Subject: Re: [baidarka] Plastic SOF Ideas / Questions
From: Peter Chopelas (pac@premier1.net)
Date: Mon Oct 22 2001 - 23:22:03 EDT
Hi Wayne,
Welcome! sounds like an interesting project. a couple of thoughts come to
mind.
because the PVC has such low stiffness I would think the only way you can make
this frame material work is to build true trusses out of it for the
gunwale/chine assembly just as Putz does with wood. Be advised that trusses
have very high stress at the joints, so you either have to make very strong
fittings or use some kind of pinned joints for the truss assemblies. though I
am not exactly sure what you are describing with your post, could you describe
it a little better? Is this just a plywood side panel (like a stitch and glue
panel) with a PVC pipe attached to the upper and lower edge forming the
gunwale and chine? If so how are they attached?
If the gunwale and chine form a truss slip fit joints (like tent poles) will
not work since the 'truss' members have to be able to take both tension and
compression loads. For others stringers that are only there to shape the skin
the slip fit shock corded tent-pole like joints work fine, I have used them
before on a lightweight folder I built.
the frames only hold the cross sectional shape of the hull but will take a
fair amount of "hoop" compression from the water pressing in on the skin. A
better connection than the cable ties I would think would be these plastic
screw type clamps purchased from tool vendors at street fairs. They are like
a 1/4 round plastic strap with threads on the radius side and a threaded
collar type "nut" assembly the each end of the strap fits in forming a loop.
You turn the nut and the loop tightens. They are stronger I think than the
cable ties, reusable, and adjustable. I do not know where to get them, I
purchased a bunch from a tool vender at a swap meet like event to use in my
own folding kayak frame. though I have not built one using them yet.
also consider using stainless through screws with wing nuts, or better nylon
"thumb" nuts, with counter sunk heads at the stringer/frame joints. simple,
strong and allows pivoting as the frame flexes. With an insert glued into the
stringer tube at this location it should not affect stringer strength either.
Also consider at these joints there will be a lot of rubbing between the
members at these joints when the frame flexes. This could eventually wear
through so consider some kind of gasket, washer, or tough rubber wrap at the
joints to prevent wear. Or sacrificial pads glued at these joint locations.
sponsons can be made the same way you make the skin but with light weight
vinyl sheet, but you may not need them. They add weight. If you simply make
the lacing system you will use to close your hull wide and strong enough to
pull the skin tight they should work as your skin adjustment too I would
think. Put similar "laces" on the fore deck and you will beable to take the
slack out of the whole skin. You will also need a way to put tension on the
skin in the fore and aft direction as well, most makers use some kind of
over-center device built into the frame.
For the coaming consider just leaving the hoop to "float" in the skin, this is
common with folders and even some traditional native designs. There is no
reason to attach it to the frame since it is not rally a structural member but
just provides a way to seal a spray skirt.
The Putz design BTW works fine but I think would be improved with a lower
seat. The way the floor beams are arrange per plan it of forces you to have a
higher CG than necessary. With your set up you can simply make a sling type
seat that puts you several inches lower in the hull and you might be able to
make it a few inches narrower without a noticeable loss in primary stability.
making it a bit lighter, faster and easier to paddle I suspect.
Great way to get tarp material BTW, I'll start checking around here with the
local banner companies too!
Good luck, keep us posted on your progress.
Peter
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