Re: [Paddlewise] paddlefloat photo

From: "Robert Livingston & Pam Martin" <bearboat2_at_attbi.com>

> > This can provide additional bouyancy to prevent sinking, but it will not contribute
> > to stability.  The water on the outside of the hull doesn't know what's inside the
> > hull.  Adding a sponson to the _outside_ of a hull can make a difference.
> 
> Speaking of what water "knows".
> 
> If you have a sponson on the outside of a hull how does the water "know"
> that it is not the outside of the hull itself?

In this case, the sponsons effectively change the shape of the hull, creating a hull
with greater stability than the original hull.  

> Say you "glue" that sponson to the outside of the hull. How does the water
> know that this is a sponson on the outside of the hull rather than just a
> bulge in the contour of the hull.

Ditto

> What if you are paddling a Klepper like the old one that I used to have that
> used an air-bag to tighten the skin and this air bag ran around the hull
> like a sponson. What does the water think of this? That air bag was "inside"
> the hull.

If inflating the sponson changes the shape of the hull, it will have an effect.  If not,
the water outside won't know the difference.  In a rigid hull, what you change inside
is irrelevant.

> The reason the water "knows" about the built in sponson is that as the
> partially flooded boat starts to tip the water is restrained from going to
> the edge of the outside hull "within" the boat. The otherwise freely mobile
> water is constrained to stay in the center of the boat because the edges are
> full of air.

Peter Rathman's also pointed this out.  It is the exception and is valid.  However, 
this represents a small portion of the total cockpit volume.  Stability comes from
the center of gravity changing at a slower rate with heel angle than the shift in the
center of bouyancy.  If the water is free to move, the center of gravity shifts faster
than would a dead load.  This reduces or eliminates stability.  Internal sponsons
don't kick in until the heel angle is relatively high and most of the water has already
moved.  I've only seen the sponsons on a Khat S - I can't imagine that Kleppers
are so different that the behavior would be favorable.

Also PeterO's comment about his Klepper brings to mind the possibility that in 
foldables and skin on frame designs, the internal ribs act as baffling and can help.
However, these also contribute only a small amount and would only affect the 
kayak if the water is not deeper than the ribs, keelson etc.  

If there is a design that significantly changes the behavior of water internally, I'd like to 
know how it works.

Mike

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