Subject: [baidarka] Frame flex, part 2
From: Hendrik Maroske (HMaroske@exmail.de)
Date: Sun Oct 14 2001 - 15:23:05 EDT
Hi again,
here is what I got, comparing flexibility of the assembled frame to that of
the completely assembled baidarka (the very same folder, frame in skin,
that
I have given you the numbers already).
Again, I have propped her up on wooden blocks under the keel, 4meters
apart, and loaded her on the seat with 5kg weight-lifter's weights, and
measured the deflecton of the keel at the seat.
During loading the completely assembled kayak, the frame has shifted a bit
withing the skin, naturally, due to the folding construction, attributing
to the interesting fact that the displacement numbers are more or less
equal,
at least at my crude measurement method.
Force; Displacement, frame only; frame in skin
[kg] [cm] [cm]
0.00 0.00 0.00
5.00 0.30 0.25
10.00 0.60 0.60
15.00 1.00 1.00
20.00 1.40 1.25
Force; Displacement, frame only; frame in skin
[lb] [in] [in]
0.00 0.00 0.00
10.5 0.12 0.10
21.0 0.24 0.24
31.5 0.39 0.39
42.0 0.55 0.49
What I see here is that the flexibility does _not_ change much with the
skin on; but this is for a _folder_ and with _small_ loads.
Anyway, it is a result that I would not have predicted before doing the
test. Still, the following remarks are true:
> From: Bob Klemick <klemick@home.com>
> [...] With the skin
> on it will still flex the same but it takes much more effort.
Right.
> From: "Peter Chopelas" <pac@premier1.net>
> The skin will make a big difference, but also the way you are loading it is
> far more severe than what happens in the water. The water supports the hull
> distributed among all the frame members.
Right.
> Also consider that the support the water gives is distributed according to the
> water displaced. so the widest and deepest part of the hull, right where the
> seat is, is what provides most of the support. Which is also right where
> most of the weight is concentrated. The ends, where you put your blocks, do
> contribute some support when in the water, but it is a very small contribution
> compared to the central part of the hull.
The reason for my putting the blocks 4m apart is to get measurable results
with
small loads; I would not load the bare frame with more than 20kg, because
some
of the folding hinges depend upon the skin to keep them from falling apart.
> There is no easy way to make this comparison unless you have a frame
> only(without the skin) of a hull that you know has the right amount of
> stiffness.
Perhaps the only advantage of a folding kayak. Although the applied loads
seem
to have been too small to generate the results I would have expected, I can
assure you the flex of the assembled folding baidarka is adequate; so, if
you
should ever repeat these measurements with your frame, if you end up with
what
I got, it is no guarantee for success, but at least a hint that you are not
completely off the ballpark. Just don't forget to show us what you got :)
> Your method of 'testing' the flex is not even remotely similar to
> what happens in the water.
Yes.
Cheers
Hendrik
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