| Coupla' thoughts:
Aluminum is a generic term; if you finally decide on using it, you
probably should do some research in order to be more specific. My guess
is that two features of aluminum (and its close kindred alloys) you may
wish to consider are corrosion resistance and hardness. The former may
vary considerably, and the latter I believe translates in practical terms
to stiffness. (Some hardnesses you can bend a 1/4" sheet by hand, though
that action then hardens it locally at the bend, making attempts to bend
it back result in a S-curve of some degree.)
One solution to antifouling aluminum is a TBT paint, *but*... As of
mid-June or July 1989, the stuff is banned for sale or use, unless:
- Applied by a licensed professional, AND
- On an aluminum hull of any length, or any material hull over
82' in length.
- Application by non-licensed people (like us) is explicitly
verboten after that date, though everyone who has a locker
full of next year's stock can get it on before the early
summer cutoff date.
If you went this route, maybe you can have your rudder painted as part of
another boat job, at a reasonable cost. Or resort to a barrier system.
Why not consider epoxy resin, with a judicious application of carbon
fiber? It's quite cost effective -- you only need a few short tows to
beat the aluminum for strength, and it'll be lighter, too. As well as
easy to paint when the hull is done. You could easily fair the shape from
a foam core to make it faster through less resistance. Meade Gougeon
(Gougeon Bros) has info on home-building with carbon-reinforced epoxy.
Aluminum seems like the difficult way to go.
J.
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| RE: .5, >>> Is there, say, a 22' aluminum keelboat?
Well, there almost was. (Windy story follows; press NEXT REPLY or NEXT
NOTE for relief.) In the early 70's I spent a year or more working
with a guy intent on mass producing a small cruising boat in aluminum.
Aluminum's advantages include high strength-to-weight (stronger than steel
per pound, more energy absorption on impact than steel albeit at the price
of greater deformation), easily worked by someone knowledgeable, just as
weldable as steel, and overall making for a remarkably tough boat. And
work-hardened as it's built, maybe...if it can be built that way, at least...
Ted Brewer designed a sweet little 24-foot canoe stern double-ender for
the project, with typical Brewer cutaway keel profile. Many of you may
actually know this design, as it eventually evolved into Quickstep 24, now
built of f.g. in RI. Anyway, we built a hull plug, from which we built a
split mold, each half of which looked rather like the rind of a slice of
cantelope. The plan was to mold two opposing half-hulls, then weld them
along the bottom seam from stem to stern. The original hull production
plan (which was successfully realized in several 1/8 scale models) was to
hold a sheet of 6061-T3 (I think it was) over the half-mold, place the
whole shebang under water to distribute the force, and then fire carefully
placed explosive charges over it to instantly force the aluminum into the
mold. The three-foot half-models were certainly fun to make! However,
after some thought and reflection, it was decided that explosively molding
a full-size hull might be unnecessarily dramatic, as well as unpredictably
hard on our valuable tooling; after some calculations, we determined that
50 lbs/sq ft water pressure would easily exert sufficient force to do the
job, and would allow more control since the process would occur more
slowly. However, several technical problems cropped up, which were never
sufficiently resolved before the investors' money (>$100K) ran out.
First, the explosive technique provided a higher percentage of stretch
molding vs draw molding. (Draw molding is where the material pulls into
the mold around the edges; stretch molding is what happens if the edges
are held tight, or the event happens to fast for the material to draw.)
That was good, as it gave smooth compound curves, and work-hardened the
metal at the same time. But these advantages were given up with the
change to hydraulic molding.
Second, hydraulic molding was much more draw than stretch. Draw molding
posed problems when the material began to form, as it wrinkled the edges,
which either: A) opened leaks, reducing pressure and stopping the process,
or B) prevented drawing at all if we tightened the mold cover down enough
to prevent wrinkling. 50 psi isn't enough to stretch mold alone, since
the material work hardens on it's way to final shape, and that requires
much more pressure to overcome, and even led to tears (...both the ripping
of metal and resultant salty water droplets in eyes!)
Third, aluminum (like most metals) is elastic to some degree, and we
discovered that over-molding is required to end up with the desired final
shape. It's a monumental problem to predict how much over-molding in
exactly what areas is needed, and then to build a plug/mold reflecting
that. And it isn't predictably repeatable, either, probably in part
because metal isn't a truly uniform material at the molecular level.
What we did learn, however, is that aluminum is remarkably easy to weld
with a bit of practice, even as amateurs like ourselves. The key is the
right alloy and sufficient thickness to absorb the local heating without
burning through -- that's also a function of practice and building up
speed. The mad inventor, Jack Winninghoff, went on to form Winninghoff
Boats in Rowley, MA, and has been remarkably successful in traditional
aluminum boat building of many non-traditional designs. If you can
justify the material weight of 1/8 or greater plate thickness, which works
out to about a 25-footer, then aluminum is grand. I've ridden in and
driven a few of Jack's boats, and they're great for what they are. Rugged
enough to be a major seller as small-scale landing craft! Yes, there are
some very real electrolysis issues, but if it's a well-tended workboat
or oft-used pleasure machine, instead of spending much time as an
expensive mooring buoy, these can be managed quite reasonably.
J.
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