Re: [Paddlewise] Primary and secondary stability

Selville wrote:

>Can you let me know how the primary and secondary stabilities of sea kayaks
>are measured?

Now that is a good question!
Primary stability is how hard the craft resists being tipped from the
upright position. If you are looking a graph of stability (see the reviews
pages of http://www.marinerkayaks.com or any kayak review at
www.seakayakermag.com ) the steepness of the angle off of zero is a measure
of the primary stability. Secondary stability is a lot harder to define (at
least for kayakers). Most kayakers will be hard put to define it but will
tell you they know it when they feel it. Sort of a seat-of-the-pants thing.
Some have tried to use the total area under the curve but this doesn't
correlate well with the "feel". One designer I know used to claim secondary
stability  doesn't exist and the kayak that is more stable initially will
always be more stable at all angles of lean. I'm not sure if he still claims
this--especially the latter claim-- since I showed him some stability graphs
(Chinook and Puffin in the Winter 1986 issue of Sea Kayaker magazine) where
the less initially stable kayak--the Puffin--had not only higher relative
stability at higher angles of lean than the Chinook but also a higher
maximum stability and a greater total area under the curve). Personally (not
necessarily scientifically) I define secondary stability as how secure you
feel when you have leaned the kayak well to one side. This makes it more
relative to the primary stability. A kayak whose maximum stability is five
times as high as its stability at 5 degrees of lean will "feel" more
secondarily stable than a kayak with a much higher maximum stability that is
only twice as stable as it was at 5 degrees. A kayak with less stability
(height of the curve at a given angle of lean) can still feel a whole lot
more secure while leaned. I'll make the comparison with rocking back in a
rocking chair (low initial stability/high secondary stability) and compare
that to rocking back in a regular four legged chair (high initial
stability/high total stabilty/low feeling of security as you teeter at the
balance point). It is hard to lean back on a standard chair (most of my high
school teachers frowned on my practice of balancing my desks on their back
legs when I was bored, especially when I would draw their attention to what
I was doing when I would almost lose my balance backwards and in a desperate
recovery crash back down loudly to the primary stable position. I don't
remember ever going over backwards but that was always the risk I flirted
with--a capsize to the rear?). Looking at the graphs I would define a kayak
with good secondary stability as one with a relatively shallow angle off of
zero (so you don't have to put a lot of energy into leaning it) but which
has the point of maximum stability (the top of the curve) at a greater angle
of lean (and maybe at a higher maximum point) than a kayak with less
"secondary" stability. Even this doesn't totally account for the difference
in "feel". A smooth progressive increase in stability out to near the
maximum allows a trustworthy "feel". Any abrupt changes would be like
putting a speed bump or flat spot on the rocking chair rockers.
These are totally my own observations and guesses regarding secondary
stability in kayaks and I make no claim to scientific validity or even at a
valid definition of the term (which probably exists somewhere in Naval
Architecture and someone will hopefully inform us of).
I and most experienced paddlers prefer lower initial and good "secondary"
stability. Easy to lean/secure while leaned.
Then there is what I define as "dynamic stability". Which to me comes down
to: "how does the kayak feel in rough seas?". Does it stay under you or is
it always wanting to slide out to the side out from under you. Does it feel
neutral, consistent, gyroscopic or are the waves knocking it around and
rocking it. Are the motions smooth or jerky. Do two waves pick it up by the
ends and make you feel like an off balanced turkey on a spit or does it
maintain a comfortable secure stability in waves. Does the high initial
stability mean the hull stays flat to the wave face leaving the paddler
constantly bending at the waist to stay vertical or does a high center of
gravity (higher seat usually) mean that even bending at the waist is not
enough to compensate for the upside down pendulum action created by the high
initial (flat to the wave face) stability. Imagine a wide flat canoe with
its high sitting position in steep seas as the stable canoe is tilted by the
wave the high seat is swung to the side, shifting the center of gravity to
that side. Any attempt by the paddler(s) to swing their center of gravity
back over the canoe will by an equal and opposite reaction also tilt the
canoe further in the direction that it was already dangerously tilted
towards. Luckily the canoeist may be able to get down on their knees to pray
while also lowering their center of gravity (after all God helps those who
help themselves doesn't she). (With John Winters on vacation who will come
forward to defend the seaworthiness of canoes after another of my scurrilous
attacks? I'm sure John would say to not go to sea in a too initially stable
canoe.)
Matt Broze
http://www.marinerkayaks.com
-----Original Message-----
From: Julia Selby & Kevin Melville <selville_at_ozemail.com.au>
To: PaddleWise_at_lists.intelenet.net <PaddleWise_at_lists.intelenet.net>
Date: Tuesday, August 10, 1999 10:40 PM
Subject: [Paddlewise] Primary and secondary stability



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