to measure these potentials but it takes a special cell.
 

Loren Beach

O34 - Portland, OR
Senior Moderator
Blogs Author
Ordinary criteria/expectations may not completely apply to Bob's port-operated marina. When Westport Yachts parks a new super yacht a couple hundred feet away, you can (slight exaggeration) darned near see the water temperature rise. And since the boat builder provides local good jobs, the port seems rather humble about enforcement.
The electrical problem does not happen for every big yacht, but is not uncommon either.
 

supersailor

Sustaining Member
Loren be correct. When they arrive, they park (dock) on the guest dock right across from my berth. These are 50 meter 500 ton monsters that are usually plugged into three or four 100 amp circuits. For those that remember, one of their smaller ones of only 240 tons took out the end of my dock and destroyed Bruce Cuttings 35-2 along with other boats.

In the first three years I had the boat here, my prop and shaft were destroyed by galvanic action in spite of the shaft zincs being changed three times a year. In 2016, I added the galvanic isolator and started using the big fish when I changed the prop and shaft. I also added a flexible coupler to the shaft. Four years later, both still look new.

One brand new 50 meter was docked next to me for a month. Within three days, all the growth on the fish disappeared. Within a couple more days all the growth disappeared off the underwater part of the cable. When I complained to the harbormaster, he said "The 50 meter can't be leaking. It's brand new." I'm sure word never got back to Westport. It's no wonder the prop and shaft disappeared.
 

Frank Langer

1984 Ericson 30+, Nanaimo, BC
We will let you know in about six months. We are installing one this weekend.
Hi,
what size and brand did you buy, and where? One of the challenges I've identified is that there is very little space in the area where the shore power enters the boat, to mount metal plates or a galvanic isolator. Where are you thinking to install yours?
Frank
 

Pat C.

Member III
You can mount it anywhere. I put mine as close as possible to the shore power inlet. That was a mistake because it's in a difficult location to test regularly. Wish I had extended the green wire to make regular testing easy.
 
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