[PUP] PPM Stability photo

[brian eiland]

I had posted this photo of a Nordhavn vessel that appears to be excessively
rolling after a power failure of her stabilizers.

<http://www.boatdesign.net/forums/boat-design/roll-stabilization-633-2.html#p
ost40251>

...and this doesn't appear to be a 'heavy sea'
___________________________________________________

------ Original Message ------
Received: Wed, 05 Nov 2008 08:34:22 PM EST
From: "Tad Roberts" <tadroberts at shaw.ca>
To: <passagemaking-under-power at lists.samurai.com>
Subject: [PUP] PPM Stability

>   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