[PUP] PPM Stability

[Ross Anderson]

Interesting but let's face it with the weather reporting available
today, even with a 7 to 8 knot speed you can minimize the risk of
capsize. Again in 75,000 miles of passaging I have never encountered
more than 25' seas with a duration of around 10 seconds and that was
east of the Straits if Lemaire. I do not minimize the possibilities of
a rouge wave but can you really design the affordable boat that will
endure all conditions? I hope I have learned the lesson of picking the
right weather window and then enjoying the passage. God Bless - Ross
on 10&2

On Wed, Nov 5, 2008 at 8:34 PM, Tad Roberts <tadroberts at shaw.ca> wrote:
>  To John on Seahorse,
>
>
>
> I'm always up for a question, fire away to tadroberts at shaw dot ca.
>
>
>
> Also I'll mention that any boat, no matter the size, shape, or ballast
> configuration, can be rolled over given the right size and shape of wave.
> Capsize tests in tanks have shown that any yacht can be rolled 180 degrees
> by a wave equal in height to 55 percent of her LOA. So a 30' wave taken beam
> on could roll the Skookum 53.
>
>
>
> In earlier postings on the Perfect Passagemaker project, a suggested
> requirement was positive stability to fairly high angles, one person
> suggested 105 degrees and another suggested nothing less than 140 degrees.
> I tried to explain that this was unlikely without major compromise.
>
>
>
> Being a curious type I decided to investigate for myself the stability of
> two well known modern Passagemakers.  I created 3D computer models of a
> Nordhavn 55 and a Diesel Duck 462.  I was able to do this from drawings
> offered by each builder on their respective websites.  Nordhavn claim a
> half-load displacement for the 55 of 97,360 pounds, my model came in at
> about 99,000, so I left it floating at that.  From pictures the DD462 seems
> to be floating quite a bit deeper than the DWL indicated on the lines, so I
> sunk her down 4", to about 75,000 pounds displacement. I also decided on a
> VCG of 18" above the waterline.  This is about half topside height and would
> be typical for a steel vessel with limited deckhouse.  For the Nordhavn I
> decided on a VCG of 3' above DWL, this is the height of the main deck
> (afterdeck) and she has two full stories of accommodation above this.  Only
> the hulls and main decks were modeled, I did not include deckhouses as they
> are full of downflooding points.
>
>
>
> Maximum righting arm for the DD is about 30 degrees, this is where the deck
> edge goes under water and righting energy decreases. She runs out of
> positive stability at about 87 degrees.  This will change with displacement
> changes (mainly liquids), and will be reduced with free-surface effect of
> the liquids in her tanks.
>
>
>
> The configuration of the N55 is unusual with her very high built-up
> foredeck.  She exhibits an unusual righting arm curve with maximum righting
> arm at 65 degrees.  But it then drops off quickly and goes to zero at about
> 85 degrees. There is a great deal of area under the curve (below zero) thus
> these boats, if they heeled to these angles, will be very reluctant to right
> themselves.
>
>
>
> An interesting result and I would be happy to send model pictures and data
> to anyone by email.
>
>
>
> All the best, Tad
>
> [demime 1.01d removed an attachment of type image/gif which had a name of image001.gif]
> _______________________________________________
> http://lists.samurai.com/mailman/listinfo/passagemaking-under-power
>
> To unsubscribe send email to
> passagemaking-under-power-request at lists.samurai.com with the word
> UNSUBSCRIBE and nothing else in the subject or body of the message.
>
> Passagemaking Under Power and PUP are trademarks of Water World Productions, formerly known as Trawler World Productions.