Re: How a keel boat recovers from a turtling.... kids, don't try this at home...

From: Kent Crispin (kent@songbird.com)
Date: Thu Feb 17 2000 - 06:38:20 PST


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On Wed, Feb 16, 2000 at 07:38:31PM -0800, Judith Franklin Blumhorst wrote:
[...]

> Case 3) If a boat is floating upside down with the keel to the sky, it will
> be pretty stable in that position, with the keel's CG positioned precisely
> above the center of bouyance of the hull. With a wide beam, it will be quite
> stable in the inverted position, especially if the sail is hanging below the
> boat like a sea anchor. (Isabelle's racing boat had a fine, pointy bow for
> low resistance and high speed, and a wide flat aft section for planing with
> humungous sails)
>
> In the latter situation, you need something like wave action or a stocky
> norseman :^)) to roll the boat over enough so the keel's CG is outboard of
> the center of bouyancy.

When the boat is perfectly level on the water the center of bouyancy is
directly under the keel. As it tilts, the center of bouyancy moves,
and the CG moves.

                 +---+ +---+
                 | x | <- keel | x | <- keel
                 +---+ +---+
                   | |
                   | <- lever arm | <- lever arm
       +-----------------------+ +---------+
       | y | <- hull | y | <- hull
-------| |-------------| |--------------
       +-----------------------+ +---------+
                   |<----------> |<--->
                   | lever arm | lever arm

"x" is the CG; "y" is the CB. I've drawn the keel abstractly, as just
a fixed weight some distance up in the air.

Just looking at the picture, it is obvious that the stability of this
configuration is completely dependent on the details of the boat design.
For some "wide body" boats this configuration is ultimately stable, and
wave action would be needed to knock it over again. For other boats
with a narrow hull, this configuration is fundamentally unstable, and
brownian motion of air molecules is enough to right it.

Kent
P19 "peregrine"

-- 
Kent Crispin                               "Do good, and you'll be
kent@songbird.com                           lonesome." -- Mark Twain



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