Re: [Paddlewise] chine and stability

Matt wrote:

(SNIP)

>
> 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?

Don't know of anything else.


> 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).

I believe Steven Killing refers to it a secondary stability (I will check on
that) but he is a Canadian designer so what doe she know. :-)

Other than that i know of no other place where it is called "secondary
stability". Since what people call "secondary stability" seems to correspond
to what naval architects call "Overall stability" it seems like one and the
same thing.


> 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?

You bet.


(SNIP)

>
> 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.

That indeed seems to form the crux of the problem. I encourage people to
paddle a numbe of different boats and compare the satbility curves. This
helps me get a better idea of what they mean when they say "good" or "bad"
overall stability. (From this day forward I shall not use the "S" word
stability)


(SNIP)

>
> 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 don't have any data to support "meaningless" and would suggest that if
peope can sense the stability of the boat then it probably exceeds
"meaningless". What do they sense if not the boat?


>
> 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.

True and I think this is what Alex tried to say when he mentioned the slope
of the stability curve. Correct me if I got that wrong, Alex.

I don't know what the paddler senses other than the physical forces of
stability acting on the boat. We can quantify those forces  and while they
don't come in a nice neat single number we can look at the package and
relate that to what we sense.

Maybe this will help.

The righting moment includes both he paddler's body weight shift, the effect
of the boat's shape, and the forces generated by the paddle(if any). The
righting force after the hull's maximum is additive. That is, boat righting
arm plus weight shift plus paddle forces.  The greater the righting moment
contributed by the boat after the maximum the less on needs of the other two
to right the boat.

Clearly the less one needs to do to keep the boat upright the more stable it
seems. So, if one righting moment curve is higher after the maximum than
another (shallower slope) then the paddler will perceive it as having more
stability. Using Matt's arm wrestling analogy one  could say that  it is a
bit like the opponent slowly reducing his effort rather than relaxing
completely.

Cheers,

John Winters






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