''Measurements: Frame Stiffness by HHM''
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Imagine yourself having now finished that wonderful craft, after several years of work, and now I suggest that you put it on blocks and heavy, dirty iron on the seat, just like the British Museum model on the pic above, that I have ''beautified'' with artificial enhancements


This here is a suggestion for a very-quick-and-dirty method of comparing kayaks. I am basically suggesting to compare apples and potatos, but that is all I was able to exchange with all you other builders until today. So be it. To be honest about what is done here, I will cite Peter Chopelas in the Baidarka mailing list:

''Your method of 'testing' the flex is not even remotely similar to what happens in the water.''

Anyway, there are several methods used, and it seems that everyone sooner or later ends up with doing one of them, and not recording the results (which is even worse!), so I'll show them here in hope of eventually getting someone to have paper and pencil ready, and in the end come up with useful data.

Method 1

''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?''

Without any numbers given, the value of the statement is minimal. But that's what I did with about every kayak I made, so at least I can say that my own creations bent noticably; in fact, so much, that I did not repeat the test too often. Next time you do this, have somebody at least eyeball the deflection!

Method 2

Suggested by
. ''[...] 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:

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 calculations of anything important. Measurements were taken between the top of the masik and the floor. If I had been more thorough, 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.


2.Bow and stern pieces lashed in, keelson clamped in place.

3. Ribs lashed in, keelson trimmed and lashed.

4. Bottom stringers lashed in.

5. Deck stringers and coaming lashed in.

Method 3

Method 2 is a very good idea, if done properly, and I should have had thought about something similar before. As you 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. 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. 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. 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). 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. I have tried to present not more digits than actually measured; only, if numbers were nearly identical, I have left more digits to show they were not exactly identical.
ForceDisplacement, frame onlyDisplacement, frame in skin
kg cm cm
0 0.0 0.0
5 0.3 0.25
10 0.6 0.6
15 1.0 1.0
20 1.4 1.25
lb inch inch
0 0.0 0.0
10 0.12 0.10
21 0.24 0.24
31 0.4 0.4
42 0.55 0.5

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
''[...] With the skin on it will still flex the same but it takes much more effort.''

From: Peter Chopelas
''[...] 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. 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. 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.''

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.

Perhaps the only advantage of a folding kayak is that I can compare the bare frame against the complete kayak any time. 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 for good paddling characteristics; 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.

Contributors to this page: Thomas Yost (TDY), Patrick Poirier (PPR), Gerald Maroske (GUM) and Hendrik Maroske (HHM)