Re: [Paddlewise] chine and stability

Jed <LedJube_at_aol.com> wrote
    >>>>Normally when a non-engineer responds to posts like yours they
usually
end-up having the hats handed to them. But since no engineering types have
responded, I'll offer a nontechnical response to tie you over.
    The cross-section of the hull determines how the boat responds to a lean
by displacing water this effect is further modified by volume displacement
along the length of the hull.

So far so good

Flatter bottomed or harder chined boats seem to
have a very clear righting effect as they are leaned slightly.  The force to
lean them further rises sharply as the angle of lean increases.  All this
boils down to the perception of stability for most novices. Also most people
understand this as primary or initial stability.

Here you are confusing width below the waterline with chine. Chine is the
turn of the bilge and a harder chine has the transition from bottom to side
around a smaller radius. soft is more rounded. Both underwater shapes below
are the same width.
\                        /      |                          |
  \                    /        |                          |
    \________/          |_____________|
Both are hard chined when compared to a hull more like the second one but
with rounded corners. The rounded cornered hull with vertical sides will be
between the first and second hulls in initial stability. This contradicts
Jed's assertion that hard chines are more initially stable. Secondary
stability consists of many things and I also don't have any exact definition
of it, and don't believe any exact meaning of the term has been agreed upon
as yet. Some FAQ's on our website http://www.marinerkayaks.com discuss one
way for a kayaker to look at it. If the sides of the two hulls above (and #3
with the rounded corners) with the same waterline width continue up above
the waterline at the same angle as below the waterlines the first hull with
the flared sides will have the greatest secondary stability by just about
anybody's perception or "feel". This contradicts what Jed just said about
the more rounded hull having the better secondary stability. As far as
dynamic stability (or ease of staying upright in waves) goes I'll put my
money on the first shape rather than the other two. This is in fact the
shape of a river dory. I want to be clear here that I'm not trying to claim
the opposite of what Jed wrote just pointing out that the chine is not the
determining factor in primary or secondary stability (but it can sure help
dynamic stability compared to a rounded hull by decreasing the sideways skid
on the face of a steep wave--as can a V-bottomed shape).

    >>>>>Softer chined hulls tend to lack this strong initial stability but
instead offer a smoother transition as you move from an even keel to tilting
the boat.<<<<<

How would one show or detect a smoother transition? What should I look for
on a static stability graph? All the graphs I've looked at seem to be
relatively smooth.


>>>>Past 10°'s or so of lean takes us into the realm of secondary
stability.<<<<

Has this been agreed upon somewhere and I've missed it all these years?
Please let me know your source or sources for this.

>>>This is where the major differences lie. A softer chined boat may seem to
have less initial stability but the secondary stability will rise smoothly
up
the point of capsize. Secondary stability is usually agreed to mean
stability
between 10° of list and the point of capsize.  A hard chined boat will seem
to have greater initial stability but reaches it's point of capsize more
abruptly and with little notice.<<<<

Why do you think that? What notice does the rounder hull give you before the
capsize point? They all seem quite capable of catching me by surprise.
although the expectation of stability of a wide flat initially stable kayak
is most likely to fake me into trusting it and not paying as much attention
as I should.

    <SNIP>
>>>>Try every boat that strikes your fancy. Let your senses and the artiste
in you make the decision. Paddling is an experience for the soul, not an
exercise in logical thinking. For ever point I've made someone will take
issue with the science, but no-one will tell you that one and only one
design
is best for all people! <<<<

I'll certainly agree with this (and have just stepped forward to take
issue), although I was hoping someone else would save me the work.
Matt Broze
http://www.marinerkayaks.com


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