On Sat, 31 Dec 2011 09:38:31 -0800, "Bruce L. Bergman (munged human
readable)" wrote:

On Fri, 30 Dec 2011 19:07:57 -0500, Ed Huntress
wrote:

On Fri, 30 Dec 2011 12:22:53 -0700, "Bob La Londe"
wrote:

I have a copy of Pollards aluminum boat building book, and I have read it.
It looks like he mostly plans on boats to be all cut pieces and welded.
That's great for big boats or even for some small boats, but braking seems
like it would make more sense if you have access to a brake big enough.
Some of the small boat designs (think shallow draft skinny water boats)
would really benefit from a mostly bent hull. Aluminum sheet can be had in
pieces large enough to make most of a hull out of one sheet. Some cutting
and welding is still needed obviously, but if you could brake the keel,
chines, and bottom of the transom only welding the front and the sides in
the back you would have an inherently stronger and more rigid boat for rough
service.

The problem of course is how do you brake a piece of metal that big?

Yeah I know a giant hydraulic brake would be a good answer for the
commercial boat builder once they have the capital for it, but how does the
backyard boat builder do it? Are they stuck with all cut and welded pieces,
or hauling their sheet to somebody with a giant brake to do it for them (if
there even is somebody with a brake big enough in the area that hires out)?

What kind of boat are you planning to make? A jon boat made of
aluminum typically has lots of longitudinal "stringers" bent into them
with a roll-forming tool, or added-on stringers made of bent aluminum
channels that were then riveted on. We had one of those when I was a
kid.

Small conventional pointed-bow boats traditiionally are formed either
by stretch-forming (used also for canoes) or with big drawing rigs to
pull the sides of the bow together. Then they were TIGed (heliarced,
actually). Today, I understand, they're MIGed.

Without componnd curves, aluminum is too floppy for a boat. When they
draw it into curves, which are simple curves, they generally back it
up with riveted stringers or those rolled-in equivalents.

I watched them stretch-form aluminum boats at the old Fairchild
Aircraft factory in Hagerstown, MD in 1957. My dad had contracted with
them to build boats for Sears after they lost some aircraft contracts.
At the time I didn't know what I was looking at, but after working at
American Machinist for a couple of years, I realized that I had been
watching a stretch-forming operation.

And when you start forming aluminum you have to take active steps to
deal with work fracturing of the metal - and that's Big Factory stuff
you can't do in your garage easily and accurately.

You have to receive the metal annealed Dead Soft, usually frozen on
Dry Ice, stash it away in a big walk-in freezer before you use it, and
that dead-soft state expires in a few weeks even when it stays frozen.
I know that's how they handle aircraft aluminum rivets - you buck them
in place while dead-soft, then the aluminum alloy naturally re-hardens
in a few weeks.

And/Or you have to have a way to re-anneal the aluminum between passes
through the benders and rollers when you are applying severe bends or
multiple operations - that requires a big furnace and a calibrated
control system and convection circulation system to prevent hot spots,
there's not a lot of room between "annealed" and "molten".

Which is why they designed boats from sheet stock and stampings for
"homebuilding" or small shops. You've got to know your limitations.

-- Bruce --

Well, as Jim says, 5000-Series is less fussy. It doesn't heat-harden,
and, unlike most 2000 and 6000 Series aluminums, it doesn't age harden
after heating. Around here, 5052 is, or used to be, the most-used
material for aluminum boats. Now I see that they're using 5083 in some
applications.

Pollard has developed some techniques for small-scale building of
aluminum boats, but they tend to be full of stringers or frames.
Unless you stretch it into compound curves, it drums and flops around,
without the reinforcement.

I can't think of many places where I'd prefer it to modern wood and
epoxy techniques for a small, one-off boat. But then, I haven't tried.
My experience in working with sheet aluminum for complex shapes has
not been happy. d8-)

--
Ed Huntress