Re: [Paddlewise] chine and stability

Matt wrote in response to Alex:

>
> You won't get high on this curve without also going steeply up.
> I didn't word that quoted passage very well. What I meant to get across
was
> that the further the kayak could be leaned before reaching maximum
stability
> the more likely an expert kayaker would say it had good secondary
stability.
> By my definition even a very unstable kayak can have good secondary
> stability as long as the point of maximum stability is shifted enough to
the
> right that a kayaker can't easily lean the kayak beyond it. This is far
> different than the "overall stability" (the total energy required to cause
a
> capsize) which can be measured by measuring the area under the curve.

Maybe I should help clear this up. The area under the curve does not
constitute the totality of "overall stability". As I mentioned in an earlier
post overall stability includes the area under the curve, the slopes of the
curve, the range of stability and the location of maximum righting force or
moment. All combine to provide what we sense as overall stability or
secondary stability. Naval architects do not have a single measure of
overall stability but utilize all factors.



>This
> feeling of secondary or final stability to a kayaker takes in to
> consideration that the kayaker wants to intentionally lean her craft. Low
> initial stability allows this to be done easily so there is no big hump
that
> has to be overcome to get the kayak leaned to there but an even further
lean
> would be resisted by a still increasing righting force.

I would not include the kayaker's objectives as a measure  or yardstick of
secondary stability. If we do then secondary stability becomes "what we say
it is" and has no meaning.

(SNIP)

>To adequately discover how a kayak will react to
> this common situation we would probably need to shift the center of
gravity
> over (to where a good kayaker could shift it) in our "model" and look at
the
> new curve that results then. Maybe I'll suggest this to Sea Kayaker it
might
> be informative to do this with the test kayaks. John has the same program
> Sea Kayaker uses maybe he can try it out for us with some of the kayaks he
> has gotten measurements for.

This is precisely what happens when the person leans to resist heel. I
discussed this in my article in Sea Kayaker to explain why a boat with
negative static stability could remain upright. By plotting the stability
curve for shifts in weight (commonly done for ships)  on finds out how the
boat will react to a wide range of conditions. Fortunately the shape of the
stability curve does not change radically on kayaks and one can simply raise
and shift the curve to the right to see what happens. This reveals why a
shallow slope on the "back" side of the curve can impart a greater sense of
security to some paddlers since it provides more righting force to slow the
capsize.


>I think they would find that
> if the ball bearing made it up the steep slope and over the break that
more
> likely than not it would also continue on all the way over the shallow
peak
> and that it would be very difficult to give the ball bearing just the
right
> momentum to get it over the transition but not also over the top. The
marble
> would be much easier to get over the top of the shallower curve from a
tippy
> kayak but it could also be much more likely to get further to the right
> (more tilt) and return back down the front again (more predictably) than
it
> would with the steeper ramp with an earlier rapid change in angle in it
> (like the Pisces). Maybe this sudden transition from hard to lean to easy
to
> lean further is what Jed means by "notchy".

We could also apply the ball bearing analogy to the speed of capsize by
comparing the velocity of the ball bearing rolling down the gradual slope of
the backside of the curve with the more rapid acceleration of the bearing
rolling down a steep slope.

One way of looking at it  that may help is to ask how much righting force do
you have to provide to keep the boat upright at each angle of heel. If the
slope drops rather steeply you have to provide more for every additional
degree of heel than for a boat with a shallow slope. One might not notice it
when up to the armpits in water but it still applies.


> I do not think any sudden changes on the back side of the curve would
> contribute to notchiness though, as John postulates, because once you have
> forced yourself over the peak (of a steep high curve anyhow) the recently
> unstuck drawer is probably going to fall out of the chest and spill all
your
> clean underwear onto that floor you didn't clean because you went kayaking
> instead. Of course if the static curves "backside" notch is no longer over
> the top because the kayaker has bent at the waist and shifts his center of
> gravity then a quick transition that had been on the "backside" could be
> shifted forward to suddenly withdraw the support you needed.

True if you "force yourself" over the maximum righting arm because you are
trying to attain a capsize by forcing yourself past the maximum righting
arm.
However, most people will try to resist the capsize and the righting force
obtained from a brace or body shift combines with the righting arm of the
hull to act towards righting the boat. The greater the righting force past
the maximum
righting moment the less force one needs to exert to return the boat to an
upright position.  Because the righting force involves both the boat and
paddler we need to look at the system rather than the individual components.

One of the most challenging aspects of discussing stability comes from
trying to verbalize perceptions. We "feel" stability and unless one has
experienced a lot of boats and related that experience to the static
stability curves they (the curves) make little sense or have little value.
In looking back over the related posts I feel like everyone has it "right"
within the context of their experience. Few of us have the luxury of
matching that experience to a full range of stability studies and we draw
our conclusions on the information we have.


Cheers,

John Winters




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