Atlantic 57 Catamaran Capsized

[Polux]

Originally Posted by Just Another Sa

I'm not limiting the discussion on RCD class A, you are.
...

Off course I am.

Class B or C have not the same stability requirements and in that case that can happen, I mean an inversion. Most of them are lightly ballasted (when they are ballasted at all) and the weigh of the crew and his position is important for their stability.

I find completely absurd to compare the stability of a 57ft catamaran with the stability of a Class B or C sailboat, that can indeed have 30ft and 4000kg.

Nobody was considering small boats not fit for offshore work when this discussion started. We were talking about the stability of a 57ft catamaran why the hell would we be comparing it with the stability of boats that can only sail on restricted or protected areas, lake lakes or bays?

[TeddyDiver]

Originally Posted by Just Another Sa

That depends on what causes the wind. Your assertion on wind shear applies when its the pressure gradients between lows and highs.
When the cause is a microburst there is practically any horizontal wind high up, and a lot of it closer to surface. Added fluid density from liquid water reverses the pressure shear, there is more pressure down low.
Hull sides are quite flat vertically these days, not much tumble home, and quite many boats have toerails to make the deck edge even less pointy. No roundness at all along the airflow.
There is also higher wind pressure below the keel than above it in those conditions, making the heeling moment a bit larger than that simple calculation. Some may call that lift, other use local flow direction and call it drag despite of being a vertical force.

Those flat vertical hull sides are horizontal in your scenario so only frictional drag there, sorry. Bilges are roundish in most boats even nowadays so there must be somekind of lift present, wanna ques what direction And how a keel it's bulb pointing to wind makes any difference. Your whole theory is based on surface pressure which is overly simplified and equally far from the reality. Thou you should get a ballasted mono to 90deg heel first with windpressure and induced drag before inverting... Good luck

[44'cruisingcat]

Originally Posted by Seaslug Caravan

My first cat teeterd on the edge in a large puff on Funnel bay , in the Whitsundays. Whilst I've intentionally flown a hull in my current cruiser I've never been close to the edge

What types of boat, and in what conditions?

[Just Another Sa]

Originally Posted by Polux

Off course I am.

Class B or C have not the same stability requirements and in that case that can happen, I mean an inversion. Most of them are lightly ballasted (when they are ballasted at all) and the weigh of the crew and his position is important for their stability.

I find completely absurd to compare the stability of a 57ft catamaran with the stability of a Class B or C sailboat, that can indeed have 30ft and 4000kg.

Nobody was considering small boats not fit for offshore work when this discussion started. We were talking about the stability of a 57ft catamaran why the hell would we be comparing it with the stability of boats that can only sail on restricted or protected areas, lake lakes or bays?

There are a lot of boats around sailing offshore or crossing oceans having no CE-certification at all. Some built outside EU, others built before CE-certification was invented.
I find nothing absurd in comparing how very different sailboats respond to very same weather conditions that capsized Atlantic 57, when said boats are in fact used for the same purpose, crossing ocean while cruising.

I'm not considering any of them non-fit, even when they flip over under extreme conditions that nobody expects to meet while cruising.
Anybody could just as well die for a lighting strike, the odds are quite similar than meeting such weather unexpected and without a warning.

[Factor]

Originally Posted by 44'cruisingcat

What types of boat, and in what conditions?

Well he used to say he owned a seawind 24 so I can understand that. Most people I know would not cruise on a 24 though I know of plenty who have including a number of bass straight crossings etc

[Polux]

Originally Posted by Just Another Sa

There are a lot of boats around sailing offshore or crossing oceans having no CE-certification at all. Some built outside EU, others built before CE-certification was invented.
I find nothing absurd in comparing how very different sailboats respond to very same weather conditions that capsized Atlantic 57, when said boats are in fact used for the same purpose, crossing ocean while cruising.
....

You mean comparing the stability and safety of an atlantic 57 with the one of a monohull with 30ft, a displacement of 3000kg with a stability that would not allow it to be certified as a Class A boat, being it a new or older boat?

Sure you can do that but it seems ridiculous to me. Such a boat would not be considered by any here as an offshore monohull, while all here consider the Atlantic 57 an offshore cat.

[TeddyDiver]

Originally Posted by Polux

You mean comparing the stability and safety of an atlantic 57 with the one of a monohull with 30ft, a displacement of 3000kg with a stability that would not allow it to be certified as a Class A boat, being it a new or older boat?

Sure you can do that but it seems ridiculous to me. Such a boat would not be considered by any here as an offshore monohull, while all here consider the Atlantic 57 an offshore cat.

There's not even a rumor of any such event happened ever regardless of boat size, not for a keel ballasted offshore monohull, old or new. So I have no problem with the boat size thou quite absurd comparison as you say.
I could imagine that with a wedge shaped light racer that could happen in rare circumstances but untill there's evidence

BR Teddy

[Polux]

Originally Posted by TeddyDiver

There's not even a rumor of any such event happened ever regardless of boat size, not for a keel ballasted offshore monohull, old or new. So I have no problem with the boat size thou quite absurd comparison as you say.
I could imagine that with a wedge shaped light racer that could happen in rare circumstances but untill there's evidence

BR Teddy

In fact ocean racers tend to have not only a bigger draft but also a bigger B/D ratio meaning that they have a huge final stability, much better than the typical cruising boat (it makes sense due to the extreme conditions they are used and also the extra power needed to sail faster).

Note that I am not saying that they have more stability, since that has to do with weight too, just final stability (from 50º to the AVS point) and that one is the more important in what regards how a boat recovers from a knock down from a huge gust of wind.

[smj]

I never thought he was comparing the stability of a 30' mono to the 57' Atlantic, just making a point that a mono can suffer from a wind only capsize. Looks to me as if this thread has gone way of track.


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[TeddyDiver]

Originally Posted by smj

I never thought he was comparing the stability of a 30' mono to the 57' Atlantic, just making a point that a mono can suffer from a wind only capsize. Looks to me as if this thread has gone way of track.


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Even in that size he's without any evidence..

[TeddyDiver]

Originally Posted by Polux

In fact ocean racers tend to have not only a bigger draft but also a bigger B/D ratio meaning that they have a huge final stability, much better than the typical cruising boat (it makes sense due to the extreme conditions they are used and also the extra power needed to sail faster).

Note that I am not saying that they have more stability, since that has to do with weight too, just final stability (from 50º to the AVS point) and that one is the more important in what regards how a boat recovers from a knock down from a huge gust of wind.

Of course you are right. I was merely thinking about JAS theory which could work in theory only if there were enough flattish bottom area exposed to wind..

BR Teddy

[Dockhead]

Originally Posted by TeddyDiver

Of course you are right. I was merely thinking about JAS theory which could work in theory only if there were enough flattish bottom area exposed to wind..

BR Teddy

Could work in theory, it seems to me, for a very small boat with a large proportion of stability from form stability. Which corresponds to Teddy's wide flat bottom BTW.

But even that's a stretch, it seems to me, because of the weathercocking thing.

I think if there's a ballasted keel, it just doesn't happen in practical reality without wave action. And certainly not boats which are fairly comparable to the A57 -- larger class "A" boats.

This lines up well with observed facts -- i.e. the lack of any observed cases of monos capsized by wind alone.

[Polux]

Originally Posted by smj

I never thought he was comparing the stability of a 30' mono to the 57' Atlantic, just making a point that a mono can suffer from a wind only capsize. Looks to me as if this thread has gone way of track.
...

Yes, I agree, comparing stability of 30ft Class B sailboats with the one of the Atlantic 57 or the response of both boats to gusts is way out of track.

Putting it on track I will take some interesting quotes from a paper from Spanish researchers about yacht stability (University of Madrid). Not only the quotes are interesting but much of it even if it points more to comercial boats than to private yachts that are mandatorially RCD certified in Europe.

"The second kind of requirements is traditionally known as the “dynamic stability”and refers to the capacity of the vessel to withstand a gust. The work done by a gust is represented by the area under the wind heeling arm curve, as the energy absorbed by the yacht which is represented by the area under the righting arm curve. The angle at which these areas are equal defines the
angle to which a vessel will be dynamicallyheeled by the gust. ....

Range of stability

This is a very important stability parameter which has always been included
in the criteria even in the new developments. With the new stability approach the effective range of stability can be found by drawing the heeling arm curve that is tangential to the righting arm curve. This situation will only be found however if the range of stability is less than 90 degrees.

That is, a yacht whose range is less than 90 degrees is particularly vulnerable whilst a yacht with a range of 90 degrees or more cannot be capsized by the wind unless if has a downward component.

The standards that come from this new approach consequently require a minimum range of 90 degrees. .."

file:///C:/Users/User/Desktop/ff/Stability_Criteria_for_Sailing_Vessels%20(1).pdf

[peter57]

If Just Anther Sa is forecasting 200 knots, I aint going out that day to test theories, I'm staying home to read other theories on CF.

I like Multi's that are fast and to use words cautiously here "slightly dangerous", so it is and interesting discussion as I have been close to undoing the velcro that held the screw driver to undo the escape hatch. (my fault as well) dark night and having a newbie on the helm as I was making a cup of tea so they would not set fire the boat (metho stove) or get scolded. Morton bay waves as well as a big wind gust that kept coming and was not forecasted. We were in 12- 15 Knots and they went to 30 plus in an instant.

33ft racing tri 29ft beam, I feel embarrassed after the wind speeds being stated in this discussion.

I might mention it was the boats design that sort of saved us. Flexing forward beams, you could stand on the ama bows and with only your body weight bounce them at least 18 inches. Strange an scary to look at from the windward ama doing around 20 knots flying the spinnaker.

[malbert73]

Originally Posted by Just Another Sa

That depends on what causes the wind. Your assertion on wind shear applies when its the pressure gradients between lows and highs.

When the cause is a microburst there is practically any horizontal wind high up, and a lot of it closer to surface. Added fluid density from liquid water reverses the pressure shear, there is more pressure down low.

Hull sides are quite flat vertically these days, not much tumble home, and quite many boats have toerails to make the deck edge even less pointy. No roundness at all along the airflow.

There is also higher wind pressure below the keel than above it in those conditions, making the heeling moment a bit larger than that simple calculation. Some may call that lift, other use local flow direction and call it drag despite of being a vertical force.

Hull sides may be flat but actually will be sloped toward bow and stern. Once a boat heels it's not flat at all and should present much less resistance to wind with curved surface. That needs to be accounted for.
And again calculations are great but there are 2 concrete examples on this thread of sailboats being knocked flat by microbursts (clipper race and my friend in a swan) without inverting. Me thinks the delta is in false assumption of hull resistance to wind when heeled. I think it would take otherworldly breeze to invert a mono if you correctly calculate resistance of below water line part of a typical hull


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