Re: [Paddlewise] A Secondary Definition!

Nick wrote;

(SNIP)


>
> There seems to be a growing consensus that because we are having
> difficulty defining "secondary" stability that it must be meaningless
> and if it means anything, it is the same as "overall" stability.

I hope not. :-0

I may have failed to make myself clear but I thought I said that the term
"secondary stability" seemed to be a  canoe and kayak word that
corresponded to what naval architects call "overall stability". I hope no
one got the idea that I thought it meaningless.

>They can feel a secondary
> stability and know a boat that has it. Take an extreme example of a
> boat with a distinct "V" bottom and low flat deck (almost diamond
> shaped in section). This boat could have low "initial" but high
> "secondary". With the low deck, it may also be quite easy to turn
> completely over in which case the "overall" stability may not be
> great. In this case there is something between "initial" and
> "overall" which is significant. This is what people call "secondary".

Overall stability as I understand it does not refer to any specific point
but to stability as a whole. This is why one has to look at the slopes, area
under the curve and range of stability to get a feel for how the curves
relate to what we feel.

Imagine a boat that had a concave stability curve that steeply rose to a
maximum and then fell as steeply to the limit of its range. Such a boat
would probably have a strange and unstable feel at all but a couple of
degrees of heel and yet it would have a high maximum and a high range. Now
imagine a boat with a lot of area under the curve but one that goes up
nearly vertically at say 5 degrees and drops quickly at 10 degrees. Lots of
area maybe but low overall stability. Now imagine a boat with a range of
stability 40 degrees with a maximum at 40 degrees but only 1 pound of
righting moment. Once again it has two of the conditions for "good" overall
stability but not the third.

Defining stability by a "point" or single factor only seems to work with
initial stability .

> If the stability curve crosses zero at 60 degrees, a paddler sitting
> bolt upright and doing nothing to recover at 59 degrees of lean will
> return to the upright position. The recovery might be slow, but if no
> other forces are applied, recovery is inevitable. This does not seem
> to me to be related to "secondary", but must somehow belong in the
> "overall" camp. Previously John has suggested the area under the
> curve as being indicative of overall stability.


I suggested it as part of overall stability (see above). I hope no one got
the idea that I said it formed the only part.

> the amount of work or energy required to capsize the boat until the
> point of no return, with an inert passenger. An inactive paddler is
> not realistic, but people are hard to predict, so we are stuck with
> it. By using area a boat with a high narrow stability curve and one
> with a wide flat curve are both shown to be hard to tip upside down.
> This seems to be what we want to know about "overall" stability.

(SNIP)

> The possible characteristics to look at for this form of "secondary"
> stability are: the slope of the curve at the inflection, the height
> of the curve at the inflection, the angle of lean at the inflection
> or the area of under the curve until the inflection point.

YEA TEAM! I would like to say that my arguments persuaded Nick but I know
better. He certainly phrased it better than I.


>
> After a little thought, I am going to say a boat with "good"
> secondary stability will be one where the angle of the inflection
> point is the highest. This will be the boat where you can lean the
> boat the most before the ability to recover starts to fall apart.
> Beyond the inflection point, less and less effort will be required to
> create the same effect on the boat until you reach the top of the
> stability curve where the slightest increase in effort will cause a
> capsize.
>

I would like to add to this. Once past the maximum the shallower the slope
the lower the effort required to restore positive righting. For example,
suppose a boat had 20 lbs maximum righting moment at 20 degrees of heel as
shown by the static curve. At 21 pounds of heeling moment the boat begins to
capsize. But if the paddler applies some righting force either with a brace
or by shifting weight she can restore positive righting moment.  If the
curve just dropped vertically (not likely but what the heck)  she would have
to apply 21 pounds (or more) of righting force instantaneously to effect
righting. If the curve continued on at such a slope that at 25 degrees of
heel the static curve showed 10 pounds of righting moment then she would
only have to supply 11 pounds of righting force at 25 degrees of heel to
prevent a capsize. The more the boat does the less the paddler has to do.

Now, the paddler may not take time to "measure" the forces and probably
overreacts a bit but in any case if she has derived more help in keeping the
boat upright she will perceive the boat as having more stability at that
point. (Choose the name you like :-0 )

Passing the peak of the static curve does not mean inevitable capsize and
the boat's contribution after that point makes it easier to avoid capsize.

In any case, defining what a person feels seems a tough one since few of us
feel the same thing in the same way. The stability curve  and its magnitudes
provide a visual way to relate our perceptions to quantifiable forces.

Cheers,

JW






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