Subject: Re: baidarka stability
From: Robert Livingston & Pam Martin (bearboat@bazillion.com)
Date: Mon Dec 11 2000 - 14:25:30 EST
On 12/11/00 8:14 AM, "Kirk Olsen" <kolsen@imagelan.com> wrote:
> Chuck <Sparky@TheWizardWorks.com> pondered:
>
>> I'm a bit confused as to the differences between initial and secondary
>> stability. Might someone clear this up for me?
>
These terms have never been formally defined. They originated (in my
opinion) as marketing terms to try and imply that there were boats that were
slender and fast that still had good "secondary" stability. Even if you
tipped fairly easily, you "felt" secure.
Since these terms have never been defined explicitly they are terms that can
be applied quite freely. If I say that a boat has good secondary stability,
you cannot really formally prove me wrong. If I say that this boat has
better secondary stability than that boat, again there is no explicit way to
"prove" this because of the lack of definition. Some people think that this
makes the terms meaningless. Others are happy using them as subjective
"impressions"
There is such a thing as a stability curve that plots the righting force
against the degrees of heeling. The righting force is zero when a boat is
floating flat and starts increasing as the boat heels. Eventually the
righting force returns to zero. This is the point at which the boat tips
over.
Some people have offered as a definition of primary stability, the slope of
that curve at its beginning. If you accept this, then there would be a firm
definition of primary stability. You could exactly determine the primary
stability of any boat as a number (assuming that you could agree where to
consider the center of gravity).
For secondary stability, I could imagine a diverse set of definitions
(maximal height of the stability curve; angle of heel at the maximal height;
angle where the righting force returns to zero; slope of the curve where the
righting force returns to zero etc) In any case, I have never seen anything
approaching a consensus on this. It is used generally as a feeling of
comfort as one heels the boat that you are not going to abruptly tip over.
For any of the definitions of secondary stability that I mentioned above it
is very hard to establish any "separation" of primary and secondary
stability. That is to say that any conventional kayak hull that has a good
primary stability will have a good secondary stability. It would take a
really weird hull design to separate the two. Marketing hype, however, likes
to separate the two. There is some sort of magic implied for a given kayak
that has low primary stability and high secondary stability. It is hard to
propose any definitions based on stability curves that would separate these
two however. A boat with marked flare above the waterline could (by most of
the proposed definitions above) have more secondary stability than a boat
with marked tumblehome. But again, just add an inch of width to the boat
with tumblehome and see how its "secondary" stability starts to beat the
flared boat that is one inch narrower.
Primary stability (if you accept the commonly proposed definition) is based
almost entirely on waterline width and height of center of gravity and to
some extent on how much the boat waterline is squared off (ie how much the
maximal waterline width tends to be maintained for and aft of the midline)
Other factors (the specifics of the cross-sectional shape: rounded, chine
square tend to be totally insignificant.) So it is very hard to design a
boat that is 23" wide at waterline that has more primary stability than
another boat that is 24" at waterline. VERY HARD.
I think that one reason the attempts to bring serious definitions to these
terms fail (beside marketing and Emperors new clothes) is that stability
curves are not the be-all when it comes to perceptions of stability and any
definition that relies on them will be faulty. Kayaks are different from
most boats in that the center of gravity is easily moved around relative to
the hull. The paddler contributes heavily to the center of gravity and he
can lean one way or the other. If the paddler has "time" to lean, he will
perceive that boat as stable even though it may have unimpressive stability
curves. If you take a 15" wide boat and imagine it to have a 3 foot very
thin daggerboard hanging down, it will feel very stable. The daggerboard
resists sudden heeling forces although its presence (it very thin) will not
show up on the stability curves. The resistance to sudden healing, allows
the kayaker to unconsciously adjust his lean for any heeling that starts to
develop. My impression is that rounded cross-sections do not resist sudden
heeling the way that hard-chined or squared cross-sections do and therefore
the later seem more stable even though they may have an identical stability
curve. It also seems likely to me that as a kayak moves through the water
there are probably hull shapes that create a form of dynamic stability as
water moves along a chined surface.
Sea Kayaker provides stability curves for the kayaks that they review. The
individual reviewers almost always refer to a skinny boat as having "good
secondary stability". If there were a formal definition for secondary
stability, the reviewers comments would be useless (which they may in fact
be in this regard. I do not recall a description of a kayak having poor
secondary stability). The informed reader would simply look at the curves or
the magazine could simply state as a number what the secondary stability
was.
My biggest objection with the stability curves in Sea Kayak is that they use
seat height to determine center of gravity. Since the height of the center
of gravity is so important, you can game the system. Just build a boat will
no seat or a very thin seat. Get a great stability curve and rely on the
purchaser to add padding to lift himself up a little. Most people are more
comfortable in the "kayaking" position if their butt is a little higher than
their heels but they pay a big cost in terms of stability curves.
Years ago, I tried to get people to stop using the term secondary stability
unless they were willing to formally define it. I obviously failed.
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