Quote:
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.
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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.
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