Re: Capsize ratio

From: Marilyn Dimson-Doyle (mddoyle@mediaone.net)
Date: Tue Feb 15 2000 - 07:38:59 PST


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        West Wight Potter Mailing List maintainer
                dfarrell@ridgecrest.ca.us
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Hi Judy,
    This is Alan. Marilyn is my wife. Sorry for the confusion on the
e-mail address. We have been talking about changing it since I spend most
of the time using it, just haven't got around to it.
    All that you have said follows exactly what I have been reading. What I
was confused on was the vulnerability to capsizing versus after capsizing.
Thanks for that clarification. Could you recommend a good text that does
talk about stability and vulnerability to capsizing? Or just a good text
about boat construction, particularly within our class of boat? The snow
and ice isn't melting very quickly.
     Thanks for putting up with us novices.
Alan
P-19"?"
----- Original Message -----
From: Judith Franklin Blumhorst <drjudyb@pacbell.net>
To: Marilyn Dimson-Doyle <mddoyle@mediaone.net>; potter
<wwpotter@tscnet.com>
Sent: Monday, February 14, 2000 6:24 PM
Subject: Capsize ratio

> Alan (or is it Marilyn?)
>
> Virtually all trailer sailboats have capsize screening ratios over 2.0.
> According to the screening test, they are "vulnerable in the event of a
> capsize" (The number does not imply that they are vulnerable to capsizing,
> but vulnerable after capsizing)
>
> The capsize screening formula has nothing to do with the stability of the
> boat. It was developed 10 years ago, after the Fastnet Race Disaster - to
> screen for boats that wouldn't return upright easily after a 180 degree
> knockdown.
>
> It is a "merit" measure of the boat's ability to come back from a 180
degree
> capsize. It should be ew-named the "capsize recovery metric" : ) It is
a
> useful metric for ocean-crossing boats that are likely to get caught in
> Force 8 or Force 10 Gales with huge waves (high wind in a storm isn't
> dangerous to the good skipper, but large waves can flip any boat). It
> really isn't a useful metric for most trailer sailboats intended for
coastal
> cruising.
>
> Most sailboats depend on a combination of keel ballast and a wide beam for
> stability. Wide boats have good initial stability, but once they're
upside
> down 180 degrees, they are also quite stable in that position due to the
> wide beam. That's a characteristic of almost all sailboats under 26-28
feet
> long and under 4000 pounds (which would be very lightweight-to-average for
a
> 28 foot cruiser).
>
> The capsize screening formula is:
>
> Beam dimension in feet divided by the cube root of Displacement expressed
in
> cubic feet of sea water (64 pounds per cubic foot).
>
> Therefore, a 1200 pound boat displaces 18.75 cubic feet of seawater and
the
> cube root of 18.75 is about 2.65. In order to have a good capsize
screening
> ratio value under 2, the beam would have to be 5.3 feet. That's too
narrow
> to for the boat to be comfy or have a usable cabin.
>
> Another way to look at it: Say you want a boat with a beam of at least
7.5
> feet. In order to have a "capsize recovery metric" of 2 or less, it would
> have to displace 43 cubic feet of seawater and weigh a minimum of 2700
> pounds.
>
> Truth is, there aren't too many trailerable boats that can measure up to
> this metric. The only one I know of is the Flicka, a 20 foot, 6000
pounder
> with a deep, full keel. I'd call that transportable, not trailerable.
You
> gotta keep that heavy beauty in the water most of the time or have a very
> big truck and a crane. In fact, until you get into the 34 foot range (or
at
> least 28 feet), there are almost no production trailler sailboats that can
> pass this "screen".
>
> The Potters aren't true ocean-crossing boats -- and we knew that. They
> don't measure up to the capsize recovery metric. Don't sail a Potter in
> conditions where you will be hundreds of miles off-shore in 50 knots wind
> with 30 foot, breaking seas :)
>
>
> Fair winds,
> Judy B
>
> > -----Original Message-----
> > From: Marilyn Dimson-Doyle [mailto:mddoyle@mediaone.net]
> > Sent: Monday, February 14, 2000 9:32 AM
> > To: potter
> > Subject: Form posted from Microsoft Internet Explorer.
> >
> >
> > - - - - - - - - - - - - - - - - - - - - - - - - - - -
> > West Wight Potter Mailing List maintainer
> > dfarrell@ridgecrest.ca.us
> > List hosted by www.tscnet.com
> > - - - - - - - - - - - - - - - - - - - - - - - - - - -
> > lengthwaterline=16.9
> > lengthoverall=18.9
> > displacement=1225
> > beam=7.6
> > sailarea=135
> > category=racer
> > capsizeratio=2.84
> > speed=5.5
> > SailAreaToDisplacement=18.86
> > DisplacementToLWL=113
> > LengthWaterLineToBeam=2.224
> > MotionComfort=7.21
> > description=Capsize Ratio: A value less than 2 is considered to
> > be relatively good; the boat should be relatively safe in bad
> > conditions. The higher the number above 2 the more vulnerable the
> > boat. This is just a rough figure of merit and controversial as
> > to its use.
> > OwnerName=Alan Dimson-Doyle
> > BoatType=Potter 19
> > BoatName=Still Looking
> > mail=wwpotter@tscnet.com
> >
>



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