Can you try to give an example of what you mean by "notchy" just so we are on the same page here. In my "inflection point" theory I was thinking of looking at the slope of the curve at the inflection point for some reading on the relative "secondary" stability. Looking at how quickly the slope of the curve changes would probably help indicate how noticeable the stiffening is. My example: http://www.guillemot-kayaks.com/Example.gif would have a quite noticeable point where it feels like it is getting more stable. I suppose this could be called notchy. At any point on the curve you have several things that may be interesting to look at: The righting moment - just how much force is being applied to right the boat; the area under the curve - how much energy it took to get the boat to that point; the slope of the curve - how hard it will be to tip it farther; and I am adding: the rate of change of the slope of the curve - the trend of how hard it is to tip the boat farther. i.e getting easier or getting harder to tip a little farther. "notchiness" sounds to me like: The rate of change of the rate of change of the slope of the curve (follow that?) - or how quickly the trend of how-hard-it-is-to-tip-farther changes. While this is quantifiable, I am not sure how important it is. By this definition a "notchy" boat might be able to tip fairly easily until a point where it suddenly becomes hard to tip farther, but if you do it quickly becomes easy again. And the notchiness is the suddenness of the change. Worded that way it sounds interesting - this feeling would be quantified by a relatively high value of the 3rd derivative. And that is why I don't really feel comfortable with it. What is the 3rd derivative really. If the stability curve corresponds to a plot of distance a car travels from home over time, the first derivative is the velocity of the car. The 2nd derivative is the acceleration of the car which pushes you back in your seat. I suppose the 3rd derivative corresponds to that funny feeling you get when go over a hump in the road while going fast and you stomach does a little back flip. I think there may be some merit to the idea of normalizing the curves to the maximum stability. Like you suggest it would help identify boats which have "good secondary" relative to their overall stability. Nick At 8:37 AM -0500 11/20/00, 735769 wrote: >Nick wrote: > >(SNIP) > >> I am also seeking a definition that does not depend on the paddler's >> skills. With practice, a skilled paddler can keep just about anything >> upright. A novice paddler may have trouble keeping a bath tub >> upright. Stability curves are determined assuming a rigid hunk of >> meat in the cockpit. This has the advantage of being skill >> independent, any idiot can by a rigid hunk of meat. >> >> My suggestion of looking at the inflection point is an attempt to >> quantify the feel in some boats that they get harder to lean at some >> point. Maybe this feeling should be called something else: "reserve" >> stability or something. > >What about the rate at which stability changes. It seems that a boat that >has a rapid change of stability would fit what some paddler's call "notchy". >What could you add to the inflection point location information that would >reveal this? > >I think the slope in combination with the area under the curve has the >potential to tell us this. Finding some kind of "number" might prove >difficult, however. I recall that the U.S. Navy has a method by which they >determine "safe" stability at high (above initial stability) heels. I don't >know how that will apply but maybe in some modified form it will. > >One thought. Suppose you normalize the stability curve (i.e. make it non >dimensional by dividing the righting moment at every heel by the maximum) >and then compare the curves. It would seem that the boat with the greater >area under the curve beyond the initial stability realm would feel like it >had the highest stability. One could easily check this if you had access to >a lot of paddlers and differing boats. In this way you might see relative >rather than absolute values and paddlers seem to like "relative". -- Nick Schade Guillemot Kayaks 824 Thompson St, Suite I Glastonbury, CT 06033 (860) 659-8847 Schade_at_guillemot-kayaks.com http://www.guillemot-kayaks.com/ >>>>"It's not just Art, It's a Craft!"<<<< *************************************************************************** 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 Mon Nov 20 2000 - 12:31:32 PST
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