Subject: Re: [baidarka] Shear, gunwale flare, stability
From: wolfgang brinck (nativewater@yahoo.com)
Date: Tue Dec 17 2002 - 21:18:12 EST
--- Matt Bowman <mbowman@dietechs.com> wrote:
> Hello again from the newbie.
>
> I saw an old thread from last year about Kirsten
> Corkey's baidarka (the "AC"
> from Zimmerly's site) and its complete lack of
> stability. I gleaned that the
> problem eventually turned out to be a hull which was
> too deep. But it got me
> thinking (uh-oh) about the value of having some
> flare above the waterline.
> Looking at the hull sections of MAE-593-76, the
> sides look fairly vertical,
> particularly at the widest parts of the hull.
>
At 17 inches width at the gunwales, the sides have to
be fairly straight to give your butt enough width to
reach the floor of the boat. You need some minimum
width there based on your anatomy. If you flare out
from there rather than going straight up, you will end
up with a wider boat.
> Isn't it true that the absence of flare goes along
> with a flat shear line?
> My understanding (from Morris' book) is that the
> greater the flare of the
> gunwales, the more the shear line will curve upward.
> MAE-593-76 has a shear
> which looks dead flat to me, whereas the Lowie boat
> has some definite upward
> shear, especially at the bow. This seems to
> correspond to the hull sections
> looking more trapezoidal - am I right in thinking
> this translates to more
> secondary stability?
Yes, the Lowie museum boat in the Zimmerly and Heath
drawings has about 1.5 inches of shear. That's not
really much. The truth is that you can build the boat
entirely flat if you like. The small depth of the
guwales would allow you to counteract sheer by putting
more tension in the keelson.
As for secondary stability, I've heard that term used
but never have seen a definition of it. I have seen
the term initial and reserve stability. The first one
means how stable a boat feels, that is, how resistant
it is to tipping. The second means how far you can tip
a boat over before it capsizes. Narrow kayaks don't
have much of either kind of stability at least not
intrinsically. They are a lot like bicycles in that
they depend on a skilled rider to keep them upright.
As John Goodman, the designer of the Nordkapp once
pointed out at a seminar, the only stable statically
stable position of a kayak with a paddler in it is
when it is upside down. An upright kayak has mostly
dynamic stability provided by the paddler.
>
> I'm apprehensive about modifying a time-proven
> design. The idea of a boat
> with a learning curve appeals to me. If I add some
> flare (and hence shear?)
> to this design, am I asking for a frankenstein boat?
> Or is some amount of
> tweaking considered normal?
The thing about this boat is that it is rather long
and narrow with little prestressing of the gunwales
which makes it more flexible overall than its longer
wider cousins. This makes it a fairly reasonable boat
even in waves despite its length. I would question the
wisdom of bringing either the nose or the tail of this
boat up very much by increasing the shear since this
could negatively affect the boat's resistance to wind
Wolfgang
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