Advice needed about aluminium hull in sea water

[thatmandan]

I've recently bought an ex-lifeboat (built 1954) with an aluminium hull. My hope is that she can be happily moored in sea water as a live aboard. Problem is that I've been getting lots of conflicting advice. Some people say that the salt in the sea water will be the end of her (as old lifeboats weren't designed to sit in the sea full time) and that sheathing isn't an option because she's covered in rivets which are 'proud'. Other people seem to think that I could get away with the right primer + paint but that I will need to get a few sea water anodes (to protect against electrolysis) that will need to be replaced on an annual basis (sounds costly?). Still others have suggested painting the inside with tar or similar. The only thing that people seem to agree on is that I will need to replace the rivets that have oxidised already. Any help with this would be much appreciated - It's all getting a bit confusing!

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[DeeGee]

Ovni boats are aluminium and v nice too.
You have two galvanic problems. The first is to protect your boat against itself. Wherever there are dissimilar metals, corrosion may occur. Which of the two corrodes and how fast, is determined by their position in the galvanic table. The lowest will corrode, and faster the bigger the gap. If you can bond the hull to another metal, lower than Al (that will be Zn) then it will be that other metal, a sacrificial anode which is protected. If you have any 'higher' metals in through-hull like your rivets, then your anode should protect the al if the electrical bonding is good. The size of the anode is determined by how much material it is protecting, so I would expect the surface area of the anodes to be quite large, probably a fair number of anodes will be required. The more you have, the less often you will have to change them.

The second galvanic attack may come from other boats. If you connect yourself shoreside, you can set up a galvanic couple with other boats, or shoreside metal structures... the way to stop that is to isolate your boat from the shoreside supply, either expensively with an isolation transformer, also large, or cheaply with a galvanic isolator, which goes in series with your earth connection.



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[Jaguar]

Be wary of aluminium livaboards if you plan to moor up close to anything made of steel (i.e. piles, keyside etc.). Even without touching, galavnic attack will soon eat the aluminium. Good paints do help, but do not use anything conatining copper, lead or mercury. Anodes are not strictly neccessary with aluminium (unlike steel), but they may help. I would not bother painting the inside. Just keep it spotlessly clean. Do not drop anything of copper in the bilges or it will soon eat its way through. I am surprised the rivets have corroded through, unless she was moored up next to something steel or if the aluminium used was not of a maruine grade. Read Corrosion in Metal Boats for detailed advice.

JJ

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[thatmandan]

Thanks massively for that - very helpful + reassuring as to my next steps forward!

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[thatmandan]

A big thank you to you too - it's extremely refreshing to get advice from people that seem to know what they're talking about and are able to point someone in the right direction. The amount of people out there that feel the urge to offer advice when they don't really know what they're talking about is incredible!

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[DeeGee]

A little illumination on what happens to Al boats near steel pilings etc.

Galvanic erosion occurs when two dissimilar metals are immersed in an electrolyte (thus forming something we will loosely call a 'battery' **see below).
Now, just like a battery, nothing will happen unless you create a circuit in some fashion. This can be caused by the common earth on a shoreside power supply or something similar. If the aluminium hull and the steel pilings or another vessel's steel hull touch - then a circuit is formed. But without the circuit, there will be NO erosion. Put your boat on a mooring buoy in the middle of a load of steel pilings, and no circuit will be made - hence no erosion.

There IS one exception (there always is, isn't there). If your vessel has two metals (maybe rivets) of which the other one is higher up the galvanic scale than the steel pilings. Then we have two 'batteries'. One is connected in series with the other, and v rapid corrosion may occur. Answer is to have aluminium rivets.

What are you rivets, Aluminium alloy, monel, steel? It will not be the rivet which erodes, but the metal around it, ie the aluminium. However.... good news. If you had a little ally plate stuck on a plate of steel - that plate would be like a sacrificial anode 'protecting' all that steel, and would vanish in two ticks. If you have a vast ally plate, with little bits of steel attached to it, then the ally plate, being so much larger, would erode only slowly. This is roughly the situation you have here, so erosion, yes, fast, unlikely.

(** a battery is an assembly of cells, so strictly we are talking cells here, but it is easier for non-techy to think battery, a more familiar object).

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[Jaguar]

By 'erosion', I think you mean 'corrosion'. It is a fact that one does not need a metal link to create a galvanic cell (e.g. between an aluminium boat and a steel boat). Sea water is sufficiently conductive to facilitate galvanic corrosion. Of course, if the other boat was clad in copper (even more noble), the situation would be far worse. Thus, as a general rule, one should not park an metal boat next to one that is more noble for extended periods of time, otherwise one may find ones pride and joy acting as the anode and cathodicaly protecting the other boat! This advice can be found in many books that dela with the subject of corrosion in metal boats.

JJ

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[MainlySteam]

<<<This advice can be found in many books that dela with the subject of corrosion in metal boats.>>>

I have never seen an authoritative one that does say that.

You do need a conductive link between the boats, apart from the electrolyte, and that link is normally the shore cable earth conductor (and sometimes the neutral, if ones boat is incorrectly wired with the neutral remaining connected to the ground conductor on board when the shore switch is made). That is the point of using an isolating transformer or galvanic isolator in the shore earth conductor on a metal boat.

John




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[Jaguar]

Try reading Corrosion in Metal Boats by Nigle Warren.

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[MainlySteam]

Maybe he is the only one who thinks so. The belief is marina legend.

I project manage the design and construction of large aluminium vessels, mainly for commercial clients, and have to say that no one I know has the same idea or concern. A commercial aluminium vessel would not be of much use at all if it had to be careful about not mooring beside steel piles or steel boats. I am sure you will also find that managed aluminium yachts (super yachts for example) do not fuss about having berths clear of steel.

Our own pleasure vessel is steel and we have spent long periods with aluminium boats only feet away from us in marinas with no one being at all concerned about nor suffering any corrosion.

John

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[DeeGee]

Sea water is acting as the electrolyte in the battery, as this is a galvanic erosion, not a chemical corrosion, and the current will only flow one way in the electrolyte.

Perhaps the books you read are wrong, or oversimplifying what is not a very complicated subject. Just work out what is anode and cathode and where is the circuit. Remember that current will only flow one way through the seawater. I agree with the sentiment, don't park near piles, steel boats etc, but nevertheless, you will not have a problem until a circuit is completed in some way.

I meant erosion, the difference is subtle as some people reserve corrosion for chemical attack eg. acid attack,oxidation; erosion for electrolytic, others don't. I am quite happy with 'corrosion'. -)


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[MainlySteam]

I agree, and can I add to your comments DeeGee that if the claim that non electrically connected boats of different metals (or a boat and a steel pile, for example) acted as a battery, then real batteries would never be any good.

If one considers any type of battery made with plates in an electrolyte, if the negative and positive terminals are left open circuit then the rate of self discharge is very small. In such batteries the two terminals are quite close together (mm to cm's for typical batteries) and the plates are almost touching in a very conductive electrolyte (when compared to seawater).

Two non electrically connected boats in seawater and of different metals are going to "self discharge" at a very much lesser rate than an open circuit battery will discharge.

Generally, if ones boat is electrically isolated from other vessels or structures then the rule is that if you have a corrosion (or erosion) problem then the problem is entirely ones own.

In a related manner, a very common error that one sees on boats is the placing of anodes on shaft/prop when that is electrically isolated from the rest of the boat with an insulated coupling. A good bronze prop material and stainless steel shaft will live quite happily connected together and have no interaction with the other metals on a boat (because they are electrically isolated from them). However, if one then puts an anode on the shaft one immediately creates a battery with the shaft and propellor implicated so the anode erodes. The boat owner happily thinks he is protecting something, wheras in fact he is just eroding zinc for no purpose.

John

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[DeeGee]

Unfortunately, that is not quite true.
The propellor is made up of an alloyof materials (i.e. not a chemically single molecular compound) with each material giving its mechanical properties into the whole, structurally.

Brasses and manganese bronze tend to dezincify due to the creation with the seawater of millions of 'microcells'. These are little closed circuits of the dissimilar metals which couldn't be better bonded, and the seawater electrolyte. So they become cells in their own right, and a further cell with the s/s shaft forming a bigger and more obvious cell.

It is likely that a prop will dezincify to the extent of bits dropping off in less than two seasons. If you want to check your prop, hit it with a hammer (genlty) and it should ring. If it doesn't then it is dezinced and on its way to becoming brittle (hece gently!).

This is not a theory, please don't post contradictions. I know exactlywhat I am talking about, and can cite references if anyone doubts me, and can PM me for them.

This is well-established fact, and you MUST protect your prop. I am sure someone will post a story "Well, mine has been in the water for umpteen years with no anode.....", please ignore. -)

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[MainlySteam]

Well I do have to say many vessels, including my own (which is steel), have been in the water for many years and as long as the correct materials are used it is true that protection is not necessary.

Please note, I importantly limit my comments to shafts and props which are electrically isolated from the rest of the boat by means of an insulating coupling and then only if an appropriate bronze and shaft ss are used. I agree completely with your comments about the dezincification of brasses and manganese bronze, but neither of those are what would be regarded as good materials for propellors and will suffer failure (in fact they, especially brasses, are probably in the class of inappropriate materials). As for almost every metal in seawater there will, of course, even for good bronzes be slow failure at the microscopic level, but not at a rate significantly different to as if the metal was immersed by itself and which is not important within the lifetime of the vessel. As you correctly point out, for poor bronzes (or any other inapproriate alloy) failure at the microscopic level will be much quicker.

I know many boats are built of inferior materials, often with the owner being unaware of that being so, and I completely agree with your comments in that case. In fact some owners may make a concious decision to go with inferior materials and protect them, but even so in that case the poor bronze will erode faster at the microscopic level than an unprotected good one. Sometimes materials are accidently inferior when produced at the mill (although not so commonly now), and I have come across marine grade aluminium plate which has failed because of a known incorrect metallurgy problem.

Regards

John

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[DeeGee]

I was referring to aluminium hulls, which are low down on the galvanic scale and thus likely to end up as anodes for someone's party.

I am afraid you are wrong about poor materials, I am referring to the best bronze propellors.

I do not wish to end up in an argument. I have some expertise in this area, and have explained what happens if you leave your prop or prop+shaft isolated and without third-party insurance (ie an anode). I feel I can add no more. Anyone reading this with a personal interest should clearly seek independent advice elsewhere.

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[DeeGee]

From one of the world's leading experts.

"Brass should never be used in saltwater, and those with 15% or more zinc
will rapidly dezincify. Bronze alloys are better, with aluminum bronze and
nickel-aluminum bronze being favorites for marine propellers. Certain of
these alloys will de-aluminify, so alloy selection is still important. The
Navy uses cast nickel-aluminum bronze alloy C95800 to specifications such as
ASTM B148 and a heat treatment such as that in MIL-B-24480 (use the 1300F
heat treatment to minimize corrosion). A wrought equivalent is alloy
C63200. I'm unsure of the best aluminum bronze alloys to use in seawater,
but C95400 or C62400 are possibilities. Stainless steels are also popular
for small propellers.

Any of these propellers must receive cathodic protection from a
well-designed system. To ensure minimum corrosion and to protect your
bearings, a propeller shaft ground strap should be installed to short the
shaft to the structure to which the anodes are attached. Without cathodic
protection the stainless steels can fail rapidly and even the bronze alloys
will fail eventually.

Finally, in choosing a propeller alloy make sure that it is galvanically
compatible with the shaft material or that the resulting galvanic couple can
be overcome by the cathodic protection system. And never, never use a
copper-based alloy propeller with an aluminum hulled boat."

Harvey Hack
Northrop Grumman Corp., Oceanic Systems

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[MainlySteam]

Fine DeeGee. I think we are really on fairly common ground with only a small point between us.

However you cannot claim I am wrong without expecting a response.

You finish up by saying that people should take independant advice, but it is a fact that I am a person who international clients come to and pay for such independant advice with respect to the design and construction of their vessels, mainly aluminium ones. To establish that I am, for example, currently managing the design and construction of two 23 m fast (2,100kW each) aluminium catamarans currently under construction in Australia to go to a foreign client, and am currently working at the concept development stage with a foreign client to have designed and built an approximate 35 m fast catamaran (about the same size as Red Jet 4 recently delivered to near you to serve Isle of Wight). I have also been, for considerable number of years, closely associated with assisting a company which employed 35 surveyors of ships.

The fact remains that I, and others, have seen good bronzes (and Manganese Bronze is a very poor performer because of its zinc content) survive quite happily without protection in the situation as I have set out. There is no reason why they should not. I am the first to agree that if one does not know what materials were used in their own vessel they should proceed conservatively.

Don't get me wrong DeGee, I respect your views greatly, you have spoken a great deal more sense that most others have on the subject. I trust no hard feelings, certainly I hold none, and I only wish we could share a beer together sometime.

Regards

John

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[DeeGee]

OK, we are getting closer....

Unfortunately, we don't have much control over what prop manufacturers make their props out of. It seems the material is chosen for easy machining, cost etc, and longevity and resistance to corrosion are not necessarily high on their factor-list.
I am an ELECTRICAL engineer with a special interest in electrolytic corrosion, and I could be really mean and point out that BOAT designers do get it wrong a lot of the time. You are probably working in the sort of league where you DO have a say in what materials are used, but I have a lovely example on my desk right now, of a prop about 2 years old that a child could break with a pair of pliers... all the classic pinky chrystallinw structure of dezincifying... if people want to err on the side of safety, they will listen to me, and stick an anode on their propshaft, or get some brushes on, so it can be connected to the main bonding. That can do no harm, but if you are wrong, then leaving off such anode can do significant harm. It is not really such 'a small point'.


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[thatmandan]

I think you both know an incredible amount on the subject and am truly grateful for the knowledge that I am gaining from the discussion. The one gripe that I have is with myself and that is purely over wishing that I'd paid a bit more attention during science lessons at school!

I'm sure that all will work out fine in the end though. My main fear (about buying a 50 year old ex-lifeboat and being responsible for causing her one of the most unworthy endings imaginable) is no longer keeping me awake at night, and this is largely due to you guys.

I remind myself that all solutions were once unsolvable problems and look forward to learning more. And for all my grievances with computers, I am wearing a wry smile at the moment about how it has proved a great way of bringing like minded (and more importantly like spirited) people together from all over the world.

And before I really go off on one...

I'll clearly keep this thread posted of any developments, but any other general tips (re: maintenance or live aboard) will always be much appreciated.

Keep smiling, sailing and speaking,

Dan


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[vyv_cox]

Quite a few years ago I had the pleasure of berthing alongside War Baby, a well known long-distance aluminium cruising yacht, at Crosshaven. Her crew placed 8 or 10 anodes on wire cables all around the hull. It is probable that these were zinc, which is lower in the galvanic series for seawater than aluminium despite having a higher electrode potential. The information given <A target="_blank" HREF=http://www.abdaluminumyachts.com/pdfs/why_alum.pdf>here</A> suggests that a large mass of zinc is required.

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