The Offshore Voyaging Reference Site

Protecting Our Boat’s Underwater Metals From Corrosion

There are few boat maintenance subjects surrounded with more bad information and general confusion than “electrolysis*”.

*In most cases the above common term is used incorrectly, which is why I put it in quotes.

Anyway, here’s what we really do need to be concerned about:

Stray Current Corrosion:

Current flowing through our boat’s underwater metals, either coming from our boat’s own batteries, or surrounding boats in the same marina that are in some way connected to our boat, usually via the ground wire of the shorepower cable.

Dissimilar Metal Corrosion:

The immersion of two metals in a conductive liquid (think salt water) with a connection between them in some way (think the boat’s bonding system) generating a current (amperage) flowing through the water between them resulting in the less noble metal corroding away.

Speed of Damage

In extreme cases stray current can turn our underwater metals to mush in days. That said, dissimilar metal problems can be just as destructive, but it does take longer.


It’s near-impossible to totally eliminate immersed dissimilar metals, so as we all know, our boats must be equipped with sacrificial anodes made from a less noble metal than those we are protecting, most commonly zinc, but magnesium for boats in fresh water and increasingly aluminum, which will work in both brackish and salt water.

The right anodes will also protect against stray current, although not for long since that modality will eat them up quickly (think days, or weeks) and when they are gone the next least noble metal starts to erode.

Anyway, sacrificial anodes are a good thing but the trouble is a huge number of boats have them installed incorrectly, the wrong type, or poorly maintained, so they don’t work.

How many times have you heard:

My zincs have been on the boat for years and are perfect, so I don’t have a problem with electrolysis.

The deluded boat owner

Just the use of two incorrect terms—can you pick them out—is a warning flag, and the fact that the anodes are not eroding is most often a warning of problems, not a cause for complacency.

Does your head hurt yet?

What Not To Do

Maybe it would be best to turn to the “professionals” at our local boatyard, or perhaps a marine electrician, to help sort this out? Or maybe a surveyor.

Generally, not a good idea. In my considerable experience (30 years of aluminum boat ownership), the vast majority of these “professionals” are woefully ignorant about:

  • how immersed metals corrosion works,
  • how to test for it,
  • how to fix it.

So they tend to fall back on FUD (fear, uncertainty and doubt) by repeating oft-heard bogus memes, rather than digging in and properly analyzing a problem.

Don’t believe it’s that bad? Check out the next two photos:

The “professional” boatyard in Connecticut where our new-to-us J/109 was stored, had painted over the one and only zinc on the boat, rendering it ineffective.

Over the years I have seen this done a bunch of times, and on several occasions the boat was launched in that condition.

Some other bright spark (pun intended) left a live 12-volt positive cable lying in the bilge under the water hose to the right (I moved it for the photo) “protected” by a few turns of white insulating tape.

I don’t know for sure who was responsible for that one, but given the boat’s maintenance history I suspect another “professional”.

(The reason that person disconnected the cable in the first place, together with the breathtaking ignorance indicated by that action, is a subject for another article. Spoiler: the original screw-up goes back to when the boat was built 17 years ago, and then was compounded later by someone else who clearly couldn’t read a simple wiring diagram.)

What To Do

So what do we do?

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Philip Wilkie

Some other bright spark (pun intended) left a live 12-volt positive cable lying in the bilge under the water hose to the right (I moved it for the photo) “protected” by a few turns of white insulating tape.

When pulling out some cables in my steel boat one afternoon I heard this mysterious ‘splat’ sound from under a normally inaccessible floorboard. Much to my shock I found exactly the same scenario, only sans white tape. The rather large cable had neatly welded itself to a rib.

Tracking it back I found the cable unfused, but had avoided a fire because another even more crappy connection had failed first. At this point I lost confidence in the existing wiring and determined to re-wire the whole boat. Which given my skill set has proven a totally worthwhile exercise.

Matt Marsh

I decided this past summer that our battery cables, which appeared by my reading of the code book to be a couple of sizes too thin, were looking rather corroded at the battery end. OK, let’s pull them out…. hey, what’s this wrap of electrical tape for, down in the bilge? Oh, that’s where the unfused positive cable chafed against its mate a decade or so ago, and melted, and was then just taped up and tossed back down there.
At least I know they’re done properly *now*…..

Alan Sexton

Hi John,
these are a good “quick fix” for battery fusing

Stein Varjord

Hi John,
This is an update I’m doing now too. The best I’ve found is an integrated battery fuse and distribution bus bar unit for both the plus and the minus side. It makes it all more tidy and protected. The product is the Victron Lynx Distributor. A video describing its function and comparing it to bus bars:
A video describing the whole system:
However I’ll only use the one item.

To make the status lights work without using other products you probably don’t need, here’s a 10 dollar way to fix that:

One can also use the Victron Lynx Power In, which is 50€ cheaper and looks exactly the same but doesn’t have the lights etc. It needs the addition of fuses and some bolts to do the same job. Video description:

You might have noticed that all the videos are from the same guy. That’s because he’s the one who described it the best, among the many sources I’ve searched. Anyway, I’m going for the Victron Lynx distributor. Since I don’t have Victron batteries, I don’t use their Lynx BMS, but I have the Victron Smartshunt, which I’ll bolt on the battery side of the Distributor – connector. Any other battery monitor shunt, like Victron BMV 712, would work fine.

Stein Varjord

Hi John,
I was thinking about that problem too, and I try to follow the ABYC standard too, even if I’m in Europe. It’s not a bad idea to follow rules developed to take care of us and our property, even if it’s not mandatory… 🙂

At the moment I can actually get it just close enough to satisfy the 7 inch rule. I will later build a lithium bank from 4 separate 3,2V 400 Ah Winston cells, 12V 400Ah bank. It’ll perform better than a 1400 Ah 12V AGM bank, mainly due to the superior charge efficiency. Winston cells are inside the Victron and Lithionics batteries too. Those two brands are the only drop-in type lithium batteries I would consider, if money didn’t matter…

Those cells probably can’t all have their studs close enough to the Lynx. To remedy that, I’ll use oversized copper bars to connect the cells in a balanced way. Then the Lynx will go straight onto the bars. The bars and all connections will be tinned and insulated by strong rubber hoses. This is definitely good enough for me. I think a similar solution could be made with lead batteries too. I think the bars can be seen as the actual battery connection points, but haven’t verified that with the ABYC rules yet.

Alex Borodin

Does anyone know of a European source of comparable quality electrodes?

Alastair Currie

This is an interesting subject. I have an ongoing worry over ths as my previous propellor dulled out and had dezinced. My new, replacement propellor now has swirls on its surface, which I have been told is caused by electrolysis. I have shore power and a galvanic isolator.
A Uk supplier of a similar system. I can’t vouch for quality or credibility.

Alex Borodin

Thank you for the link, Alastair

Bryan Keith

Like Alastair I cannot vouch for quality or credibility but check these guys out also,

Alex Borodin

Hi John,

I know they are shipping to the EU, but with their stated shipping costs and the expected customs duties, I would have to pay about $260, which is nearly exactly double their price. This amount of overhead (~100%) seems over the top to me. That’s why I’m looking for a more local source.

In fact, I seem to have found one industrial supplier here in Germany, from whom I can get an Ag/AgCl electrode with a 5m cable with banana plug for EUR135 delivered to my door. I’m just waiting for an order confirmation from them now.

As to the booklet, I’m going to have to learn from other sources, unfortunately.

Markku Mäki-Hokkonen

Hi Alex,

Could you please post a link to the mentined German supplier?

Alex Borodin

Hi Markku,
Will do, as soon as I’ll get the confirmation that they do indeed fulfill orders from private individuals.

Alex Borodin

Hi Markku,

Here’s the electrode I’ve ordered
It can be configured with a couple options for connector or a fixed cable of customizable length. I’ve been promised a delivery in the first week of January – this shop is not an option for the impatient.

I’ve also ordered The Boatowner’s Guide to Corrosion and still have change from 230 it would cost to order an electrode from boatzincs. Time will tell, if this was a good decision.

Marc Dacey

I found this book quite illuminating on the whole subject. As an aside, I have started to purchase aluminum “Diver’s Dream” flat plate anodes for our steel boat after a discussion on the phone from their manufacturer. and

Alastair Currie

I am thinking that is the way to go. I have friend down near Galveston that would purchase for me and then post to the UK. A good article John and one that has me thinking a lot.

Andrew Dickenson

Great article that inspired me to buy the Corrosion Reference Electrode from Boatzincs. Unfortunately, a change in US/UK tax law has meant a big admin overhead for US shippers (having to collect VAT for UK shipments and sent these to UK customs). As Boatzincs aren’t setup for that level of admin overhead they’ve currently stopped shipping to the UK.

Dick Stevenson

Hi Andrew,
When in Europe, I did testing using a silver/silver chloride as a reference electrode following instruction from Steve D’Antonio’s article ( among other resources. I do not remember where I purchased the reference electrode.
My best, Dick Stevenson, s/v Alchemy

Marc Dacey

I just spoke to Tyler at on the phone as I need various obscurities. What a pleasure to talk to someone who seems reasonably well-informed on the products he is selling. Shipping to Canada outside of B.C. is estimated at two weeks.

Tommy Holewinski

I had the worst Electrolysis problem that anyone has ever seen when I was in Shelter Bay Marina in Panama a few months ago.

There was about 40A going through the ground wire on both my 110v and 220v shore power cables. This went directly to the saildrives, not the zincs for some reason.

I saw bubbles coming from under the boat. I asked other people what they think and they said it was probably a diver in the water. But I didn’t see any? So I jumped in the water and shocked what I saw…

Giant crystals sticking out 6-8” inches were growing out of the saildrives anywhere the paint flaked off. You could literally feel the power in the water, it felt like pins and needles on my face. When I brushed off the crystals to get a look, they immediately started growing back quickly.

Through a process of elimination, we eventually tracked it down to the ground wire cables. We disconnected the ground and I got to work cleaning off all the damage. There were large pits burned away in many different spots that were about 2mm deep. The raw water inlets were completely sealed with crystal corrosion and had to be chipped away with a screw driver underwater. The propellers were completely burned and no paint at all was left on them. The zincs weren’t painted over but for some reason, none of the bubbles or electrolysis touched them at all???

We disconnected the ground wires permanently on both cables. Then hauled out for few weeks while I ground down everything off. Luckily the damage wasn’t permanent. While it doesn’t look pretty, the functionality wasn’t damaged at all after cleaning it up.

The mechanic and Electrican at the marina have never seen anything like it in over 30 years. We never found the exact source of the problem and didn’t have any test tools like the ones mentioned in this article. Too bad… it would have put my mind at ease if we found the problem and root cause.

Dick Stevenson

Hi Tommy,
Am I correct in thinking that you may be lucky to be alive?
What a story! Dick Stevenson

Marc Dacey

Yeah, I cringed when I read “so I jumped in the water”. Our old boat club in Toronto forbids swimming in the vicinity of the docks because they cannot guarantee swimmers won’t be electrocuted, although the lack of dissolving saildrives suggests it’s generally OK.

Steve D

John: An incredibly helpful thread, and a subject with which all boat owners must have familiarity. I receive corrosion-related questions from readers on a near weekly basis.

I routinely tell boat owners, especially those who own metal boats, and especially those made from aluminum, and aluminum outdrives and stern drives, ‘you need to be your own corrosion expert, because the advice you get from others, the usual “It’s a bad ground” or “it’s a ‘hot marina”, the latter an all too common myth, is often, as you say John, just plain wrong, and this includes professionals. When I hear advice like this, and I know it’s well-intentioned, but incorrect, instead of arguing with them I say, “show me the path he electrons and ions are taking, draw it out so I can understand it”. Few people can do this, and fewer even know electrons travel through conductors and ions through water.

One of my favorite corrosion faux pas examples, I have so many, in the case of corroding aluminum tanks is, “it’s corroding because it is/is not bonded”. This is almost never the case, aluminum tanks nearly always come to grief because of poultice corrosion, from standing in bilge water or from water trapped within the tank. Metallic tanks must be bonded, but this has nothing to do with corrosion.

More on the “hot marina” myth here

Galvanic and stray current corrosion

Aluminum corrosion in particular

There are only two metals more corrosion-prone than aluminum, zinc and magnesium, which means it is at a disadvantage when in contact with virtually any other metal and in the presence of an electrolyte, moisture. You really need to be on your corrosion game if you own an aluminum hull, stern, sail drive etc.

If you need help diagnosing a corrosion problem, go to he ABYC website and search for an ABYC Certified Corrosion Technician. With anyone else you simply have no way of knowing their level of expertise.

Corrosion is the most commonly misunderstood issue with which boat owners, builders and yards, contend, bar none.

3% of the world’s GDP is expended on the effects of corrosion.

David Lochner

This summer we moved our boat from Lake Ontario to the ocean. Over the years on the Lake, we used aluminum anodes that were changed each season. The anodes showed a modest amount of decay each year, enough to show the anodes were sacrificing, not enough to warrant a lot of concern.

In preparation for the change to salt water, the bronze prop and SS shaft were painted with a high zinc content anti fouling paint made by a reputable marine paint company.

After about 2 months with only a week or so in salt water the boat was hauled. I was surprised to find the aluminum prop anode was serious degraded, worse yet, the anode was eroding near the mounting screws. Another week or two in salt water and the anode would have fallen off. The shaft anode also showed decay, but not as severe.

My suspicion is the aluminum anode and the zinc based paint reacted causing the anode to erode. It was worse on the prop anode because there is more zinc in the vicinity than on the shaft. The boat now has zinc anodes.


Timely article for me! I just moved to a new marina and one of the 1st things my new neighbor told me, after he got out of the water where he had been replacing his zincs, was that it was a ‘hot marina,’ and he had to replace his zincs every two months. I just ordered the corrosion reference anode from Boatzincs and look forward to diagnosing my new slip! Thank you!


Thanks John. I agree, and I do plan to offer my neighbor the use of the reference anode and manual.

Terence Thatcher

Steven D mentioned something I hope he or John can clarify. Steven said that all metallic tanks must be bonded. A few years ago, I installed an auxiliary diesel tank made of aluminum. I made sure it would not be sitting in water. But I did not “bond” it. What does Steve mean? Does he mean there should be a sizeable wire running from the tank to the deck fill? Or something else, such as having it bonded to the prop shaft? Thanks.

John Hunt

Bonding systems on boats. European standards on bonding (or not bonded) verses the American standards are different. Both systems work; but the last thing you want to do is apply a solution of a bonded boat, to a non bonded boat. I spent some time this past year working on my Nauticat 35, cleaning up stray currents and installing an isolation transformer. The reference Anode discussed above was incredibly helpful.

Andrew Hudson

My boat is in the Hudson River about 25 miles north of the battery in Manhattan. Salinity varies according to the amount of rain in the summer. No rain barnacles. Lots of rain zebra mussels. What anode(s) should I use?

Philip Aston

This looks like a reasonable option for those in the UK:

I’m going to order one.

Ee Kiat Goh

Hi John, during a recent hual out, a fellow boater advised me to polish my zinc anode until it is shines before putting her back to the water because zinc oxidises when exposed to air and will render the anode useless. Is it true?

Alex Borodin

I am not John, nor claim to be as knowledgeable as him, but this assertion seems to fail basic plausibility test.

I guess you haven’t seen many people polish anodes before splashing their boats, have you? If it were true that unpolished anodes are useless, then how many boats would be unprotected and therefore constantly sustain damage to underwater metals? I think people would notice. Also, those anodes wouldn’t get used up if they weren’t providing protection.

Correct me if I’m wrong, of course, but I’d say your fellow boater doesn’t what he’s talking about.

Alex Borodin

On the second thought, this idea may be a broken phone-like distortion of the recommentation to abrade mating surfaces of anodes with very fine sandpaper to improve electrical contact with the metal being protected. This latter technique is recommended by Rod Collins (, which is enough proof that it totally makes sense.

Ernest E Vogelsinger

But whatever you do, NEVER EVER polish or abrade your zincs with material containing metal (wirebrush, e.g.). Contamination with iron molecules will render the zinc utterly useless.

Alex Borodin

Hi Ernest,

care to explain the reasoning or provide a source?

Ernest E Vogelsinger

Maybe “utterly useless” is a bit overemphasized. It is what I often read in forums (mostly german), or on specific websites as well – they tend to warn against using iron stuff on zincs as introducing iron oxide particles into the anode would make it less effective, up to the point where the protective effect is said to be reduced to useless levels.
I haven’t tested this, but I use plastic brushes or sandpaper to clean my anodes.

One example:

(but maybe this is just old wives tale 😉 )

Alex Borodin


So far, I’m not convinced that the effect size, if it exists, is more than negligible.

Ee Kiat Goh

Thank you John and all for the very interesting discussion. I should get myself one of the meters! I have also made one observation. My stainless steel rudder post is supported at the bottom of the rudder by a bronze boot. I have a zinc on my stainless steel rudder post and another zinc on the bronze boot. The zinc on my stainless steel rudder post wears out in about 6 months but the zinc on the bronze boot doesnt seems to wear out at all. Is this normal? The rudder post is also connected to the underwater “earthing strip” to all metals underwater which is in turn connected to the negative side of my battery bank. (This earthing strip was recommended by a marine electrician to have all underwater metals at the same potential as the engine is negatively earthed.)

Paul Fleming

John thank you so much for this excellent practical discussion and links to product. I have owned a Goderich (Brewer) 35 steel boat for 15 years. When I first bought it I had it sandblasted and repainted. It has served me well. It is freshwater boat on Lake Ontario. As you know steel boats rust from the inside out and this year, after haul out my number was up! A small leak in the hull, aft, in the engine area. Time to pull out the welder etc. this spring. This event, plus your timely article has spurred me on to address corrosion testing more systemically going forward. Thank you so much. It is my first year of membership with you and I have been delighted. I am generally self taught and you have been like the “Dad, I never had” in regard to all things boats. Sincerest thanks for making a difference in my life.