Subject: RE: baidarka Aluminum North Alaska Recovery Kayak
From: Peter A. Chopelas (pac@premier1.net)
Date: Tue Feb 20 2001 - 01:41:19 EST
Paul Raymond wrote:
> I was considering a thin piece of plywood between the two
> parallel gunnels, for stiffness. Either that, or maybe I would use a
piece
> of wood here (I caught a thread in which Mr. Dyson was considering that),
> but aluminum for the rest of the stringers, keelson, and deck beams.
I do not think it would gain you anything: the idea is to create a kind of
I-beam or truss with the wood as a web with a tube as the top and bottom
members but the loading properties of wood and aluminum are so different
the aluminum tubing would load up in pure bending faster than the wood web
could carry the shear loads to create the desirable truss action to put the
upper tube in compression and the lower tube in tension. virtually all
baidarka frames but all of the members in pure bending with the larger
gunwales taking most of the bending loads (i.e. without diagonal members or
a web to transfer the shear from the upper members to the lower members
there is no truss action of the frame). With 11 aluminum tubes in bending
over only a 9 foot or so long span it will be stiffer I suspect then a wood
frame, if you bond the skin to the frame you will get some shear transfer
to and it will make the hull that much stiffer.
> I also thought that with an aluminum frame like this, you wanted to
minimize
> flex. Isn't aluminum tubing kind of like a paper clip, where if you flex
it
> enough it would eventually weaken? I would think a shorter boat would be
> better in this regard. Of course if flex broke tubing it must happen over
a
> very long time, as I have not seen any major reports of such things with
> Folbots or Feathercrafts.
Well I have news for you, ALL metals and most composites get a little
weaker with each loading cycle when you load them beyond their fatigue
limit. The idea in designing with highly loaded structures is to keep the
load below this fatigue limit. this is the stress level where no matter
how many times you cycle the material, there is no loss in strength. Flex
really does not have anything directly to do with the loading, the amount
of "flex" you get is a function of the bending moment on the member, the
elastic modulus of the material and the physical dimensions of the cross
sectional area of the member, not the stress on the material. Though the
maximum stress in a function of the bending moment and the dimensions of
the cross sectional area alone.
The loading on the 1/2" tubing on a Retrieval kayak I would expect to be
way below that of the working stress of aluminum, regardless how much the
frame flexes during use.
Apparently you have never watched how much the aluminum wings of commercial
jets flex up and down as it goes through turbulence, if your idea was
correct you would see the wings falling off the aircraft due to fatigue
cycling.
> I really want to try lashing here. I don't know whether to go with
aluminum
> frames, or if it is possible to lash aluminum stringers to ply frames. If
> there's one thing I learned from my other kindling project is how to use
a
> coping saw to cut ply frames. Plus I'm too cheap to buy power tools, or a
> welding set for that matter.
Plywood would be easy to work and inexpensive, I used a similar frame set
up on miniature kayak I made for my daughter using salvaged plywood (i.e.
free). It only took about an hour to cut out six plywood frames with a
hand held jig saw I bought at a garage sale for $4 (how is that for cheap
tools?). The only disadvantage would be that the 1 to 1.5" of each frame
depth would take away interior space inside the hull, which is already
pretty tight if you intend to carry cargo. But if positioned properly a
plywood frame would make a pretty good foot rest. Since you are buy the
aluminum tube anyway I would use tubing frames, try and see if you can
borrow a tubing bender from someone just for the project.
Let us know how it comes out, I would be interested to know how much it
weights when you are done.
Peter
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