Protecting Aluminium hull in seawater

[John_Hampton]

I have been told that a rivetted aluminium hull (ex-ships lifeboat/tender) will rot in seawater, is this true and if so what is the best way to protect it?

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[TheBoatman]

John
In a past life I owned an ali ex ships lifeboat and I did a complete refit on her. In doing this I contacted Blakes Paints and asked them the best method of protecting the hull. They supplied me with a complete painting programme which I followed and it served me well. It basically meant a complete stripping of all original paint, etching the hull, priming, undercoating and then top coating. You also need to pay very good attention to the anodes to be fitted.
I never had any problems with corrosion?

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[BarryH]

As Boatman says a proper paint job. The most important coat, imho, being the etch prime. Its this coat that all the other coats are layed over.

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[andyball]

perhaps rivetted ones are more trouble ?

we've a welded ali boat with fairly "easy peel" factory paint top & bottom. Bottom is antifouled & has been in continuously since dec 2001 with no problems at all. I fitted two anodes, but neither have corroded at all

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[circumnavigation]

Being based in France (west coast), there are a lot of Ovni's around here. You will see that they are not painted. What is of interest and this applies to all Ali boats is that they leave a cable hangling in the water (anoid attached) for protection purposes. Perhaps there is someone out there that can elaborate on this?

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[extravert]

I don't know where this requirement to paint aluminum boats comes from. For properly made welded aluminum boats it is not necessary to paint them to avoid corrosion. Maybe it is different for rivetted boats, but I have seen unpainted rivetted workboats in seawater.

I used to sail an Extravert which is an all welded aluminum 7m boat. Most Extraverts are unpainted and are over 20 years old and are doing fine.

There are precautions you need to take to avoid corrosion (like using the correct antifoul and anodes) but my experience is that painting them is not one of them.

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[DeeGee]

I am not an expert in aluminium hulls, but I know a bit about galvanic corrosion.
I suspect the dangling reference is because the anode has to protect a very large area of Al, so has to be high area itself, and will also waste quickly.
The rules are that the lowest metal around - if all metal elements are directly and effectively bonded, including your rivets - will galvanically erode. The erosion rate is determined by how much higher up the galvanic scale the 'nobler' metal is, and the relative areas. Now Al is not much further up than Zn, which is good news, but a lot of Al is bad news from the area point of view. I suggest plenty of anodes will be required, as the rivets are probably pretty much higher up, and will burn a hole in the Al if there were no anode to take the brunt. The Al will still be damaged, but not as much as if there were no anode.

If I were you, I would also fit a galvanic isolator, certainly if you are going to connect up to shoreside for power, not if you are on a swinging mooring. It is the earth connection you have to break.

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[richardandtracy]

When on holiday on the Gt Barrier Reef I went on the 'Quicksilver' cruise out of Port Douglas. The Quicksilver cats were made from unprotected 5086 aluminium. The vessels were 3 years old at the time, and not showing the slightest signs of corrosion despite the high temperatures (speeds corrosion up significantly - every 10C increase in temp roughly doubles the corrosion rate).

If there is any way you can find the alloy used, do so. If the alloy is anything other than 5086 or 5083 (not so good), it must be protected. Preferably by paint. If you can't find the alloy, assume the worst & paint. An etch primer is an absolute necessity. One to BS X32 will give a good bond, but tends to be picky about how soon the next coat is put on (24 hrs max) and the application temperature. With this paint system the rivets shouldn't be a problem if they're solid - BS X32 is designed for [solid rivetted] aircraft but used elsewhere too.

Regards

Richard.


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[Heckler]

must say i tend to agree, especially after reading the road test of those frog ones in one of the mags a while back, they stated quite categorically no paint necessary and the only reason it is used is because owners specify it for aesthetic reasons
stu

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[circumnavigation]

Remember, this forum does go worldwide, and those of us that prefer to have French administration, also means we are without that of the UK!

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[Heckler]

mais oui mon brave, mais mes amis francais me dit rost bif et je dit grenouilleur. pas de problem. quand je visite le SIB l'autre semaine, les hommes de beneteau invite mois et ma femme a diner sur le Royal Southampton Yacht Club, une bon nuit passe et nous faissons encore les amis francais.
ils remarquent un anglais qui parle francais? j'explique je ne suis pas anglais je suis un gallois, rost bif non, nous faissons l'amour avec les moutons!!

stu

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[TheBoatman]

Mine was a rivitted hull and I bought her off an "older" man who couldn't really keep her in fine fettle so she was showing signs of corrosion, but as I say I followed the painting programme and she was then no bother. BTW Blakes also said that the etching/primer coats were the most important. If I remember correctly we had to have 2 of us doing the job, 1 etching and 1 priming directly behind as it is important that the primer goes on to "kill" the eching coat and make it stick to the hull.

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[qsiv]

It certainly is an anode. The philosophy is that in order to get a decent bond and surface you need largisih hull anoides, but that they interfere with waterflow and slow you down. Also there is an increased risk of corrosion in most/many marinas/harbours (lets ignore earth leaks from mains, there are almost certainly mild steel posts holding it all together and Al cats as an sacrificial anode to steel). My boat has 6 large (12") dinner plat anodes scattered around the hull, but they are recessed into 'sockets' so that the hull itself presents a pretty fair surface to the water.

And yes galvannic isolation is a MUST if you have shore supply, and if you want to do it properly, fit leakage current detectors to the DC installation with a suitable test switch.

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[richardandtracy]

I'm surprised you were able to get hold of them separately. Etching & priming is usually a 1 coat operation these days in an etch-primer.

Regards

Richard.


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[TheBoatman]

Richard
I could be mistaken, it may have been etching/primer with the undercoat being the "killer" but what I do remember is that we had to do 2 operations one after the other?

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[Birdseye]

vous et votre femme, ou vous et votre mouton?

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[Heckler]

ma femme, on faire l'amour avec les moutons quand on est jeune et n'ais pas une femme.
stu

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[John_Hampton]

Thanks for your replies, they were for the benefit of a young man who recently bought my boat and was told by somebody that it would dissolve and sink if put in salt water! The posts in French are not much use (French is a foriegn language to me!) but I'm sure they are very meaningful to the people who posted them.

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[Heckler]

sorry about that, showing that the use of the word frog was not meant in a derogatory way and when a brit speaks french the french they take it in good part, they also know that when they call us french speakers roast beefs we take it in good part as well, and then because i explained that i was welsh and that we shag sheep here allegedly some one asked did i take my wife or sheep to dinner with the french at the sbs and i explained that we only shag sheep when we are young and not when we have a wife.
stu

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[vyv_cox]

Speak for yourself!

We have some very attractive sheep in Treuddyn, far superior to the ones around Llangollen.

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