Re: [Paddlewise] Stability Discussion

Matt,
  Thanks for your comments. The discussion we had here motivated me to 
have another go at an article I'd been working on for a while. The 
discussions here have helped me create a more fully realized 
understanding of some of the concepts which were kicking around 
loosely in my brain. Reading other's explanations here on Paddlewise 
(and wave~length before) and having the opportunity to try putting 
words to my thoughts has been invaluable for my own understanding.

One of the concepts which is still rattling a little loosely is 
secondary stability. As such I tried to put that moniker on something 
I do think is important. The area under the curve to the maximum 
point is a good indication of how much energy the boat can absorb 
before it gets scary. If some was to paddle by and punch you in the 
shoulder, this is how hard he would have to hit you to push you over 
the hump.

I agree that this is not exactly the same thing as what a skilled 
paddler feels as "secondary" but I still think it plays a part. The 
reason I didn't include a curve-slope derived definition into 
"secondary" is I don't have any fully realized thoughts on how to do 
it. So, my definition was a bit of a cop out.

I will need to spend more time playing with the first derivative 
(slope of the curve). To plot out the first derivative from point 
data, just divide the vertical difference between two adjacent points 
by the horizontal difference. Plotting this out for a variety of 
boats should help show if there are any patterns which might be 
useful.

Maybe you will have more influence with Chris C. than I did. :-)

Thanks, Nick


>At 11:35 PM -0800 2/15/01, Matt Broze wrote:
>While the above may be a possible definition for "secondary stability" (if
>we could all agree on one) it wouldn't make me happy. For one thing it is
>trying to combine two different things into one number, height and angle of
>lean at the maximum height (area under the curve). Greater height to the
>stability curve usually means a greater angle of the slope getting up to
>that height. If a kayak is too hard to lean then the paddler must lean
>further out to the side (using his weight hanging by his knee to overcome
>that righting force) and that puts one into a very vulnerable position
>hanging out over the water, if his knee slips he's going to get wet. So for
>me just increasing the height of the curve (beyond some necessary minimum)
>has little or nothing to do with my perceived feeling of "secondary
>stability" (when leaning a kayak up to as much as I want to lean it to use
>the change in the underwater shape to help control it--without hanging out
>over the water). The angle at which maximum stability occurs works somewhat
>better for me but it too has its drawbacks. If that angle is too far out to
>one side, I can't bend enough to get near it and it is therefore wasted on
>me. Or the slope is made too shallow too soon for me to feel much righting
>moment to give me a feeling of security. Maybe a ratio of the slope of the
>curve coming off zero (initial stability) to the slope of the curve at some
>angle (some reasonably easy for the average paddler to bend too angle like
>20 or 25 degrees) might work better to meet what I "feel". I'm just
>speculating here. Personally, I think the slope of the curve determines what
>one perceives, not the area under it up to that point. Maybe a ratio between
>the slope of the curve and the angle of lean would work. I'll have to think
>more about that.
>An expert kayaker defines "secondary stability " by the feeling of security
>he has while leaned a good bit. Because he bends at the waist the actual
>righting moment (height) isn't critical to that feel, but the angle of the
>slope is (as you explained very well in your discussion of it). So I guess
>what I'm saying is I'd be very hesitant to define "secondary stability"
>without some discussion included about what expert paddlers feel. I'd then
>try to figure out what are the parameters that influence that feel and see
>if we can't come up with a way to calculate it from the stability curve
>graph.
>
>Great article, by the way, accurate and understandable. I'm going to ask
>Chris Cunningham to look at it on your website, maybe Sea Kayaker would like
>to publish it (or an abridged version). I hope all the back and forth we did
>about secondary stability on Paddlewise had something to do with your
>inspiration to write it.
>
>P.S. Could you also redo the graph keeping the overall widths (maybe by
>using wider widths) of the shapes the same so they all have some positive
>stability?
>
>Matt Broze
>http://www.marinerkayaks.com
>
-- 
Nick Schade
Guillemot Kayaks
824 Thompson St
Glastonbury, CT 06033
(860) 659-8847

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