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

From: Rob Pettibone (rpettibone1@home.com)
Date: Thu Feb 17 2000 - 03:21:58 PST


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        West Wight Potter Mailing List maintainer
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For those who want to delve further into the problem I'd recommend "Improved
Keelboat Performance", Green, Fox. 1974. An easy enough read along with the
basic math involved. Geared toward cruising and not racing. Unfortunately
it's out of print but your library might have it.

Rob Pettibone
P-19 #1094 Gail Marie

----- Original Message -----
From: "Judith Franklin Blumhorst" <drjudyb@pacbell.net>
To: "Potter Email List" <wwpotter@tscnet.com>
Sent: Wednesday, February 16, 2000 10:38 PM
Subject: How a keel boat recovers from a turtling.... kids,don't try this at
home...

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> West Wight Potter Mailing List maintainer
> dfarrell@ridgecrest.ca.us
> List hosted by www.tscnet.com
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> Lars,
>
> Thanks. I stand corrected on the story of the Edmund Fitzgerald. Thank
you
> for giving us the correct details. Others folks wrote me too and made me
> wonder about encroaching senility.
>
> You friends with the 38-footer are describing something other than
> recovering from a turtle, I think. I think your friends are talking about
> Case 1 and Case 2 that I describe below. I was talking about something
> different. So I'll clarify my post.
>
> Wind usually doesn't knock a ballasted boat down past 90 degrees or so,
> because the sail dumps the wind when the profile is reduced. However,
> confused seas and breaking waves CAN and frequently do knock boats down
past
> 90* and even past 120-140 degrees during fierce gales.
>
> Case 1.) If a boat is rolled over, and if the sails are not rigged (ie
> during a storm), the momentum of a heavy keel will keep her rolling past
> 180, completing the roll to 360, and voila! the boat is back on her feet.
> If the sails are rigged, the drag from the sails probably will prevent the
> boat from completing the second half of the role and she'll stay inverted
at
> 180*
>
> Case 2) I you knock a properly designed, ballasted off-shore monohull
> cruiser as far as 120* from vertical, the weight of the keel/ballast (plus
> forces from hullform stability) will be sufficient to reverse the knock
down
> and set her back on her feet. 120* recover from a knockdown is in the
loow
> end of the generally accepted range for a safe open-water cruiser.
>
> In both cases, your friends are correct. But the question focused more
> specifically on the differences between righting a turtled dinghy vs a
> turtled keelboat that's sitting complacently, upside down in the water.
>
> 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. Once the CG is far enough outboard, the wieght of
> the keel exerts a moment arm sufficient to right the boat.
>
> At least that's how I understood the stuff I've read. I'm no expert, just
a
> motivated student, so I'll accept it graciously if someone corrects me.
>
> If anybody needs more explanation than that, they'll have to go research
it
> themselves, because that just about exhausts my knowlege of the topic.
>
> Fair winds, Judy B
>



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