Subject: Re: baidarka stability
From: James Mitchell (mariner@seanet.com)
Date: Mon Dec 11 2000 - 22:51:27 EST
OK, since I started this thread, it's time to 'splain whaddaye meant.
Primary and secondary stability to me have to do with the gut feel of the
hull's stability. A hull with low primary stability feels tippppppy, dicey,
tender, that woooo woooo feeling that you are soon going to be swimming.
Secondary stability is what really decides whether you are right about the
wooo wooo.
Robert, I have given up looking at the stability curves. I used to follow
them as religiously as Doonesbury and Dilbert, but now I just use them to
get a laugh out of a hull I just tested. Let's try some new categories.
"Whale". "Barge" "Tanker". "Washington State Ferry". Those are the hulls
that hang all their beam at the waterline, spend it all down low. That's
what Kirk is warning about, they feel great (stability wise) on flat water,
but can get "interesting" in rolling waves -- broaching, bucking, flipping.
"Tender", "twitchy", and what I prefer, "nimble" -- those are the hulls that
either feel light are responsive, or just plain scary when you first get in.
When you get over on that gunwale, the issue for me is more one of
predictability than of absolute stability. I hate it when a hull heels over
about 20 degrees, and then goes bloop -- that's what I see in the tumblehome
hulls, hence my comments. A deep vee hull will be very tender at the top,
but as you dip that gunwale in she will tighten up nicely.
Great example, although I have not had a chance to paddle her (obviously) is
the Cape Espenberg in the Sitka Museum. Here you have a beamy hull (Qajaq
1, Fig. 76) at about 25", but with a very steep vee (and reverse sheer...);
I know this from copious photos, the kind you take of that lovely creature
with whom you first fall in love. Also in Adny and Chappelle, Bark Canoes
and Skin Boats of North America, see page 198 for the King Island hull. My
guess is that these hulls will be just plain scary at first, fast for short
boats, but with gobs of secondary and carrying capacity. Same reference,
compare Fig. 181 with Fig. 178. Or look at Arima, Fig. 20 vs. Fig. 19.
Harvey, have you tried any of these hulls?
Anyway, Robert, I quit looking at the curves in Sea Kayaker. I'd rather just
look at the curves of the hull itself. I just guess what the hull will feel
like, and then verify that on the water. I know it fails the objectivity
test, but expressions like primary and secondary stability serve me somewhat
better than tippy vs. barge.
I also am living proof that you can learn to paddle anything if properly
motivated -- as in having dumped six months of your life into building a
boat that scares the beejeesus out of you. How do you think I learned what
little I do know? There's hope...
----- Original Message -----
From: "Robert Livingston & Pam Martin" <bearboat@bazillion.com>
To: <baidarka@lists.intelenet.net>
Sent: Monday, December 11, 2000 11:25 AM
Subject: Re: baidarka stability
> On 12/11/00 8:14 AM, "Kirk Olsen" <kolsen@imagelan.com> wrote:
>
> > Chuck <Sparky@TheWizardWorks.com> pondered:
> >
> >> I'm a bit confused as to the differences between initial and secondary
> >> stability. Might someone clear this up for me?
> >
> These terms have never been formally defined. They originated (in my
> opinion) as marketing terms to try and imply that there were boats that
were
> slender and fast that still had good "secondary" stability. Even if you
> tipped fairly easily, you "felt" secure.
>
> Since these terms have never been defined explicitly they are terms that
can
> be applied quite freely. If I say that a boat has good secondary
stability,
> you cannot really formally prove me wrong. If I say that this boat has
> better secondary stability than that boat, again there is no explicit way
to
> "prove" this because of the lack of definition. Some people think that
this
> makes the terms meaningless. Others are happy using them as subjective
> "impressions"
>
> There is such a thing as a stability curve that plots the righting force
> against the degrees of heeling. The righting force is zero when a boat is
> floating flat and starts increasing as the boat heels. Eventually the
> righting force returns to zero. This is the point at which the boat tips
> over.
>
> Some people have offered as a definition of primary stability, the slope
of
> that curve at its beginning. If you accept this, then there would be a
firm
> definition of primary stability. You could exactly determine the primary
> stability of any boat as a number (assuming that you could agree where to
> consider the center of gravity).
>
> For secondary stability, I could imagine a diverse set of definitions
> (maximal height of the stability curve; angle of heel at the maximal
height;
> angle where the righting force returns to zero; slope of the curve where
the
> righting force returns to zero etc) In any case, I have never seen
anything
> approaching a consensus on this. It is used generally as a feeling of
> comfort as one heels the boat that you are not going to abruptly tip over.
>
> For any of the definitions of secondary stability that I mentioned above
it
> is very hard to establish any "separation" of primary and secondary
> stability. That is to say that any conventional kayak hull that has a good
> primary stability will have a good secondary stability. It would take a
> really weird hull design to separate the two. Marketing hype, however,
likes
> to separate the two. There is some sort of magic implied for a given kayak
> that has low primary stability and high secondary stability. It is hard to
> propose any definitions based on stability curves that would separate
these
> two however. A boat with marked flare above the waterline could (by most
of
> the proposed definitions above) have more secondary stability than a boat
> with marked tumblehome. But again, just add an inch of width to the boat
> with tumblehome and see how its "secondary" stability starts to beat the
> flared boat that is one inch narrower.
>
> Primary stability (if you accept the commonly proposed definition) is
based
> almost entirely on waterline width and height of center of gravity and to
> some extent on how much the boat waterline is squared off (ie how much the
> maximal waterline width tends to be maintained for and aft of the midline)
> Other factors (the specifics of the cross-sectional shape: rounded, chine
> square tend to be totally insignificant.) So it is very hard to design a
> boat that is 23" wide at waterline that has more primary stability than
> another boat that is 24" at waterline. VERY HARD.
>
> I think that one reason the attempts to bring serious definitions to these
> terms fail (beside marketing and Emperors new clothes) is that stability
> curves are not the be-all when it comes to perceptions of stability and
any
> definition that relies on them will be faulty. Kayaks are different from
> most boats in that the center of gravity is easily moved around relative
to
> the hull. The paddler contributes heavily to the center of gravity and he
> can lean one way or the other. If the paddler has "time" to lean, he will
> perceive that boat as stable even though it may have unimpressive
stability
> curves. If you take a 15" wide boat and imagine it to have a 3 foot very
> thin daggerboard hanging down, it will feel very stable. The daggerboard
> resists sudden heeling forces although its presence (it very thin) will
not
> show up on the stability curves. The resistance to sudden healing, allows
> the kayaker to unconsciously adjust his lean for any heeling that starts
to
> develop. My impression is that rounded cross-sections do not resist sudden
> heeling the way that hard-chined or squared cross-sections do and
therefore
> the later seem more stable even though they may have an identical
stability
> curve. It also seems likely to me that as a kayak moves through the water
> there are probably hull shapes that create a form of dynamic stability as
> water moves along a chined surface.
>
> Sea Kayaker provides stability curves for the kayaks that they review. The
> individual reviewers almost always refer to a skinny boat as having "good
> secondary stability". If there were a formal definition for secondary
> stability, the reviewers comments would be useless (which they may in fact
> be in this regard. I do not recall a description of a kayak having poor
> secondary stability). The informed reader would simply look at the curves
or
> the magazine could simply state as a number what the secondary stability
> was.
>
> My biggest objection with the stability curves in Sea Kayak is that they
use
> seat height to determine center of gravity. Since the height of the center
> of gravity is so important, you can game the system. Just build a boat
will
> no seat or a very thin seat. Get a great stability curve and rely on the
> purchaser to add padding to lift himself up a little. Most people are more
> comfortable in the "kayaking" position if their butt is a little higher
than
> their heels but they pay a big cost in terms of stability curves.
>
> Years ago, I tried to get people to stop using the term secondary
stability
> unless they were willing to formally define it. I obviously failed.
>
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