Re: [Paddlewise] chine and stability

Alex Ferguson <a.ferguson_at_chem.canterbury.ac.nz> wrote:

>>>>Matt wrote -

 >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.

By that do you mean the flatter the peak the more "secondary stability"
(not having the Pices curve handy at present)?

I suppose one point to really emphasis to everyone is steepness (or not) of
curve, not total height that we're talking about.<<<<<<<

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. 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.

Once beyond the peak righting force it is hard to stop a capsize because
each degree further you lean the less the stability becomes. The original
stability curves Sea Kayaker's John Dawson did back in 1986 were done with a
torque wrench (now sea Kayaker models this with a computer program).
Interestingly the testers quickly learned that the only way they could
measure the righting force on the downhill or "backside" of the curve was by
starting at the capsize point and working backwards up the curve taking
measurements as the kayak came more upright. If they moved from left to
right along the curve (closer to capsize) just the slight added force moving
the kayak closer to the capsize point once it reached the peak would throw
the boat over into an immediate capsize This happened even when they were
trying to be as gentle as possible with the torque wrench. For that reason I
think one can pretty much ignore the shape of the curve past its peak (for
any feeling of static stability anyhow). Of course a kayaker/kayak combo
isn't static and by shifting the center of gravity (by bending at the waist)
a greater angle of lean can be achieved in the kayak before reaching the
maximum righting moment. 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.

The Pisces curve can be found on the XL review from Sea Kayaker (that I got
permission to put on my website www.marinerkayaks.com). What is interesting
about the Pisces curve is that it climbs fairly steeply, has a break but
then still climbs at a much more gradual angle for awhile before reaching
its peak. It isn't entirely clear to me but that break might actually be the
real beginning of the unstable "backside" range because the rate of increase
drops off so suddenly after the break. That was why I was using the stuck
drawer analogy earlier (and the bump on the rockers of a rocking chair in
the FAQ's about stability on our website. Imagine someone trying to roll a
ball bearing up ramp the shape of that curve. 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".

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.

There, in trying to explain what I really meant I have probably confused
things even more.
Matt Broze
http://www.marinerkayaks.com



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