29 Aluminum Boat Care Tips—Part 1

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Morgan's Cloud, our 56-foot aluminum cutter, turned 30 this year, and she and I are coming up on our twenty-sixth year together. One of the amazing things about the boat, and the material she is built of, is that, at least as far as I can see (I look pretty carefully and regularly), she is just as strong today as she was the day she was built with no appreciable wastage.

That said, boats built from aluminum do require care and attention to detail. To help with that, in this three-part chapter I'm going to provide practical tips.

And let me emphasize the word practical. I'm going to focus on what we actually do that has worked to keep our boat in good shape for a quarter of a century, rather than get into a lot of theory.

Here we go:

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Mike hiscock


Thank you for removing some of the mystery behind caring for aluminum boats. An Ovni is high on my list for future cruising, and over time I’ve heard horror stories about caring for them (and similar boats). So these articles are very helpful.

Brian Engle

That VDO gauge looks nice. Found it at two other French retailers as well. Have asked Summit Racing if they can order it (part YC310) from their VDO supplier. Will ask same of VDO directly and ping thread if responses are noteworthy.

Ian Wright

Great article , very timely , have just relaunched my 1989 Ovni after a 2 year refit and are in the process of sorting the electrical system . Could not source the VDO product so using a local Australian made product called Seabis . Can’t wait for part 2 !!!

Marc Dacey

Damn, John, inset zincs? It’s so obvious, but evidently not, as I’ve never seen it before. Aside from the obvious reduction of drag, do you find any other benefit? I have a steel boat and it’s probably not worth doing as “going faster” is a low priority, but I’m willing to listen to persuasion.


Thanks John, great info!
Just replaced my inset zincs with alloy anodes, as the boat spends most of the time in fresh water. The old zincs had turned brownish and was told its an indication they stopped working, but not much show of wastage. My isolation transformer gets quite hot (50″C?) is that normal, does the temp. go up and down with the amount of amps use.?
Certainly will try to obtain the VDO gauge you mentioned. Also like to know how often do you haul your boat? I understand yours is always in salt-water. Mine apparently will soon be due for a new barrier coat.
Thanks again.

Brian Engle

Our steel vessel had an “Acme” branded isolation transformer aboard, which to my understanding was engineered for land-based construction purposes. It got way too hot for my liking just sitting there with no load. That and other factors spurred me to do an extensive electrical refit. I replaced the Acme with a Charles unit designed for the marine environment — night and day difference. The Charles runs cool, has a powder-coated case, and to the best of my recollection features epoxy potted or transformally-coated innards. I think the Acme transformer was widely used in the 80s as a cheap alternative to the marine units. I have seen photos online of its use on other boats. However, having witnessed the drastic effects of salt air on non-tinned wiring and connectors, the thought of keeping the Acme scared the hell out of me.

As for zincs, ours are teardrop shaped and I “reactivate” them by diving on them twice a year with a stainless steel wire brush to remove any scale and brighten them up.

Matt Marsh

Many (perhaps most) of the boaters who don’t completely neglect their anodes tend to throw them away far too early. Probably more than three-quarters of the anodes I see in scrap bins still have most of their useful life in them.

What matters, for this purpose, is total exposed surface area. If the anodes look brown or gunked-up, just give them a quick scrub with a wire brush to expose some fresh metal. The bulk of the mass is just there so that they won’t waste away to nothing during the intervals when you aren’t checking them.

Re. the isolation transformer. AC power hardware designed for use on land really does not belong in this application. It’s usually not protected from corrosion in any way, and there’s a good chance it won’t be wired properly for marine use (where the ABYC standards are different from the Electrical Safety Authority codes, it’s for a good reason). And while transformers do get warm, the kinds found on boats do not get dangerously hot. If you burn your finger on the case, unplug it and call the vendor.

Dave Hopkins

Hi John, Just remember though if a 2 bob piece finds its way to the bottom of the hull, you’ve had it. Seriously though, our solid fibreglass cruising hull will see us out. In our next lifetime though with even more resultant enlightenment an aluminium hull will be just the shot. Cheers Dave & Ruth.

Svein Lamark

HI John, this is a very important article if one has a aluminium boat or a boat with aluminium in the hull. I have an old oak icebreaker with a 2mm aluminium skin. This thin protection skin lasted 60 years. We followed your rules. If you do not, I know such a thin skin will vanish fast. To mix oak, steel, copper and aluminium in a hull is very dangeous. You can get a lot of corrosion. And the corrosion will start in the aluminium parts. But doing it your way even a 2 mm aluminium hull can last 60 years. If one can manage to protect such a thin aluminium skin, steel, iron and oak will also be in a perfect condition.


Thank you John and Matt, and my comment re : the hot isolation transformer was overstated, normally it feels warm , but there have been times, maybe when the hot-water tank was heating up, it felt much warmer. Certainly will check it more frequently.
The reason we changed the zinc annodes, was advised alloy ones would work better in fresh water, and will try to keep them shiny as mentioned.
Time will tell.

Svein, like to know more on how your alloy skin was applied to your boat, maybe under a different heading.

Svein Lamark

Hi Rene! Your question is to me very interesting, but I think the editor will find it off the headline. So John, as an editor you must find another option to discuss this matter if you think it is important. I have studied ice skin of wooden boats fore a long time , but you must deside how to continue or not on this matter.


Hi John,
As to Sveins comment re: alloy skin, if you dont like to pursue it at this time and if your R&R
permit to do so, you may forward my e-address to Svein.
Many thanks,


Useful post, thanks John.

And yes Alu alloy anodes do seem counter intuitive dont they! Nonetheless our Allures 45 is factory fitted with 2 on the skeg and one on the centreboard and applicable for all waters. Interestingly, they are mounted on rubber isolation gaskets and their Alu fixing bolts are set in silicone sealant by way of protection. We have had very little wastage indeed over the last 3+ years.

The prop and bowthruster are also similarly protected.

Phil Streat

Hi Neil,

We recently had anodes mounted on our older Garcia at the Allures/Garcia yard so I know exactly what you are seeing. I think the rubber gasket is just between the anode and the hull but the mounting bolts pass through it. The bolt heads & nuts are covered with silicone. The idea, I believe, is to limit anode erosion to its working surface rather than on the back or around the mounting bolts, which might make it disintegrate or fall off.


Hi John,
4 of my inset alloy anodes are about 8´ in diam and bolted down with one bolt in the center.
Should the hull have been scraped clean of all coatings before mounting anodes?
Am not sure if this was done.


Thanks again John for, what now appears to be a very important topic, confirmed by Svein and his 2mm alloy skin. Always knew the role of anodes, but never realized how vital role they play on alloy boats and looking forward to your Part 3.


This is a very timely article for me. I am the (relatively) new owner of a 12 year old Garcia and in the midst of anode replacement. The anode arrangement is much like described above for the Allures with a rubber gasket between the boat and anode. In my case, there is a stud welded into the hull and a nut that holds the anode on. Electrical contact is made from stud to nut to anode. The nut was completely covered in sealant. Additionally, the previous owner had coated both sides of the gasket with adhesive sealant. Needless to say, that made getting the old anode separated from the boat a real pain in the ***.

I don’t really see the point of making the back of the anode a 100% watertight connection and prefer not to reinstall with sealant behind the anode. I’m not even so sure sealant over the stud and nut is necessary. Can anybody else using this type of anode mounting system chime in as to exactly what you do and why?

As an aside, the anodes are aluminum, this seems pretty standard in France. They are evidently alloyed with indium and magnesium to both adjust the potential voltage and prevent the aluminum oxide layer from forming.

Philip Streat

Hi Martin,

I have an older Garcia and recently had the Garcia/Allures yard add anodes to the hull. They have a rubber backing pad and sealant over the nuts; I don’t know if there is sealant on the backing pad.

My understanding is that there are two reasons for doing this. First you want to confine the anode erosion to the front surface and limit it on the back and around the mounting bolts because if it wastes away there it will fall off, or at least loose its secure electrical connection, before the end of its useful life. The second reason is that water trapped behind the anode becomes very caustic (high pH) and this causes paint damage. Using sealant as well as the backing pad may well be overkill but I think using the pad and sealing around the bolts is probably good practice.



Thanks Philip, that is useful info. I agree that sealant on the back is overkill but want to know if anybody has a reason why it is necessary. Ironically, the sealant on the back of the anode indirectly damaged my paint in that I accidentally dug through the paint trying to remove the sealant. I’ll put sealant over the nuts but not on the back when I put anodes back on.

Dan Manchester

Hi John,
Surely one of the key points here is that your aluminium hull is painted. This is the primary reason that your zinc anodes have so little corrosion – they are only protecting the hull in any spots where the paint is damaged and the Al exposed, which would be few given your obvious attention to detail. The primary corrosion protection on any vessel or marine structure should be a protective coating. These unpainted Aluminium boats are fine in theory, but as soon as you throw your steel anchor in you become one big anode.

Dan Manchester

Hi John,

I agree, there are always parts that are unpainted or hard to get to to repaint, and the anodes will/should protect them, but you will need your anode relatively close to the point to be protected, sea water still has a resistance of 0.2 Ohms/M which becomes significant over a boat length.

Not sure I agree 100% on whether having an unpainted hull above the waterline won’t make a difference. Yes, aluminium oxide is a good corrosion inhibitor in air and if you are permanently moored in a sheltered harbour that might be fine, but in use, boats have a tendency to be wet a lot of the time above the waterline, and thus you have an electrolytic path. For my money you’re better off painting as much of the vessel as possible and saving your anodes for the bits you can’t get to or the scratches you can’t see.

Stephen Actor

Due to the difficulty & expense of replacing a shaft log I applied epoxy barrier coats inside mine. Easily done by pushing a paint saturated rag through. The coverage was not 100% each coat so multiple coats & drying periods were
required. Using different colors each coat allows you to see when it is fully covered. The initial aluminum preparation inside the shaft log was done with sandpaper attached to a slot cut in a 1/2″ diameter 5 foot long aluminum rod, spun by a hand held drill. This was followed by the chromate conversion. It took a couple of hours.
My Van de Stadt Samoa was built in Japan and the anodes are in flush/recessed pockets, bolted to an aluminum stud that is welded to the hull. They were not bedded in sealant, but do have the rubber pads and full barrier coat on the hull behind them. I had always wondered about that. The nut holding them on is never loose, and they always show just the slightest amout of activity to assure me that they are working.

Steven D'Antonio

John, a very good piece on a poorly understood subject.

While I understand why you advocate isolation (it’s common among European builders), Grounding the hull to the AC safety ground, and DC negative, is a requirement for compliance with ABYC Standards.

The reason this arrangement often goes astray is because there should be only one connection between the hull and the vessel’s DC negative/AC safety ground system. If there are multiple point so contact, the hull can become a conductor.

If the hull is not grounded, particularly to the AC safety ground, it could become energized with 120 volts without tripping a circuit breaker. If a person then touches the hull and something that is referenced to the AC neutral/ground, a generator or engine block, or the chassis of other electrical gear for instance, an electrocution could be the result. Again for ABYC compliance, the DC negative and AC safety ground should be common.

On the other hand, from a corrosion perspective, if you do have a DC positive fault to a grounded hull, rest assured, the corrosion is unlikely to persist years, the boat would probably sink long before then, as stray current corrosion is among the most aggressive and fast acting, it will eat a hole in the hull in weeks if not days. The question is, what’s more important, electrocution prevention or corrosion prevention? Having wired a number of steel and alloy boats, good attention to detail and making certain those doing the wiring understand the importance of isolation, and chafe protection, can for the most prevent such faults.

Silver Silver-chloride reference electrodes are available from a wide range of sources, simply Google it. I’ve used on from Mercury Marine for 25 years.

I’m an advocate of aluminum anodes, they are truly a free lunch. Aluminum is amphoteric, however, which means it, a hull, is susceptible to attack from alkaline solutions, which, as you note, are in turn produced by using too much cathodic protection, i.e. too many zincs. It will cause paint failure and it can indeed attack the aluminum, I’ve seen it on several occasions. I’m also a strong advocate of isolation transformers, and consider them a prerequisite on any metal boat. More on these here


And while I agree that an isolation transformer virtually eliminates the issue of outside corrosion sources, there’s no such thing as a “hot dock” because corrosion is primarily a DC phenomenon, and docks are energized with AC. While possible, AC corrosion is very rare, and requires especially high current. When you plug into a dock, even if your power isn’t on, without isolation from a transformer or galvanic isolator, you are instantly connected to every other boat in the marina, and their associated (DC) electrical/corrosion problems. Blaming the docks, therefore, is laying blame in the wrong place.

Transformers can be wired in such a way that they may not provide isolation and still be correctly wired, these are referred to as polarization transformers, the only difference is the manner in which they are wired. They still offer in water electrocution and other protection, just not isolation, some believe these to be safer from an onboard electrocution hazard perspective. They must be used in conjunction with a galvanic isolator. For metal boats, transformers should be wired in isolation mode.

Your guidance regarding ABYC compliance (which is voluntary for diesel recreational vessels btw in the US) is welcomed and excellent. When I was in college I worked part time for an electrician, all of his vans had a bumper sticker that said, “Wiring is not a hobby, call a professional”. Canada (Transport Canada), Australia/NZ (AS/NZS 3000:2007) and the EU (Recreational Craft Directive and ISO) have mandatory marine electrical standards (Australia’s/NZ’s is among the most stringent I’ve encountered). As an ABYC Certified Electrician and Master Technician, I’m in the position of identifying faults on a near daily basis, either in person or on line, many the result of incompetent professionals or unwitting do it yourselfers. It keeps me up at night.

Charles was one of the few manufacturers making a transformer designed specifically for the marine market, it is UL Marine approved (different than just UL approved) and ABYC compliant. They stopped offering boosting transformers and now only offer a straight transformer. Since then a few others have become available, Atlas, Asea, and Wards, and others, with varying features and cost. The only drawback to a non-boosting transformer is when plugging into 208 (or less), you get 208 and 104 (or less) on the secondary, with the latter being hard on refrigeration and other AC motors. This is a real problem for 120/240 VAC vessels.

Steven D'Antonio


OK, thanks for the clarification, and I see what you are doing.

ABYC, if you choose to comply, does actually mandate a connection between AC safety ground and DC negative. And ABYC E-2, 2.5.10 says, “A metal hull shall be connected directly to the engine negative terminal” i.e. DC ground. This is a stray current prevention requirement. Then add the AC to DC connection requirement and you have a AC/DC bonded hull. Again, you can choose not to do this, it isn’t mandatory, none of ABYC material is, just pointing out the Standards as they are written.

I’m not a ABYC Standards adherent for the sake of compliance. There are some standards with which I disagree, this just doesn’t happen to be one of them. When it comes to electrocution potential I can’t advise others to take chances.

Having said that, and not to belabor this but it is important for any boat owner to understand, metal or FRP hull, there is a good reason for tying the AC and DC grounds together. Any system that is “floating” if energized with 120 VAC, has the potential for electrocution if it is touched along with something that is referenced to the AC ground. This is why ABYC guidelines effectively prohibit floating systems. If an AC fault develops to the DC wiring system (think of AC and DC wires melting together, this isn’t uncommon, it was the source of Kevin Ritz electric shock drowning case), and the two do not share a common ground, it’s possible for you to touch a component in the DC system, which is now energized with AC, and complete the path to the AC source when touching the hull or something connected to it.

I think the risk of corrosion is low if the vessel is wired properly, and is equipped with an isolation transformer. Add a permanent reference electrode/meter and you have full-time monitoring and the risk lowered even further.

It is true, steel bulkheads and pilings, provided they are connected to the shore power ground system, it must be part of a circuit, just being nearby has no effect (bulkheads perhaps, pilings probably not), adjacent to a vessel that’s plugged in to shore power, can create a galvanic corrosion potential. I’ve had experience with this on aluminum stern drives located near steel bulkheads. This is why Mercury Marine, manufacturer of many aluminum stern-drives, was a pioneer in corrosion prevention, introducing one of the early galvanic isolators and impressed current systems.

An iso xfmr would negate the issue of interaction between steel bulkheads/pilings and hull, as would a galvanic isolator. Again, every metal-hull vessel should be equipped with an iso xfmr.


Can anyone recommend engine mounts that isolate the engine electrically? I checked with the maker of the mounts on my aluminum boat, and they say that the mounts are not designed to do that. The boat is now on the hard and it’s time to replace those mounts and add a flexible shaft coupling, too.

I had an electrician on my boat a couple days ago and he wants to REATTACH everything I have disconnected from the hull back to the hull. He doesn’t like the isolation switch on my engine at all. All of his advice runs counter to everything I want to do to eliminate electrolytic corrosion, but I’m ignorant. I don’t know how to proceed — not that I have much choice because finding techs to work on the boat is nigh impossible (severe shortage of labor here in Vancouver, it seems).


Thanks John. Exactly. I heard from the electrician again and he insists on bonding for ABYC safety standards. This is a guy who crows about building aluminum boats. But he thinks it’s normal to replace zincs ever six months. Good for makers of zincs.

I’m planning to add the flexible shaft coupling, although I worry a tiny bit that it might shift potential electrolysis from shaft to hull. I don’t think I can isolate everything, but my objective is to get there. A tiny worry is that I’ll make things worse; there are no signs of corrosion on my boat, so one is tempted to do nothing. But I got a Seabis and that scares the crap out of me.


Alas, Aquadrive doesn’t have a dealer in Canada and the Canada yard I’m using says they cannot get these mounts. Does anyone reading this know of an alternative — engine mounts that isolate electrically? Many thanks for any help!

Marc Dacey

I got my AquaDrive, adapter plates and soft mounts from Mermaid Marine. They are very popular on fishing boats and the “soft” mounts and the CV unit itself have been great at reducing noise and vibration (and, of course, concerns about alignment) on our steel sailboat. I hope this helps.


Thanks again John. Just to be clear, the electrician here (from a very fancy-pants company) wants to violate your Rule 1: “All aluminum boats should have their DC (battery-based) electrical systems isolated from the hull.” He wants the DC system grounded to the hull. After some back and forth he has said he’s willing to not do this, but he doesn’t like it. A lesson for everyone is not to assume that electricians with lots of experience on (commercial) aluminum boats will know how to do it right.

Also, this guy kept saying “galvanic corrosion” for every kind of corrosion. That’s normal among laypersons, but it made me wonder how knowledgeable he is of electrolysis.

Steven D'Antonio

The number of professionals in the marine industry who truly understand corrosion, its causes and prevention, and can explain to you what’s occurring, is very small indeed. As my corrosion training prof. used to say, “show me the path the electrons are taking”. If a pro can’t do that, he or she is simply guessing.

You can increase the likelihood of getting someone who knows what he or she is talking about by using only an ABYC Certified Corrosion Technician. You can find them, and confirm those who say they are, on ABYC’s website, http://www.abycinc.org

Among corrosion professionals, and within ABYC circles, the word “electrolysis” is frowned upon (whenever a claimed corrosion pro uses it, it immediately catches my attention, because most corrosion training programs and respected texts discourage its use), as it does not accurately describe the corrosion process. Here’s what ABYC has to say about on the subject, “Electrolysis – The breakdown of an electrolyte resulting from current flowing in an electrochemical cell that includes that electrolyte. Example: the breakdown of water into hydrogen and oxygen gases in a supplied-current electrochemical cell. ‘Electrolysis’ is often used loosely to describe corrosion in general, or the operation of supplied-current cells in particular. Its use in this respect is often confusing, and should be discouraged.” Corrosion specialists jokingly say “Electrolysis has more to do with hair removal than corrosion”

Connecting the hull to the DC negative, and AC safety ground is mandated for those wishing to comply with ABYC Standards, so it’s not surprising that the electrician insisted on this. While there are other schools of thought that John has detailed, according to these widely-accepted ABYC guidelines, all be they voluntary, that is the “right” way to do it.

The two primary (but not only) types of corrosion we encounter in the boat small boat marine industry are galvanic and stray current. The former occurs between dissimilar metals, the latter is the result of a current “leak”, most often DC, into bilge water or the water in which the vessel floats, or both. Aluminum is also susceptible to poultice corrosion. If he was referring to the type of corrosion that is mitigated with anodes, then that is galvanic. Either can be affected by the various hull grounding/isolation techniques. Don’t get me wrong, I’m not defending this tech, I have no idea if he knew what he was talking about, I’m simply defining the terminology and practices.

I recently wrote this article http://stevedmarineconsulting.com/wp-content/uploads/2014/03/Corrosion-CRWHS18_HOS4_FINAL.pdf in an attempt to concisely explain these two common types of corrosion, and in the hope of dispelling many of the myths and urban legends swirling around them. And this one to explain aluminum corrosion scenarios specifically http://stevedmarineconsulting.com/wp-content/uploads/2014/03/Aluminum-Corrosion-Cruising-World-May-2017.pdf

If someone puts forth a theory about a corrosion scenario aboard your vessel, ask him or her to draw you a picture, literally, tracing the electron path. If that person cannot do this, then yes, you should be suspect.

Steven D'Antonio


Understood on all counts, and you’ve made your reasoning for a floating DC ground clear. As an aside, as a young mechanic/electrician, I used to regularly work on Jim McCurdy’s boat in Oyster Bay. It had two battery switches, one for positive and one for negative…

Also, on the subject of compliance, your clarification is appreciated, however, for the AC system to be ABYC compliant, its safety ground must be common with the DC negative, and hence the hull, so for those who don’t wish to do this, direction to professionals is necessary. I’m not debating the floating ground issue with you, I understand your reasoning and you’ve made its advantages clear, just pointing out how the Standards approach this issue.

It would be helpful for those taking the floating ground approach you advocate, if you wrote a brief summary on how and why this is done, one that could be shared with electricians carrying out work aboard these vessels.

Kors Slot

Hi John & Steve,

I have read your comments on floating or Isolated DC system with lots of interest.
Maybe a silly thought but in order not to compromise on safety, why not connecting the DC ground via a galvanic isolator to the vessel’s AC safety ground? In that way it’s blocking up to 1.4V DC but allows AC fault current directly to the AC’s safety ground wich is also connected with the hull.




The electrician I got really wants to ground DC to hull. Takeaway: follow John’s advice to be wary of electricians.

Does anyone know if the VDO device John recommends is essentially doing the same thing as the “Seabis” device? I’m guessing that the latter is just a fancier version of the former (and more frightening, with lots of colored lights labelled “critical” and so forth).


Thanks John.

Do you know if the VDO device you recommend is doing more or less the same as “Seabis”?

Ann Bainbridge

It looks like the VDO leak tester is no longer available. Is there any alternative, other than SeaBis?

Ann Bainbridge

Thanks so much! I have ordered one…


Has anyone tried the corrosion reference electrode from boatzincs.com (http://www.boatzincs.com/corrosion-reference-electrode-specs.html)?


Yes, I bought one about a year ago. I have not compared it to another reference anode but it does give readings about as expected. Parts of the booklet it comes with are also useful.

Steven D'Antonio

I’d chat with the folks at ABYC before you jump to the conclusion regarding their take on, and experience with, metal boats. Some very savvy folks sit on the Electrical Standards Project Technical Committee. In fact, I’d suggest you speak with Capt. Dave Rifkin, I can put you in touch with him if you like.

Have you been to Seattle/Pacific Northwest lately, alloy boats everywhere, and there are several manufacturers in the US building small aluminum craft for military, police, Coast Guard etc. Of those I inspect, and I inspected one last week that belongs to NOAA, most at least claim ABYC compliance, i.e. a common ground.

Again, you have a system that works, and it’s embraced by some alloy boat builders, more in EU than NA. It’s worth pointing out that the EU approach is taken with FRP as well as alloy vessels, so it’s not specific to metal boats.

Bottom line, ABYC mandates a common ground primarily for electrocution prevention, if everything is at the same potential, it makes it difficult for a person to complete the path.

Steven D'Antonio


It’s a worthy topic to be sure.

The small aluminum boats to which I referred are not classed per se, however, many of the agencies ordering them do mandate ABYC compliance. Not all end up fully compliant, however.

I’m currently working on an inspection project in Australia, however, once I have free time I’ll review in detail the material in your post.

Steven D'Antonio


I have a wx delay here so some time to respond to this.

Simply put the crux of the floating DC ground approach means if AC hot comes into contact with a chassis that is only bonded to DC ground, and not the hull, an engine or genset block for instance, it’s energized, you touch it and the hull and you complete the path to the source, and get shocked, and maybe killed if no GFCI is present. This is why ABYC guidelines call for a common ground system, electrocution prevention.

Also, you must think about the required B/C grounding requirements (both AC and DC grounds connected to case of metallic chargers, failing to do this means the chassis can be come energized and not trip a fuse or breaker). This is in fact the way many boats unintentionally or unknowingly comply with the AC-DC grounding requirement.

You and others can choose not to do this for corrosion prevention reasons, I’m not debating that, I’m simply saying it’s not without risk.

Ernest E Vogelsinger

Hi John,
I found a handy online calculator for capacitators (https://www.elektronik-labor.de/OnlineRechner/Kapazitaet.html albeit german, scroll down to “Kapazität berechnen”). 1000 µF would allow for 2.4(4.8)A to pass at 120(240)V, and I doubt that a breaker would trip at this amperage. If my calculations (read input into said online calculator) are somehow correct you would need 2080µF for 240V (4160µF for 120V) to allow currents up to 10A.
I found Elcos from Jianghai up to 1000µF, so you could put them in parallel to allow for sufficient current in case of an AC to DC leak.

What I am wondering however is how would the DC wires react to such a shortage – they most probably would not be sized for this power.

Ernest E Vogelsinger

As far as I am concerned (and I’m by no means a professional electrician) my main concern for the above mentioned scenario – AC hot faulty connects to DC positive on a non-floating DC system) would be the power that would tranverse the DC cables until the shortage gets abandoned by the breaker. These wires would by no means be sized to carry that wattage so I believe danger of fire would be eminent.


I’m in BC now and right in the middle of having electrical work done on my boat. I’m getting massive blowback from electricians about wanting to isolate DC and the engine. It’s not just about safety/ABYC standards; they insist that doing this will actually increasing damage from electrolysis/stray current. I’m going completely insane. I’m really starting to doubt the wisdom of all the advice to isolate, and I’m worried that I’m actually making my boat more likely to suffer corrosion. I just don’t know what to do and whatever I do has to be done now now now while the boat is on the hard.


Hi John,
Keep up the great articles. They are helping us enormously with the build of our aluminium catamaran. With some difficulties we managed to secure a VDO leakage test instrument through Kent Marine but no instructions were supplied as to exactly where to fit it. Could you perhaps please shed some light on this?


So a question about the Ovni rudder anodes. The previous owner of the boat has installed the round rudder anodes on a length of stainless threaded rod through a hole in the rudder – one on each side. The anodes sit on top of the primer and antifoul. Can anyone tell me if this is how Ovni rudder anodes should be fitted? Seems to me they should make contact with the metal of the rudder by scraping back the antifoul and primer to bare metal where the anode sits. Can anyone with an Ovni shed any light on this and how it should be done?



The brand of the vessel aside, no anode works if it does not have a very (no more than one ohm) resistance connection to the object it’s protecting, the cathode, the rudder in this case. You can check this with an ohm meter, while hauled out. If the anode and rod are installed over a painted surface, that’s problematic. Typically, in the case of a metal boat, the stud to which an anode is attached is either welded (for steel) or fastened (for aluminum) to the hull. Again, the connection must be very low resistance. If anything, the rod should make good electrical contact with the rudder, or the surface over which the anode is placed must be clean, free of paint and bright metal when the anode is installed. Never use a wire brush, even if it’s stainless steel, to clean anode connection surfaces, always use non-metallic abrasives such as ScotchBrite pads.


Thanks guys much appreciated

Sebastian Hjort

Hi there,
Bought a 30 yo aluminium boat 3 months ago.
Survey shoved no signs of galv. corrosion.

Today though i spotted that my negative poles are connected to the engine.
There is no transformer or other device that makes this okay right??

Sebastian Hjort

Hi John,

Thank you for the quick reply.
I have read and re-read your thorough explanation.
I contacted my surveyor whom I trust and he replied that as long as the motor is isolated from the hull, there should be nothing to worry about.
I think I will need to pay a professional to have a look. As mentioned the boat is 30 yo and shows no sign of corrosion which I guess it would if something was very wrong..

Ernest E Vogelsinger

As to my (very humble and illiterate) opinion: the isolation of the (most probably non-aluminum) power train needs to be _really_ good. Is the engine block sufficiently isolated? This means are there really no “bridges” where current can flow, such as salt water bridges? How isolated is the engine/gearbox from the (most probably VA) shaft? As I see it the shaft cannot be considered isolateable from the hull as it protrudes to the outside into the sea water which is a perfect electrolyte of its own. So if there is any, even inadverted isolation breach which might easily happen in an engine department, such as water vapour, salt deposits, etc, there _will_ be electrolysis if the drive train is electrically active – and it is active if connected to the battey negative. For me this would be something I’d try to avoid even on fiberglass or steel boats.
But on the other hand as you said there has obviously been no electrolysis for the last 30 years, so who knows why 😉
I wish you ultimate success and joy with your new/old boat (a bit in envy, but I hope this is ok)!

Rob Gill

Hi everyone,
Our 3 day winter haul-out on our 47 foot fibreglass (Beneteau 473) yacht is nearly compete – we splash tomorrow. Just one problem encountered – both the end cone anode on our max-prop and the split-ring anode attached to our stainless shaft were almost gone (to mush) and this after just 6->7 months.
We sit on a marina when not cruising, but have a galvanic isolator which, according to my marine electrician friend is connected properly and working fine. We can detect no stray currents around the shaft or engine block.
Paint wise, we could easily manage 12 months between hauls, but the anodes are disappearing too quickly, so I need to haul after 6 months – just long enough to wash down and change the anodes.
As you would expect, we have a few boats around us on the marina, but they have changed over time, but the problem hasn’t gone away. I suspect the base of Coppercoat may be contributing to the issue, but last year we over-coated this with epoxy primer and then anti-Altex #5 anti-foul and the rate of anode attrition hasn’t slowed.
Please can anyone suggest the best way to add additional protection when stationary on the marina, so my prop and shaft anodes last better and we can return to a 12 month haul-out? When we first bought the boat we didn’t have this issue. I have read about connecting a wire from the shaft or engine block to a larger anode and dropping this over the side when in the marina adjacent to the prop. But might this over zinc the boat and have un-intended consequences?
Any thoughts or experience with this would be most welcome.

Rob Gill

Hi John, please see answers to Steve’s comment below.
As a relative newbie to this field, I will make the small investment you suggest in kit and larger investment in my electrical / protection understanding. I do have a multi-meter on the boat and I understand the the anode is positive, but what would I connect the negative terminal to that best represents my “system” cathode (sum of Copper-coat hull and bronze Max-prop)? My through hulls are all glass/nylon. I would think I have two options, the prop shaft or the engine block which will be connected through the cooling water in the system. Which would be the best proxy? Or do I use an average?
By the way, searching for an e-book or PDF version for a “Corrosion Workbook” I came across this PDF version, which has US and European (ours is a European build) standards covered: https://cdn.shopify.com/s/files/1/0658/7343/files/CORROSION_WORKBOOK_2010.pdf
Finally, regarding your suggestion of a hooker rig, I have considered one too (also for intermediate scrub-downs), but I try to resist extra “stuff”, to keep Bonnie Lass close to her optimum cruising weight. I can dive but I don’t carry the gear. I just don’t want the performance hit of any extra weight or the storage hit – something you discuss in your latest post. If I need to, I will hire dive gear when we can and go down and change the anodes over. Usually it’s about $30-$50 to hire everything.
Thanks, Rob

Rob Gill

Thanks John,
No bond system and not much else to protect. The keel, whilst steel is protected and isolated by a triple layer of epoxy paint and the keel is in remarkably good condition, zero weeping or blistering – looks like new. The keel bolts are stainless, but the bilge is dry and there are no signs of corrosion externally from above. The rudder shaft is composite/GRP so nothing there except a copper bottom bearing (more like a sleeve) which is epoxied into the stern tube. We removed this in 2017 before going offshore and it looked prefect, with little or no signs of corrosion.
The engine, a Volvo D2 55hp (a Perkins engine) has no anode protection that I am aware of. The FW side is protected to an extent by the Volvo coolant mix which has “inhibitors” according to the manual. The raw water side relies I understand on there being (no?) different metals in the system.
When we had our old Fisher Panda generator, that used almost no zinc in four years. We have since removed the generator to save space and weight!
Thanks for you help. I’ll report back with my readings. Might be a few months.
Cheers, Rob

Steve D

“Just 6-7 months”? In my experience that is reasonable, and isn’t unusual at all, depending on water temp, current and salinity.

But, why guess, I’d recommend performing a reference electrode test, pretty easy with a little investment and education, and very definitive.

Did your marine electrician friend actually test the galvanic isolator to make sure it’s wired properly and working? Is is a model that was made after ABYC’s update of this standard in July 2008 and if so is it compliant? There are some very low quality units out there, even today, that are non-compliant and don’t work well.

The only side effects from over-zincing an FRP boat are potential paint damage around cathodes, i.e. protected metals such as through hulls, struts etc. Paint can lift, leaving a surface where barnacles can strike.

My bet is anodes on your shaft are doing nothing other than protecting the prop and shaft, even if you have a shaft brush most don’t work to the level that’s required for galvanic current. And hull anodes are doing noting to protect prop and shaft, this isn’t uncommon and it isn’t harmful per se as long as all are protected.

(In Kaohsiung, Taiwan…inspecting new builds)

Rob Gill

Thanks Steve, sorry for the delay in replying – tied up in “splashing” the boat yesterday.
I am much relieved to understand that my rate of anode loss isn’t unusual. And what a wonderful resource this site is when you can get such super informed responses to questions. A few answers to your enquiries:
We are in Auckland NZ so SW water temp varies between 15 – 25 degrees C. Salinity would be average for SW on our marina, with some FW feed in rain, but also some evaporation and little tidal flow.
With my focus on the safe return to our berth and putting the engine to bed, I forgot to check the brand of the galvanic isolator – I will come back on that soonest, but it will be certified for our market, otherwise it wouldn’t be allowed for sale here. My sparky friend tested the unit and was confident it was wired correctly and working, but I wasn’t there when he did the testing – I can ask some more questions when we meet next.
I will read up on the electrode test thanks. Logic would indicate the anodes are both being eroded + effective, but as I understand your recommendation – I should check that it is in the normal range as per John’s tip #4 and reference reading. If in the normal range (around 1.0 V, then just suck it up and change the anodes at whatever frequency is required. If outside this range then I should investigate the source of this.
Many thanks again, Rob

Steve D


Understood on all counts. When you do the ref electrode test be sure to test the shaft specifically, as it likely has little or no reliable connection to the hull anodes.

(In Kaohsiung, Taiwan)

Rob Gill

Thanks Steve,
We have no specific hull anodes (none specified in the B473 boat manual). I have one cone anode on the end of the max prop that definitely connects to the shaft and one split ring anode around the stainless shaft itself, both zinc. My original questions was whether I might lower a zinc in the marina connected to the engine or shaft, (in lieu of a hull anode) but I understand now the best idea is to do the test and then determine what I may need from there. I can also definitively check if the isolator isn’t doing its job, as John suggest above.
Thanks, Rob

Sebastian Hjort

Been Reading about this for Ages and it is still a mystery to me! My negative poles are connected to the hull (previous owner) and it has been like that for 30 years with no sign of damage. You write that if a connection develops (i am currently measuring 12v from positive pole to hull) this is bad. Will my anode take Care of this? It Works just fine although i need to change it annually…

Sebastian Hjort

Took my boat out of the water after 5 months and the newly fitted anode was all gone so it seems I have the very problem you predicted on my hands… Do you have a method for hunting down whatever is causing this?

Oleg Mints

Hi John,
I own a 25 yo alu cutter and wonder what anodes would be better.
The ship stays winter in hangar and summer in freshwater (IJsselmeer in Nederland) but goes for weeks to North sea / Atlantic with salt and east of Baltic sea with brack water.
I understood the aluminium anodes (Navalloy from performance metals for example) are a right choice for the hull.
But what with VA shaft and bronze Autoprop (the shaft isn’t isolatet)?
I wonder if its would be zinc or alu anodes too?

Another question: Is it a good idea to install an (alu) anode inside water tank?
The water tanks have a lot of pitting because of previous owner used silver ions to prevent the drink water from spoil and serious damaged the tanks 🙁 Would the anodes help from further damages or I have to coat the tanks anyway?

Oleg Mints

Thank you a lot John!
I cleaned the tanks (with bleach) at the end of last year and will coat with epple-plast LS (drinking water / food grade) now as you proposed.

Louis Henry

Just great article, but can’t find you issue on looking after an aluminium sailing yacht.

Charles Guggenheim

Hi John,
Why do I need a isolation transformer if the battery are isolated from the hull?
Just got a 1984 OVNI 40 and learning about it.

Philip Wilkie


Expanding on John’s response a bit. The immediate answer why is that an transformer means that the earth wire from the shore power is left disconnected on the primary side, but the neutral on the secondary side is bonded to the hull and becomes effectively a local AC ground, independent of the shore power.

There are three reasons why isolation transformers should be considered mandatory on metal boats. (And desirable on all others.)

1. The primary reason is that earth isolation prevents circulating stray currents because the shore power AC ground and the ship’s AC ground are now separate.

2. Transformers inherently provide reverse polarity protection, which is a nice secondary benefit.

3. The third and more subtle reason is that the local AC grounding of the hull at the transformer, is inherently better than relying on an earth wire running from the ship, through multiple connections back to the dock power source. It’s this connection that current balance protection devices (RCD’s) rely on to ensure they work correctly. If the hull is not reliably earthed it is possible to have a fault that doesn’t trip the protection breakers, yet leaves the hull at a dangerous voltage.

Richard Wasilkowski

Hi John,

Quick question, which isolation transformer did you use on your ovni 435?

Richard Wasilkowski

I wonder if I could trouble you.

I have an OVNI 385 and have had her for about 4 weeks.

I did some testing and the engine appears to be electrically bonded to the hull as do some other systems on board.

Can I ask you which is the best way to discover where the fault is.
My problem may be that there are multiple faults.

But my understanding is that the engine should be totally electrically isolated from the aluminium hull.

I tried using a multi meter with the batteries totally disconnected and measure resistance between negative and hull, positive and hull, engine block and hull, but seem to get spurious reading except for the the engine block and hull are electriably in contact with each other.
I have also checked positive to hull and get 12 volts although at 2 milliamps.

What am I doing wrong and what is the best way to isolate perhaps multiple faults.

With thanks and in anticipation

Richard Wasilkowski

Richard Wasilkowski

Hi John, thanks for your reply. Can I ask how much damaged will be done if the engine is shorted to the hull. How much time do I have to fix this problem. I am quite desperate as I cannot seem to find the short

Richard Wasilkowski

Hello John, thank you for your repossessed.
I was on my yacht yesterday and managed to eliminate several items.
I disconnected the engine loom, throttle cable and gearbox cable and they were not the cause of the engine to hull short.
Also, starter motor and alternator seem to be isolated.
I eventually had a brainwave and with the batteries completely disconnected I measured the engine to hull resistance.
It’s a dead short when the negative of the meter is on the hull and the positive on the engine. However, if I put the positive meter lead on the hull and negative on the engine it’s not a short and measures 100 ohms. What could cause this, a diode and regulator? Can you help please ?