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Old 04-12-2015, 18:55   #1
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Classic Aluminum hulls

I am giving some consideration to getting an Aluminum hull for some long distance sailing due to some of the advantages, such as no hull/deck seam and very few deck penetrations etc. I appreciate the classic lines of some of the older designs so have been looking at a few boats as old as 50 years that have passed a gauging test and show no obvious external corrosion..but wonder what still might lurk under the fairing and finish? Could there for instance be a lot of pitting that has been filled that gauging would not be able to detect? Is there anyway to be sure short of removing the filler and finish to tell? Can the plating lose appreciable strength due to corrosion that does not show up in Ultrasonic testing? Should one avoid aluminum boats this old that have lived in saltwater for this many years or can they still be in good shape if well cared for? Thanks for any input.

James
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Old 04-12-2015, 21:16   #2
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Re: Classic Aluminum hulls

In my experience, older alloy hulls can have issues with isolated pin holes and pitting in a few areas that are very hard to detect with ultrasound. Usually areas like under engines, tanks and inside watertanks with the chlorine.

Reasonably easy to cut out and weld in a patch, once found. Not really a major strength issue as they are normally spread out.

Another possible issue is that the older alloys were rolled, and they can get inclusions inside the plate that can cause some internal corrosion and spalling. I've seen the odd coin sized bit that flaked off. Again not a big issue, but worth being aware of.

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Old 04-12-2015, 21:20   #3
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Re: Classic Aluminum hulls

Hi James, I won't comment on the questions about filler and thickness gauging as I have no experience with either on below water line aluminum.

My boat has been in the water (off and on) since 1982 and has had virtually no below waterline corrosion. The tiny bit of insignificant corrosion that I did find occcured on the tops of some welds that had the protective epoxy sanded off (accidentally. by my father) and not properly re-coated. Also, this area was about 20 feet away from the nearest zinc (zincs are now located more frequently along the hull).

Another area to look at is the inside of any built-in water tanks that use the hull as part of the tank. My tanks have had water in them continuously since 1976. They are un-coated and they currently have some sporadic small pits that have a depth of approximately 20% of the plate thickness. I keep an eye on them if they ever increase to 50% or more of thickness I will address by welding.

I think your concerns about corrosion being covered up (intentionally?) are valid. Perhaps even better than thickness gauging would be to somehow determine that the previous owner is honest.

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Old 04-12-2015, 21:33   #4
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Re: Classic Aluminum hulls

SnowP, I had not heard of that inclusion problem with rolling the metal. Does that occur because of some contaminant being forced into the metal by the wheel? Or?

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Old 04-12-2015, 22:09   #5
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Re: Classic Aluminum hulls

Not really 100% sure of it either, but I've heard of the rolling issues from a few sources, and seen what looks like the results, with an almost grainlike corrosion pattern. I've seen one example on snowpetrel 2, under the engine. About 15mm (3/4 inch) diameter or so.

I assume that a pit reached the inclusion area, then started corroding outwards, rather than going deeper. I was able to prise the top piece off, and since I was replating under the engine it all got removed and nice shiny new 6 mm plate replaced the old pitted stuff.

It had had a hard life under the old rusty bukh engine for 35 years with no cleaning and salt water lying in it from the leaky stern gland most of the time...

I found one small pit that extended through to the filler, in a nice 3 mm wormhole, but the rest of the pitting didn't get deeper than 2 or 3 mm on 6 mm plate. I'm guessing that a bit of copper wire might have caused the hole. It was very plugged with tight corrosion products and had 5mm of tightly adhering epoxy bog on the outside so it was still pretty solid.
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Old 05-12-2015, 00:16   #6
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Re: Classic Aluminum hulls

Sounds like the flaking might be caused by intergrainular corrosion caused by the rolling process. Also called exfoliation corrosion. http://apac.totalmateria.com/page.as...ite=ktn&NM=187

But overall a well built and looked after alloy yacht less prone to slow degradation than many other materials. Unless someone has deliberately hidden issues or areas are out of sight most big problems should be easily spotted.

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Old 05-12-2015, 05:54   #7
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Re: Classic Aluminum hulls

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Originally Posted by Snowpetrel View Post
Not really 100% sure of it either, but I've heard of the rolling issues from a few sources, and seen what looks like the results, with an almost grainlike corrosion pattern. I've seen one example on snowpetrel 2, under the engine. About 15mm (3/4 inch) diameter or so.

I assume that a pit reached the inclusion area, then started corroding outwards, rather than going deeper. I was able to prise the top piece off, and since I was replating under the engine it all got removed and nice shiny new 6 mm plate replaced the old pitted stuff.

It had had a hard life under the old rusty bukh engine for 35 years with no cleaning and salt water lying in it from the leaky stern gland most of the time...

I found one small pit that extended through to the filler, in a nice 3 mm wormhole, but the rest of the pitting didn't get deeper than 2 or 3 mm on 6 mm plate. I'm guessing that a bit of copper wire might have caused the hole. It was very plugged with tight corrosion products and had 5mm of tightly adhering epoxy bog on the outside so it was still pretty solid.
Snow Petrel, I really appreciate the input. Aluminum is new to me and it seems that information on older contruction techniques and materials are hard to come by. I saw a daggerboard for a snipe which had swelled to about 2x it's original thickness and could be peeled apart in layers as though it were laminated, not sure of the cause but perhaps this was an extreme cass of the inclusion issue that you described. One of the older boats I am considering has a 250 gallon fresh water tank. As I understand the hull framing is likely 6061 and extends into the tank. The boat also originaly had bronze seacocks along with at least some copper tubing in the underwater plumbing. Micarta was used as an insulator. I plan to make a trip out to personally inspect this boat and will bring my fiber optic tool for some of those under engine inspections. The potential for widespread pitting that has been filled is a concern to me. It there is a lot of it, should there be telltale signs inside the hull? The hull was surveyed recently and reportably did not show any exterior signs of corrosion tbough I do not know the age of the filler and finish. Thanks again. James
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Old 05-12-2015, 06:38   #8
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Re: Classic Aluminum hulls

I thought aluminum inclusions are usually from the metal from when it is in a molten state, although I can see how anything could maybe be pressed in when rolling
https://en.wikipedia.org/wiki/Alumin...loy_inclusions

In aircraft, I have never seen an inclusion, although I work usually with very thin metals by comparison, perhaps that's why, I would expect more inclusions in extrusions maybe?
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Old 05-12-2015, 14:07   #9
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Re: Classic Aluminum hulls

This report has some interesting stuff in it. Given its from around 1974 (a good year) that shows that they were developing improved tempers to reduce the exfoliation issues in the marine grades around then.

https://www.google.com.au/url?sa=t&s...EA9K-u1NhT8MPg

Definitely sounds like a grain boundary issue not really inclusions. And an issue that's been more or less solved since at least 1974.

One of the guys who told me about this issue worked on older ex maxi charter yachts, and maybe even an old 12 meter. He reckoned he had seen the aluminium delaminate, but these were possibly high strength grades that may have had poorer corrosion resistance.
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Old 05-12-2015, 14:28   #10
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Re: Classic Aluminum hulls

Hi James. One advantage of the older designs, is that they are normally transverse framed rather than longitudinal, so there are often no stringers to trap water and debris and cause local pitting. Also a good likelyhood that there is no insulation, with the same issues. So chances are the topside plating down to the bilge should be good. Decks are always going to be hit and miss, depending on how well stuff has been attached to it. Watch out for wood to aluminium joins, often a lot can go wrong in this area. Also look deep in the keel for issues. Is the ballast internal lead, or bolted on? Both can have issues but probably internal is better unless water finds its way inside.

If someone's hidden stuff inside there will be telltale signs like new paint, and the odd poorly faired bit. Outside it will be obvious if there are any issues with loose bog, it will bulge slightly and tap with a dull sound. Wet the hull and look at it from an angle to spot any bulges.

Also some of the older designs used quite light plating and lots of frames. I wouldn't want to go under 4mm or so underwater.

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Old 05-12-2015, 15:11   #11
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Re: Classic Aluminum hulls

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Originally Posted by Snowpetrel View Post
Hi James. One advantage of the older designs, is that they are normally transverse framed rather than longitudinal, so there are often no stringers to trap water and debris and cause local pitting. Also a good likelyhood that there is no insulation, with the same issues. So chances are the topside plating down to the bilge should be good. Decks are always going to be hit and miss, depending on how well stuff has been attached to it. Watch out for wood to aluminium joins, often a lot can go wrong in this area. Also look deep in the keel for issues. Is the ballast internal lead, or bolted on? Both can have issues but probably internal is better unless water finds its way inside.

If someone's hidden stuff inside there will be telltale signs like new paint, and the odd poorly faired bit. Outside it will be obvious if there are any issues with loose bog, it will bulge slightly and tap with a dull sound. Wet the hull and look at it from an angle to spot any bulges.

Also some of the older designs used quite light plating and lots of frames. I wouldn't want to go under 4mm or so underwater.

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Snow Petrel,

The hull plating is 3/16" and 3/8". I found out that the builder (Jakobsons Shipyard) was using 6061 for the framing during this era but I am not sure about the plating. Do you know what was available in 1961 and what might have been used?

Good point about the traverse versus Longituginal framing re trapped water. For sure some of these boats have wood trim so I will have to expect some issues there.

I will bring my rawhide mallet, some good lighting and my horoscope when I go to look at the boat. So basically the most difficult to get to areas are the ones to look at, sounds fun! (grin)

What is your favourite priming and finishing system to use on an aluminum hull? I have painted lots of aluminum spars/ fiberglass hull with Awlgrip but never an aluminum hull. Are any of the non metallic bottom paints for aluminum any good?

James
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