Re: [Paddlewise] chine and stability

Jed wrote:
 >>>>I do however maintain
my perception that boats with harder chines tend to have a "notchy" feel
when
they are leaned aggressively. And that boats with softer chines tend to feel
less "notchy".<<<<<

Please define "notchy". I've seen at least one hundred stability graphs (for
both round and hard chine kayaks) and have yet to see one that isn't a
fairly continuous curve without sudden changes except sometimes near the
"peak" (the point of maximum righting moment). Which boats are you
comparing? Which ones) feel "notchy"? I'm much more comfortable comparing
specific trees than trying to make sense of the forest.

Jed asks:
    >>>>>>Matt, is it not possible to back away from the forest a bit and
make some
generalizations about the current commercial offerings of boats with hard
chines versus boats with softer chines? Given that we are addressing this to
a novice curious about researching her first boat.
    I understand that your post seeks to correct inaccuracies from my post,
and I welcome the correction. But please remember that I was referring to
complete boats not the chines themselves.<<<<<

Since there are hundreds of current offerings covering a wide range of the
possible spectrum in cross sectional shape limiting the discussion to
current offerings doesn't get nearly specific enough. If you would like to
get real specific and tell us the kayaks you find these differences in I
might be able to account for them because of other reasons besides the
differences in hardness of the chine. You can't make generalizations that
would be helpful to a novice here (if one ever can) because, as I tried to
point out, other variables overwhelm the one you want to generalize about.
Even trying to say "if everything else is equal" won't work here because
everything else can't be equal here and whichever characteristics you make
equal will both effect the outcome and still probably allow exceptions to
the generalization. Therefore a generalization here will only lead a novice
astray, and in the worst possible way, by believing something that is not
true. Much better to know you are ignorant and try the kayaks for yourself.

>>>>>>>Matt wrote:
<<<<<<< How would one show or detect a smoother transition? What should I
look for
on a static stability graph? All the graphs I've looked at seem to be
relatively smooth. >>

>>>>Jed responds:
    >>>>One could easily determine the rate of change of the righting force
relative to the change in attitude. This then could be compared relative to
similar stability data from other designs. The combined data then would
allow
a reasonable person to make a judgment about one design having a relatively
smooth or less smooth transition of forces as the boat is tilted off an even
keel. Smooth or less smooth in this case would refer to the relative
steepness of the stability curves.<<<<<<<<

This is what is done. It is known as the "static stability curve" and you
can see the results for many kayaks in Sea Kayaker magazine's kayak reviews.
Hard chine or soft chine, the curves look very similar to me. I don't think,
even after studying them closely, I could pick out a hard chine boat from a
soft chine one based only on looking at the static stability curves unless I
also had more data on the kayaks.

John Winters wrote:
>>>>>Perhaps Jed had in mind the classic naval architecure definition of
initial
stability which refers to the stability range in which the metacentric
height can reliably predict stability and is represented by the slope of the
righting curve. "Secondary stability" for sea kayakers and canoeists seems
to correspond to what naval architects sometimes call "Overall stability".
I had never heard the term secondary stability until I started paddling in
Canada.<<<<<

I am in full agreement with the definitions of initial stability made by
John and the naval architecture texts. A good definition of secondary
stability is what has eluded me. It appears to be a paddlers term not a
Naval Architects term. However, good paddlers know it when they feel it. It
is my experience that what the paddler senses doesn't correlate at all well
with "overall stability". I think I can spot it on a static stability curve
by looking at the lean angle at the peak of the curve or the point where it
breaks most sharply near the peak (see curve for the Pisces in the XL review
for an example of what I mean). The larger the lean angle is at the peak of
the curve (or the break point) the more likely that kayak will be described
as having good secondary stability by a kayaker willing to test the limits
of stability to the point of capsize. The amount of righting force at the
peak does not correlate well with subjective reports and, in fact, too high
a righting force can work against the feeling of good secondary stability.
Ever try to open a stuck door or drawer gently, without going too far once
it finally comes unstuck. Similar kind of thing.

John wrote:
>>>>>True enough. Some hard chine boats have very high section coefficients
(i.e
more boxy). They have a lot of initial stability and then the stability
drops like a rock at a certain point of heel. I would sayt this kind of boat
has  a small "range of stability".<<<<<

I think this "drops like a rock" point may have more to do with the height
of the deck than the shape of the bottom. The stability curve generally
peaks about the time the deck starts to submerge during a lean. This makes
sense because the buoyancy most helping support the lean (at the widest
point) is then fully under the water with no reserve left to counteract more
lean. A higher hull will be able to lean further than a lower hull before
this happens. Of course if the low hull also has hard chines one might think
the chines were responsible for reaching the point of maximum stability at a
lesser angle. Alternately if both kayaks were wide enough then the increased
initial stability resulting from a more box like hull (higher midship
section coefficient) may be enough extra stability to make the lean
difficult and result in your drawers suddenly coming unstuck (as you fill
them with lubrication and capsize;-). Note: taken to the extreme a boxier
shape will result in a harder chine but a hard chine can also exist on a low
midship sectional coefficient hull as well if it is combined with something
like flare to the sides of the hull). (2nd note: the midship section
coefficient is the ratio of area of the shape of the boat underwater at the
cross-sectional plane at the widest waterline beam with the area of a
rectangle as wide as the waterline beam and as deep as the draft. For a
rectangular box it would be 1/1=1.00, a V bottom triangular hull would be
.50, a semicircular hull would be pi/4=.7865).

The capsize point is defined as where the righting moment drops to zero but
if one leans beyond the point of maximum stability ones momentum can also
quickly take one past the capsize point (as if the rug of increasing
stability with further lean has been suddenly pulled out from under you, or
the stability drawer came unstuck). The Sea Kayaker Magazine Review of the
Mariner XL on our website http://www.marinerkayaks.com has a very good
discussion at the end to help the reader determine the significance of the
stability graphs.

A strictly personal interpretation of "kayaker perceived secondary
stability" exists in the FAQ's of our website.

Sorry about the messed up diagrams in the last post. All I can say is they
looked okay when I pushed the send button. Jackie how do you get your
drawings to hang together? I'm definitely a newby at drawing using keyboard
characters. Do you have any generalizations that could help me next time I
try?

Matt Broze
http://www.marinerkayaks.com



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