Re: [Paddlewise] chine and stability

John Winters wrote:
<SNIP>
>>>>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.<<<

Just how much weight do they assign to each of these factors (and are these
factors all that need to be included in "overall stability") or is this
overall also somewhat subjective?
Where is "overall stability" also called "secondary stability" in Naval
Architecture literature? I've not seen this before now (and if I remember
correctly earlier in this thread you wrote you hadn't encountered the term
"secondary stability" until you had became a paddler in Canada--or something
like that).
If they aren't defined as the same thing somewhere (that we can agree is the
last word on the subject) then why not use the term "overall stability" as
defined and forget trying to make "secondary stability" also have the same
(apparently vague) meaning. Isn't one word enough?


I wrote:
>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.

John responded:
>>>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.<<<

That may well be why the term "secondary stability" apparently isn't defined
in Naval Architecture. It seems to be subjective and relates to a feeling of
security one has while leaned (not necessarily an easily measured thing). We
all have our own feel for it and it has meaning for us but for lack of a
clear definition it has no universal or agreed on precise meaning. I'm
trying to apply what I feel with what I see on the stability curves so I
might interpret them better.

(SNIP)

John wrote:
>>>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.
<<<

One need not be trying to capsize to accidentally force oneself beyond the
point where the righting force is suddenly disappearing so fast in front of
the lean angle that there isn't time to recover. Too much initial stability
makes it hard to get the kayak to lean and once leaned much past the point
of maximum stability it becomes a balancing act to ride the fine edge
between the point where you don't have to work too hard to keep the kayak
leaned and where you have gone over to the "Backside" and might be baptized
before you can return.  Using the analogy of the standard chair vs. the
rocking chair that I used on the stability FAQ's on our website. Here I'll
save myself rewriting it and just copy the relevant part for you:

I'll make an analogy with rocking back in a rocking chair (low initial
stability/higher secondary stability) and compare that to rocking back in a
regular four legged chair (high initial stability/high total stability/low
feeling of security as you teeter at the balance point). It is hard to lean
back on a standard chair (most of my teachers frowned on my practice of
balancing my desks on the 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 attempt to recover--crash back down
loudly to the "initially" 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 static stability graphs I would define a kayak with
good secondary stability as one whose stability curves show 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, but not necessarily, at a
higher maximum point) than a kayak with less "secondary" stability. (See the
Mariner XL, Arluk III, and Solander stability comparisons in the XL review
in the Spring 1987 issue of Sea Kayaker magazine). [note: this comparison
can be seen in the "reviews" section of www.marinerkayaks.com]
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. The above definition is from my own
observations and guesses regarding secondary stability in kayaks. I make no
claim to scientific validity or even at a valid definition of the term
(which may exist somewhere in Naval Architecture). I and most experienced
paddlers prefer lower initial and good "secondary" stability. Easy to lean
yet secure while leaned.
End of FAQ from website--------------

John continued:
>>>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.<<<

While what you say is true, I think it gives a wrong impression about what
we need to look at here since the power of a brace can be so much stronger
than the righting force it combines with that it renders the righting force
part of the system almost meaningless in comparison. This also doesn't take
into account our nervous system's feedback delay that slows the paddlers
reaction time.

I think the effort it takes to get a very stable kayak up to the point where
that leaning effort can be somewhat relaxed (now on the "backs(l)ide")
leaves the paddler in a precarious position, hanging out over the water as a
counterbalance to the kayaks stability and dependent on his paddle and the
security of the kneebrace to prevent a capsize that is only a few degrees, a
moment on inattention, and a little momentum away.

Imagine you are arm wrestling with an opponent you only want to tie (like
your strong child maybe) and he suddenly relaxes. The delay in your nervous
system will not allow you to slack off at exactly the same time as your
opponents relaxation so you are going to move his hand further than you had
intended (and also further than if you had been pushing a lot less hard to
begin with--an analogy for initially less stable). This nervous system delay
and the suddenness with which a capsize can occur when the resisting force
drops away in front of the lean means the paddler either can't lean that
wide gear laden kayak much at all or if he does, he risks going too far over
before his reaction time forces him to need a quick brace to prevent a
capsize.


Matt Broze
http://www.marinerkayaks.com



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