Subject: RE: baidarka stability...
From: Peter A. Chopelas (pac@premier1.net)
Date: Tue Dec 12 2000 - 12:31:29 EST
Chuck,
There as a long and very detailed discussion of this subject over on the
Paddlewise list just last month, if you want to know more (probably more
than you want to know) you could go look at the archives there.
There are a couple of interesting things that came out of the lengthy
thread:
1. The term secondary stability is unique to the kayaking community, it
appears in no other navel architecture or watercraft usage. I would
speculate this is because only in a kayak does it have relevance since you
control the kayak with weight shift and body movements, this allows the
"captain" to control the craft (which really becomes an extension of
his/her body) through rough conditions in a way not possible with other
water craft.
2. It has no precise definition and therefore really can not be measured
or rated other than subjective feel of the kayaker. Some have tried to
show correlation with the stability curves though others pointed out (me
included) there was no consistent measurable quantity that can be
determined from the curves. In other words, by looking at the curves alone
you could not say which kayak would have good secondary stability and which
would not, it is too subjective.
I personally think a measure could be developed but it would take a lot of
different kayaks, a lot of different paddlers, and a lot of testing and
cross correlation between different measurable qualities, not just the
quasi-static righting moment curves you see in SK magazine.
I suspect however that the higher heel or roll angle that one feels secure
or comfortable with in a kayak, the more most people would identify as
having good secondary stability. The perception of the angle of heel is
the important issue, if you can roll a kayak on it's edge and comfortably
hold it there, it feels like it has good secondary stability. OTOH, even
if a kayak is rock steady at low angles of heel but you find you can not
hold it on it's edge, most would say it has high initial stability and low
secondary stability.
There are similar terms used in Naval Architecture called initial and
overall stability, which do have definitions but are not quite the same
thing. Most text books define initial stability at heel angles less than
10 degrees, and overall stability as the maximum righting moment achieved
by the hull design beyond this 10 degrees. Of course in just about
everyother water craft, and certainly virtually all commercial vessels, you
can not apply a righting force as you can with weight shift and bracing in
a kayak. So the term overall stability likely has another meaning in the
mind of a hull designer I would guess, than secondary stability does to a
kayaker.
Peter
On Monday, December 11, 2000 7:06 AM, Chuck
[SMTP:Sparky@thewizardworks.com] wrote:
> Hello all...
>
> I'm a bit confused as to the differences between initial and secondary
> stability. Might someone clear this up for me?
>
> Thanks!
>
> Chuck
>
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