"Canuck57" wrote in message
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"Eisboch" wrote in message
...

"Canuck57" wrote in message
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wrote in message
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First, rivets are used on aircraft for the following reasons, not all of
value on a boat:

- low weight per attached foot of seam than welding
- cheap and manufacturing ease of use
- thermal expansion and flex
- can be drilled out for access/repair/inspection
- welding is more expensive per attached foot than rivets


Are other opinions welcome, yes.

I think aluminum welds are fine as long as they are reinforced or the
sheets are relatively thick. Thin sheet (seam) welding of aluminum is
prone to cracking when subjected to even a little flex as compared to
other metals. Maybe someone with knowledge of metallurgy can explain the
crystalline structure, etc. My opinion is based on experience in
industry.

BTW, I think flex is much more important in airplane wings than you give
it credit for. Next time you fly, note the location of a wing tip as the
airplane begins it's take-off run and watch it as you become airborne.
While flying in turbulence, watch the tip of the wing again. There's
quite a bit of wobbling around going on out there. Welded aluminum
panels wouldn't last long, nor would the wings if they were designed to
be stiff rather than being allowed to flex.

Eisboch

But my point being, do you want that much flex on a 17-19' boat?

I would think not. But on an aircraft, this is good. The wings are in
effect shock absorbers for up/down drafts and air density changes. But I
don't think this level of flex is needed nor desirable on a small boat.


The wings of an aircraft are *not* designed to be shock absorbers. They may
be, by default of their ability to bend without breaking, but that's not the
design purpose. The design purpose is to allow them to bend and flex (which
they are going to do, welded or riveted) without breaking off. Rivets allow
the bending and flexing without exceeding the yield of the aluminum.
Welding would make them so stiff that they would snap near the welds.

But, that's not the issue. It's the fact that even a small, thin skinned
aluminum boat *will* want to flex. If it can't (because the seams are
welded) it's likely that one or more of the relatively weak welds will break
whereas a riveted construction allows the bending or flex without damage.

Even large, welded steel ships have expansion joints built into the hull
structure to allow for flexing, otherwise it would crack in half in rough
seas.

Eisboch

On Sun, 13 Jan 2008 16:50:29 -0500, "Eisboch" wrote:

Even large, welded steel ships have expansion joints built into the hull
structure to allow for flexing, otherwise it would crack in half in rough
seas.

I saw a documentary on the Britannic, sister ship to Titanic, about
this very thing.

One of the more interesting aspects of the Titanic sinking were the
expansion joints. According to the documentary, the type of joint
used on the Titanic contributed to the diaster - I can't remember all
the technical details at the moment, but it was a type of joint that
was a "floating" joint. After the diaster, the Britannic was modified
to eliminate the the "floating" joint in favor of a boxed type joint.

Oddly, that type of joint contributed to Britannic's sinking when it
struck a mine off Greece in WWI.

It was pretty interesting.

"Eisboch" wrote in message
...

"Canuck57" wrote in message
news:R6vij.61672$EA5.53484@pd7urf2no...

"Eisboch" wrote in message
...

"Canuck57" wrote in message
news:wHdij.60275$EA5.19428@pd7urf2no...

wrote in message
...

First, rivets are used on aircraft for the following reasons, not all
of value on a boat:

- low weight per attached foot of seam than welding
- cheap and manufacturing ease of use
- thermal expansion and flex
- can be drilled out for access/repair/inspection
- welding is more expensive per attached foot than rivets


Are other opinions welcome, yes.

I think aluminum welds are fine as long as they are reinforced or the
sheets are relatively thick. Thin sheet (seam) welding of aluminum is
prone to cracking when subjected to even a little flex as compared to
other metals. Maybe someone with knowledge of metallurgy can explain the
crystalline structure, etc. My opinion is based on experience in
industry.

BTW, I think flex is much more important in airplane wings than you give
it credit for. Next time you fly, note the location of a wing tip as
the airplane begins it's take-off run and watch it as you become
airborne. While flying in turbulence, watch the tip of the wing again.
There's quite a bit of wobbling around going on out there. Welded
aluminum panels wouldn't last long, nor would the wings if they were
designed to be stiff rather than being allowed to flex.

Eisboch

But my point being, do you want that much flex on a 17-19' boat?

I would think not. But on an aircraft, this is good. The wings are in
effect shock absorbers for up/down drafts and air density changes. But I
don't think this level of flex is needed nor desirable on a small boat.


The wings of an aircraft are *not* designed to be shock absorbers. They
may be, by default of their ability to bend without breaking, but that's
not the design purpose. The design purpose is to allow them to bend and
flex (which they are going to do, welded or riveted) without breaking off.
Rivets allow the bending and flexing without exceeding the yield of the
aluminum. Welding would make them so stiff that they would snap near the
welds.

But, that's not the issue. It's the fact that even a small, thin skinned
aluminum boat *will* want to flex. If it can't (because the seams are
welded) it's likely that one or more of the relatively weak welds will
break whereas a riveted construction allows the bending or flex without
damage.

Even large, welded steel ships have expansion joints built into the hull
structure to allow for flexing, otherwise it would crack in half in rough
seas.

Eisboch

While I don't profess to be an aircraft engineer, I done believe the sales
hype about boats are better because we build then like aircraft. The needs
of the two are different. Even the rivets are different.

You mean like Edmund Fitzgerald? Me, I don't plan on being in anything
remotely as hazardous as Lake Superior and have no intention of amidships
being over a 35' wave. Michigan, no problem. Koocanusa, Lake of the Woods,
Rainy River, nice. But I just have a thing about Superior...used to live by
that lake and I would sooner hit the Pacific.

But how much does a 17-20' boat need to flex? If we look at fibreglass in
this size, not much hull flex at all.