Re: [Paddlewise] chine and stability

<lots of snipping and sequence reordering follow>
John wrote:
>>>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.<<<<


This seems to get to our major point of contention. John, have you read John
Dawson's piece on what the stability curves shown in the Spring 1987 edition
of Sea Kayaker? I'd love to quote it here but it is copyrighted material so
I will point to the location where it can be found on the web
www.marinerkayaks.com (in the reviews section at the end of the Mariner XL
review). John Dawson reported to me personally that they had to measure the
backside of the curves from the capsize point back up to the point of
maximum stability because (going the other direction) once the kayak was
angled past that point the stiffness becomes negative and any motion towards
leaning the kayak further (anywhere on the backside of the curve) resulted
in an immediate full capsize of the kayak as the righting moment was further
reduced the more the kayak was tilted and they were unable to release the
pressure fast enough to prevent the capsize.

So, what was to stop it from continuing to tilt further against a
continually reducing righting moment? Sure, if you can arrest the fall with
a paddle brace you may be able to hold a tenuous balance on the "backside"
of the curve but you had better be ready to brace immediately because the
kayak's stability won't help much here (like the still positive "static"
righting moment makes it look like it should).
Perhaps another analogy will be revealing. Well on second though perhaps it
isn't totally apt and might just be confusing. Heck I worked on it long
enough I hate to just throw it away so I'll make the those poor readers who
haven't skipped right by this discussion (that maybe should be taken
completely back channel) already and are still curious enough suffer through
it too see if they think the analogy is somewhat comparable.

Imagine we make two ramps in the shape of a stability curve to roll a ball
bearing up and over. The front side of the curves are identical but one then
drops straight down to zero at its peak and the other falls away like most
static stability curves do. Question: Which ball will reach the (ground)
zero point on the back side the quickest (if you were to roll it over both
ramps equally hard and with plenty of momentum to easily make it over the
hump)?

I don't believe the path (through space) the two balls would take would be
any different with these two ramps. Both balls would be in free fall the
whole way down, one would just have gotten further away from its ramp
because its ramp dropped away more suddenly. You might not notice the other
was not quite touching its ramp on the backside. Sort of like an orbiting
satellite continuing to fall past the earth due to its speed. Of course if
you could get the force just right so the ball barely clears the peak the
backside ramp would slow the fall a bit.
Leaning your body will also shift the maximum point of the curve (but the
equal and opposite reaction, if that were done too late, (that would further
tilt the kayak) also might push the system into the ......"wetside" of the
stability curve.

John wrote:
>>>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?<<<<

That's my point, how many of us can sense the stability of the backside once
our momentum has put us there and into the virtual freefall we call a
capsize.

John wrote:
>>>>>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.<<<<

While I can't disagree with anything you say here it is essentially
irrelevant because that little righting component is so small compared with
the righting moment that you can generate with your paddle blade that it
would be inconsequential. In other words, it won't make the difference
(whether you capsize or not) once in a thousand times.


John Winters wrote earlier:
<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.<<<

As best as I can figure this out then "Overall stability" includes initial
stability ("the slopes of the curve") and secondary stability (and
apparently everything else about a hull's stability). Since it includes
initial stability I don't see how it can also be equated with secondary
stability unless secondary stability also includes initial stability. I have
been unable to find a text at hand that includes this term so I am relying
on the definition of overall stability John gave. This seems to be a very
vague definition that essentially includes everything (and I was hoping to
define out the various parts of "Stability" and understand how they differ
from the other parts). Can somebody find a textbook quote for "overall
stability". I assumed it was a term used to refer to the total energy
required to capsize a static hull which I believe is a number that can be
determined by the area under the curve. Would that area under the curve be
called "total stability" or is there some other term that refers to the
total energy expended to cause a capsize?
Searching with Google I found what appears to be a US military website (thus
not copyrighted) at
http://www.fas.org/man/dod-101/navy/docs/swos/dca/index.html that says this:

"FUNDAMENTALS OF STABILITY

Stability is the tendency of a vessel to rotate one way or the other when
forcibly inclined. Stability can be broken down into several categories,
each of which are alternatively emphasized in designing and operating Navy
and Coast Guard ships.

STABILITY

INITIAL STABILITY - The stability of a ship in the range from 0° to 7°/10°
of inclination.

OVERALL STABILITY - A general measure of a ship's ability to resist
capsizing in a given condition of loading.

DYNAMIC STABILITY - The work done in heeling a ship to a given angle of
heel."

It does say "overall stability" is a measure so it must somehow be
quantifiable.
Area under the curve? John seems to say it is much more than that but also
includes that. Anybody know how it is measured?

Well, that's certainly not how I've been using the term "dynamic stability".
I've been using it more as a component of "Seakindliness" in rapidly
changing (rough) wave slopes, wave angles, wave peaks and troughs. Maybe
I'll have to make up my own word;-) Any suggestions. Maybe "Lateral
Seakindliness" (as opposed to "Pitch Seakindliness" and Yaw Seakindliness")
which all blend together into, you guessed it, "OVERALL SEAKINDLINESS".:-)

Further on in the lesson on stability the text describes what info can be
gotten from a Static Stability Graph:

"Much information can be obtained from this curve, including:

Range of Stability: This ship will generate Righting Arms when inclined from
0o to approximately 74o. (This curve usually assumes that the entire
superstructure is watertight.)

Maximum Righting Arm: The largest separation between the forces of buoyancy
and gravity. This is where the ship exerts the most energy to right itself.

Angle of Maximum Righting Arm: The angle of inclination where the maximum
Righting Arm occurs.

Danger Angle: One half the angle of the maximum Righting Arm.

This "Danger Angle" sure sounds interesting I wonder why it is defined as
1/2 the angle of the maximum righting arm. Anybody out there understand
"Danger Angle"? I couldn't find much on Google.

Matt Broze
http://www.marinerkayaks.com



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