Subject: [baidarka] Frame flex.
From: Hendrik Maroske (HMaroske@exmail.de)
Date: Sun Oct 14 2001 - 13:36:45 EDT
Hi,
first, best wishes and good luck for your new family!
> [...] I placed two 2x4's on edge underneath the boat,
> one about a foot and a half in from either end. Then I stepped into the boat.
> [...] I understand
> that these boats are supposed to flex, but how much are they supposed to flex?
> Will it not seem to be so much in the water?
Ahhh, experiments :)
actually, we _do_ have conducted experiments, and that's what we got; I
have
corrected my previous typo errors here to make the numbers come out right
(as compared to the original post; have added other typos just in case :)
> Subject:
> Re: baidarka flexibility
> Date:
> Tue, 05 Jan 1999 20:45:28 +0100
> From:
> HendrikMaroske <HendrikMaroske@swol.de>
which is now hmaroske@exmail.de
and
ve_hengda_smith@yahoo.com (you still there?)
> Ve Smith wrote:
> >
> > and wondered how flexible they really are. I measured deflection under
> > load at each stage of construction and; here's the procedure I
> > followed and what I found:
>
> This is a very good idea, and now I found the time to come up
> with the data I found. As you might already know, I built folding
> baidarkas, the most recent being a mixture of Aleut and Greenland
> styles and having a wooden frame. My design target is to have as
> rigid a frame as a normal kayak, so my data is not necessarily
> valid for commercial folding kayaks.
>
> > The gunwales were supported the outside of the first deck beam at
> > either end and a 40 lb load was applied at the masik, which slightly
> > overstates the deflection that would occur with the driver in the
> > standard position; I figure this doesn't matter because the masik is a
> > convenient reference point and the data aren't being used for
>
> I placed the supports 4meters apart and put the load right onto
> the seat at its front edge. The load were pieces for weight lifters :))
> 5kg each (10 pounds). I measured at the upper surface of the
> keel stringer directly in front of the seat, since where the
> rigid baidarkas have the masik, I made a hinge joint in the keel.
>
> > calculations of anything important. Measurements were taken between
> > the top of the masik and the floor. If I had been more thouough, I
> > would have recorded the distance between supports and a lot of other
> > relavant data; maybe next time. Lashing was done with a flat, waxed
> > nylon tape called "synthetic cat gut" (available from Tandy
> > Leather)where it would contact the skin and with nylon seine twine
> > where it wouldn't. All lashings were pulled as tight as I could get
> > them, and the seine twine was epoxied after completion.Measurements
> > should be accurate within +/-.031 and are expressed to the nearest
> > 1/16th.
> >
> > 1. Assembled deck-deck beams lashed, end blocks temproarily screwed in
> > place with four #10 wood screws at each end.
> > Deflection=2.0"
> >
> > 2.Bow and stern peices lashed in, keelson clamped in place.
> > Deflection=1.312"
> >
> > 3. Ribs lashed in, keelson trimmed and lashed.
> > Deflection=1.125"
> >
> > 4. Bottom stringers lashed in.
> > Deflection=0.562"
> >
> > 5. Deck stringers and coaming lashed in.
> > Deflection=0.312"
> >
>
> My Greenland Baidarka results are:
>
> 4. and 5.: Deflection with and without deck and bottom stringers:
> in metric units:
> 0kg 0cm
> 5kg 0.3cm
> 10kg 0.6cm
> 15kg 1.0cm
> 20kg 1.4cm
>
> and here for the rest of the world :))
> 0lb 0cm
> 10.95lb 0.12in
> 21.91lb 0.25in
> 32.86lb 0.41in
> 43.82lb 0.57in
>
> > The surprise here is the big contribution made by the chine stringers.
> > I expect that it's a combined effect of the stiffness of the stringers
> > themselves and the fact that they prevent the middle ribs from
> > flattening as much as they could with just the keelson in place.
>
> Well, compared to what you got, your frame is twice as rigid as my
> foldable. My frame does not get stiffer when the stringers are added.
> Without stringers, both are comparable...
>
> > A review of the figures makes it obvious that the baidarka frame is a
> > very well integrated structure, with all the elements contributing to
> > it's strength and stiffness. This is by no means the case with all
> > kayaks. The 4" deep gunwales of Greenland kayaks are approximately 12
> > times as stiff as the 1 3/4" gunwales of baidarkas ( beam flexure
> > varies as the cube of depth, so that a beam that is twice as deep is 8
> > times as stiff; unlikely, but true.) This is probably why you don't
> > hear anyone touting beneficial effect that flexibility has on
> > Greenland kayak performance. Actually, baidarkas don't seem to be all
> > that flexible either.
>
> Right. See above :))) It all depends on how the gunwales are
> designed; in my case, not of a single block of wood but rather
> several components.
>
> > I didn't keep track of torsional rigidity, but was surprised at how
> > little of it the finished frame had. Skinning it seems to have
> > provided plenty of it, once again emphasizing how well integrated the
> > whole structure is.
>
> That's also my observation.
>
> > If anyone else has gathered this kind of data, I'd sure like to hear
> > about it.
Until now, I haven't heard of anything new here. Two years of baidarka
list have passed, and still only two guys find the time to show real data
8()
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