Re: [Paddlewise] Stability Discussion

Marvin <skylakeboatworks_at_yahoo.com> wrote:

<SNIP>
I appreciate and like the work that went into the stability curve
comparisons.  I would really like to see one more graph that takes the 5
shapes you compare and give the widths for the same stability curve (really
just getting the line to cross back at zero at the same point for all 5
shapes.  I would use the rectangle as the baseline and then do the rest as
rectangle-x or rectangle+x in order to compute the comparative results.  It
would be nice to see how much wider a V would have to be to get the same
stability (till capsize) as a round bottom.<<<<<<SNIP>

Back about 20 years ago when Cam and I had just started designing kayaks, We
met Robert Livingston (same one now posting here on stability) who was also
designing sea kayaks. We became friends and surf kayaking buddies. Robert
wanted to find out about stability (probably because his first design didn't
have any to speak of). He programmed his computer to calculate the relative
stability of various midship section shapes and placed the paddler's center
of gravity the lowest a seat that could fit an average butt could go down
into that particular shape. If I recall correctly Robert had done about 50
curves of various shapes and widths (and I have the printouts still on file
but unfortunately the program apparently went out along with an old computer
several generations ago).

Why I'm bringing this up is because I think those who want to know which
shapes of equal width will have the most stability (or how much wider a
shape has to be to have the same stability) are barking up the wrong tree.
What I was looking for most in analyzing Robert's graphs back then was not
how to get the most stability from the narrowest kayak or even the narrowest
waterline (although I paid attention to that as well). What was more
important to me was which shape gives the most stability for the least
amount of wetted area. So what I'd like to see are some stability curves
that hold wetted surface as the constant for the differing shapes. Those
with considerable flare should do well by this measure (as well as doing
well on the equal waterline width measure). Lots of flare will also satisfy
the expert who wants an easy to lean kayak but also a secure feeling once
leaned (what most define subjectively as "good secondary stability"). By his
third design Robert took flare to the extreme (rather than just
concentrating on minimizing wetted surface--as with his first design) and
came up with a very successful little kayak that had a lot of influence on
the design of our Coaster model.

Also I wouldn't worry about the angle of final capsize, I'd look at which
shape produces a curve with the steepest rate of increase in the 20 to 30
degree range (with an empty kayak).
Matt Broze
http://www.marinerkayaks.com


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