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?
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.
Scary story! Good point that once you get to certain point with wiring the best option is to tear it all out and start again. I had to do that with my old Fastnet 45 and I was well rewarded for the work with a safer and more reliable boat.
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*…..
Scary stuff. It’s easy to forget that fusing batteries at, or close to, the posts is a comparatively recent advance. Even our J/109 built in 2004 does not have fuses on the batteries. Definitely on the fix list.
these are a good “quick fix” for battery fusing
Looks nice, but I don’t think I can get it to fit given that I’m height constrained in the battery locker. That said, I’m a big BlueSea fan so I will be looking at all the different ways I can get the batteries fused including that one.
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: https://youtu.be/548aRhZMN-g
A video describing the whole system: https://youtu.be/1oNMwSDrts8
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: https://youtu.be/8h-E1lkYCKg
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: https://youtu.be/LIVh7lZ5IT0
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.
That does look like a nice system, particularly since I’m a huge Victron fan-boy.
That said, one problem I see with the fuse block for North American boats is that using it would make it impossible to conform to the ABYC requirement to have fuses within 7″ of the battery post. That said, I have long thought that requirement is a bit arbitrary, so not a deal breaker. However it is worth keeping in mind that a boat so wired would not be strictly ABYC compliant, which could have implications at resale.
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.
That makes sense. If I were still doing a system for our last boat with her larger bank and loads I would be all over this. I just love the neat way that Victron have designed this. I’m guessing that on that boat the entire system would take up 25% of the space that it did with normal buss bars.
But I think for the J/109 I can do fine with the BlueSea stuff like Timothy suggested, since I’m trying to keep my natural proclivities to go for super-cool in check!…But then again, Victron is so cool…
Does anyone know of a European source of comparable quality electrodes?
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. https://www.edt.co.uk/reference%20electrode/cathodic-protection-reference-electrode
Thank you for the link, Alastair
Like Alastair I cannot vouch for quality or credibility but check these guys out also,
Just checked with boatzinc.com and they are shipping to the EU without any problems. Well worth getting their product since it comes with the booklet.
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.
Could you please post a link to the mentined German supplier?
Will do, as soon as I’ll get the confirmation that they do indeed fulfill orders from private individuals.
Here’s the electrode I’ve ordered https://www.meinsberg.de/shop/details.php?anr=SE23
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.
I get that. That said, the booklet is worth that and more compared to what underwater metals damage can cost. That said (again), this will help: https://www.morganscloud.com/category/aluminum-boats/care-tips/
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. https://www.amazon.ca/Metal-Corrosion-Boats-Nigel-Warren/dp/0713678178 and https://canadianmarineparts.com/product/anode-divers/
Yes, you definitely need to get to the bottom of that one. A reference anode will almost certainly make that a fairly simple process, although it would be better if you had boatzincs.com’s little book to take the mystery out of the whole process.
The point being that the simple explanations on the page you link to is not going to help you trouble shoot what is almost certainly a stray current problem.
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.
Good call, you need to get to the bottom of the prop discolouration issue and the little book that comes with the boatzinc anode is pure gold.
If you have any questions when you get into trouble shooting it, make a comment here and I will try and help.
And, when you do find it, I know we will all be interested in the cause.
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.
When in Europe, I did testing using a silver/silver chloride as a reference electrode following instruction from Steve D’Antonio’s article (https://stevedmarineconsulting.com/reference-cell-testing-know-thy-corrosion-protection-level-editorial-old-vs-new/) among other resources. I do not remember where I purchased the reference electrode.
My best, Dick Stevenson, s/v Alchemy
Hum, I think boatzincs have some bad information. I think I’m right in saying that the requirement to collect VAT at source for out of country vendors only applies to intangible goods such as software services, not hard goods like the anode.
In the case of a hard good the person receiving is responsible for paying the VAT as part of the clearance process. That’s certainly the way it works here in Canada where I paid our 15% HST (sales tax) on the anode when we picked it up at the post office. I have written to Tyler to let him know this.
I just spoke to Tyler at boatzincs.com 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.
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.
Am I correct in thinking that you may be lucky to be alive?
What a story! Dick Stevenson
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.
Wow, that’s a scary story.
Note to others: while Tommy got away with it, if you suspect a stray current issue (which you can confirm with the above recommended testing rig) the very last thing you want to do is get into the water. (I’m not getting on Tommy’s case here in that I have done the same thing before I knew the dangers.) If you must get into the water to check the damage, and you should, leave the marina and go anchor some place else well away from other boats and AC sources and then make sure that all AC sources on the boat are turned off and locked out before doing so.
Tommy: one other point. The fact that your zincs did not erode is very concerning and almost certainly indicates they are installed wrong or the boat’s ground and/or bonding system is screwed up in some way. You definitely need to find this issue and fix it.
The best source for figuring out what’s wrong and fixing it right is ABYC, so well worth joining to get access to their standards.
Turns out that’s good information: https://blog.stamps.com/2020/12/22/shipping-from-the-u-s-to-uk-with-2021-vat-rules/
Crazy, but true!
I suggested that Boatzinks.com appoint a UK distributor and have a possible suggestion for them.
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 https://stevedmarineconsulting.com/wp-content/uploads/2019/09/Hot-Marina-Myth-CW-AugSept-2019.pdf
Galvanic and stray current corrosion https://stevedmarineconsulting.com/unraveling-the-corrosion-mystery/
Aluminum corrosion in particular https://stevedmarineconsulting.com/understanding-and-preventing-aluminum-corrosion/
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.
Great comment. You are clearly more patient than I am since I tend to throw a screaming fit every time I hear the hot dock myth. Thirty years of aluminium boat ownership and being told BS stories has clearly unhinged me. Love your test of asking them to draw out how it’s occurring.
Just one thing I need to clarify for others:
In fact aluminium when properly managed is probably the least prone to corrosion of all commonly used marine metals.
For example, a steel boat is always corroding if not perfectly covered by paint (pretty much never happens), not so aluminium.
And further, proper management of aluminium is pretty easy: https://www.morganscloud.com/category/aluminum-boats/care-tips/
I know you know this, but it’s amazing how many people don’t, so I like to clarify the situation as often as I can.
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.
That’s interesting, and I think you may be right about the cause.
One other thought, I’m not a fan of painting the prop and shaft and have never needed to do that. Over the years all I have done is give them a spray of Boeshield, which last an amazingly long time, as long as we are using the engine regularly.
Next year we are going to try Propspeed on our J/109 saildrive and prop and I will report at season’s end.
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!
Sounds good, but do start off by knowing there is pretty much no such thing as a “hot marina” so you berth is probably fine. If there’s a problem it’s probably on your boat. And it’s a lot more likely that your new neighbour has a wiring problem on his boat that’s causing the problem than anything external, so when you get done it would be neighbourly to lend him your new reference anode and the booklet that comes with it. That might also save him from killing himself by going swimming in a marina with a lot of plugged in boats around. https://www.morganscloud.com/2017/10/09/dont-swim-near-marinas/
Thanks John. I agree, and I do plan to offer my neighbor the use of the reference anode and manual.
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.
Bonding is a complex subject and there are even credible people who question if it’s the right way to go in all cases. So the first question is does your boat even have a bonding system, many do and some don’t. Take a look and see if all the sea cocks are connected together with heavy green wire or copper strapping, if so the boat has a bond system.
If it does have a bond system, then just connect the tank to it with a piece of say #6 green wire. However, if your boat does not have a bond system then this becomes a much more complex question.
That said, ABYC code requires bonding and specifies in detail how it should be done, so if you want to get this right, your best bet is to become a member and then dig into their papers on the subject. At first glance ABYC stuff is a bit intimidating, but when we get into it, it’s all pretty well explained.
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.
Sure, we certainly don’t want to mix standards, and if we do decide to bond, then we need to go the whole hog with one standard.
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?
In that case Aluminium anodes will probably be best.
This looks like a reasonable option for those in the UK: https://www.edt.co.uk/cathodic-protection-reference-electrode
I’m going to order one.
That does look good, particularly since they provide some documentation with it, albeit not quite as good as that provided by boatzinks.com, but still adequate. Thanks for the heads up.
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?
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.
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 (https://marinehowto.com/anode-installation-best-practices/), which is enough proof that it totally makes sense.
We advocate for the same treatment. If the zinc is not connected it won’t work and I have often found at least one of the hull zincs on our aluminium boat disconnected: https://www.morganscloud.com/2017/12/04/27-aluminum-boat-care-tips-part-3/ (#4)
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.
care to explain the reasoning or provide a source?
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: https://forums.ybw.com/index.php?threads/cleaning-zinc-anodes.495555/#post-6348822
(but maybe this is just old wives tale 😉 )
So far, I’m not convinced that the effect size, if it exists, is more than negligible.
I had never heard that, and to be honest, I’m more than a little sceptical that a brush with a good quality stainless steal wire brush would do any harm. That said, I could be wrong.
Anyway, the key point in all of this is that if we regularly test the boat with the rig detailed above and the voltages are high enough, the anodes are working, end of story. Dispelling FUD and mythes is one of the reasons I advocate for the this.
Hi Ee Kiat,
No, that’s not true. If it was zincs would never erode away, as Alex points out
Anyway, as long as the boat passes the testing above the zincs are working.
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.)
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.
Glad it was useful. As a metal boat owner you will also find this series helpful: https://www.morganscloud.com/category/aluminum-boats/care-tips/
It’s mainly about aluminium boats, but the same principles apply to steel.
And thank you for the kind words, made my morning.