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Steam Source We
begin with a Coleman stove, a 2-gallon camp pot, a piece of plywood covered
with aluminum foil for a lid and a long radiator hose. Look closely and
you will see a chimney made from 12 inch aluminum flashing stock around
the outside of the boiling pot. This arrangement provides about one inch
of space between the pot and the chimney and allows waste heat to more
effectively heat the water.
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Steam Box I We
thought that steam bending a few ribs might be a bit tricky. But just how
challenging it would become, was not immediately evident. Ribs and steamer
are all part of the same story, but it's easier to separate them and talk
about our steamer adventures first.
To build a steamer, all you need is a heat source, a vessel, a duct, and some insulation. Working examples are described and pictured all over the Internet. So, how difficult can it be to build a steamer? We made an 8' long by 6" square duct from a sheet of Owens Corning 5/86" pink foam (closed-cell polystyrene) insulation board. We fired it up and had a great plume of steam rolling out the business end in less than five minutes. |
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 The
foam buckled and the PVC fittings warped after just ten minutes of steam.
We had to abort the whole mission.
We know of others who have made ducts of (what we thought was) the same stuff and they seem to have worked just fine. But, Mr. Bob Place, an engineer at Owens Corning® (phone 419-248-6476) has since told us that the maximum allowable temperature for Pink FOAMULAR® Rigid Foam Insulation is 165F. He definitely doesn't recommend it for a steam box. Furthermore, they recommend that it not be used where temperatures can exceed 150F. Maybe they used another material that Owens Corning recommended to us. Their 700 Series Insulation foil-covered fiberglass board will work in temperatures as high as 450F. It is available in thickness from one inch to four inches, but only through industrial distributors. To locate an Owens Corning distributor in your area call 1-800-372-8801. A Johns Manville's® Spin-Glas Board is a similar product. |
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Steam Box II Others
talked about using PVC piping for their steam ducts -- even showed pictures
of them at their web sites! So, we went to the hardware store and
got ourselves a nice economical 4" PVC drain pipe, repaired our foam duct
a bit, and lined it with our drain pipe. Again we fired up, and again we
had steam quicker than we could make coffee. But, as soon as the steam
started rolling out, our PVC pipe took deep a bow. And it never stood up
again. Menu |
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We knew PVC softened at modestly elevated temperatures. We were even aware
that the National Plumbing Code forbids the use PVC piping for household
hot water lines. So this surprise was embarrassing!
Being engineers, you would think that we would have had enough sense to check the allowable working temperature for PVC piping first, wouldn't you? Especially after our pink foam disaster. Alas, we figured it worked for everybody else, so why bother? Why indeed. According to people who make the stuff, the maximum allowable service temperature of Schedule 40 PVC pipe is 180F. That is a full 32F below the temperature of steam at atmospheric pressure! (A PVC manufacturer told us that they get this question frequently from people who want to pipe steam for sterilizing systems). Now we wonder how others got theirs to work. Did they really use PVC? Or might they have used CPVC. CPVC has a working temperature limit of 210F. That might be just high enough since the outside of the pipe would probably never reach that temperature. |
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 Maybe
PVC needs Viagra to work in Cincinnati?
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Steam Box III Knowing
that even the mildest steel retains appreciable strength at 800F, we were
damned sure that galvanized stove pipe would work. So we trotted down to
the hardware store and came back with 15 feet of 5" diameter galvanized
stove pipe. (We decided to go to 15 feet so that we could steam stringers
and gunwales, too.)
We extracted the remains of our buckled PVC drain pipe, replaced it with the stove pipe, and fired it up. |
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 After
10 minutes of steam. Mr. Murphy appears to have left the building.
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 Steamer
III really does works! HALELUJAH!
(Test hoop is of 3/8" square straight-grained spruce.)
We are pleased to tell you: It works! It works fine! [ED.NOTE: It is still working!] Our recommendation? Don't screw with marginal materials. Avoid all of this nonsense and build yours along the lines of our Steamer III. Steamer III is like Steamer I with a liner of 5" diameter galvanized duct pipe and iron fittings. In fact, skip the foam altogether. Just seal your intermediate pipe joints with duct tape and overwrap the pipe with several thicknesses of newspaper to insulate it. To keep the paper dry, dress it back from the ends of the pipe and slope the pipe up a little so that condensation will drain back to the boiler. |
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Rib WoodThe canoe plans specify that the ribs should be made from straight-grained ash sap wood that is still green or from oak of similar quality -- or we could laminate them. Ash has a reputation for strength and for ease of steam bending, so we opted to go with ash.Big ash trees grow everywhere around Cincinnati. You can hardly drive anywhere without seeing hundreds of them. In fact, if you close your eyes and take your hands off the wheel, you stand a good chance of running into one. So, finding ash around Cincinnati, should be a piece of cake. Right? Wrong! We spent days and days looking for suitable source. We talked to virtually every lumber yard, hardwood supplier, and saw mill within a 50 mile radius. We talked to everybody we could think of that might have some, or might use some, or that might know of someone who might. We talked with cabinet makers, carpenters, antique boat restorers, hardware store managers, guitar makers, timber truckers -- even a retired surgeon who runs a portable one-man mill.
"I know," responded the plumber, "I used to be your doctor." Could it be that the saw-milling business is now becoming more attractive to expatriate physicians then plumbing? |
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Preparing Ribs After
all of our searching for green ash, we had discovered only one place in
our entire region that had any ash for sale -- Paxton
Lumber. They specialize in exotic hardwoods for discriminating cabinet
makers from all over the world. They had a little kiln-dried ash on hand.
Desperate, I paid $30 for two of the clearest, straightest-grained rough-sawn
2X8's that they had. One was 14' long, the other was 16'. Needless to say,
it wasn't sapwood, it wasn't particularly clear, it wasn't very straight-grained,
and it certainly wasn't green. Using a table saw and a planer, we quickly
turned the lot into sticks for rib stock (with 1/4"X1/2" cross-sections).
But, we weren't happy with the results. The planer scarred the surfaces of the dried ash rather severely, and the grain frequently ran out along the edges of the small sticks. Fortunately, we still had a bit of surplus stock on the rib stock. So we took a thin cut off the affected surfaces with a hollow-ground planer saw blade. That and some diligent hand scraping improved the surfaces markedly. Of course, it didn't do anything for the grain runout. |
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Bend Failures
We decided to try steam-bending a few ribs to see what would happen. Our
very first effort resulted in several fractures. A gradual bend caused
failure along a minor grain run-out. Grain parallelism may be as important
as freedom from knots. The ash cracked everywhere there was the smallest
knot. Knots, being far stiffer than the surrounding wood, would tear out
when the containing ribs were mildly bent. The ribs then fractured because
the remaining wood had insufficient cross-section to withstand the bending
moments.
Fractures were also frequent in places where the annual rings were inclined to the axis of the rib (i.e., at grain run outs). Some failures occurred where the grain slope was close to 1 in 16. (That's the same slope that we are using for scarf joints! Gawd!) |
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 We
noted that fractures often started at a track in the surface that was left
by a passsing saw-tooth during the ripping operation. We saw this often
enough that we became convinced: it is important to sand or scrape the
rib surfaces smooth before attempting to bend them.
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 Occasionally,
we would get an amazingly tight bend. In one early experiment with the
1/4" thick kiln-dried and straight grained rib stock, we got a bend that
had a 2" bend radius . That is a radius-to-thickness ratio (R/T) of 8/1;
the tightest rib-bend on the canoe only has an R/T of about 20/1. If we
could only make 20/1 bends reliably, we'd be home free.
As it turned out, that was really lucky. Mostly the ribs broke before they were bent enough to use in the canoe. |
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 We
thought we might be able to salvage our kiln dried ash by cutting out significant
irregularities and splicing the good sections back together with epoxied
scarf-joints. We prepared a rib blank with three such splices, taking care
that the scarf pitch was equal to or less then one in sixteen. First, we
coated both halves of each joint with epoxy. Then we added about 20% sawdust
filler (by volume) to the remaining epoxy, mixed it well, buttered each
joint with it , assembled and lightly clamped the joints, and let the spliced
rib cure for two days.
Before steaming, we scraped the exposed surfaces of the spliced rib level and smooth (thereby removing surplus cured epoxy and joint ridges and smoothing the adjoining surfaces). Then, after ten minutes of steaming, we tried to bend it in a two-foot diameter circle. All three scarf joints failed cleanly along their join lines. We think that the epoxy deteriorated from exposure to 212ºF during steaming. Whatever the cause, we concluded that scarf joints would not get us out of our ash quality dilemma. Lee followed this with more Internet research on steam bending. Most of the reports found were from hobbyists like ourselves whose research time and funds are severely limited. So, their conclusions (like ours) are generally drawn more from practical experience than from a rigorous application of the scientific method. While the conclusions that they drew may not always be entirely correct either, they are certainly based on far greater success with steam bending than either of us has had to this point and we value their input highly. Here are some good links that Lee's research turned up: |