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@shaw.ca> To: <passagemaking-under-power@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