Capsize Factor ? I'm Confused !

[Jim Cate]

G'Day all,
for Steve: Exactly what do you mean by capsize? Full inversion, 90 degree knockdown or what? If the former, capsize from wind gusts are near impossible. Mast in the water, perhaps, but not further. If the latter, then the AVS seems quite important in determining the speed or even probability of recovery.

I'm no NA, but it seems (from some considerable sea time) that the incidence of seas whose actual breaking height is more than perhaps half or less of their peak to trough height is pretty rare when in deep water. I suspect that this parameter is the one that matters. If so, even if the situation that you are worrying about is correct the odds are much better due to these monsters rarity.

It is worth noting that huge waves are relatively common when rounding the great capes, and none of the storied voyagers who have completed such passages have had boats of great beam. I think that common seamanship tends to avoid being beam-on when in such conditions.

As to the catamaran capsize referred to above -- I suspect that the poster is thinking about the Lightwave 38 that pitchpoled on the Wide Bay Bar. In this case, the boat was surfing on a large but not breaking wave and managed to stuff one bow into the bottom at a reported 16 knots. A soectacular cartwheel resulted, and Terry and Jan were so very fortunate to escape injury. The boat, incidentally was in remarkably good nick at this stage, although the rig was gone. Terry reports that most of the damage was done in righting the boat with the aid of a friendly car ferry barge. Remarkable...

At any rate, Steve, I'll agree with MarkJ in saying that such theoretical issues may be of academic interest, but impact only slightly on the real world of cruising. I hope that you can satisfy your curiosity and get on with selecting an appropriate real world boat.



Good luck and cheers,

Jim

[GordMay]

Originally Posted by tropicalescape

I think Ted Brewer could help with your question..he invented the or a formula...did he not ?

No he didn't. He did invent the "Comfort Ratio".
Ted Brewer Yacht Design

[svHyLyte]

Originally Posted by stevensuf

From what I've read, a breaking wave equal in size to a boats beam will capsize the boat, if taken ahull, regardless of length or displacement, So why does the capsize factor equation have displacement in it?

Now i can see why beamier boats will be harder to initially capsize but on the other hand slower to self right, makes sense easily, kinda takes away the argument for greater avs through narrow beam, yeah your boat will self right much quicker, but hey it will be capsized by a small wave.

im focusing on capsize by waves here not by wind.

can anyone put me right?

Your first proposition--and I assume you meant a breaking wave taken "abeam" (perhaps while lying 'ahull")--is erroneous. Resistance to capsize is a function of Roll Moment of Inertia which is greatly influenced by both length and displacement. In one of the post-Fastnet studies, there was a statement to the effect that there was a greater propensity for capsize produced by breaking waves of approximately 1.5x the beam of a given yacht and that produced a "jet" of water off the crest but that was not a flat statement of causality.

Your second statement is also erroneous. Greater beam for yachts of equal length and displacement produces greater resistance to heeling due to greater GZ. However, as discussed in C.A. ("Tony') Marchaj's tome "SEAWORTHINESS--The Forgotten Factor", rightly cited by Neelie in his earlier post (although I disagree that it is "dry") the propensity to capsize is a function of the Rolling Energy that can be transferred to a yacht.

Rolling energy (Er) transferred to a boat due to the action of wave slope can be expressed mathematically as: Er = (∫[from 0-ΔT] Mrdt)^2 / 2* (Ir+Ia). The magnitude of the Rolling Moment (Mr) for a given displacement depends upon wave slope (a) the righting lever (GZ) and the time interval (ΔT) of the wave.

Accordingly, Mr for a given displacement is directly proportional to (a, GZ, ΔT) of which only GZ—the righting lever of the yacht--can be controlled. Accordingly, decreasing GZ results in a decrease of rolling energy and the risk of capsize. GZ, of course, is proportional to beam, hence for the same displacement, a yacht of lesser beam has greater resistance to capsize; and, a greater propensity to recover if capsized.

Stability curves should not be thought of as merely representations of angles of recovery from heel. The areas under the curves are, in fact, measures of the energy that the yacht can absorb before capsize occurs. Accordingly, one wants as great an area under the curve as possible with the yacht in an upright position--even if heeled--and as small an area under the curve as possible when the yacht is "inverted" so that its recovery is assured. A catamaran for example, has nearly symmetric stability curves.

FWIW...

[John A]

Here's somemore studes on stability of boats, the short answer is it depends.
http://koti.kapsi.fi/hvartial/stab/stab.htm#XX3

[stevensuf]

Hey im still trying to figure it out, i thought the upper part of the gm curves was stability and it is, but not capsize resistance, after seeing some english university water tank model studies, they found that side on it was the beam that was most important to resist capsize from beam on waves.

the upper part of the gm curve shows how stable it is in the upright position , the bottom ie negative part shows you how stable it is turned turtle position, the other thought im having is boats with a very high avs, while they will immediately self right it turned turtle will do it under rapid acceleration, i wonder if this would more likely cause them to loose the rig, some boats capsize and self right with rig ok, many loose it, granted the strength of the fittings will play a huge part, but a boat with a very high avs, will accelerate back to upright far quicker than a lower avs boat, meaning more likely to tear rigging to bits, but less likely to sink taking in water while turned turtle.

im only trying to understand this from an academic point of view, i do realise capsizes are rare, helmsmanship and storm tactics are a greater part in a storm survival situation, series drogue and run if you have the sea room..

[Adelie]

Originally Posted by stevensuf

Hey im still trying to figure it out, i thought the upper part of the gm curves was stability and it is, but not capsize resistance, after seeing some english university water tank model studies, they found that side on it was the beam that was most important to resist capsize from beam on waves.

the upper part of the gm curve shows how stable it is in the upright position , the bottom ie negative part shows you how stable it is turned turtle position, the other thought im having is boats with a very high avs, while they will immediately self right it turned turtle will do it under rapid acceleration, i wonder if this would more likely cause them to loose the rig, some boats capsize and self right with rig ok, many loose it, granted the strength of the fittings will play a huge part, but a boat with a very high avs, will accelerate back to upright far quicker than a lower avs boat, meaning more likely to tear rigging to bits, but less likely to sink taking in water while turned turtle.

Regarding the question above, while a higher AVS would likely lead to greater acceleration and higher loads while righting, the forces involved would be a fraction of the impact forces the rig was subjected to during capsize. If the rig didn't come apart during the capsize it probably won't during righting.

On the other hand the higher the AVS the sooner the boat is likely to right itself. While the difference between 1 and 3 minutes to right might not mean much to people in the cabin but to anyone on deck, especially if they are harnessed to the boat and can't get loose to get to the surface, the faster the better.


My understanding of stability in general and capsize resistance in specific are as follows.

Stability is a boat's static resistance to overturning forces. The OP seems to already have a grasp of how to interpret a stability curve so I won't belabor the topic

As previously mentioned a capsize is defined as when the turn upside down or at least far enough to reach the area of negative stability which would then complete the job, pretty much the same thing.

Angle of Vanishing Stability (AVS) is the angle of heel (other than 0 or 180) at which righting moment is 0, any less heel and the boat wants to right itself, any more and the boat wants to turn turtle.

Almost universally wind phenomena can not cause an 'offshore' monohull sailboat to capsize, the overturning moment of the wind on the sails and rigging decreases steadily as the boat heel increases while the static resistance to heeling is steadily increasing, on top of which, as soon as the boat passes 90* there is a tremendous increase in resistance because of the high drag of water on the very long lever arm of the mast and rigging. I would say universally but I have never heard of anyone discussing the actual results of a tornado passing directly over a sailboat in waters with little fetch and therefore little wave action. Boats that have a low AVS (90*-110*) are at risk from capsize due wind action, once the boat is slightly passed a knock-down state, it wants to keep going. A monohull example of this would be a J24. Multihulls are also subject to capsize from predominantly wind action but they are a different beast not subject to the 'Capsize Screening Formula'.

Following the Fastnet disaster in 1979 a lot of research went into stability and capsize resistance. During tank testing they figured out how to get boat models to consistently roll over under a breaking wave. The key finding was that models without masts are easier to roll over than the same models with masts.

This was against expectation. A boat or model with a mast in it has a lower peak righting moment, less positive area under the curve and a marginally lower AVS, all things that decrease stability. It was at this point it became apparent that capsize resistance in ocean going sailboats is not related to general stability. The key thing that was ultimately understood was that capsize is a dynamic event so resistance to it is dependant on inertia, whereas stability is dependant on the static forces of the water acting on the hull, whatever it's shape.

The research derived a formula relating size and roll moment of inertia to capsize resistance. Because of the difficulty in calculating roll moment inertia or measuring it, a simplified formula was developed that used just beam and displacement, called the 'Capsize Screening Formula'. This formula is a gross simplification, but apparently was checked enough to beconsidered a decent approximation.

Please note that beam and displacement are the only two values in this formula. Length does not contribute to greater capsize resistance. People talk about how longer boats are more safer, but really length is only a proxy for greater weight and more importantly greater roll moment of inertia. Consider that few ultralights are taken cruising until you get up to about 50'. Part of this is the more mundane consideration of load carrying capacity, but some of it is that increasing roll inertia does not catch up with increasing area presented to waves and increasing beam which give waves a longer arm to act on until about 50'.

The most prominent alternative to the 'Capsize Screening Formula' I know of is the European STIX (Stability Index) developed by the ISO. I am not willing to fork over the $500-1000 to get see and use the copyrighted procedure but from what I have been able to learn it primarily depends on the positive area under a stability curve and the AVS for that boat. Given that these are values based on the static stability of a particular boat's hull shape and weight distribuiton, I am dubious about it's value in predicting that boat's resistance to capsize under dynamic forces such as a breaking wave. The other capsize formulas or numbers that have been developed are similarly dependant from what I have been able to learn.

There is one thing the stability curve is useful for in a capsize situation, determining AVS which is useful in predicting how long a boat will remain inverted.

Finally in previous discussions of this issue on this forum people have questioned the value of tank testing vs the 'real world'. My response is that they should consider several things.
A) Tank testing is testing a small bit of the 'real world' under controlled conditions. The physics are the same in the tank or on the ocean, the only arguements that can really be made against the tank is that all of the ocean conditions have not been replicated or that the results do not scale well from model size to life size.
B) The only alternative is to say it has always worked in the past. But for a new design that pushes the envelope just a little more than the previous there is no past there is only engineering judgement and the things you can test for ahead of putting the boat in the water and seeing how it does.
C) Consider the next time you get on a plane that most newly designed large airliners are approved for transoceanic flight from their maiden revenue flight because of the large amount of testing that goes into their parts and design whereas in previous decades it was some years or even a decade in service before a plane was allowed very far offshore.

In conclusion to a small extent they are right, there is plenty of testing that can go into something, but until boat hits the water you can't really know how it will do. The point of tank testing is to help point out the designs with gross problems so only the more subtle problems make it onto the water. When those more subtle problems become obvious testing is done to figure out how to cure them.

[James S]

Originally Posted by Cheechako

would the longer keel make it roll more?.... the water on the front of the wave is moving up, so more lateral area would push the keel up the face of the wave right?

I would think that yes, apples for apples (keel/balast) the longer keel (fore and aft) would have more lateral resistance as the water at the face of the wave acts on it...add to that the gravitational pull of the rest of the boat in the direction of the trough....
Just my seat of the pants think-un.

[goboatingnow]

Stix takes into account far more then just static stability variables. Like all such " magic numbers " it has it's drawbacks

Here's a link that gives you an idea of the variables. http://www.boatdesign.net/forums/att...ix-formula.xls

[bcn]

And here a link with some more argumentations:
SetSail » Blog Archive » Evaluating Stability and Capsize Risks For Yachts

[LauderBoy]

Originally Posted by MarkJ

In the real world you will not capsize.

So instead of worrying about stats I feel folks should buy the boat most comfortable for them.

I just lost a post where I described how the 5 yachts I personally know sank. None of them were by capsizing at sea. All were driver error, one thru-hull failure; 2 solo sailors slept through alarms with wind vane steering and put onto beachs/reefs; one following a buddy boat into a channel instead of navigating himself.

5 boats lost and NONE by what everyone always worries about when buying a boat

This is a great point. How many stories are out there of a mono hull turning turtle in 60ft waves and going down because of it?

Then compare that to how many are lost because of a bad through hull, improper anchor setup, poorly maintained chain plate or just up and crashes onto a beach because of the idiot behind the helm.

[rtbates]

Originally Posted by Rakuflames

But since "stuff happens," what if (assuming you are able to) you turn your boat into that wave, so it breaks over the bow or the forward quarter instead of abeam?

My boat is 31' long. Is it possible for my boat to survive a 30' breaking wave without capsizing? Yes, I'd do everything in my power to not be in such a situation. On the other hand, in a bad situation it isn't possible to know too much.

What if you have a drogue out, or some other significant drag mechanism? Can you turn the boat enough to avoid a beam wave?

You'd want your STERN tethered to a drogue , preferably a Jordan Series

[sailorboy1]

Originally Posted by MarkJ

5 boats lost and NONE by what everyone always worries about when buying a boat

Good point! In the end when I got my current boat I just stopped thinking I knew more than the boat designers. I think we get too caught up the various "ratios" etc. In fact I think that a lot of the older formulas we read about don't apply well to more moderm designs.

If I believed in the old formulas my boat would sink after I had filled the water tanks and had more than a littlwe boy on the boat because it has a low DLR.

In the end forget the formulas and decide if you want a:
1 - racer
2 - racer/cruiser
3 - cruiser/racer
4 - cruiser (for the most part this will be an older boat as now days also all are more cruiser/racer. ie a fast cruiser)

[Adelie]

Originally Posted by goboatingnow

Stix takes into account far more then just static stability variables. Like all such " magic numbers " it has it's drawbacks

Here's a link that gives you an idea of the variables. http://www.boatdesign.net/forums/att...ix-formula.xls

So I worked my way thru the spreadsheet and the formulas going into the final number. Some of the values calculated do depend on weight and beam so I was wrong that it did not address them, though they seem to be a small part of the calculation.

As Goboatingnow says it is a magic number and it has some drawbacks. The one that occurs to me looking at this spreadsheet is that it looks like a committee came up with the formula. It appears that the ISO attempted to incorporate all the pet theory's of the big names in naval architecture into the formula and weight them somewhat to come up with a single number that is supposed to be predictive of a number of related but different sailboat behaviors. The fact that the angle of down-flooding is considered indicates that this formula isn't just about capsize resistance. I would rather deal with the behaviors individually and know where a boat's weaknesses are likely to be rather than have a single number that is indicative of a number of issues lumped together.

I would like to see the underlying research and a summary of the discussions that went into formulating the formula.

I played with the spread sheet with the aim of determining how it would treat loosing the mast. To that end I adjusted the displacement down by about 125kg, the CE went to 2m, the AVS to 122deg, and sail area was dropped to 5 from 208 (the area of the cabin and topsides) and the Gz increased by .02 m which together almost doubled the STIX value. (Bigger is better) But from the Fastnet research, loosing the mast decreases capsize resistance. STIX is not very indicative of capsize resistance is my conculsion.

That said, I did note that the formula did look at the depth of the CLR which tells me it might be looking at the lever arm between the resistance offered by the keel and the impluse of the wave on the topsides and cabin. I would see this as a positive step towards dealing with roll moment of inertia directly as a means of evaluating capsize resistance.