Re: [Paddlewise] Secondary stability... another way to think about it

Nick wrote:

(SNIP)

>
> "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.

I see it the same way. The boats that I have heard people describe as
"notchy" have curves that show abrubt (OK, how much is abrubt?) changes in
curvature both up and down slope.  I once paddled a canoe that flopped from
side to side at small angles of heel quite easily then got stiffer and then
a bit further on just about tossed me out of the boat. Others who paddled it
called it notchy. I called it weird. It had a concave curve for the first
ten degrees or so (if I remember correctly I no longer have that info or the
lines to that boat) rose quickly to its maximum and then dropped just as
quickly to the maximum range. Very disconcerting and reminded me of the feel
of a sprint canoe. The boat had a steeply "V"ed bottom with a small radius
at the turn of the bilge and tumblehome coupled with relatively low
freeboard. I wish I had kept the lines.


>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.

Beats me. Never understood that stuff

>
> 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.

I did some last night and it looked interesting but, as an old friend once
said, "I don't understand everything I know about that." Unfortunately we
now have hard water and I can't test anything.

Cheers,
John Winters
Waveform Plastics Technologies Ltd.
15 Ena Ave.
South River, Ontario
P0A 1X0



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