Atlantic 57 Catamaran Capsized

[Just Another Sa]

Originally Posted by Dockhead

Once the boat is not square to the wind, then all bets are off -- lateral resistance will fall off drastically, as will wind force, plus you start to get form stability.

Changing from beam to wind into headwinds due to yawing is the reason why it is likely the boat will not do 360, just 180. Of course given enough wind it can do multiple 360 and one 180 at the end.

[Just Another Sa]

Originally Posted by Dockhead

Heeling to 30 degrees under bare poles doesn't tell us anything at all about heeling moment at 90 degrees. The bare poles and rigging have a lot of drag, the force of which is imparted over a very long lever arm.

No but the calculation presented already twice does tell that.

Originally Posted by Dockhead

We can agree, however, that saying that heeling moment of the hull is "negligible" for all boats and under all conditions is an overstatement.

But pointing that out does not prove that it is practically possible in real life for a ballasted keel monohull to be inverted by wind action alone, at least, not a typical one. It is indeed not.

Not a typical situation agreed, but this thread is not about typical winds either, but about the extremes that did in fact capsize Atlantic 57. Would have done the same on the small monohull.

[Just Another Sa]

Originally Posted by Polux

You are only considering the RM at 90º when you should be considering the RM at all points of heel till the AVS, that on a class A bout should be about 125º or superior.

The energy needed to capsize the boat has nothing to do with the RM at 90º but with the RM corresponding to the positive area of the RM stability curve between 90º and the AVS point (about 125º). That is much more than the RM at 90º.

Most 30ft boats built so far and still used are not class A and do not have AVS of 125 degrees or more. I'm sure you know that.
The amount of energy from the wind is practically unlimited. If it produces enough heeling moment to exceed RM the energy is there for rolling energy automatically. It can invert even if there isn't. For that you need to analyse the energy conversion as well, but not then HM<RM for all the way up to 90 degrees, because in that case it will remain so upto AVS as well.

[Just Another Sa]

Originally Posted by Polux

For someone that is studding all this matter you make a lot of very erroneous assumptions regarding stability, obviously you are referring here to Max RM related to safety and overall boat stability.

Regarding the boat stability, that is proportional to the area behind the positive area of a RM curve till the AVS point, you can see here, on a concrete case (HR 48MKII) that the area of the stability curve till 30º of heel is around 4 or 5 times smaller than the area that goes from 30º to the AVS point at about 128º.

It is incomprehensible that you have such a small grasp regarding monohull stability.

Name one.

[Dockhead]

Originally Posted by Just Another Sa

Even if the boat is accelerating at 90 degrees, it wont remain so forever and not even close to the time intervall a short duration microburst last (minutes, not seconds). My analyses applies after the boat has ended lateral acceleration, because that is what accounts, not how soon it will happen but if it will happen during that weather event.

This doesn't make any sense to me at all. Your calculation assumes infinite lateral resistance, but there will never be that. The fact that the weather event is short duration doesn't change anything -- the wind force can be converted to a short duration lateral sliding just like it can be converted to short duration heeling. The key fact is that not all and probably not most of the wind force will be converted to heeling moment. The calculation just doesn't work without a value for lateral resistance, unless I am deeply misunderstanding something (which is a possibility of course ).

And don't forget about weather-cocking -- you also can't assume that the boat will stay square to the wind -- part of the wind force will be absorbed by rotation of the boat in the horizontal plane, which will neutralize windage of the hull and lateral resistance of the keel (and even deck) and kill heeling moment.

Originally Posted by Just Another Sa

In case of anchored boat during predictable hurricane or typhoon the situation is totally different.
The boat is closer to head on situation, but still swings around. It means less windage due to less area and lower cd as well. It also means only a fraction of windage is sideways to the boat, the other component producing pitchpoling moment backwards that is mostly irrelevant for monohulls.
And last the most important factor is that the center of effort of windage is rather close to centerplane of the boat, where the anchor rode or mooring rope is tied. Hence the leverage is less than 0.2 m instead of 1.5 m as in the case the lateral resistance is acting on the immersed deck. All combined the capsizing windspeed is 10...20 times greater without taking into account conversion of linear to rotational energy.

Indeed! And a boat not anchored, hit square on by a massive microburst, will try to rotate in the horizontal plane -- as Poiu pointed out. This will tend to prevent any knockdown, much less inversion.

[Just Another Sa]

Originally Posted by Polux

We all are discussing cruising boats and I have been repeating that I am referring to RCD class A boats, that is what almost all cruising boats are.

Most 30ft crusing boat on the market don't even get the minimum stability characteristics to be certified as class A boats.

Certainly the ones that are discussing this with you are not thinking that your observations regarding the possibility of a cruising boat to be inverted by the wind regards only to light 30ft small cruising boats.

Regarding 4000kg not to be a light 30ft monohull maybe you don't know that a First 30 (9.81m) weights 3750kg?

and that an Halberg rassy 31 (hull length 9.62 m)

weights 4500kg?
Remember that on those calculations 4000kg regards a cruising boat sailing, in sailing loaded condition. On that condition even a light First 30 will displace well over 4000kg and probably a Halberg Rassy over 5000kg.

first bold:
It is not my responsibility if people do not read what I write and draw false conclusions due to that.

Many cruisers store some weight even on deck, like fuel canisters.
That reduces RM, not increases it at 90 degrees of heel. Not all weight inside increase RM either, the added Center of Gravity is much closer to added Center of Buoyancy at 90 degs than the empty boat condition. That means GZ of the boat typically decreases with added weight at 90 degs of heel such a way that RM remains approximately the same.
The added weight does however significantly increase RM at less angle of heel.

I'm aware there are boats like HR and some made of steel that re heavy. I don't consider them as small boats, just short.

The point was and is that not all monohulls are immune for wind induced capsize, and that the same conditions causing Atlantic 57 to capsize are enough for some mainstream roomy monohulls to do the same.

[Just Another Sa]

Originally Posted by neilpride

BS>!! Seems to me you never experienced a hurricane.....

The laws of physics are the same regardless of what I have experienced and what not, and apply to small monohulls just fine.

[Polux]

Originally Posted by Just Another Sa

first bold:
It is not my responsibility if people do not read what I write and draw false conclusions due to that.
....
The point was and is that not all monohulls are immune for wind induced capsize, and that the same conditions causing Atlantic 57 to capsize are enough for some mainstream roomy monohulls to do the same.

So you were talking about relatively light 30ft class A monohull boats.

You certainly now that there is not a single Cat that can be certified as class A boat with 30ft? That the smaller multihulls that can reunite the stability criteria to be certified as such are trimarans and even so above 30ft and that the smallest (and very rare) cats that can be certified as class A have about 35/36ft?

So why talking about 30ft monohulls in what regards stability when we are discussing the capsize and the stability of a 57ft cat? Do you think that there are any similarity? I don't think so.

[Just Another Sa]

Originally Posted by Dockhead

This doesn't make any sense to me at all. Your calculation assumes infinite lateral resistance, but there will never be that. The fact that the weather event is short duration doesn't change anything -- the wind force can be converted to a short duration lateral sliding just like it can be converted to short duration heeling. The key fact is that not all and probably not most of the wind force will be converted to heeling moment. The calculation just doesn't work without a value for lateral resistance, unless I am deeply misunderstanding something (which is a possibility of course ).

And don't forget about weather-cocking -- you also can't assume that the boat will stay square to the wind -- part of the wind force will be absorbed by rotation of the boat in the horizontal plane, which will neutralize windage of the hull and lateral resistance of the keel (and even deck) and kill heeling moment.




Indeed! And a boat not anchored, hit square on by a massive microburst, will try to rotate in the horizontal plane -- as Poiu pointed out. This will tend to prevent any knockdown, much less inversion.

No part in my cals assume infinite lateral resistance. Just that lateral acceleration * mass of boat << lateral resistance of the hull at 90 degrees.
That happens either before 90 degrees is reached or less than one second later.

Windage (force) = Dpress * area * cd = 1525 * 20 *1 = 30500 N
Weight of boat 4000 kg

From those it results than lateral acceleration < 30500 N / 4000 kg = 7.625
m/s^2.
How long do you think that kind of lateral acceleration can take place without force of lateral resistance growing to the same as windage?
Thats 15 knots sideways in a second, 30 knots in 2 seconds during 90 degrees of heel and the boat didn't end up with no lateral motion during heel going from 0 to 90 degrees. In reality that acceleration period last less than a second and allows energy conversion from linear kinetic into rolling to make it flip in cases than it otherwise would not have done so. It makes things worse not better.

It's not going to last long enough to make any positive difference to the outcome. Once the acceleration period is over the calc applies and if HM > RM it will flip over. The duration of microburst is not short enough to end before that happens.
The rotation about vertical axis takes longer than the inversion. There is no chance at all it being otherwise.

[Just Another Sa]

Originally Posted by Polux

So why talking about 30ft monohulls in what regards stability when we are discussing the capsize and the stability of a 57ft cat? Do you think that there are any similarity? I don't think so.

I'm not limiting the discussion on RCD class A, you are.
The similarity is the weather. And the false claim that it can't happen on any monohull made by someone in this thread from where this all begin.

When ever some claim is made in this thread I consider to have right to disprove it until proven otherwise by the authority of this site.

btw, the situation is the same on RCD A class, only the windspeed required can slightly change, but not by much. Once the mast goes under with high lateral boatspeed the drag goes suddenly up and it flips over regardless how high the AVS is even without any heeling force from windage, which in reality is still there. It only effects on what happens after the inversion, does it stay there or continue rolling back upright.

[Dockhead]

Originally Posted by Just Another Sa

No part in my cals assume infinite lateral resistance. Just that lateral acceleration * mass of boat << lateral resistance of the hull at 90 degrees.
That happens either before 90 degrees is reached or less than one second later.

Windage (force) = Dpress * area * cd = 1525 * 20 *1 = 30500 N
Weight of boat 4000 kg

From those it results than lateral acceleration < 30500 N / 4000 kg = 7.625
m/s^2.
How long do you think that kind of lateral acceleration can take place without force of lateral resistance growing to the same as windage?
Thats 15 knots sideways in a second, 30 knots in 2 seconds during 90 degrees of heel and the boat didn't end up with no lateral motion during heel going from 0 to 90 degrees. In reality that acceleration period last less than a second and allows energy conversion from linear kinetic into rolling to make it flip in cases than it otherwise would not have done so. It makes things worse not better.

It's not going to last long enough to make any positive difference to the outcome. Once the acceleration period is over the calc applies and if HM > RM it will flip over. The duration of microburst is not short enough to end before that happens.
The rotation about vertical axis takes longer than the inversion. There is no chance at all it being otherwise.

Ok, that makes some sense. I'm not sure how confident we can be in the time frames, but I do get it and am convinced that at some point, lateral sliding will stop absorbing wind force. Plus if the boat is sliding laterally and heeling more and more, and the rail catches the water surface -- that would trip the boat and flip it over if the CG of the boat is higher than the center of hydrodynamic resistance (?). I can see that in THEORY.

But in practice, what are the chances that the boat will stay square to the wind as it slides laterally? Zero? We all know that the center of wind pressure is not aligned with the center of hydrodynamic resistance, which causes every cruising boat to rotate in the horizontal plane when hit by a blast of wind. And with the boat not square to the wind -- your calculation is out the window.

[Just Another Sa]

Originally Posted by Just Another Sa

Most 30ft boats built so far and still used are not class A and do not have AVS of 125 degrees or more. I'm sure you know that.
The amount of energy from the wind is practically unlimited. If it produces enough heeling moment to exceed RM the energy is there for rolling energy automatically. It can invert even if there isn't. For that you need to analyse the energy conversion as well, but not then HM<RM for all the way up to 90 degrees, because in that case it will remain so upto AVS as well.

A typo, should read HM > RM

[Just Another Sa]

Originally Posted by Dockhead

Ok, that makes some sense. I'm not sure how confident we can be in the time frames, but I do get it and am convinced that at some point, lateral sliding will stop absorbing wind force. Plus if the boat is sliding laterally and heeling more and more, and the rail catches the water surface -- that would trip the boat and flip it over if the CG of the boat is higher than the center of hydrodynamic resistance (?). I can see that in THEORY.

But in practice, what are the chances that the boat will stay square to the wind as it slides laterally? Zero? We all know that the center of wind pressure is not aligned with the center of hydrodynamic resistance, which causes every cruising boat to rotate in the horizontal plane when hit by a blast of wind. And with the boat not square to the wind -- your calculation is out the window.

Zero in the event of sudden increase of windspeed into extreme values > 40 m/s that do happens in some (but not all) microbursts for the boat values in the calc, not for all boats. It doesn't have to stay exactly twa=90 degs either, 70...110 degs have practically the same outcome, perhaps 1 m/s more wind needed. In fact if twa is initially 100 degs, the small rotation taking place will only make it worse.
That calculation was not meant to be exact, as I have repeatedly stated. It has correct order of magnitude even if twa is initially 130 degs. The different properties of different boats have much greater effect than small changes in initial twa. Therefore the calc is not out of the window, it's just an estimate and reality is worse not better, due to energy considerations and wave action. If any sails are up it will rotate quicker and/or lose the mast in the process. As far as I'm aware there is no warning in case of a microburst in the night -> sails up probable.

The important issue remains that the windspeed needed depends a lot on boat size, in particular RM values, and not RCD A compliance or STIX values. Some bigger and heavier ones will not roll over to 180 degs even in 200 knots of wind without wave action, even if AVS is less than 100 degs. Those are vulnerable to wave action though.

[Just Another Sa]

Originally Posted by Dockhead

I'm not sure how confident we can be in the time frames ...

You are free to send email to someone you know would be able to calc this and who you trust.
Just make sure they know how much wind can exist out there in rare events.
Expert in boat design is not expert on meteorology, but might still know.

Then you can compare results with estimated windspeeds needed to capsize Atlantic 57 under the reefed sails it had.

[Dockhead]

Originally Posted by Just Another Sa

Zero in the event of sudden increase of windspeed into extreme values > 40 m/s that do happens in some (but not all) microbursts for the boat values in the calc, not for all boats. It doesn't have to stay exactly twa=90 degs either, 70...110 degs have practically the same outcome, perhaps 1 m/s more wind needed. In fact if twa is initially 100 degs, the small rotation taking place will only make it worse.
That calculation was not meant to be exact, as I have repeatedly stated. It has correct order of magnitude even if twa is initially 130 degs. The different properties of different boats have much greater effect than small changes in initial twa. Therefore the calc is not out of the window, it's just an estimate and reality is worse not better, due to energy considerations and wave action. If any sails are up it will rotate quicker and/or lose the mast in the process. As far as I'm aware there is no warning in case of a microburst in the night -> sails up probable.

The important issue remains that the windspeed needed depends a lot on boat size, in particular RM values, and not RCD A compliance or STIX values. Some bigger and heavier ones will not roll over to 180 degs even in 200 knots of wind without wave action, even if AVS is less than 100 degs. Those are vulnerable to wave action though.

Sure, but if the boat slews right around, then that's the end of the tendency to heel over. The slewing will be just as rapid and violent as any of the other motions generated by the sudden wind force.

I do think that this is probably -- and I think you agreed in one of the posts above -- why microbursts do not in fact capsize ballasted keel monohulls, in any recorded event that I have ever heard of. Just doesn't seem to happen.


If your boat is 30 feet and light then you probably "feel" more vulnerable to this and this might explain some of your instincts. For us with bigger boats -- and I have been in hurricane force winds in my 54', 32 registered tons vessel -- we just feel and know that it can't happen.


As you say, wave action is a totally different ball game. All bets are off if you're dealing with breaking waves. I've been knocked down by a breaking wave even in this boat. It was not fun.