Nick Schade <schade_at_guillemot-kayaks.com> wrote: >>>>>I've put together a discussion on stability and would like to get a critique. Check http://www.guillemot-kayaks.com/Design/StabilityArticle.html and let me know what you think.<<<<< Typo in this line:extra words in brackets >>>side will cause the center of buoyancy to move toward [that towards] the side you are tipping. If the boat is shaped to be stable, the CB will move out to the side faster than the CG.<<< Much later the word "how" is missing before "much": >>>>>Raising or lowering the CG will effect the stability in a predictable manner. The exact change in stability can actually be calculated based on much the CG is changed, but<<<< Word "be" is missing below before "sure" >>>>You can never sure of the personal preferences of someone describing a boat. A salesman <<<<<, Nick wrote: >>>>>>>The peak of the curve is where the stability starts to diminish. Having this point either be higher or at a greater angle of heel will make the boat feel like it has more secondary stability. By looking at the area under the curve until the peak of the curve (the dark blue area) you can get one value for the secondary stabilty.<<<<<<< Note: last "stability" misspelled. (and said similar things elsewhere in the article) 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 *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************Received on Thu Feb 15 2001 - 23:32:21 PST
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