Re: [Paddlewise] 2ndry stability (and more)

Nick wrote:

>>>The Mariner XL does not have distinctive secondary by my proposed
definition. http://www.guillemot-kayaks.com/MarinerXL.gif shows a
replotting of the data, traced (crudely) from the web site and then
put into Excel. The first derivative is just the slope of the curve
at any point. The Magenta line shows this by taking a simple
rise-over-run calculation. The Red line does the same thing again to
get the rate of change of the slope of the curve.<<<<
<snipped lots>

Below is much of what my back channel response was to Nick (I'm on the
digest and didn't realize he was posting this to the list because I thought
the graphs wouldn't show up (but now I see why he posted them to the web
rather than just send an attachment to me).

To put this all in layman's terms, it looks to me that what the paddler
feels is
the stiffness (slope of the curve) and when that stiffness starts to
decrease significantly is when the paddler is going to get nervous. This
pushes the critical angles for "secondary stability" (or whatever we choose
to
call it) even further to the left than I had imagined (and I have been
moving
to the left from how I first looked at secondary stability on the curve
progressively). First I thought the capsize point had some significance but
then John Dawson's
explanation convinced me to retreat to near maximum stability as the
critical
point. Then his explanation of how the test boats capsized uncontrollably
beyond
the maximum righting moment caused me to believe that the critical point was
probably
somewhere between the point of maximum stiffness and the maximum righting
arm. In other words, if one moved beyond the break in the Pisces' curve's
steep upward slope (where the stiffness drops off quickly) they will have
gotten into an area (beyond an angle) where you couldn't trust that any
momentum would be absorbed by the increasing stability. (If you read it,
this
is what I was trying to point out with the first ramp example where I
suggested it would be hard to roll a ball bearing up and over the break in
the Pisces curve without also going all the way over the top). The curves
can be found in the XL review at http://www.marinerkayaks.com
I think early on I was fooled by my own experience with how far I could lean
a hull and
still feel secure. I probably wasn't allowing enough for the effect of
shifting my CG to keep it over the kayak. This is why I'd like to compare
curves that include the shift in center of gravity that a kayaker can make
with the existing static
curves. I think they would show us better what the kayaker really feels out
near the
edges of comfort and by measuring the kayaks tilt then also see to what that
angle relates to on the stability curve (corrected for CG shift). The
ability to lean will probably mean that the area up to the original maximum
or a little more are important to the stiffness (on the static stability
curve).
I think it is the rate of change with a further lean (from the existing
balance between the healing and righting forces at a given lean angle) that
is important to what the paddler senses.

Now I'll comment on some other things that weren't in my e-mail to Nick. I
like John's suggestion that the ratio of height at a given angle to the
overall height of the curve (or some other ratio) might be valuable. One
caveat, too much initial stability will mean that having a good feel of
secondary stability is difficult to achieve because one has difficulty
getting the kayak up to that angle of tilt without shifting some weight out
to one side (over the water) to gain the leverage needed to tilt the stabile
kayak in the first place. Thus you must first lower the stability of the
kayak by leaning out over the water to get it to tip very far at all (and
one result will be reaching the capsize point at a lower angle than might be
indicated on the static stability curve). Even then this lean still takes a
lot of effort with a loaded stable kayak. The same paddler in a much less
initially stable kayak can lift a knee and at the same time shift his weight
in the direction that makes the kayak more stable rather than less stable.
The paddler in the stable kayak must at some point shift from the "lean to
help the kayak to tip" mode to a "balancing the kayak on edge" mode.
Probably this happens when the upward slope of the stability curve starts to
decrease its rate of climb--this would be looking at the curve that takes
into account how he had to shift his center of gravity to get the kayak to
tip at all.  At this point the kayaker will be able to find only a tenuous
balance point between capsizing or having the kayak flop back down hard due
to its high initial stability. The high force needed and the unfavorable
body lean give this stable kayak a very insecure feel when leaned to higher
angles. If anything slips (like the knee on the kneebrace) the kayak may
right itself but the paddler may also fall overboard. Contrast this with the
initially tippier kayak that was easy to tilt just by lifting a knee and
that at the same time allowed the paddler to move his center of gravity in
the direction that increases his stability right from the start (rather than
have to try to switch the direction of his lean at some tenuous balance
point later). If your knee slips with this less initially stable kayak you
are still directly over the kayak and the righting moment simply rights the
kayak below you. Your center of gravity will shift slightly lower but will
not have to shift to the side to regain equilibrium.
If you want to lean your kayak it seems pretty obvious it had better also be
unstable enough initially that it is easy to do so. If it takes too much
effort you just won't lean it much. A tippy kayak with little or no
"secondary stability" will require better balance skills than one that is
tippy at first but progressively stiffens up. The paddler learns they can
trust that the initial looseness will be absorbed by the increasing
stability at greater angles of heel and will catch the kayak before it tips
too far.
BTW the XL is probably the quintessential high secondary stability kayak to
a paddler who "knows it when he feels it" so I'd look closely at that curve.
I think you can find secondary stability on a static stability graph by
looking for a relatively shallow but consistent rise in the stability curve
(changes to the slope of the curve is probably what gets described as
notchy, and humans appreciate consistency). The further to the right the
point of maximum stability is the further you can lean such a kayak before
feeling unstable (but this area of feeling unstable may still come well
before the maximum righting moment is reached--probably it is where the rate
of increase lessens significantly--like the break in the Pisces curve). If
however, the curve is too shallow initially you won't pick up the feeling of
secondary stability soon enough in the lean to give you a feeling of
confidence even if the point of maximum righting moment is further to the
right than the kayak with a slightly steeper curve. Essentially if a paddler
can't easily tilt the boat far enough to finally feel the secondary
stability kick in they will feel that the kayak is unstable even if it
ultimately has good secondary stability for those willing to lean it that
far.
The best feel will come from a consistent slope that doesn't start to round
off until at an angle just a little further than the paddler will want to
lean it. Given curves with the same height of the maximum righting moment,
if the maximum is reached too quickly the paddler might accidentally go
beyond it and capsize (or have to brace), if the maximum is too far to the
right the slope will be shallower and the stability won't be felt as much as
it could have been had the slope been steeper. The higher part of the curve
beyond that point will be wasted on most paddlers unwilling to lean far
enough to take advantage of it.
The kayak that is (or with a gear load becomes) too initially stable will
not allow a strongly leaning paddler to feel secure for the reasons given
above, but more likely the paddler won't lean it much at all because it is
too much work. That paddler is therefore unable to enjoying the benefits
that come from leaning the kayak.

Oh, yes I disagree with whoever said a low backrest (or being free of the
backrest) makes one feel more stable in waves. While there are several
advantages to just high enough in a backrest (like rolling ease for a lay
back type roll) I always feel far more in control in waves if I can use my
spine against a firm backrest to hold the kayak level as I keep my body
vertical over the kayak (if the kayak is not too stable to allow this). Now
the difference may be that he uses a very stable kayak, and I use a less
initially stable one. I think in a very stable kayak in chaotic seas, I
would want to keep my head level and let the kayak rock as it wants below me
while staying loose at the hips. In a tippy kayak I want to keep it level
(as well as myself vertical above it).

Awhile back, John Winters made the case for why a stable kayak being tipped
by the swell would not destabilize a rigid paddler. While he is right about
this I wonder if the resulting tipping side to side of the (stable kayak and
paddler) unit (and the momentums that make the tipped paddler's seat of the
pants feel be balanced--with his eyes closed) might not also give the
paddler a disorientation that could lead to sea sickness or kayak angst as
he tries to integrate the horizon with his seat of the pants (inner ears)
feelings.
Matt Broze
http://www.marinerkayaks.com



***************************************************************************
PaddleWise Paddling Mailing List - Any opinions or suggestions expressed
here are solely those of the writer(s). You must assume the entire
responsibility for reliance upon them. All postings copyright the author.
Submissions:     PaddleWise_at_PaddleWise.net
Subscriptions:   PaddleWise-request_at_PaddleWise.net
Website:         http://www.paddlewise.net/
***************************************************************************