Ovni AVS?

[jbeam]

hi guys , dum quetion for you , if a onvi has a AVS of only 110 degrees ,
what happerns if you go passed 110 ?

i know this is not a tank tested avs and is done on a computer and is theory , but in theory what happens at say 140 , 160 ,180 ?



cheers

[Cormorant]

Once it's inverted, it will be much less stable than in the upright position. So even though on a flat calm sea, it might actually stay upside down once you go past 110, in the sort of sea that would roll you, the next big wave should easily roll you back upright. Not a fun ride, but good to know you'll see daylight again.

[goboatingnow]

Once it's inverted, it will be much less stable than in the upright position.

Only asking, how do you know that, have you seen the elusive stability curve, since OVnis have their ballast in the hull rather then the kneel, I wonder how much of the stability curve is "below the line", which is the only way to judge the recovery situation.

Dave

[Cormorant]

Sorry -- I should have prefaced that by saying it was an intuitive guess rather than a scientific analysis. I haven't seen the stability curve. Just going on the assumption that the boat's lower half (when upright) is heavier than its upper half; that even without a deep lead keel, CG is still down low. If it can self-right from 109 degrees, you just have to wonder how much force it would take to shove it from 180 to 109. Again just a guess, it it seems unlikely that it needs as much force as it took to roll it from 0 to 110. Hmm. . . . I don't sound very convincing. . . .

[Cormorant]

I didn't realize there was so much history to this sort of question. Here's one relevant CF thread from a couple years back:
http://www.cruisersforum.com/forums/...ere-10438.html

And Jimmy Cornell has a lot to say about Ovnis and their suitability as blue-water cruisers. Though he is a bit fuzzy on the math / stability curves as well. . . .

[goboatingnow]

Oh I know about the debate and Cornels Views, I just thought that you had some ( new) info

Dave

[Cormorant]

Yes, I had a feeling that you've followed this for a while. . . . The link and the Cornell reference were really more for the OP's benefit, in case he hasn't Googled it to death already. . . . Interesting stuff. I think I'll go Google it a bit more.

[GordMay]

Boat Stability is the resistance to capsizing; and the Angle of Vanishing Stability (AVS) is the degree the boat can heel, and still right itself.
Boats which have a stability angle of less than 140 degrees, may be left floating upside down once capsized. Boats with a higher angle will usually right themselves.

[stevewrye]

I don't think you can use conventional stability math for these boats. They function completely differently in all types of seas. Can they roll? You bet they can but it is not easy, most likely a steep wave, pitch pole situation. If seas are that big she will right again. Remember these boats slide sideways if caught beam too in a breaking wave, even better than a fin keel will.
As a new buyer in a Boreal, I have many nights staying awake picturing myself at the helm in 5 to 7 meter storm driven waves and wondering how she will handle. My images feel and look pretty good, better than my images of our Mason 44 in those seas. By the way the Mason sailed well in storm seas but you always had the feeling there could be instant disaster in steep big seas.
I'm looking forward to the day crossing the Bay of Biscay in the new boat and seeing what she does but until then it will be a test drive once in awhile when we go visit the factory every 5 or 6 months and the rest of the time my imagination.

[Cormorant]

The one Ovni capsize I saw mentioned (off the coast of Norway) came back up in 3-5 seconds. I'm sure it was the longest 3-5 seconds ever recorded, but still nice to know it righted itself.

[SaltyMonkey]

You also have to take into consideration the coach house shape in righting, as well as the beam. A flat deck is going to FX righting.

[goboatingnow]

Originally Posted by stevewrye

I don't think you can use conventional stability math for these boats. They function completely differently in all types of seas. Can they roll? You bet they can but it is not easy, most likely a steep wave, pitch pole situation. If seas are that big she will right again. Remember these boats slide sideways if caught beam too in a breaking wave, even better than a fin keel will.
As a new buyer in a Boreal, I have many nights staying awake picturing myself at the helm in 5 to 7 meter storm driven waves and wondering how she will handle. My images feel and look pretty good, better than my images of our Mason 44 in those seas. By the way the Mason sailed well in storm seas but you always had the feeling there could be instant disaster in steep big seas.
I'm looking forward to the day crossing the Bay of Biscay in the new boat and seeing what she does but until then it will be a test drive once in awhile when we go visit the factory every 5 or 6 months and the rest of the time my imagination.

I see nothing in an ovni that suggests she's exempt from the normal stability curves, nor any particular reason why she may side slip any more or less then any similar cross sectional fin Boat.

It has always been strange that Ovni have declined to release stability figures. The calculated ones by others suggest an AVS of 100 degrees. By contrast , Southerly who put all the ballast in the drop keel, are very forthcoming with their numbers.

Most boats even with a 100 degree AVs will quite happily role back , because the conditions in which the role occurred tend to also cause the easier role back.

I'm sure that you'll be delighted crossing biscay. Just avoid it from end of sept on.

bTW surely the European STIX figure had to be available for the Ovni. It would provide a reasonable comparison

Dave

[noelex 77]

This is only stability curve of an Ovni that I have seen (from Yachting monthly)

It looks a bit scary. The stability inverted is the same as the right way up. if this graph is correct if it inverts I think you would be waiting for a similar magnitude event to the one that capsized it in first place before it rights itself. I would think it would sink first.

In practice Ovnis are very capable offshore boats with many having made long voyages.

[Doodles]

Originally Posted by noelex 77

This is only stability curve of an Ovni that I have seen (from Yachting monthly)

It looks a bit scary. The stability inverted is the same as the right way up. if this graph is correct if it inverts I think you would be waiting for a similar magnitude event to the one that capsized it in first place before it rights itself. I would think it would sink first.

In practice Ovnis are very capable offshore boats with many having made long voyages.

Anyone know or care to venture a guess as to how this would compare to a Southerly or Feeling (swing keels) where the ballast is in the keel versus the hull of the Ovni?

[goboatingnow]

a quote from YBW in 2004

"the choice is between a french ovni and a british southerly, and what is worrying me is the stability curves. the ovni has an avs of 107 degrees and a max righting moment of 3.5 tonne meters at 50deg. stix 30.8 By contrast, the southerly claims an avs of 160deg, a max righting moment of 6.5 tonne meters at 67deg and a stix of 57. "

this is due to teh 7 foot drop keel and the ballast in it.



( the whole web page is on stability of southerys)http://www.troldand.dk/en/?The_Yacht:Stability
Dave