Sailing boats' Stability, STIX and Old Ratios

I will post the STIX Formula and the Stix factors.

Guillermo says that the “ GZ curve… as well as the Avs value (as usually published at the magazines and quoted as being data provided by designers or manufactures), use to be get by calculation methods on estimated COG position, and not after an stability test, as it should be”.

But that information contradicts what I have been told (and read) and that is that the GZ curve is obtained with data taken from inclination trials on the prototype, conducted by an independent and certified body (Naval Architects cabinet). Those data are integrated with measures taken on the prototype and with computer simulations. They are not data provided by the manufacturer or by the boat designer.

The precise way to find the GZ curve and all factors in STIX are described in the ISO 12217-2, but it is an expensive document and I can not find it in the NET.

Guillermo, do you have it? Somebody does?

 

The only way of getting a trustable GZ curve is having an accurate body forms plan, layout plans showing all tanks (even tanks plans, if necessary) and knowing COG position by means of a carefully performed inclining experiment.

I have ISO 12217, but at the office, not here at home. Yes, it can be downloaded from the net, but it has a price.

Notified Bodies are not persons, but companies, many of them also being Classification Societies. Among the many NBs I have reference of, I do not know any NA's gabinet, although probably several exist out there. Boats testing and assessment can be performed by people hired by those companies, being not necessarily NA's. They may even have not studies at all, just know how to do the thing.

GZ or RM curves published at magazines are provided by designers, NAs or manufacturers (Many of them state there so) and not by the Notified Body. The Notified body does not provide anything but the certificate after a satisfactory assessment. Necessary calculations are performed by an NA or designer and submitted for approval.

 

Guilhermo, those are not STIX factors and don’t enter in the STIX formula. What is their meaning?

Humm, the FDF (downflooding factor) is also substantially better on the Red boat.

Don’t see anything wrong in it. The penalization seems right to me.

I don’t fully agree, I mean, if the boat is big enough, eventually it could be Certified as Class A boat, but its STIX would be very low compared to its length and that would indicate a very poor boat in what concerns safety. Guillermo, I don’t see how the FIR (inversion recovery factor) can be calculated without taking into account the AVS and the inverted stability.

A clarification for the ones less familiarized with the STIX number. That indicator was conceived to give not knowledgeable people a fast and reliably information about the safety and stability of the boat. So that number was calculated to follow the length of the boat in feet. With a higher STIX than the length of the boat, you have a better boat (regarding safety); a lower number than the length means a not so good boat.

No, Guillermo, you are not reading well your numbers, the Red FKR (knockdown factor) is 1.146 and the Blue is 1.508. They are a lot bigger for the Blue. The FKR as you said, is the factor that takes into account the recovery from a Knock down (90º) and the other factor that takes into account the capacity from recovering from an inverted position, the FIR (inversion recovery factor) is also bigger in the Blue boat (1.010 to 1.240). So it seems that the correction factors of the STIX are doing their job.

Guillermo I have been following STIX numbers for a long time and I believe that they work well as a generic information about the boat’s safety (better than anything that was done till now) and I believe that they should be mandatory in every sailboat catalogue.

I agree with you that the complementary information about the force that the boat is making to right itself after a knock down, the total force needed to capsize the boat, the AVS and the total force needed to bring it back from an inverted position, is an important one. Those are important figures to understand the boat, but you don’t get any real conclusions with GZ numbers or curves (the ones that normally are published) but only with RM curves, the ones that are made from the GZ, but that doesn’t give you abstract values (in meters/Ton) but the real forces for that boat, taking in consideration the displacement and length of the boat you are looking at. The RM curve is the only that can give you the real forces that the boat can make to recover from a knock down or the force to resist an inversion.

 

You mean that inclining experience is not made with a real prototype, but only in the paper and in the computer?

Are you kidding me? I was talking about Naval Architect cabinet, meaning offices with Naval Architects (poor English). I see in internet lots of advertisements from that kind of offices for doing the Certification of boats.

Do you mean that it is only a paper thing? You talk like if it was only a project approval, not a prototype approval. They don’t perform inclining experiences with the boat to see if the paper work matches with the reality?

About the GZ curves, obviously the Notified bodies don’t give them to magazines, but I have been several times waiting for GZ curves of new boats and the manufacturers refuse to provide them (I mean the ones from the project) till the prototype is made and subjected to tests, the ones that provide the certification. Obviously they don’t want any discrepancy between the projected curves and the real ones.

What I was meaning is that these curves are what the manufacturers provide to the magazines and not the ones from the project.

 

If I'm understanding correctly, a boat with narrow beam will nearly always perform worse in terms of downflooding, and yet, a finer hull typically has less inverted stability and greater dynamic stability.

But if it's at the expense of downflooding...but really, how hard is it to make a companionway watertight?

 

They enter the STIX formula as part of STIX factors. Study the formulae.

FDF is 1,25 for both boats. Read post number 11

Please, it would be useful for the sake of discussion if you learn first how STIX factors are calculated (I've posted an STIX spreadsheet calculator, so you can follow things there). Avs and Mmsc are the only input data for this factor. GZ curve's negative zone (What you call inverted stability), is not taken into account.

I would like to know on what numbers your statement is supported. Do you have any examples we can follow through the influence of the several STIX factors?

No, you're the one who's not reading well. Please don't try to mixtify what I wrote. Read it again carefully.

Don't make me laugh. You should tell this to all NA's, national authorities, IMO, Classification Societies, etc., etc., in the world, who assess stability using the GZ curves, instead of RM. Probably we are all stupid and do not know about our work. Please teach us.

 

Yes I guess you can say that about dynamic stability even if I would change your "near always" by normally. About the downflooding I would say that it is more difficult to make a generalization. A narrow beam boat does not have necessarily a worst downflooding angle than a beamier boat.

About the companionway, I would say that it is more important to have it closed when the boat capsizes. In many cases it is open and the severe downflooding that it origins is really dangerous and can prevent the boat to return to its normal position.

I will post two images that explain the differences in dynamic stability and in the RM curves between beamy boats relying more on form stability and narrow boats, reeling more on Ballast.

 

From the paper :”RORC Safety and Stability Indices (SSI) SSSN and ISO 12217 Part 2, STIX”Developed in Co-operation with The RYA.

“In developing the new ISO 12217-2, Small craft, Stability and buoyancy assessment and
categorisation, Part 2, Sailing boats of hull length greater than 6 metres, the ISO working group has developed a stability index for assessing the stability and safety of sailing monohulls, known as STIX.
STIX is a development of SSS, but uses more detailed stability data, including some from a righting moment curve.

Yes, you are right, and I apologise, my mistake… but I don’t understand why you say that I try to mistify what you wrote. You have said:

I guess that as you seem to have a somewhat low opinion about the utility of the STIX number, I have wrongly assumed that the sarcastic expression was related with the attribution, by the STIX rules to calculate FKR ,of a worst number to the boat that normally should have the best recovery factor from a knock down. well, as I have said I am sorry, but I have to say that this way I don’t understand your Sarcasm, nor why you are talking about “mystifying”. That doesn’t seem to make any sense to me, I did not have any intention of doing any wrongdoing.

I didn’t notice the post nº 11, but let me see if I got it right:

You Begin this thread Quoting an example referring a published article comparing two boats and saying : “The numbers were taken from the article written by Rolf Eliasson, one of the fathers of the STIX number, in Professional Boat Builder magazine, February/March 2003. “
Then you alter the numbers that you say that have been published assuming that Rolf Eliason has made a mistake in the calculation of the STIX numbers that he has published in that Article, and with a help of a spreadsheet that has been repeatedly modified, corrected the StIX numbers of both boats.

Sorry Guillermo, but that doesn’t seem right to me.

By the way, the calculated FDF can be bigger than 1.25, and that number you get is the correct calculated FDF for the boat. For calculation of the STIX number, if the FDF is superior than 1.25, it is this number that should enter in the calculations. Both numbers appear on the files that are presented for the boat certification, at least in the ones I have.

Here you are very disagreeable, to say the least. I don’t have the pretension to teach nobody, I am here to learn, but it seems that you have already learned everything.

I have explained why you should use a RM curve to understand the real stability of the boat (comparing it with others) instead of a GZ curve (and you didn’t explain why we should use a GZ).

A GZ curve is the tool to see if a boat is well designed or not, and it is that the reason that makes it a priority tool in the design of a boat. But for the buyer and to compare different boats regarding stability, it is the RM curve that matters. I have already explained why in a previous post.

Perhaps you care to read it again, or if you prefer it, I can put it in the words of Leif Angermark Marin, an Architect of international prestige and the designer (among others) of the Malo Yachts, that are reputed by his seaworthiness and concern with safety stability:

I will post also the GZ and RM curves of the boats he is talking about.
To sea the boats click on the link that I will post.

Obviously the GZ curves are the ones that look very similar. The RM curves are the ones that look very different, as different as the big real differences in stability between the boats to which they refer.

"G – Z curve versus real stability.

When we talk to "ordinary" yachtsmen, we frequently notice that it is very easy to misunderstand the meaning of the usually published stability curve that indicates the righting lever ( G-Z ). This curve alone is no measure of a yacht’s real stability.

To illustrate this statement we have prepared stability data for three different boats, and it is very common that the G-Z curves are interpreted as indicating that all 3 yachts have roughly the same capacity to resist a capsize.

In fact it is the curve of righting moment that illustrates this capacity.

Every wave contains a certain amount of energy en every vessel requires a certain amount of energy to be turned over. The energy required to capsize a ship is represented by the area of curve above the baseline, and energy needed to return it to an upright position is represented by the area below the baseline. NOTE: This is only valid for the curve representing the righting moment, NOT the G-Z curve.

Thus we can see that it takes more than 4 times more energy to overturn the MALÖ 43 as the 33 footer.

… A much better assessment of the real stability is thus to compare the curves of righting moment, and also look to the STIX number as defined in the ISO standard".

http://www.maloyachts.se/Portals/0/STABILIT.PDF

 

Vega,

Thanks for clarifying. I will agree that I overgeneralized in my previous statement. Those illustrations helped me to really understand the concept.

This may again be too far of a generalization but would you suppose that a finer hull will require more ballast than a beamier variant due it's diminished positive rm curve to be seaworthy and comfortable?

 

Inclining experiment is always done on the real thing, but Annex C of the standard does allow the option of using calculated values.

Of course I'm not kidding you. It's not mandatory an NA tests and assesses boats under RCD. Not even to design them.

Yeap. Many times.

 

Thanks for the clarification.

Dam bureaucrats:

 

Excuse me, but that's incorrect. Probably a typo or the like. RM curve IS NOT used, but the GZ curve. Just have a look at STIX input data.

About the GZ and RM issue, I'm not sure if you fully understand stability issues. Righting moment is only GZ * Disp, so knowing GZ curve for every displacement condition and heel angle is what matters. Curves of dynamic stability are mandatory to be developed and assessed also for commercial vessels, but even those are based on the integration of the static GZ curves.

The energy of the boat to resist capsize is given by the righting moment, absolutely, and, for two boats with a similar GZ curve, the bigger is able to better resist heeling forces. But as sizes and forms are so variable the only way of stablishing simple all around rules is to make the criteria "dimensionless", so avoiding displacement. As an example IMO rules are the same not matter what the size of the vessel (over 24 m length, although for smaller fishing vessels are also applicable, as recommended by IMO).

That's why what it is MANDATORY UNDER MOST REGULATIONS to analize the GZ curve to assess stability. All international criteria, including the RCD, stablish such criteria based on the analysis of the GZ curve. Even STIX uses only righting arms levers and areas under the GZ curve, but no values based on RM. But here, contrary to what happens with commercial vessels, the size of the boat is introduced in the criteria: Length roles a very important part, as the first term of equation by itself depends only on it and has the bigger weight in results (so favouring size), and displacement is thoroughly used around to calculate remaining STIX factors.

On the size of vessels and stability problems related, maybe you would like to read something I've wrote about commercial fishing ones at: http://www.gestenaval.com/estabilidad.htm

I perform many dozens of stability tests and assessements all years, including RCD ones. As a matter of fact I earn my life with this. Excuse me for sounding disagreeable, but honestly I'm upset with you, because several times you have contradicted my posts as if I were not knowing what I'm talking about. You have even told that to me, word by word. I have been polite to you up to now, what you have not been to me. Or that I understand.

The least I could have said to you is that you sound to me like an 'aficionado' having read some ideas here and there, but without a deep knowledge of matters. That's not bad, it happens all the time to everybody when acting out of his/her field of expertise, including myself. What is annoying is to cheat professionals with irrespetuous statements like I understand you've done in several ocasions. I have not said this to you before and I regret to have to say it now, but maybe it's better to clarify things. Sorry.

Cheers.

 

I suppose that by finer you mean a narrower boat?

If it is the case I would say yes, if it has not really to do with comfort. The narrow boat will be more comfortable, but to carry the same quantity of sail (to have the same RM between 10º and 30º of list) of the beamier boat, the narrow one will have to have more ballast.

That's why in a lot of modern boats (that are beamier) the Ballast/Displ. ratio can be less than 30% while on old narrow boats that ratio was between 40% and 50%.

Generally speaking, it is also why modern boats are lighter, faster and more uncomfortable than older boats. Unfortunately modern boats tend also to have a worst AVS and a bigger negative stability.

 

Slightly off topic perhaps, for that I'm sorry.

Where might I find statistics for boats that capsize or pitchpole while NOT racing?

Of the cases that sailboats that capsize or pitchpole while not being pushed under marginal conditions (racing), how many of them could be prevented by a decent grasp of weather, good forecasting, and a bit of seamanship?

In other words, how remote is the possibility that a cruising sailboat will be in danger of capsize?

In my mind there is a similar situation with autos. There is a large effort to increase the chances of survival *after* a crash, but little effort to improve the crash avoidance ability of either the vehicle or the driver.

Taken to extremes, that would mean that less attention would be paid to seamanship in a boat with a favorable STIX number. This is proved daily by the driving behaviour of people in "crash proof" cars.

I'm all for not selling boats that will sink or capsize on their own after little or no error is made by the crew, but rules and regulations that would force boats to be built that no idiot could capsize make no sense. Sounds like we are going to see some pretty dull boats.

 

No, it’s not about that. A Bavaria 30ft, is certified as a Class A boat, and that is the ultimate category, meaning Oceangoing boat. It has not really to do with excessive protection but with the possibility of giving to the buyer a valuable information about the seaworthiness of the boat he is interested in, and I mean the STIX number, and that one is not even compulsory on the boats catalogue (it should be in my opinion).