© Attainable Adventure Cruising Ltd and the authors, all rights are reserved.
Nothing on this website or in direct communications received from us, or in our articles in the media, should be construed to mean or imply that offshore voyaging is anything other than potentially hazardous. Dangers such as, but not limited to, extreme weather, cold, ice, lack of help or assistance, gear failure, grounding, and falling overboard could injure or kill you and wreck your boat. Decisions such as, but not limited to, heading offshore, where you go, and how you equip your boat, are yours and yours alone. The information on this web site is based on what has worked for the authors in the past, but that does not mean it will work for you, or that it is the best, or even a good way for you to do things.
It had never occurred to me that running my 12 volt electronics from a DC-DC converter from the 24 volt battery bank, as I do, would effectively isolate them from the leak detector on the 24 volt system. I guess I could install a two-pole switch so that I can switch the leak detector between the 24 volt and the 12 volt systems.
Philip
Hi Philip,
Yes, I think that would be a good idea.
John
Thanks again for the 3 part article. Very informative though I have one questions I have been pondering. When corrosion appears under the paint in the form of blistering, what is the urgency to fix and what are the options? I have an Ovni which I have now had for 18 months. At time of purchase the top side paint was in pretty good condition though now there are increasing areas of blistering. Will leaving the blistering sealed under the paint increase the corrosion and as such should I remove the paint on the offending areas as soon as possible. Some areas I plan to total remove the paint and leave bare though if I start by just removing the blistering areas until I can remove the lot will make the side look scrappy. Though better a scrappy looking boat if that reduces pitting.
Appreciate your thoughts and advise.
Chris
Hi Chris,
As far as I know, and based on my experience with our boat, it really does not matter much either way. The great thing about aluminium is that as soon as it is exposed to the air it self-anodises by forming a film of aluminium oxide and that film prevents further corrosion.
I have heard it said that corrosion is increased under a blister because the area will not dry out due to the remaining paint, but I have never found that to be true, or at least not enough to measure.
I’m not at all educated on this topic, but my impression is that accelerated corrosion under paint seems to happen with moulded aluminium parts and way less, or maybe not at all, on marine grade aluminium plates.
I guess the moulding process might make the metal more porous or something like that, and most likely, a boat is built in way more corrosion resistant alloys. There are huge differences between aluminium alloys in corrosion resistance, strength, stiffness, and much more.
If I was getting myself an aluminium boat, I’d put a lot of nerdiness into learning more about this issue, but I assume that the serious builders have already done just that.
Hi Stein,
Yes, I think you are right. This is probably one of the reasons people are so scared of owning aluminium boats: they don’t realize that the alloys are way more robust than those used for other stuff they have owned.
I concur. Just saying “it’s aluminum” really doesn’t tell you much, without knowing the actual alloy being used. Would you say “it’s wood” without specifying the species, or even softwood vs. hardwood?
In aluminum metallurgy, even tiny changes in alloy composition can completely alter the properties of the material. Alloy 5083 is about as closely related to alloy 7075-T6 as pine is to oak.
The 5000-series plate alloys used for boat hull framing & plating are specifically designed for very high corrosion resistance in a salt-rich environment. In the absence of a stray current source, they are extremely durable and have no inherent tendency to corrode away.
Hi Matt,
Thanks for coming up on that. I think your last sentence really sums up what aluminium boat owners need to know: watch out for stray current and then stop worrying.
I have a 20 year old Ovni 32. Last winter I finally gave in to the scruffy appearance and stripped the decks of the failing Treadmaster and cut back all the areas of blistering or otherwise failing paint on the decks and coachroof. Wherever I got through to the underlying aluminium it was perfect. I did not find any pitting. So from that experience, I would say don’t worry. My boat now looks smarter, but in terms of structural integrity, I have made no difference whatsoever. But as someone who has previously owned wooden boats, I cannot get used to not having an even extending To Do list, so have to find jobs to do.
Thanks for some great information. We are looking to purchase an OVNI 445 evolution. The vessel was built in 2010 and there are numerous patches of blistering where the deck coating meets the unpainted hull. In addition, blistering is evident around a number of the fittings eg hinges. Moreover, in both cases, there does not appear to be any obvious underlying mechanism causing the corrosion. It seems that its a bit of a lottery.
If I understand you correctly, blisters under the paint are more of an aesthetic problem then something that requires immediate rectification.
Appreciate clarification.
Regards
Pat
I haven’t owned an Ovni, but every last one that I have seen has been exactly as you describe. I once saw an Ovni document recommending to repaint the hull at very short intervals. I cannot recall the time period, but I do remember being shocked at the frequency (I seem to recall that it was annually, but maybe it was every five years). The point is that I believe even Ovni would say that this is normal for a 2010 hull. One of the reasons I decided against Ovni is i just didn’t want the headache of keeping the paint (top of topsides, coach roof, deck, cockpit and coaming always seem to be painted) looking OK. If you don’t care what it looks like, and the boat is otherwise what you want, then I don’t think this should be an obstacle.
I’m a bit suspicious of painted aluminum hulls regardless. Often the paint is there to cover up filler that was used to fair the hull. Nothing wrong with filler if that’s what you like, but it’s just more to go wrong eventually and make the boat look uglier. And if the filler is used under fittings, well, then it’d get really messy — but still not necessarily a problem with the basic integrity of the hull.
Go to any fishing port where aluminum boats are the norm (at least where I have been). You’d be hard pressed to find any paint on a hull. It’s not there for a reason. The only reason to paint an aluminum hull above the waterline is because one wants it to be a certain color — to make it pretty on one’s mind’s eye. But when that paint peels and cracks, it will be far, far from pretty. A really pretty aluminum boat has not paint.
Excellent series of articles, as always. Using capacitors to lead spurious signals to earth/ground is certainly the way to go. However, a cation for anyone, unwisely, using galvanic isolators in preference to an isolation transformer (or the non-ABYC, Ovni option, of disconnecting the earth/ground lead when in harbour).
As the number of grounding capacitors, in use, increases you eventually reach a point where the galvanic isolator stops isolating and starts conducting. There are lots of “hidden” grounding capacitors in modern switched-mode power supplies (almost everything these days from phones to iPads) so it can happen quite easily.
So, don’t rely on the cheaper “Zinc Saver” option, fit a proper isolation transformer as recommended in John’s part 1.
Good point re the Ovni method of disconnecting the earth. I was never happy with that, probably o.k/good for the boat protection, but pity any poor swimmer who happened to be close to this hull if there was a fault. Doesn’t take much current flowing through the water to earth to incapacitate a swimmer. Also re isolation transformers, just had to replace my 10yr old mastervolt gi3.5 after they tell me it cannot be repaired, because they wouldn’t have the parts. Seriously pissed off.. I spent many years at sea repairing Radars/transmitters that were at least double that.
Changed times I guess.
Garry
Hi Garry,
I was not aware that the Ovnis had such a “feature”. You are right, truly scary, and not just for a swimmer in the water, also for anyone working on the boat if an equipment case goes live. The more I think about this the more I say either have no shore power or have an isolation transformer, there are no other good options.
Sorry to hear about your transformer. As you say, pretty disappointing for a piece of gear that has no moving parts to die in 10 years.
I believe Ovni’s/Alubat argument is that when afloat. The best earth path is though the hull to the water. So when afloat a short from live to the case will trip the RCD on the 220v panel, but when the boat is out of the water/ashore then the shore earth is required to trip the RCD or ELBC.
Garry
Hi Garry,
Hum, I see what you mean, but I still really don’t like that idea. Salt water is not that conductive and what if there is more fresh water around than one expects? Bottom line, the more I think about this the ever more convinced I am that an isolation transformer and then connected the boat side ground and neutral to the hull is the only way to go.
Hi Frank,
Absolutely. I’m no fan of galvanic isolators and certainly not on a metal boat.
I think your articles are very informative. I have experience with underground fuel tanks and like aluminium the steel ones have anodes on the outside to protect them from corrosion. Many owners don’t know that the corrosion protection is only for the outside skin since that’s where the reverse charge is. In the underground tank world we make sure people know to keep all the water out of the bottom of the tank because the worse corrosion is between the water fuel interface.
The other thing is I think areas of corrosion under paint is going to corrode faster if you don’t get the paint off because the failed paint is holding water in that area. And the water will become low on O2 so the protective layer will not be on the aluminium. So I would think you should remove the failed paint and corrosion as soon as possible so it won’t continue to spread and get deeper. It is best for the surface to have well adhered paint or no paint. Not in between. I have been always interested in paint and keeping it good since I was a BM3 in the Coast Guard 30 years ago. Keeping a 378 foot ship’s paint in good shape was a crew’s full time job.
Hi Ted,
I hear you on theory that aluminium will corrode more quickly under loose paint (i believe it’s called poultice corrosion) but the fact is that I have just never seen this to be a problem in the 26 years I have had the boat. This might be because (as Stein points out) the alloys used in boat construction are really corrosion resistant.
Poultice corrosion is common to see in areas without any paint but where there has been constant damp from seawater or urine. I saw this on several boats when I was searching for my own. It looks just dreadful — like a layer of fluffy puss — but I think it can vary from completely insignificant to potentially serious.
Related to this, I think it’s important to point out that not all standing water in the bilge (or elsewhere lacking limber holes, as John found) is the same: it matters if it’s freshwater, say from a leaking water pipe, or seawater, say from a dripping stern gland/shaft seal. I think standing freshwater won’t matter at all — OK, maybe after 50 years or more. Standing cruddy water — the normal combination of drippings from stern gland, engine cooling hoses, maybe a tiny bit from an errant battery a decade ago (even sealed batteries discharge sometimes), etc. That’s the stuff that’ll cause poultice corrosion — maybe more so if the proverbial copper penny is dropped in the bilge (just kidding, mostly).
One addition to my comment on poultice corrosion: For me, when searching for an aluminum boat, poultice corrosion was not a sign of an inferior boat, but rather of an inferior boat owner. It was a real turn-off because it got me wondering about what other stuff the owner didn’t care about.
Hi Paul,
I wouldn’t necessarily let a bit of white powder and goop from corrosion put you off that much. In my experience, although it may look bad at first glance, once the debris are cleaned up it’s rare for there to be any significant damage.
After 26 years of caring for an aluminium boat the biggest take aways for me are that as long as we keep stray current under control the material is pretty much indestructible and if I got worked up every time I saw a paint bubble or a bit of white powder I would have long ago driven myself insane.
To second what John says: if you are going to have an aluminum boat build, avoid paint, and for goodness’ sake don’t paint the bilge. My boat has a painted bilge, so I speak from the same experience as John. I can see the temptation: paint the bilge white and then you can keep it spotless. True — until the paint starts to come off. Ain’t worth it.
One of my top criteria for buying an aluminum boat was “no paint!” I can accept it on deck if it’s for non-skid purposes — not so tough to work on that (just get on hands and knees to scrape, sand, tape, paint, etc.). In the end I did tolerate paint in the bilge to get an otherwise cool boat. The bilge paint does no harm at all to the hull, and nobody can see it peeling there! Not so with peeling topsides. Almost the worst thing for me about owning a fiberglass boat was the insane amount of work that went into keeping the topsides looking acceptable. (I don’t even consider myself to be fussy about such things.) Give me an aluminum hull, sans paint, with a nice layer of aluminum oxide any day. Want to scuff or smudge my topsides when you come alongside. Go ahead. I just don’t care. It’s the joy of an unpainted aluminum hull. It may be ugly until you think about all the work you never need to do.
The takeaway message: Unless absolutely essential, avoid paint except for surfaces that get fouled (i.e., outer hull below waterline) and potentially nonskid on deck.
Hi Paul,
I agree!
I agree too. If it were not for the work involved, I would take my boat back to 100% bare aluminium except where anti-foul is needed. As It is, I look after the Ovni topside strake of paint and the deck/coachroof and so far the workload has not been excessive. It seems to be a matter of using good epoxy primer, undercoat system and two pack topcoat. Lasts a lot longer than a paint job on a wooden boat. And you know if it does get a ding, I does not matter. You just have to learn to live with the appearance, but as Paul says, the freedom of wondering about scuffs when coming alongside is a liberation.
Another thing I love about my Ovni is the relative lack of concern about drying out. I know that even if I do get it very wrong and sit on a rock, the worst that is likely to happen is a dent in the bottom. It will scrape off some paint, but that can wait until the next haul. A few years ago we were dried out in a small Cornish harbour and sat on the massive chains the local boats use to keep themselves off the wall in dirty weather. We left some anti foul on the chains, but could not see a scratch on the boat next time I was able to look. Meanwhile a fibreglass lifting keel yacht anchored outside and rolled heavily all night. When we ask them why they did not come in, they said they were worried about the harbour bottom. With good reason, if they had sat on the chains, it would have crushed gel coat and needed immediate attention before the after penetrated the laminate.
Ah, Colin, you list another reason for aluminum that you simply cannot get with any other material: the freedom to dry out without worry. Of course with steel you could do the same thing without any structural damage, but you’d have to attend to the paint pronto. And to do that would require a haul out to get at the area needing the paint.
While I’m on the topic, having the ability to dry out was at the top of my list of must-haves when searching for a boat. If one stays in the world of Travelifts and doesn’t mind often-extortionate fees for lifts, the ability to dry out isn’t a big deal. But if one goes off the beaten path or just hates being tied to marinas/yards/yacht clubs, having a boat that can dry out without fuss (meaning an ALUMINUM boat that can dry out) is fabulous. I love the Ovni hull, but my boat has twin keels. There’s no perfect boat (as John rightly reminds us often), but one not-so-little advantage of twin keel over lifting keel (which has its own advantages over twin keel) is that when you do dry out, the amount of bottom paint (including epoxy barrier coat) subject to being ruined is just on the bottom of the keels (and, in the case of my boat, the bottom of the skeg). In such a boat one can dry out often and the worst that happens is a sea farm growing from the bottom of the keels, instead of from the bottom of the hull (probably a larger affected area overall).
The upshot is that the only good way to do this is with an aluminum hull. With any other material one is likely to avoid drying out just to avoid the subsequent maintenance issues.
Yes there are always compromises and the flat bottom of the Ovni does mean that there are areas of the bottom that cannot be accessed until the boat is hauled. But, it does mean I worry less about finding somewhere level for three hard points to sit on.
Another plus about aluminium (as compared to steel) is that the swarf from drilling holes does not cause rust streaks. That is a nightmare with steel.
I built an alloy Dix 43, starting in 2009, launched in 2016. I painted the bilges and am very happy to have done so. I was concerned primarily with poultice corrosion in the bilges along the weld beads, stringers and frames. If you are not familiar with poultice corrosion it is created when organic material (sawdust, dog dander, skin cells, food particles, bacteria the usual unmentionable soup ) is mixed with a bit of water (condensation usually), leading to an acidic situation that just sits there in a crack or crevice and eats away at the bare aluminum. I used a Pettit system designed for painting aluminum. It involves three different coatings, the first being the acid etch primer, followed by a tie coat, then a top coat. The metal needed to be abraded mechanically just before the first primer was applied, then there was a strict schedule of following coats. It’s been about 5 years since I did this coating and not one flake of paint. If I did it again I would just use the same barrier coat system I used on the underwater part of the hull, that grey epoxy paint is tough stuff. The etching primer was the same in both systems. I did use this in the anchor locker, which I sandblasted prior to coating. Over the years (since 1983) as a blacksmith, machinist, fabricator and sculptor, I have learned that paint performs best when one follows the directions to the letter. The prep work is the key.
Hi Brian,
Each to their own, but I would still stick with my recommendation of not paining the bilges of an aluminium boat.
Our boat is now 30 years old and I have not seen any appreciable problems with poultice corrosion.
In fact, based on my own experience, I would worry more about water getting under the paint and being held on the plate by the paint.
That said, marine grade alloys are pretty resistant to poultice corrosion, so it probably doesn’t matter much either way.