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Roger Long Roger Long is offline
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First recorded activity by BoatBanter: Jul 2006
Posts: 172
Default Steel hull and aluminum superstructure

If you are worried about rust, I would suggest making the deck of
aluminum as well as the cabin trunk and having the flange arranged at
the deck edge like a fiberglass boat. Any leaks that did develop
would then be out where they would most likely just run down the side
of the hull. The area under the cabin edge would be very clean and
easy to finish off. A bedded guard rail would protect the edge of the
gasket and keep water away from the area where two kinds of metal are
in close proximity.

The best thing of all would be the fact that sandblasted aluminum
plate without paint is an excellent non-skid surface.

--

Roger Long



"André Langevin" wrote in message
...
Hi Roger,

Here is the quote i've received. Indeed quite costly for an
amateur. My Roberts 43 would need at least 100 feet of this so
about 6000 $ of material.

Item Qty Thickness Width Unit Price USD /lin ft Total Price
USD /item
1 12' 0.75 * 2" $62.13 $706.56


The more i think to your proposal the more it make sense. And it
would but my Roberts 43 to a very high level of stability given that
i would save me about 2000 pounds and the equivalent lever-arm since
it is farm from the CG. Another advantage is that it is so easy to
keep the rust out of the hull but much less on the decks since there
is a lot of holes, fixtures and contact with items like anchor, spi
pole and so on.

It definitely need more reflection !

Thanks !


"Roger Long" wrote in message
...
You would have plenty of electrical connection through the bolts.
They could be isolated but there would be little need to since they
would be inside and dry. You would probably want to use stainless
bolts anyway.

Even if you had the house isolated, lightning current would jump
over the small gap at the gasket like it wasn't even there.

The people wringing their hands over dissimilar metals forget that
stainless and aluminum are quite far apart but the millions of
highly stressed and critical stainless rigging fittings attached to
the millions of sailboat masts out there function for years and
decades without a problem.

--

Roger Long



"André Langevin" wrote in message
...
Roger,

I'm still discussing with people at SPURIND and another company.
I thought of using a joint and it would work but what if lightning
strike and i don't have electrical connectivity between the
superstructure and the hull ? The mast could still be
interconnected to the hull but it is a risk....

Better have electrical bonding between the two. i'm currently
discussing price, i'll post the findings.

cheers

"Roger Long" wrote in message
...
That is the stuff.

For homebuilding, I would just build a flatbar flange at the base
of the superstructure. Bolt and aluminum flatbar to it and then
build the aluminum superstructure on top. Unbolt after it's
done, lift, and insert a suitable gasket material. Then bolt it
back on.

Being able to remove the superstructure would have a lot of
advantages if you had to do major repair on the interior.

Actually, I would build the whole boat out of aluminum. Stronger
at the same weight, more likely to deform in a way that stays
watertight in event of major damage, and easier to drill for
temporary patches with hand or battery powered tools. I'd much
rather go up on a reef in a far away place in an aluminum boat
than a steel on unless the latter was large enough to carry a
full welding outfit.

Also, less compass issues with an aluminum boat.

--

Roger Long



"André Langevin" wrote in message
...
Very interesting Roger what you bring. As you said, i've
searched also in internet but there is not much reference. I
will talk with a local machinist also and keep you posted.

There is this company though: www.spurind.com but it might
turn out very costly since it seems to be a specialty.

André

"Roger Long" wrote in message
...
Sorry, but the following is completely wrong. The explosively
jointed bimetallic strips have a long and successful history.
The aluminum is welded to the aluminum side and the steel to
the steel side. The strips simply seem to have become hard to
locate, at least via the web.

Regular steel and stainless steel are often joined. Problems
can occur, especially if submerged in salt water but you'll see
mild steel / stainless joints on fishing vessels that have been
going for years and years.

True, you can't weld aluminum directly to either.

How do I know about aluminum superstructure on a steel hull?

I did it on this boat:

http://www.bbsr.edu/About_BBSR/Facil...herbird_ii.htm

--

Roger Long



wrote in message
...
I have just double check with my friend who worked as a welder
for 35 years in a shipyard that build commercial, coast guard
vessels, battleships and drilling platforms.
If you have an aluminums structure welding steel plates on or
doing the reverse was not in practice. Aluminums and mild
steel or cold rolled steel are not compatible. The same thing
applies to welding stainless steel. What takes place is a
white inter granular corrosion that is hardly visible to the
naked eyes. Given time the white corrosion will cause a
structural failure. Not to mention the saline atmosphere at
sea that will accelerate the process.

"André Langevin" wrote in
message ...
Hi to all,

I am a newcomer on this newsgroup and you'll see me around as
i'm starting the construction of a new boat. I currently
have a 34 feet powerboat in aluminum and my nest boat will be
a 44 or 45 steel sailboat. I'm looking at building a Bruce
Roberts design and i would like the deck superstructure to be
in aluminum. I've seen many commercial boat done this way
and even old Coast Guard patrol boat of 30+ years old without
any corrosion problem so it is something i'd like to do. But
i can't find any industry that carries the special strip that
isolate both metals but still permit to weld them both.

Does someone ever see this ?

André