Re: [Paddlewise] stability of folding vs. hardshells

rdiaz_at_ix.netcom.com wrote:
> 
> Leander wrote: [?]
> >
> > At 11:44 AM 09-04-99 -0700, Philip Torrens wrote:
> > >
> > >Okay, I can't express this in formal technical terms, but I think Ralph may be
> > right (upright?) in feeling that a sponsoned folding boat could be more stable
> > and self-righting than a hardshell of equal chine and beam. (I mostly use hard
> > shells so I'm not biased in favour of folding boats.)
> > >The displacement of a hardshell is essentially static, changing only as the
> > entire boat moves. The sponsons of a folder, in contrast, are squeezed at the
> > bottom as they are pushed deeper into the water, and therefore expand into
> > greater width and stability in the higher parts.  ...snipped...[snip]
> 
> Philip's and Leander's comments remind me of something interesting about
> the way a sponsoned folding kayak behaves when it has taken on a lot of
> water.  If you turn that folding kayak on its side, it will rise on the
> sponson on that side and spill a lot of the water out, up to about the
> inside level of the sponson.  (It is a neat way to begin partial
> emptying of a folding kayak that most people don't know about.  The
> phenomenon is even more pronounced if you also have flotation bags fore
> and aft as you alway should in any folding kayak or non-bulheaded
> kayak.)
> 
> If there were no different in the displacement effect between a
> hardshell and a folding kayak with sponsons, then this float-up
> phenomenon would also happen with a hardshell laid on its side.  To my
> knowledge, the hardshell would not at all rise that way to spill out the
> water, only the sponsoned kayak would.  That column of compressed air in
> the sponson is fighting its way to the surface.

Ralph, this is completely accurate to this point.  The [emphasized] part of
your next sentence (see below) describes an effect which is not physically
possible until the inflated tube is **completely** submerged, thereby
displacing a volume of water equal to the sponson's volume, giving a
buoyant effect equal to the weight of the water displaced.  Any restoring
force, as Philip points out, which acts to right the kayak,  *before*  the
sponson is *completely* surrounded with water (both inside and outside the
yak), is due to the *form* of the outside of the yak, and can not be
affected by what is inside the yak.  OTOH, I think Philip may have
correctly identified the source of the "feeling" you and he describe -- it
is due to local deformation of the *outside* of the yak's surface, owing to
the flexible character of the hull.

>  In a corollary way, **it also resists being submerged.** [emphasis added]
>  Philip's idea of a dynamic as opposed to
> static displacement certainly has a ring to it that shows itself in real life.

Yes.  The *dynamic effect* could be genuine.  The "resists being submerged"
can not.  I love my folding boat, but it can not violate principles of
physics or buoyancy.

-- 
Dave Kruger
Astoria, OR
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