Capsize Ratio

[Adelie]

Originally Posted by Polux

The capsize ratio does not take into account the boat CG and that is a main factor in what regards resisting capsizing.

Since the mid 90's all boats have stability curves that are needed to certify the boats in the EC. If you want to access a boat stability use those curves.

I disagree, research following the 1979 Fastnet debacle showed that righting moment, height of CG, AVS and stability curve were vastly less important to capsize resistance than roll moment of inertia.

The testing showed that a sailboat without a mast was much more likely to be rolled over than one with a mast. The boat without a mast had more area under the RM curve, a higher AVS and a lower CG. All of which gave it more stability, but not more capsize resistance.

The researchers ultimately determined that capsize is a dynamic event so values related to static equilibria do not apply to a great extent for normal vessels.

Following a capsize the RM curve, AVS and height of CG ARE useful in predicting how long a boat will remain inverted. This is an important consideration to those on the boat, especially those on deck, and most especially those on deck that can't release their tethers to swim to the surface.

What is happening in the EU, STIX (Stability Index), is not an evaluation of capsize resistance, but a weighted balancing of a whole bunch of factors related to sailboat safety in heavy weather: AVS, energy under the righting moment curve, length, beam, mass, draft, down-flooding angle, hull strength .... It is not an evaluation of capsize resistance specifically. I have looked at a spreadsheet showing the calculation. There are are a few things in it that might be a proxy for roll moment of inertia but what I saw was not well documented so I can't be certain.

This is not to say that the CSF is a great proxy for roll inertia, but it is better than anything that relies strictly on evaluation of static forces. The underlying formula would be a much better one to use, but it would require a naval architect to calculate or a giant testing apparatus to actually measure the roll inertia.

[Polux]

Originally Posted by Adelie

I disagree, research following the 1979 Fastnet debacle showed that righting moment, height of CG, AVS and stability curve were vastly less important to capsize resistance than roll moment of inertia.

The testing showed that a sailboat without a mast was much more likely to be rolled over than one with a mast. The boat without a mast had more area under the RM curve, a higher AVS and a lower CG. All of which gave it more stability, but not more capsize resistance.

....

After a thread with 18 pages i will not continue this nonsense. AVS is an important factor regarding seaworthiness and stability in any sailboat. AVS is greatly dependent on the boat CG.

A boat without a mast will be at the mercy of the seas. Modern boats are made to sail, even with bad weather. A boat sailing on heavy weather and high winds under storm sail, even on bare poles (depending on wind intensity) will be "tied" to a side by the wind. Roll moment of inertia is only relevant when the boat is bouncing around and not sailing.

Regarding the former case the more important factor to stability is stiffness. Stiffness will permit the boat to keep sailing while the force of the wind keeps him steady with little heel. There are two ways of increasing stiffness in a sailboat: increasing beam and lowering CG.

One of the more important safety measures taken by ISAF after the Fastnet 79, following the advise of research studies conducted by Na was the implementation of minimum AVS requirements for race boats.

[sandy stone]

Originally Posted by wolfenzee

It tells you nothing in comparing those two boat....but what it does tell you about both of those boats is that neither would be a good choice for offshore use.

I think you are putting way too much faith in an almost meaningless formula and an arbitrary cutoff point.

[TeddyDiver]

Originally Posted by Polux

After a thread with 18 pages i will not continue this nonsense. AVS is an important factor regarding seaworthiness and stability in any sailboat. AVS is greatly dependent on the boat CG.
Not so simple, more than CG the bouancy of the superstucture, downflooding angle into the cockpit and such are affecting to AVS
A boat without a mast will be at the mercy of the seas. Modern boats are made to sail, even with bad weather. A boat sailing on heavy weather and high winds under storm sail, even on bare poles (depending on wind intensity) will be "tied" to a side by the wind. Roll moment of inertia is only relevant when the boat is bouncing around and not sailing.
[COLOR="rgb(139, 0, 0)"]Nonsense, and again a lot of other factors included in the outcome[/COLOR]
Regarding the former case the more important factor to stability is stiffness. Stiffness will permit the boat to keep sailing while the force of the wind keeps him steady with little heel. There are two ways of increasing stiffness in a sailboat: increasing beam and lowering CG.
[COLOR="rgb(139, 0, 0)"]You have misunderstood something you have read thou the last sentence is allmost true[/COLOR]
One of the more important safety measures taken by ISAF after the Fastnet 79, following the advise of research studies conducted by Na was the implementation of minimum AVS requirements for race boats.

What comes to the usefullness of th CSF we have to remember many of the boat builders don't release the data conserning the approval of their boats so we don't have the stability curves nor their STIX calculations available for anyone of us. Hope this would change someday. And no worries if you didn't understand stability thoroughly, it's pretty complicated..
BR Teddy

[Andrew Troup]

The propensity to capsize in breaking waves doesn't have much to do with stability, however the angle of vanishing stability does have a definite bearing on whether a vessel, having capsized, will come back upright in a realistic timeframe.

(In other words stability has little to do with prevention but a lot to do with cure)

In many ways the optimum solution for certain types of survival conditions (midwinter Atlantic storms of long duration and fetch; Southern ocean ditto) is perhaps counterintuitive: it might in fact be a boat featuring a hydraulic lifting keel, one which can be retracted into the canoe body to alleviate the problems of tripping and excessively low CoG which can increase the lever arm of white water hitting the topsides, tending to rotate the boat as it is launched sideways into the trough.

If it lands on its side (or worse) it's very likely to continue into a roll.

If keel retraction proves insufficient to prevent a capsize in the prevailing conditions, or the conditions have not the necessary grasp of physics <WINK>, the ideal solution keel would have a gravity-actuated handle on the keel hydraulic valve, so the keel would redeploy of its own volition as the boat went upside down.

A strategy of prevention, followed (if unsuccessful) by cure.

This would naturally require a stored source of hydraulic fluid under system pressure (ie one or more accumulators)

[Polux]

Originally Posted by TeddyDiver

What comes to the usefullness of th CSF we have to remember many of the boat builders don't release the data conserning the approval of their boats so we don't have the stability curves nor their STIX calculations available for anyone of us. Hope this would change someday. And no worries if you didn't understand stability thoroughly, it's pretty complicated..
BR Teddy

British magazines have been publishing the stability curves of all boats they test on the last 15 years. This gives you a big library. They have a system that allows you to download the test of any boat... for a price.

I always got all stability curves I have demanded to boat builders. normally the dealer don't have them and he has to ask them to the shipyard. In some smaller brands I did not only receive them as I even exchanged some nice emails with the NA.

Some brands like Elan, Southerly, Malo or HR among others have their stability curves on their site or paper catalogs.

There are enough information on the net to provide a good comprehension and interpretation about the information provided by stability curves.

[Polux]

Originally Posted by Andrew Troup

The propensity to capsize in breaking waves doesn't have much to do with stability, however the angle of vanishing stability does have a definite bearing on whether a vessel, having capsized, will come back upright in a realistic timeframe.

....

I would say that it has all to do with stability: Static stability and also Dynamic stability. Many factors in static stability are important and relevant to dynamic stability.

[Andrew Troup]

Originally Posted by Polux

I would say that it has all to do with stability: Static stability and also Dynamic stability. Many factors in static stability are important and relevant to dynamic stability.

I await your list of those factors with interest. I'm struggling to think of any factor which assists static stability, which does not become a liability when you consider capsize from a large breaking wave on the beam.

[TeddyDiver]

Originally Posted by Polux

There are enough information on the net to provide a good comprehension and interpretation about the information provided by stability curves.

I don't agree. Thou some of the information online is good there's more 'not so good' and it's hard to distinguish them. Before you know what you didn't know..
Fair winds! Teddy

[Polux]

Originally Posted by Andrew Troup

I await your list of those factors with interest. I'm struggling to think of any factor which assists static stability, which does not become a liability when you consider capsize from a large breaking wave on the beam.

Consider the stability curve of an Open 60 with a very low CG. This boat will have all negative characteristics you seem to imagine in a boat with a huge static stability and a low roll moment of inertia...and experience shows that this is one of the most seaworthy sailboats ever built, able to go with 40K at over 20k on autopilot with the solo skipper sleeping, breaking waves and all. Something should be wrong about your reasoning.

Have a look at a VOR 70 taking not one but two huge breaking waves: By your theory this boat has everything to have been capsized. This is a boat with a very low CG, a huge stability and a low roll moment of inertia.

You can determine the energy a breaking wave has to produce to capsize a boat measuring the area behind the positive area of its stability curve. A boat with a small stability would have been capsized by that wave.

Obviously those two big breaking waves had not enough energy to capsize a boat with the huge stability of a VOR 70 and all negative points you seem to find in boats with a low CG.

http://www.youtube.com/watch?v=UrKQgJykdO4

[Polux]

Originally Posted by TeddyDiver

I don't agree. Thou some of the information online is good there's more 'not so good' and it's hard to distinguish them. Before you know what you didn't know..
Fair winds! Teddy

Maybe you are right. I am just saying you can find solid useful information on internet about it. It may take sometime to understand what is relevant and not. Just some examples of good information about stability curves and how to read them:

http://www.rya.org.uk/sitecollection...ty%20Intro.pdf

Stability

s/y Troldand - Stability

Interesting Sailboats - Page 237 - SailNet Community

http://www.hiswasymposium.com/assets...osium-2004.pdf

[TeddyDiver]

A good start to study stability but limited only to static stability curve. Thou somewhere they mentioned that other factors lie behind.
Richard Birminghams paper is good, he has also written a very good book about boat building, one of my favourites, thanks for him.
Rya is pressing the boat manufacturers to publish their data but concentrating only to GZ curves, when the STIX calculation data is forgotten. STIX number is a product of several formulas all concentrating in separate aspect of the stability and seaworthiness as you might know. Much left in the dark.
And then there's dynamic stability..
BR Teddy

[Andrew Troup]

I don't think I know how to respond to your points because I'm not sure what they are.

Open 60s and Volvo 60/70s seem to me to have little relevance to the discussion: they do not have a particularly low CoG in relation to their beam, particularly with the keel canted.
What's more important, they are not in any way relevant to the requirements of a cruising vessel, being completely unable to look after themselves in survival conditions.
Instead, they have to be very actively managed to avoid capsize in the very unhappy circumstance where they are forced to travel slowly side-on to dangerous breaking seas (which they try at all costs to avoid): sometimes this may even involve inclining the keel the 'wrong' way (as in the 14m breaking seas encountered by the 2008 Volvo fleet in the Luzon Strait, south of Taiwan)

Regardless of this, they do capsize, generally down in my part of the world.

Finally, I'm not sure why you would think the video you posted shows anything remotely resembling survival conditions.

[Polux]

Originally Posted by TeddyDiver

A good start to study stability but limited only to static stability curve. Thou somewhere they mentioned that other factors lie behind.
Richard Birminghams paper is good, he has also written a very good book about boat building, one of my favourites, thanks for him.
Rya is pressing the boat manufacturers to publish their data but concentrating only to GZ curves, when the STIX calculation data is forgotten. STIX number is a product of several formulas all concentrating in separate aspect of the stability and seaworthiness as you might know. Much left in the dark.
And then there's dynamic stability..
BR Teddy

STIX has also only to do with static stability and other factors but all in a static way. Stix can eventually be useful for someone that don't know nothing about sailboats or stability curves but if you have some experience you can do better accessing a sailboat seaworthiness than trusting blindly on STIX.

The formula is heavily dependent on boat size and is known to give sometimes weird results. I don't like particularly the factor that considers sail area as detrimental to seaworthiness. That would be true if the sail area could not be reefed. In my opinion and regarding to that, the boats should not be penalized by sail area (providing it is appropriated) but regarding the difficulty or easiness to reef and the number of available reefs (3 versus 2), with a bonus for a small 3th reef. Also in what regards in mast furling versus other types of reefing it should be giving a bonus to all systems that can be operated from the cockpit.

Regarding also sails, boasts with removable stays or cutter rigs with a small and efficient jib should have a bonus face to boats with only a big genoa that as you know will perform very badly when heavely furled, not to mention associated risks of a big sail furled as a sail for use in bad weather.

Initially, in the late 90's I thought it was a great toll, now I think it should be perfected to give more meaningful results.

[boatman61]

And here's a totally irrelevant one in this thread...
Depends who you send up the mast...