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Old 02-04-2016, 16:12   #31
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Re: So, what can go wrong with aluminum?

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Originally Posted by bdbcat View Post
Folks...
I've owned and cruised a large aluminum power-cat for 16 years.

FAQs About the Boat

Two points to make:

1. The number one corrosion issue is caused by small particles of of stainless/steel/copper accidentally dropped into a wet bilge. Especially copper, as from shards of stripped wire which inevitably result from electrical projects.
The corrosion is known as "exfolitiate corrosion", and occurs mainly in rolled Al plates. It is as though the foreign metal is "melting" into the Al plate, creating a coin-sized patch that eventually flakes off. The result is locally reduced plating thickness.
So, one must be scrupulous about clean, and preferably dry, bilges.

2. Antifouling: This problem is essentially licked by using an epoxy barrier coat to electrically isolate the hull. Normal high-copper antifouling may then be used over the top. I have seen much more internal corrosion than anything attributable to galvanic action with copper paint.

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Dave,

Very good information here

Can you elaborate on how the epoxy coat is applied. What type of epoxy, prep, and application method. Why not epoxy coat inside bildges the same way?
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Old 02-04-2016, 16:53   #32
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Re: So, what can go wrong with aluminum?

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Steve, how is the lead isolated from the hull? Friends with French alloy boats have had serious issues with such interfaces and corrosion... their internal lead ballast was "potted" in coal tar epoxy mastic and it failed.

Jim
Jim,

My father coated the inside of the keel with some kind of paint prior to adding the lead and cement. I was only 6 years old at the time but I am quite certain he did NOT do a 'proper' job of prepping the metal (no blasting, no etching).

The idea was for the lead to be separated from the metal by the cement. However, in at least one place, I can see where he was having trouble getting the trapezoidal shaped, 50 pound 'pigs' to fit. This resulted in the lead touching (or very nearly touching) the painted metal.

Of course I cannot inspect everywhere beneath the cement but I have chipped small areas of cement away and have NOT found corroded metal.

I believe the success of this installation of cement and lead is a result of all of this remaining DRY. The perpetually wet 'sump area' that I mentioned previously, is located below and several feet behind the lowest ballast area.

Someday, I would reeeeeeeeely like to chip all this cement away, and recast the lead into shapes that fit the keel properly. I would do a proper sandblasting of the keel, paint, and then encapsulate in epoxy. Might even weld cover plates on top and inject more epoxy to fill the void. My primary motivation would be to consolidate the lead in a lower and more centrally located position. It would also look a hell of lot more tidy and would increase bilge storage.

Was the ballast of your friends boat at the 'bottom' of the keel and therefore wet?

Steve
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Old 02-04-2016, 17:13   #33
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Re: So, what can go wrong with aluminum?

Steve, the boats in question are Via 39s (IIRC). They are typical Trismus derivitives, with a very flat shallow bilge and two centerboards. I can't speak for all of them that had the problem, but our friend's boat is well maintained, has a saildrive and I expect that the bilges were normally dry. As in all boats, there likely were times when salt water got in and wet things down, but surely not chronically damp.

It's interesting that your Dad's kinda low tech concrete method has succeeded where the "modern" epoxy based stuff failed! Your proposed refit sounds like one hell of a job, but I can understand the desire to have it done... but not to DO it!!

Jim
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Old 02-04-2016, 17:28   #34
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Re: So, what can go wrong with aluminum?

@pbmaise...

Check this out....
SetSail » Blog Archive » Aluminum Paint System Update

Works for me, too...

I decided not to coat the bilges with an epoxy barrier for the simple reason that the space is very intricately framed. Scraping, sanding, and recoating every few years would be a real chore. Spraying would be extremely disruptive to life aboard. Better for me to inspect the bare metal periodically, sponge out the usually dry bilges, and vacuum as necessary.

Dave
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Old 05-04-2016, 00:06   #35
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Re: So, what can go wrong with aluminum?

Corrosion with aluminium is quite often because it is a very reactive metal. However, for our industrial corrosion protection requirements we can adopt some powder coating or liquid coating methods.
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Old 05-04-2016, 05:08   #36
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Re: So, what can go wrong with aluminum?

Greetings and welcome aboard the CF, richar.
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Old 05-04-2016, 07:48   #37
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Re: So, what can go wrong with aluminum?

I find it fascinating reading that CIA article and all of the other information on Ti. Thanks to everyone who has contributed on that topic in this thread!
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Old 05-04-2016, 08:13   #38
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Re: So, what can go wrong with aluminum?

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Originally Posted by bdbcat View Post
I decided not to coat the bilges with an epoxy barrier for the simple reason that the space is very intricately framed. Scraping, sanding, and recoating every few years would be a real chore.
We did not paint or bilge for the same reason . . . but we DID paint the bilge sump. It was a smaller simpler space and that's where water would sit if any got in the boat. We really heavily epoxied it, and it in fact only had to be done once - never got any blisters or peeling.

I might comment that ALOT has to do with the specific ally used. Much has been written about the pitfalls of copper filings or stainless screws lying in the bilge. But with out hull, 5083 H113 aluminum, those things in fact did not seem to cause any noticeable problem. I tried not to have them happen, but several times they did and I found much later, and never saw any aluminum problem/pitting in the areas.
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Old 05-04-2016, 08:24   #39
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Re: So, what can go wrong with aluminum?

I am no hull expert, but when I first saw this thread, I thought.
You ought to see my aluminum tanks, they are my biggest concern about my boat, I sure wish I had plastic tanks, the holding tank lasts about 10 years I think before a hole gets eaten through
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Old 05-04-2016, 08:24   #40
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Re: So, what can go wrong with aluminum?

Dave nailed it. I have a 47 yo aluminum boat. Made from 5000 series alloy. If built strongly and not abused, cracked welds and fatigue are not a real concern any more than a hose popping off a thru hull. Can it happen? Absolutely. But proper use and care go along way towards preventing these incidents. In the grand scheme it should not keep you up at night.

Paint is an issue. Aluminum does not like to keep paint on it unless you go through meticulous prep work. I'm busy using the boat so I just do my best and figure I'll be repainting every seven years. Usually I need to do touch up way before that from scratches accumulated while using the boat.

I sandblasted the hull 16 years ago and applied high build epoxy from the waterline down. This has virtually eliminated all exterior corrosion issues where they matter most.

The inside of the hull is what needs watching most, as pointed out previously. I'm so paranoid, i throw all pennies (US) in my pockets into a special jar so they cant accidentally make their way to the bilge even through there is very little real copper in them at this stage.

An Isolation transformer for the AC side is also good investment.

With some basic maintenance, my boat should outlast me.

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Old 05-04-2016, 09:12   #41
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Re: So, what can go wrong with aluminum?

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Originally Posted by estarzinger View Post
We did not paint or bilge for the same reason . . . but we DID paint the bilge sump. It was a smaller simpler space and that's where water would sit if any got in the boat. We really heavily epoxied it, and it in fact only had to be done once - never got any blisters or peeling.

I might comment that ALOT has to do with the specific ally used. Much has been written about the pitfalls of copper filings or stainless screws lying in the bilge. But with out hull, 5083 H113 aluminum, those things in fact did not seem to cause any noticeable problem. I tried not to have them happen, but several times they did and I found much later, and never saw any aluminum problem/pitting in the areas.
This is one of the things that concerns me on homebuilds. A commercially built aluminum hull should be made from at least two, but preferably three seperate alloys. They really do have different corrosion resistance, stiffness, and strength. When properly done you get a much better boat than just using the same alloy everywhere.

5083 is much more corrosion resistant than 6064 for instance, but 6064 is much stiffer. One goes under the waterline to prevent corrosion, the other becomes the bridgedeck to stiffen the hull. By mixing and matching the alloy to the usage you get a lot of advantages for minimal cost.
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Old 05-04-2016, 21:02   #42
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Re: So, what can go wrong with aluminum?

We have a 6 metre power cat that is 36 years old. For the past 10 years it has had the continual problem of corrosion, tiny little holes that mostly follow along the hull in a line. The hull has not been able to be welded for some time due to the condition of the aluminium, it just blows out. We have used Sikaflex to keep it afloat, but a month ago we bit the bullet and purchased a much newer cat of almost identical size.

We also had a weld fail at the top of the inside starboard hull about 10 years ago. Very frightening when the hulls filled with water. Luckily we were inside a large (but relatively protected bay) and the floatation material kept us afloat till we reached the boat ramp.
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Old 08-08-2017, 04:26   #43
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Re: So, what can go wrong with aluminum?

Fastener, bilge, stray current, inability to use copper based antifouling, difficult to paint and paint doesn't last. Unlike steel which retain 90% of its strength on its welding joint, aluminum only has slightly more than half its strength.

With all these drawbacks and despite steel rusts, would you guys say that steel is easier to maintain than aluminum alloy?

I understand that aluminum boats retain value better than its steel counterparts, how about in terms of insurance cost?

There are more super yacht build in steel, or some with aluminum structure and steel hull, would this be a good compromise even for the smaller boats?
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Old 08-08-2017, 07:21   #44
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Re: So, what can go wrong with aluminum?

Well not to much really as long as care is taken with electrics and dis-similar metal fitments, and i think has many advantages as:

1) For longevity. For good resale value. For the benefit of being able to create a custom design and build it economically, without the huge penalty of having to build a mold first, as with fiberglass. For freedom from the stench of fiberglass, and from the dread fiberglass boat pox. For repair-ability. For lightness and strength. For the competitive edge in performance. And most importantly, for the security of safe cruising.

2)Fact: It takes over 60,000 pounds per square inch (psi) to tear apart a chunk of mild steel, and 30,000 psi to deform the same piece; to make it yield. With aluminum, around 45,000 psi will tear it apart, and around 35,000 psi will deform it. Yes, you read that correctly: size for size, aluminum has a higher yield strength. In these facts lie the extreme benefits of metal for hull construction: The "plastic range" of either metal is quite high, so the material can take a terrific beating without failure.

3) Another significant advantage with aluminum is that there is no need to sandblast or paint the interior (or perhaps even the exterior !)

Disadvantages:

1) Aluminum is subject to electrolysis, pitting and crevice corrosion, but these liabilities can be managed as long as the installation of dissimilar metals and electrical items are correctly done. After that, it is a matter of attending to these matters during the life of the boat.

2)Aluminum's extreme lightness can introduce a faster pitching and rolling motion in some hulls

3)You do have to insulate an aluminum hull

4) Building a bare hull costs just under twice as much as the mild steel to build the same design.
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Old 08-08-2017, 09:09   #45
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Re: So, what can go wrong with aluminum?

The other thing about aluminum is that it is far less stiff than say steel. It's "bendy" because it's modulus of elasticity is less than half that of steel IIRC. Thus the boat flexes a lot. Flex then creates cracking... in welds and also parent metal too. It's not usually a huge deal, but on some designs can be rather rapid in occurring in specific places.
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