Aluminium hulls

[nik]

Hi, I hope some one can clarify this for me.
I had a look at an Aluminium hulled rib some months ago, there was signs of corrosion above the waterline (bubbled paintwork), which could have been fixed. But what concerned me more was the possibility of corrosion inside the hull.
1. How can you detect it
2. How can you fix it
I have no idea if there are inspection hatches or the like, but I do know that unprotected aluminium in salt water can corrode quite quickly. There is marine grade aluminium, but if this boat was made of it, then it will still corrode.
I would imagine that there could be problems with electrolysis as well.
Thanks, Nick.

[Garygee]

Hi Nick

Suggest you talk to Ocean Dynamics in St Davids.

They build all their RIBS from alloy.

The other person is Crazyhorse on Ribnet.

Think the RIB he was looking at had a alloy hull so he may have done some digging and be able to help.

Regards Gary

[thewavehumper]

PEPE Boatyards in Hayling Island also make Ali boats, they might give yo advice

[Cookee]

Originally posted by Garygee

The other person is Crazyhorse on Ribnet.

Think the RIB he was looking at had a alloy hull so he may have done some digging and be able to help.

Regards Gary

He's having one built in Canada

[Searider]

Nik,

1. Its very difficult to detect corrosion inside hulls without getting access. If there are inspection hatches or if the deck unbolts then once you have got access corrosion is very easy to see.
Ultrasonic inspection from the outside is an option but is unreliable at finding localised pitting - which can lead to holes etc.

2. Fixing requires the corroded sections to be cut out and new sections to be welded in.

3. Anodes are normally used to protect the exterior hull surfaces - ne reason why they can't be used in the bilge as well but the point is to keep the water out of the bilge.

4. Finally, so you dont feel too bad - everyone with an aluminium, and to a lesser extent stainless steel under deck fuel tank is at risk of hidden corrosion should water sit in the bilge for extended periods.

When was that last time everyone inspected their fuel tank?

Duncan

[nik]

Thanks for the replies.
These issues put me off the boat and someone else bought it. But I have seen it back up for sale again at around the same price??? I wonder why.
Nick.

[Steel Dev]

Just thought you may be interested in this web site if you come across any problems with corrosion, its very informative & helpful

http://www.corrosionsource.com

[nik]

Thanks Julie, I am constantly amazed at what you can find on the internet. Curiously, one tip would appear to be not to paint your hull below the waterline, but maybe impractical due to fouling.
Thanks all the same, Nick.

[thewavehumper]

was this a 7 metre ish jet drive project rib that needed a bit of work doing to it?

[nik]

No, it is a NZ cabin rib 6.8m sterndrive. I have just noticed it was for sale in june 2001 as well.
Nick.

[eksrae]

bubbled paint work is nothing to worry about. it is normal on an aluminum boat. paint will last and not bubble as long as all the holes are drilled prior to painting. unless there is actually pits i would not worry. aluminum is far superior than fibreglass. no need to woory about chipped gel coat. do check for cracks around any stress points as well.

[nik]

Hi Eksrae,
In its original condition, I am sure that aluminium is far superior to fibre glass & resin. But I have seen examples of aluminium that have been reduced to the consistency of a cheese cracker because of corrosion.
Now, it maybe that this has been pure aluminium and I am pretty sure you use aluminium alloys for boat building, which may corrode much less. But I dont really know, which is why I am asking the question.
The fast seacat ferrys are aluminium so I guess it may not be so much of a problem, but these are proffesionaly maintained boats.
I may be being paranoid, but at the moment I dont know.
Another thought has just occured to me with respect to the paint blisters. The paint has not cracked, but there is corrosion between the paint and aluminium. How did the corrosion start? was the seed of corrosion there when when the hull was painted?
Dunno
Thanks for the reply, Nick.

[eksrae]

if proper prep work was not done or the wrong paint/primers were used than the paint has no hope in lasting. alot of stuff here gets powder coated. it only seems to look good for the first year or two than it bubbles/blisters and looks like crap.

I have not seen a painted ali boat that looks good years down the road. the aluminum likes to oxidize and once it gets a place to start it will keep going.

thing to check for would be nothing is grounded to the hull. this is hard to do but worthwhile. some do ground items thru the hull and this will lead to holes where you do not want them.

a good check on the companys history will tell alot about there product.

[nik]

this was a new zealand make. I have tried emailing them a couple of times about this, but I have had no reply so far.
But as far as I can tell they are a big company, at least they make big boats.
This is an example of what I found:

[IMG]C:\pics\boats\rust.JPG[/IMG]

There were other patches similar to this.
So, if I wanted to take on a repair job involving somthing like this, I would need specialist paint. I have used a two part aluminium primer in the past, but there is a limited time to use it and it was cyanide based.
But the main point is, can you tell if your hull has been eaten away from the inside, or is it even likely to happen.
Nick.

The layoput is a bit different to what I intended cos I cant get the hang of these bloody pictures
Its meant to be up there where it says [img]

[eksrae]

i would check at the stern on the inside. water that sits in there with a bit of stray current will eat holes. anywhere else should be fine. unless there is alot of water sitting in there than i would check more. i know of a boat that corroded bad cause it sat on a bunk trailer and the carpet
was always wet. got rid of the carpetand the boat is now fine.
led pellets from your weight belt getting in the hull is bad as well

the picture looks like typical blistering from moisture getting behind the paint. nothing to worry about.

[Neville]

Nik,

I have had a couple of alloy ribs, both ex north sea standby where they have been worked hard. Full of dents etc and corrosion.

As you may remember from your physics days aluminium is quite a reactive metal, it wii slowly sacrifice itself for other less reactive metals such as steel, stainless etc.

I found alot of the corrosion on ours was close to lifting hooks and other fittings that were made from a dissimilar metal. Also remember that the dissimilar metal do not have to be touching each other, the anodic / cathodic process will work over a distance, so technically if you have a supertanker parked next to your aluminium boat for a few years....

And yes, sometimes it has been known to corrode or delaminate from the inside out - but i havent seen this before, this will probably be due to impurities inside at the time of manufacture.

For another prime example of aluminium corroding, just look at your average outboard motor.

You can always have your hull gritblasted with a very light expedable grit and then painted with a fairly sophisticated scheme if you are that worried - as for me, I welded the repairs on ours, and that will give them another 5 years good service. You could also look into different and slightly more reactive anodes for extra protection... good luck

[nik]

Ok, I feel we are getting to the nub of this now.
So, aluminium is relatively safe from corrosion apart from where a dissimilar metal has been introduced (or bolted on) and I think I have heard of magnesium anodes being used.
So, what about the bubbled paint? either there was a corrosive product there when the metal was painted, or the paint is porous in some way, or the paint is damaged (not obvious), or somehow moisture can creep between between the paint and metal through capillary action.
Any thoughts, Nick.

[Neville]

Nik, yes either the paint has been damaged - doesnt look like it in your photo, or if you rule out electrolytical corrosion, possibly the surface has not been primed / prepared properly before it was painted. On any surface the preparation & priming is the key to a proper coating. Forget all this one coat , paint on top of rust etc.

Aluminium is quite a slippery metal and doesnt take coatings well compared to steel so it needs a good primer. Some that are used include an etch primer which chemically etch the surface giving it a good key, others include epoxies which stick like poo to a blanket.

I suppose the only way for you to get to the bottom of the corrosion is to scrape it off and see how bad it really is.

A safer way to do this would be to get a surveyor who has alot of experience in aluminium hulls to check it for you.

In most cases the designer will have some corrosion allowance built in to his design. This is commonly put down as a percentage. So if its lost a millimeter or so it might still have alot of life left in it.

Also remember that some insurance companies will want a survey depending on the age.

[nik]

Thanks for the input, for myself, this was an academic exercise as I walked away from this one. But it is good to be better informed.
And for some others reading this it may be a bit more than academic.
Nick.