Re:[Paddlewise] Stability

> I wondered about that too so I picked out four boats (not completely at
> random) and found that they had waterplane coefficients that varied from
> 0.674 to 0.609. Not sure what "fairly similar" means so maybe this much
> variation doesn't matter. "Fairly" similar sounds a bit "mushy" to me
> though. Of course beam  does have more impact but does that mean you ignore
> the waterplane?

I would still consider Nick to be "correct". If you define primary stability
to be the slope of the stability curve at 0 degrees (i.e. the slope between
0 degrees and (0 + Small Delta) degrees) it is a function of the shape of
the water plane and more importantly the waterline width and the height of
the center of gravity.

You can hold the center of gravity constant (distance to the surface of the
water or, if you prefer, the bottom of the boat. The latter will assist the
narrower and V bottom boats a little). Then it is a function of the width
and the shape of the waterplane.

Now, as a designer, try the following. Increase the waterline width of a
design (any design) by 1 inch. Now try and attain the same increase in
primary stability by changing the waterplane. You cannot do it for all
intents and purposes. The impact of that 1 inch is SO great and the impact
of changing the waterplane shape within the "family" of shapes seen in
commercial designs does not approach it. Certainly if the waterplane
coefficients are between .6 and .7 If you think that one inch is a lot, you
will find that it applies to 1 cm as well for all intents and purposes.

Therefore, I would state that there is no commercial kayak out there with
any arbitrary waterline width (X) that has a "primary stability" greater
than any other commercial kayak with a waterline width of (X+1). And that is
irrespective of shape of the cross-section or the waterplane.

You can expand this statement to any design that is not "truly bizarre" So
play with your curved and squared off and V cross-sections to your hearts
content.

As for "secondary stability"since that has no "scientific" definition, I
cannot make the same statement. But the effect of width on the entire curve
of stability is so paramount that again it is hard (impossible?) to find any
"commercial-like" design where the stability curve for 0 to 45 degrees will
be greater at ANY point than a commercial-like design with a waterline width
that is 1 inch greater. The other important factor that comes into play as
you move out the stability curve is the degree of flare of the "out of
water" portion of the design. This can be roughly estimated by considering
the width of the seam. So I would state that NO design X with a waterline
width 1 inch greater than another design Y when the seam width of X is equal
or greater than Y's will have a stability curve that is less than Y's at any
point between 0 and 45 degrees.

I would invite people with computers to try this out with whatever program
they prefer. Bearboat Pro is available for free on the web and it will
calculate stability curves. Submit your examples...

I can conceive that the slope of the stability curve could impact something
that might be perceived as "secondary" stability. If you are pushing on a
door with 20 lbs of force and someone on the other side is pushing with
roughly equal force and you push the door open another inch and suddenly the
other person is pushing with a force of 10 lbs then you might "fall into"
the room. And the same might occur if you are pushing with 40 lbs of force
and suddenly the push back is 25 lbs. So the steepness of the reverse slope
of the stability curve after you have reached the hump might be a definition
of "secondary" stability. But my saying so makes no difference because their
is no universal agreement.

As I noted in a previous post, I believe that the shape of the cross-section
DOES have an impact on the perception of stability at 0 degrees of tilt
because of dynamic resistance to having the kayak rotate around its long
axis. But this factor does not appear in a stability curve and is neglected
in the literature that I have read. Also, in the real world, as kayaks are
being flung around by waves and currents, I think that the shape of the
cross-section (particularly flare) IS important in affecting one's ability
to remain upright. There are dynamic forces that the viscosity of water
imposes on the kayak.

As the debate is currently framed with an actual definition of primary
stability out there, reviews that talk about "poor" or "weak" primary
stability are a little weird in that they say no more than one can basically
judge by knowing the waterline width. Sea Kayaker reviews contain the
numerical data so why would the reviewer be making these statements about
the primary stability at all?

I have also read about designs with poor initial stability but good
secondary stability to the point that it is almost a clichi. But where are
the reviews about kayaks with good primary stability but poor secondary
stability? What are they comparing themselves to? 
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