# Sailing boats' Stability, STIX and Old Ratios

Discussion in 'Stability' started by Guillermo, Sep 3, 2006.

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### GuillermoIngeniero Naval

Beneteau Oceanis Clipper 473

Some data for this boat, with the 2.10 m keel, 3300 kg ballast version (Antonio Alcala's one):

Overall Hull Length = 14,16 m
Length Waterline = 13,35 m
Flooded Buoyancy (Y/N) = N
Beam Waterline = 3,9 m
Beam = 4,33 m
Displacement MSC = 12000 kg (estimative. Light:11500)
Height of CE above DWL = 6,47 m
Height of CLR below DWL = 0,64 m
Angle of vanishing stability = 126 deg
Downflooding angle = 110 deg (estimative)
GZ at downflooding angle = 0,323 m
GZ at 90 degrees = 0,569 m
Sail Area 114,38 = sq.m
Area to flooding (Agz) = 87,77 m.deg
Area to AVS = 91,5 m.deg

With these numbers STIX goes up to almost 60, very far away from the 48 figure provided with the info from the manufacturer. Lowering downflooding angle to 90º (not probable) and correcting GZ lever and area under GZ curve for this value, STIX comes down to around 50 (Another example of the utmost importance of this angle for the STIX calculation). We realize here the difficulty to check these calculations against the boayards' ones, as they don't provide enough data.

'Old' ratios and estimatives for her:

Length/Beam Ratio L/B = 3,14
Ballast/Disp Ratio W/Disp = 0,28
Displacement/Length Ratio D/L = 140,68
Sail Area/Disp. Ratio SA/D = 17,99
Power/ Disp. Ratio HP/D = 3,78 HP/ton (powerful engine)
Hull speed HSPD = 8,87 Kn
Potential Maximum Speed PMS = 9,73 Kn
Velocity Ratio VR = 1,18
Capsize Safety Factor CSF = 1,91
Motion Comfort Ratio MCR = 26,55 (pretty low)
Heft Ratio HF = 0,88
Angle of Vanishing Stability AVS = 115 º (Real: 126º, due to volumes over deck)
Roll Period T = 2,76 sec (better around 4,5 sec)
Roll Acceleration Acc = 0,16 G's (quite high)
Stability Index SI = 0,64 (quite low)
Dellenbaugh Angle DA = 13,56 º

Initial Metacentric height GMo = 1,40 m
Righting Arm 10º GZ10 = 0,24 m
Righting Arm 20º GZ20 = 0,44 m
Righting Arm 30º GZ30 = 0,60 m (coincides pretty well!)

So, although these last 'old' numbers are not a great deal for a boat intended to be a passagemaker, STIX goes high (relatively much higher than the other Oceanics), because of the low center of gravity (bulbed keel).

As we've seen before, STIX favours big positive areas under the GZ curve (due to low VCG in this case, as in all bulbed keelers), but does not take into account the negative-positive areas ratio (in this case over the maximum of 20% recommended by Andrew Blyth) and also, because of this and because it doesn't take into account inertias, categorises as blue water boats ones with very short rolling periods, as this one, which not only makes for uncomfortable movements, but also calls for dangerous situations when in breaking waves.

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### Antonio AlcaláOcean Yachtmaster

Is the 473 the best choice for speeding blue water cruiser?

Hello to all.After of conversations maintained between Guillermo and I, believe that serious a good idea commentaries on my decision. I will accept all and I will listen to you. After to review all the STIX, I bought Oceanis 473 because wanted a new boat, wanted to release it and I did not want a old model . But the reason for which me it buys it was, because he was the one that better quality price tapeworm, nonsingle of the market but also in relation to its STIX of all the Beneteau. By a STIX between 48-52 it is necessary to go away to nordics designs or yachts of very high cost.

Best winds

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### Antonio AlcaláOcean Yachtmaster

Another question

Do you really think Oceanis 473 is the one of beneteau models that getting a real class A ( under 50 feet)?

Best winds

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### GuillermoIngeniero Naval

Just a quick note to realize RCD is in fact influencing consumers minds (Took from IBI e-magazine):

"The RCD applies to any craft between 2.5m and 24m built or offered for sale in the EU and EEA (European Economic Area). It has been with us for a decade now, and has succeeded, according to the Department of Trade and Industry (UK), in creating a level playing field for marine manufacturers and traders across all 25 member states plus the EEA states of Norway, Iceland and Liechtenstein. It has also, says the DTI, raised the level of consumer confidence by addressing the essential aspects of a boat's design, construction and safety, evidenced by the CE Mark on the builder's plate."

The least that I can say is that consumers are trusting the RCD and, if a manufacturer categorizes a boat like A, that means, for the average consumer, it's an oceanic boat, so able to go anywhere. Risky business.

P.S: May somebody (other than myself) give Antonio an opinion on the Oceanis Clipper 473? (His post has that intention)

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### fcfcSenior Member

No. You are on error on that. The potential buyer of a category A boat knows according to RCD that his boats will be able to survive over force 8 winds and over 4 m waves. But he also knows that RCD addresses neither range, nor comfort. In any RCD papers or category delimitation, all categories are listed by wind and vawe, not range or amenities.
And simply looking quickly at the market, given the difference between catgory A boat, from mini transat 6.5m to 30m swan, he will understand what category A means.
Now, if he is a complete dumb ***, you can put a law to forbid complete dumb *** to kill themselves, that wont prevent dumb *** suicide.

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### GuillermoIngeniero Naval

No, my friend: He is induced to asume that! It is not truth at all that every single categorized A boat is able to resist over force 8 winds and over 4 m waves (7 m waves and force 10 winds as top limits).

Let's have a look at the Multichine 28 GZ curve (attached): This boat is said to have an STIX of 34.3 by its designer. So, if her structure fulfills also the requirements (not difficult to attain), most probably it will be Categorized as A (I wouldn't like to be the designer if his statements on this are not true).

But (in my humble opinion of course), the GZ curve presents an excessively big negative area, so the ability to recover after a B2 knock down is seriously threatened. With a Capsize Safety Factor of 2.1, a roll period of 2.03 sec (against a beam of 3.18 m), an Stability Index of 0.64 and a GMo around 1.39 m (so an steep angle of the curve at 180º) this boat may be posed in serious risk when encountering breaking waves of only 3-3.5 m on top of being probably uncomfortable and a crew punisher.

Are you sure the potential buyers of this boat are going to think like that? In my opinion they just will trust the RCD and the designer, that's it. And they will go happily globetrotting!

Cheers.

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### Antonio AlcaláOcean Yachtmaster

Most of buyers in my opinion are looking for yatchts with high level of security, and of course marcation CE "A". I mean most of the ocean yachtmaster of Spain. I mean too, most of ocean yachtmaster prefer a new boat and released it that find problems in old designs, wich could be safer according to classic rules, but with problems in another places of the boat. That means money and time.Moreover, most of my fellows prefer a speed cruiser with higher roll periods but with the ability of escape of low pressure systems. Speed is safe too if the helmman is equally fast. But the question is: What is in your opinion a true security number of STIX for blue water cruiser? And secondly: Is the marcation CE the right form of measuring the capacities of a yacht in heavy weather? Why boats wich are A, don´t have in any case these capacities mainly in bad weather with medium waves?
And finally are we in a great mascarade? if outside thus, we should claim before official organisms : people begin being satiated with deceits

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### VegaSenior Member

Olá António,

Your post is not big but to answer to all your questions it would be necessary a huge one .

That’s a myth. Some of the old boats are still good boats but most of them are just old and outdated designs. Most of them are slow, heavy, undercanvased boats, with old rigs (that in many cases need a complete substitution) and they have reefing systems that need the presence of a crew man at the foot of the mast, a situation that can be dangerous in troubled seas. It is true that they are normally more comfortable (because they are heavier) in a seaway.

The difference in speed between one of those old and heavy boats and a fast cruiser can be very considerable and in that case there is some truth in that. But comparing with your boat that difference will be no more than 2 knots, and I don’t think that it will be important, regarding the ability to escape low pressure systems.

Regarding the roll periods of those boats, they would be a little higher than in your boat, but not much, and that has to do with comfort and not with seaworthiness (unless you get seasick at sea). If the difference in roll period can be considerable between your boat and an old and heavy one (making the older one more comfortable), I don’t believe that you would feel a significant difference in comfort, comparing your boat with a sport cruiser, like the First 47.7, in what concerns rolling.
In this kind of (modern boats) the amount of difference in forefoot is more important regarding comfort.

As I have shown in this thread (see the comparison between the Jeanneau 44), regarding seaworthiness, normally for big production boats, and for the same range, the sports boat is not less seaworthy than the cruiser boat and sometimes it is even more. On the other hand, the superior sailing area will make it a boat more difficult to handle and a boat that will need a lot of reefing work (not that it will be more difficult to reef, just that you will need to do that a lot more times).

I would say 40, with a minimum weight of 5T and a minimum AVS of 120º.

Stix number, with a minimum AVS and a minimum weight is the best general rule they could provide to make an evaluation of sailing boats stability and seaworthiness.
Personally I find that displacement has not the importance it deserves in the STIX calculation.

The Dehler 29 is a very good boat, but with a weight of around 3T do you really think that the boat will be safe sailing in force 10 winds and 7m waves? I would like to put all the guys responsible for the actual criteria that define Class A boats, inside a Dehler 29 in those weather conditions .

Probably not, because you are a sailor, but if you where a kid buying his first cruiser boat, coming from dinghy sailing and knowing very little about the ocean or boats, why should he not believe in that official certification? And armed with that false assurance expose himself (and others) to dangerous situations that could end in tragedy?

Yes, certification of anything regarding safety should be a serious matter (and in my opinion this one is not credible).

But one thing is the certification which advises you about what is officially considered safe, another thing is the freedom of setting sail in a boat of your choice.

I am for the responsible advice, but also a defender of the individual liberty. The advice should be one that you follow or not, and should have nothing to do with the freedom and possibility of sailing in the boat of your choice to where you want .

Saludos

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### Crag CaySenior Member

Hi Vega, Guillermo et al. I really do mean to write something about the nature of the information that should be provided to the buying public with all new boats. I will post it in Guillermo's new thread about this.

However, a couple of points you made here does illustrate the limitations of the current system. Vega here has prioritised displacement in a 'seaworthy vessel' where others might put more emphasis on something else. But by lumping all these criteria into one correct 'answer' (either Category or even STIX, etc), we must all either concur what these qualities should be, or be happy to allow other 'learned people' to prescribe an answer for us.

My alternative, which I will expand on in the other thread, is to keep the information about all the qualities of the boat separate. Give the buyer details on the boat's strength, resistance to knockdown, comfort factor, weatherliness, recovery from 90 degrees, behaviour if rolled, susceptibility to crash damage, etc, etc. Then the buyer can choose where to make his own trade offs.

This system works with cars: Basic legal standards prevent total junk being placed on the market (an equivalent role to the RCD) but then the buyer is left to judge, with the help of reviews and third party standards (such as NCAP safety), which of the models offered by the manufacturers is the most suitable for their needs. Some will decide a 'safe car' has lots of air bags, while others might prefer more active safety with better handling and brakes.

If we are happy that the public can make these sorts of judgments with something as dangerous as driving, we can certainly allow them to make a similar judgment with boats. After all, sailors are still haunted by the 19 deaths in the Fastnet disaster 27 years ago, but society as a whole is not unduly troubled by the same number who have died on UK roads since yesterday.

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### Antonio AlcaláOcean Yachtmaster

Vega, we have something in common. The necessity to prioritize in the security of a yacht beginning by its same design. Active, passive security, but also design. The design is very important. Design save lives.I am convinced of it because I sail 28 years ago, and the last 8 sailing alone.
I enterely agree with you except in the STIX number and weight. For me would be 50 the right number for maximum security and 8.5 T at least.
You fall to me well, but don´t know if it is right my expression :?: in english.
Tell me more things

Best winds

A.

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### fcfcSenior Member

It depends.
I have sailed for some years. In race. I can tell you from experience that force 8, 4-5 m v=waves, in a IOR half tonner, can be very very different depending on condition.
One was a saturday afternoon race, with the usual crew of the boat. In tropical waters. Force 8 tradewinds. Heavy ocean waves. Water temp 26°c, outside temp over 30°C. All crew on deck. The memory I have tacking with 2 reefs, jib n°2 was wet, but fun. Surfing under spi was even more fun. (we did not blast the spi, and we did not put the spreaders in the water ...)

The second remind I have was also a race. A "long range" race, about 200 nm. It was in April, in britanny. The crew was an assembly crew gathered for that race. I pulled the shortest straw to be on deck at 2 AM, in the night. The deck was slippery/icy on the early morning. One member of the crew was seasick by lack of training after winter. The wind was force 8, wave 4-5m. The boat was also an IOR half tonner.
I do not have at all the same memory from this race.

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### VegaSenior Member

I agree, but unless you are a very powerful man in the “system” and have a lot of back up, the possibilities of implementing that, should be near 0.

That was possible with cars AGAINST the initial will of almost all manufacturers because people started to get conscious about it and mainly because Volvo had made a banner of the safety of their cars and were rewarded in sales. All this together and decades were needed to arrive to the comparative safety tests between several models from the same segment.

It is obvious that what you want is against the interest of the companies that sell 90% of the market sailboats.

Of course, the situation would change if a middle-sized company started a commercial sales strategy based in safety, like the one Volvo has done in the 70’s and 80’s, and if that strategy paid in sales. Anyway you would have to wait many years.

And they would have to produce safe boats for a price that would not be more than the double of the prices of their market competitors (as it is now) and, as you know, that would be difficult.

Giving the odds, I think it is much easier and effective to stick to the actual system (STIX, minimum displacement and minimum AVS), but demanding more credible and responsible criteria. I think this would be a lot easier to manage and without being perfect, would provide a base for future and more demanding improvements.

You have misunderstood me. I have not, “prioritised displacement in a 'seaworthy vessel”

You should know by now that I don’t prefer heavy boats.

What I have said was:

This does not mean that I prioritise displacement, and in fact I do not, but as you, I prioritise Righting Moment (“resistance to knockdown, recovery from 90 degrees”) and the righting moment is obtained multiplying GZ x Displacement.

I have said that the displacement has not the importance it deserves in the STIX calculation. That’s because, for the calculation of the Dynamic Stability Factor (part of the STIX formula that represents the global boat stability), it is not the area behind the RM curve (the one that measures the amount of force needed to capsize the boat) that is considered (that one is obtained multiplying the values from the GZ curve by the displacement of the boat) but the area under the GZ curve. That value, instead of being multiplied by the displacement, is multiplied by the length of the boat. The length of the boat has little to do with the boat’s stability.

I prefer light boats, but I try to be impartial and it is obvious that heavier boats have STIX numbers that don’t represent their true stability. This is the case with, for instance, the Island Packets or with the Swan 46, a boat that obviously has bigger RM values than his racing brother, the lighter 45 and this one, nonetheless has a bigger STIX number.

Take a look at Post 121, where the GZ curves of both Swans were posted and calculate the RM values for both boats. You are going to see what I mean.

See also the post 122 where Guillermo presents some justifications for this situation (quoting Eliasson).

But facts are facts and I am not the only one saying that STIX number relies too heavily on the boat’s length, other more knowledgeable persons have said the same.

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### Crag CaySenior Member

Vega - I'm sorry if I misrepresented you, and I agree with most of what you say. However, all the problems you describe are a function of trying to reduce too much information down to one number. It always amuses me that boat designers have this fascination with these complex formulae and their inherent trade offs. I suppose as we work with rating rules, handicaps, scantlings schedules and the like, on a daily basis we think this sort of thing is 'normal'. But I think it alienates ordinary boat people who don't share our love of finding the most 'efficient' way of reaching some arbitrary target value.

For all this information to be accessible to the buying public, it should be presented to them in answer to all the natural questions they have when looking for boat. The figures are available anyway, so wouldn't require more disclosure by the manufactures, but at the moment the relevancy of this information is lost when stirred together into the 'pot purri' of STIX or Categorisation.

So, as I mentioned before, we need to answer, in an accessible way, these types of questions: How strong is this boat, how stiff, how weatherly, how comfortable, how fast, how self righting, how susceptible to damage, etc?

The other problem with having one defined standard such as 'It's Cat A', is that in the short term it may lift standards but then, why would a manufacturer go to the expense of exceeding the base line value? After all, Cat A is Cat A. My 'sub sets' of information would all be open ended scales which would recognise excellence. It would let some manufactures demonstrate the particular qualities of their boat, and if the market responded, it would put pressure on the others (your Volvo syndrome). But lesser boats would not fail, as the consumer could still choose the level of prevision that suited them. In the same way plenty of people still buy cars without ABS and only one air bag. Simply raising the bar for everyone just increases costs across the board and stigmatises 'failures', which otherwise might suit some people's needs perfectly well.

My dissatisfaction with these 'trade off' formulae originated with the RORC Safety and Stability Screening Numbers. My race boat had a Base Value below the minimum required for some offshore races in Scotland. I was however allowed the maximum Adjustment Value to bring her up to compliance as I was able to impress the Measurer with various modifications we had made. However, none of these modifications actually addressed the short comings of the boat. They were 'fudges' that beat the system - but done at the behest of the safety authority. Needless to say, the safety regime I instigated as a responsible skipper, hardly relied to any meaningful degree on the 'required features'.

So I think the RCD should be left alone. As the Dept of Trade said, it's done its job in easing the hassles of selling a boat throughout all the EEA countries . It's also claimed to have began to help the consumer. So let's build on this second point by developing an information system that is accessible, meaningful, workable and allows the consumer to really make sound choices in the boats they want to buy. The consumer's needs should be the driving force for this process and not just to allow another oppurtunity for boat designers to fiddle with formulae.

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### GuillermoIngeniero Naval

YES!

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### VegaSenior Member

Fcfc, I was talking about force 10 wind and 7m waves. These conditions have nothing to do with the ones with force 8 and 4/5m waves.

With force 8 and 4m waves the Dehler would not have any problem.

A friend of mine sails regularly with a Beneteau 27 on conditions like that...solo. I have sailed some times with those conditions on my old traditional wooden 25ft sailboat.

Those conditions are the upper limit for Class B boats.

The conditions that a class A boat should be capable of handle are very different.

From the RECREATIONAL CRAFT DIRECTIVE:

ANNEX I: Essential Requirements

ESSENTIAL SAFETY REQUIREMENTS FOR THE DESIGN AND
CONSTRUCTION OF RECREATIONAL CRAFT

1. BOAT DESIGN CATEGORIES

Article 3 of the Directive : Essential Requirements

The products referred to in Article 1 (1) shall meet the essential safety, health, environmental protection and consumer protection requirements set out in Annex I.

Design Categories

A. OCEAN: Designed for extended voyages where conditions may exceed
wind force 8
(Beaufort scale) and significant wave heights of 4 m and
above
, and vessels largely self-sufficient.

B. OFFSHORE: Designed for offshore voyages where conditions up to, and
including, wind force 8
and significant wave heights up to, and including, 4m may be experienced.

The Design Category parameters are intended to define the physical conditions that might arise in any category for design evaluation….For Category A, extreme conditions apply as they reflect that a vessel engaged on a long voyage might incur any conditions ]and should be designed accordingly, excluding abnormal weather conditions e.g. hurricane."

Note: the bold marks are mine.

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