Sailing boats' Stability, STIX and Old Ratios

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

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GuillermoIngeniero Naval

Jeff promised, a long time ago, to open a Forum about Stability issues. Because he can always transfer this thread to such a Forum, if ever created, I post this thread to discuss Sailing boats' Stability, their STIX and the correlation with 'old' ratios and parameters.

To begin with, here the numbers for two cruisers, one light and more 'modern' and the other one more conservative. 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. Let's call the boats RED and BLUE, as they appear in the article. I will consider only displacement as Mmsc (Some kind of medium load), as this is the one giving the lowest figure for STIX in both cases, which is the mandatory one to be asigned to the boat.

Let's see:

RED's BASIC DATA:
Overall Hull Length = 11,98 m
Length Waterline = 10,57 m
Flooded Buoyancy (Y/N) = N
Beam Waterline = 3,27 m
Beam = 4,1 m
Tc (HD) = 0,55 m
T = 2.38 m
Displacement MSC = 5165 kg
Displacement Max = 6190 kg
Ballast = 1300 kg (guess; does not influence any ratio or factor other than W/Disp)
Height of CE above DWL = 6,87 m
Height of CLR below DWL = 0,95 m
Angle of vanishing stability, Avs = 123 deg
Downflooding angle Afl = 137 deg
GZ at downflooding angle = -0,23 m
GZ at 90 degrees = 0,58 m
Sail Area = 71,1 sq.m
Area to flooding (Agz) = 74,46 m.deg (The one to be used in this case, as Afl >Avs)
Area to Avs = 75,6 m.deg

RED's 'OLD' RATIOS AND PARAMETERS
Length/Beam Ratio L/B = 2,75 -- being L = (Lod+Lwl)/2
Ballast/Disp Ratio W/Disp = 0,25
Displacement/Length Ratio D/L = 121,99
Sail Area/Disp. Ratio SA/D = 24,18
Power/ Disp. Ratio HP/D = 4,74 HP/ton
Hull speed HSPD = 7,89 Kn
Potential Maximum Speed PMS = 9,78 Kn
Velocity Ratio VR = 1,24
Capsize Safety Factor CSF = 2,39
Motion Comfort Ratio MCR = 15,2
Heft Ratio HF = 0,5
Angle of Vanishing Stability AVS = 116 º
Roll Period T = 1,93 Sec
Roll Acceleration Acc = 0,3 G's
Stability Index SI = 0,47
Upright Heeling Moment UHM = 19635,03 Ft*pound
Heeling Moment at 1º HM1º = 1306,88 Ft*pound
Dellenbaugh Angle DA = 15,02 º

RED's FACTORS AND STIX
Base Length Factor = 11,040
Displacement Length Factor (FDL) = 0,914
FL = 1,001
FB = 2,519
Beam Displacement Factor (FBD) = 0,815
FR = 3,066
Knockdown Recovery Factor (FKR) = 1,146
Inversion Recovery Factor (FIR) = 1,010
Dynamic Stability Factor (FDS) = 1,361
Vaw = 1,000
Wind Moment Factor (FWM) = 1,000
Downflooding Factor (FDF) = 1,522
Delta = 0
STIX = 42,546

ASIGNED DESIGN CATEGORY: A
Wave height max 7 metres (significative)
Windspeed max. Force 10

BLUE's BASIC DATA
Overall Hull Length = 12,2 m
Length Waterline = 10,52 m
Flooded Buoyancy (Y/N) = N
Beam Waterline = 3,2 m
Beam = 3,65 m
Tc (HD) = 0,80 m
T = 2,00 m
Ballast = 3600 kg (guess; does not influence any ratio or factor other than W/Disp)
Displacement MSC = 10319 kg
Displacement Max = 11769 kg
Height of CE above DWL = 6,65 m
Height of CLR below DWL = 0,8 m
Angle of vanishing stability Avs = 147 deg
Downflooding angle Afl = 114 deg
GZ at downflooding angle = 0,4 m
GZ at 90 degrees = 0,67 m
Sail Area = 72,5 sq.m
Area to flooding (Agz) = 57,42 m.deg
Area to Avs = 66,83 m.deg (The one to be used in this case, as Avs>Afl)

BLUE's 'OLD' RATIOS AND PARAMETERS.
Length/Beam Ratio L/B = 3,11 -- being L = (Lod+Lwl)/2
Ballast/Disp Ratio W/Disp = 0,35
Displacement/Length Ratio D/L = 247,21
Sail Area/Disp. Ratio SA/D = 15,54
Power/ Disp. Ratio HP/D = 2,37 HP/ton
Hull speed HSPD = 7,87 Kn
Potential Maximum Speed PMS = 8,26 Kn
Velocity Ratio VR = 1,05
Capsize Safety Factor CSF = 1,69
Motion Comfort Ratio MCR = 35,35
Heft Ratio HF = 1,24
Angle of Vanishing Stability AVS = 123 º
Roll Period T = 3,66 Sec
Roll Acceleration Acc = 0,07 G's
Stability Index SI = 1
Upright Heeling Moment UHM = 19074,33 Ft*pound
Heeling Moment at 1º HM1º = 697,53 Ft*pound
Dellenbaugh Angle DA = 27,35 º

BLUE's FACTORS AND STIX
Delta = 0
Base Length Factor = 11,080
Displacement Length Factor (FDL) = 1,033
FL = 1,001
FB = 1,781
Beam Displacement Factor (FBD) = 1,047
FR = 7,170
Knockdown Recovery Factor (FKR) = 1,508
Inversion Recovery Factor (FIR) = 1,240
Dynamic Stability Factor (FDS) = 1,040
Vaw = 1,000
Wind Moment Factor (FWM) = 1,000
Downflooding Factor (FDF) = 1,267
STIX = 52,100

ASIGNED DESIGN CATEGORY: A
Wave height max 7 metres (significative)
Windspeed max. Force 10

Well, thinking CE marking is not conceived for racing boats, but for the recreational market, where most of cruising boats are short handed and boats should look after their crews, in my humble opinion, I think it's worrying that the RED boat can be labelled as Category A, taking into consideration it has what has been commonly understood for many years, among designers, NA’s and boat owners, as 'cruel' and even dangerous ratios and parameters. The boat is too stiff by all means, with a low motion comfort ratio, too high accelerations (And so quite punishing for a short handed crew) and with a Capsize Safety Factor well above 2, widespread considered a safe limit.
BLUE boat seems to be much better suited for ocean crossings than RED, both from the point of view of 'old' ratios and the STIX.

As we can see STIX provides not enough information about the seaworthiness of a boat (It was never intended to be a clue to this, but this idea is spreading around quickly) and may even be a tricky and dangerous number. Seaworthiness is a complex matter, involving stability, all around scantlings, quality of movements, and a long etc.

I think manufacturers/designers should at least be obliged to publicize the STIX Factors and not only the number itself (Which is not even mandatory!). And even better, publish also the 'old' ratios and parameters, for the people to have a more complete view and understanding of the boat.

Rolf Eliasson himself expressed some serious concerns about the STIX number and how it finally 'came to life', in the aforementioned article. He even suggested minimum STIX for Categories A and B should be 40 and 28, instead of 32 and 23 as it is now. But even rising the level provides not enough guarantee as to define a boat as seaworthy, as we could see per numbers above.

Again in my opinion, most probably a great pressure from modern mass (and light) boats producers (and their designers) was put into the process. Those manufacturers produce very nice boats for Club racing and coastal cruising in fair weather (what most of users do) and fun to sail, but of course they want not many of their models to be obliged to be labelled as Category C, what they should be in most cases.

Long range racing is quite a different thing (with full trained crews aboard), than family oceans crossings so, in my opinion, CE Category's assigning criteria should be revised and adequate to the RCD’s own reason of existing. If racing is intended, a new special Category or Note should apply in addition to the ‘familiy’ Category.

I recommend the reading of Rolf Eliasson's article. Here a LINK to buy the magazine issue. Search for number 81. Also here a LINK to a very interesting document written before 1996 by Dr. Peter van Oossanen, another STIX father.

I attach again SailDesign's STIX spreadsheet, with a further correction to allow for downflooding angles greater than AVS.

Attached Files:

• STIX formula.xls
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Last edited: Sep 8, 2006
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GuillermoIngeniero Naval

A last correction (And I hope the very last one!) of the STIX spreadsheet.

Attached Files:

• STIX formula-R3.xls
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GuillermoIngeniero Naval

Interesting....
No comments at all?

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M&M OvendenSenior Member

Guillermo,

I'll jump in for a comment. I find all the stability discussion very interesting and have enjoyed reading most of your postings on the subject. It can be sometimes a little uncomfortable to post comments and questions as we feel we are dealing we some big boys of boat design.
So about the STIX. I find the index really interesting and quite well thought threw even though it doesn't, as you point out, on its own mean enough to decide of the seaworthiness of a boat. I don't think the problem is the index itself but more that tendency of our modern societies to want everything simplified into standards. The standards themselves are not the problem but more the habit of many to blindly rely on them. Standardization tends to deresponsibilize individuals by making them feel safe without anymore questioning. On an issue as boat stability there are way to many variables to bring it down to a simple binary answer. But most people want simple answers, they don't want to understand why a boat is considered stable and safe.
This is unfortunate because there is no perfect boat and understanding the specific characteristics of a vessel will lead to different ways to face weather and make the boat safer by taking advantage of its own abilities.
So the problem with any index is the blind confidence most people put into it as soon as some authority starts using it. Coming up with a different index for cruising boats wouldn't make much difference, a ballasted log would probably qualify. Understanding of each specification and good common sens is the best index and I agree they should be published.

Heres a question about a variable used for STIX. What is considered for down flooding angle?
Does it stop at watertight opening portholes, or the same porthole if they have deadlights, or at any possible opening? Or is it left to the designers judgment? I couldn't find a clear answer.

Cheers
Murielle

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Tim BSenior Member

STIX is intended to allow classification, it is an indication of a yacht's stability, not a Quick-Fix answer. Thinking that STIX will give a be-all-and-end-all answer is foolish. No single number can sum up any boat. They are too complex for that.

Taken to it's greatest extreme it could be argued that the RAOs and full stability data are made public. How many sailors fully understand RAOs or would be able to use them. Similarly, how many would back off well before GZmax. There are people on this forum who don't like 20 degrees of heel, let alone 70!!!

What information is divulged is largely down to the designer. And generally they get it about right.

Tim B.

PS. Will explain Downflooding later when I've had chance to look at ISO again.

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VegaSenior Member

Interesting post Guillermo, and I am with you in what concerns the need of a specific forum for stability discussions.

I agree 100% with this:

And I don't agree with this:

In my opinion you mistake comfort with safety. I agree that the Blue is a much more comfortable boat than the Red, but may I point out to you that the Red has a downflooding angle of 137º with an AVS of 123º and that the Blue has an AVS of 147º but a somewhat poor downflooding angle of only 114º.

That means that, for instance, at 116º of list the Blue is being downflooded, and that is going to have a fast and nasty repercussion in his AVS, while the Red has no problem at all.

As you know, for the calculation of the STIX number there are some factors that take into account the boat dimensions but there are others that are taken from real trials with the boat (RM and AVS) and that and the experience and savoir faire of the ones involved in the studies make it by far the best criteria to judge the Safety of a boat and I say safety and not seaworthiness, because it doesn’t take into account comfort. Of course, if you know enough you can have a better idea with more information, but I am quite sure that 99% of the buyers will not have a clue about all those numbers.

I disagree with you in what concerns the importance of the capsize screening formula.

As you know that formula does not take into account the vertical position of the CG and because obviously the Red has a Bulb in the end of a very deep Keel (draft is missing on the boat’s data) the CG of the boat will be lower than expected and if that was taken into account the Real Capsize Safety Factor would be a lot better.

I would say yes, but Blue’s downflooding angle should be revised. But that doesn’t make the Red a particularly dangerous boat. A STIX of 42.5 is not bad and my only concern is the AVS. As you know there is a big discussion about what is acceptable as an AVS for an oceangoing boat. The actual norm is a minimum of 120º. I think that it should be at least 125º and personally I would not have a boat with less than 130º.

In what concerns comfort, I would gladly give away the Blue more comfortable motion for the pleasure of sailing a fast and more exciting boat…and to shorten time passages for days…but that is a personal choice (I don’t get seasick, no matter what).

I agree with you that people should be correctly informed about the type of boat they are interested in and mostly they should try the boat.

I believe you put too much of your personal preferences when analyzing a boat. The choice is wide, as people tastes; some like comfort, some prefer speed and the excitement, but that has not necessarily to do with the safety of the boat.

I agree with you and with Rolf Eliasson that the actual parameters to define the Class A category are dangerously low, but I think that the real problem regarding safety is all the Certified Class A 30ft boats (and smaller) in the European Market. If you know little about boats and buy a boat that is government certified to go Offshore , it is normal that you think that it is safe to do so.

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GuillermoIngeniero Naval

Sensible post, Murielle.
Downflooding angle for this kind of boats is the one at wich water may enter the boat trough a non watertight opening under ISO rules. So watertight portholes or hatches under the ISO rules, even if may be left opened, shall not be considered in the calculations, but the "lowest" opening not being watertight under them. Usually one of the the top corners of the companionway is the considered downflooding point as this companionway uses not to be watertignt (This is the "lowest" point as top corners are upside down when heeled in sailing boats having a downflooding angle greater than 90º).

Relevant information on watertightness shal be looked at in EN ISO 12216:2002 "Small craft - Windows, portlights, hatches, deadlights and doors - Strength and watertightness requirements" and EN ISO 11812: 2001 "Small craft - Watertight cockpits and quick-draining cockpits"

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GuillermoIngeniero Naval

Precisely! The problem is that as people understand almost nothing about these matters, they tend to fix to a simple rule, as it couldn't be other way. And STIX is becoming dangerously a kind of 'magic' number among boatowners. What is worst, this Design Categories scheme is tending to confuse people's judgement on a boat's ability to survive bad weather, in my opinion. I think nowadays Category A means in a growing number of people minds, that the boat is able to go anywere, which is not true even under the RCD. Categories are a clue but not a 'Safety Act'. In my opinion this misunderstanding is being consistently encouraged by manufactures; and designers as well....

Taken from the "Recreational Craft Directive and Comments to the Directive Combined":
"The directive does not include any navigation or usage rules and there is no link between the design categories and any such rules; taking into account construction safety, the user is only clearly informed of what the boat was designed and built for in relation to certain parameters of significant wave heights and wind speeds."

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GuillermoIngeniero Naval

In my opinion I don't.
Comfort is just one of the factors in safety, specially important with short handed crews.

Downflooding point has been taken as being one of the the top corners of the companionway for both cases. As you point out, the BLUE boat has a much better AVS than the RED, but flooding angle is lower as the RED is beamier and lighter. This only gives the RED an advantage in the Dynamic Stability Factor (FDS) as FWM and FDF are the same (*) as well as length correction (FL), which is not actually a factor in itself but a part of the FDL factor. BLUE boat is better in all other respects except the Dynamic Stability Factor (FDS), where RED has an advantage. The reason for this is that in all remaining factors (FIR, FKR, FDL and FBD) the displacement plays an important part, and the Blue boat is the heavier of the two.

The FDS is calculated using the positive area under the righting arm curve, up to the Avs in this case (as Afl>Avs). So the entire positive area under the curve is used, which is big thanks to the wide beam. The usable positive area for the BLUE boat is limited to the downflooding angle (Afl) and this, joined to the narrower beam, gives much less area, so penalizing FDS.

But it has to be noted that, since STIX gives no consideration to the importance of the negative areas, it is fully possible to draw a boat that's wide and flat with a low center of gravity, with a GZ curve that reaches to the sky quickly, and then drops again as quickly as it rised, just passing the minimum Avs requirement, and producing a negative area almost as big as the positive one. Such a boat could still receive a category A rating, which it clearly does not merit; it would be unsafe in the conditions outlined in that category. (Rolf Eliasson)

Not exactly. RM curve is not considered to calculate STIX, but GZ curve. And this 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.

I agree 100% . But market should be educated. It's designers, NA's, manufacturers and authorities' responsibility.

Draft is not missing. You can find it as "T" for total draught and "Tc" for body draught.
And yes, RED is bulbed.
About the safety of the boat and CSF: Although symplistic, CSF gives a clue, and RED's value is quite high. And what this high value of CSF highlights, is supported in this case by the RED's much bigger negative area under the GZ curve than BLUE's (curves available at Rolf Eliasson's article), being BLUE inherently more safe than RED because of that.
On the COG's position, the RED boat, even having a bulb, has it located 5 cm above the waterline, while BLUE's is 2,5 cm under it.
As you can see, CSF seems to be quite right in this case.
(Interesting to note also that neither draught nor ballast are used, as direct input data, to calculate 'old' ratios or the STIX).

In my opinion Avs has become also one of those 'fixing factors' intended for widespread and easy understanding among not specialized people. Being important, of course, and generally speaking the higher the better, it should be used not alone but in conjunction with GZ at 90º value. A high value here is to ensure that the boat is able to right herself from a knockdown to an angle of around 90º. This is accomplished in the STIX calculation by means of the FKR factor, which (surprise!), is bigger for the BLUE boat.
Also GM at 180º or GZ max in the inverted position should be considered to have a better clue about the boat's ability to recover from a knockdown.

Yes.

I again agree 100%

Quoting Rolf Eliasson again:
"...It's always possible to add more factors (To the STIX) without disturbing the end result, which should hover around a factor of 1. I'm thinking of Stability Loss due to Speed Factor, Transverse Inertia Factor, and GZ-area Ratio Factor (positive vs. negative areas), among others...."
1000% agree!!

(*) Please note, there is a mistake in FDF factor for both boats, as it cannot be greater than 1,25. I've already corrected the spreadsheet R3 constant in my second post)

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Crag CaySenior Member

Whilst it may be possible to argue that one boat is better than another for offshore work, the RCD (as a trade agreement - and nothing else), is there to ensure that 'patently unsafe' boats don't come to market.

It is a 'trading standards' regulation, in the same way as kitchen appliances get a CE mark: It guarantees that an electric toaster, for example, with a CE mark shouldn't kill you in normal use, but it doesn't give you one iota of information about whether it makes good toast.

It's exactly the same with boats. Wind and waves of a stated size shouldn't kill you when a boat with that rated category is operated properly. It unfortunately does not indicated a good boat that Guillermo would enjoy sailing.

The laudable ambitions to harmonise trade within Europe by eliminating individual counties' specific boat design requirements, were under intense corrupting influences from the word go. Countries with boat use restrictions or licensing saw the RCD as an extension of those, marketeers saw the potential of 'high categorisation' of their products as a sales tool, and to be honest, a lot of designers and naval architects saw the rule, at worst, as an interesting intellectual exercise, and at best, an decent revenue stream by being a Notified Body.

Many of the current problems with the public's perception of the meaning of the RCD are only natural and predictable. A few of us warned that this would be the case, but our practical objections to the way it would be perceived and used in the real world were often confused with those who just objected to the whole thing on principal. At RINA conferences 10 or 12 years ago those who could see the selfish advantages in it had already got momentum. There was even completely fuzzy thinking by those who were involved in its conception. The top dog in the British marine industry's trade body kept going on about it being as 'significant as the introduction of the Plimsoll Line was to shipping', despite that being a 'restriction on use and practice', something the RCD was never intended to be, but might still become by de facto practice.

So any criticism of the RCD can only be made on the grounds that it is allowing boats that are patently dangerous to come onto the market. Not just indifferent, or undesirable or uncomfortable or misguided, but just down right lethal.

However, the (mis)use of RCD Categories as a marketing tool brings them under other legislation, namely the trade description regulations. Now a seller can only make claims for something that is truely 'fit for purpose'. This is open to a much broader interpretation and the courts in these case only have to decide whether you were miss-sold a product. If a boat proves in service to be unsatisfactory in any way for a purpose that the salesman and advertising had you believe otherwise, then you are entitled to redress. It's in this circumstance that the difference between the very restricted scope of the RCD and the much fuller implication of it given by the marketeers, would be revealed.

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GuillermoIngeniero Naval

Correcting FDF factor to its maximum of 1,25 for both boats, STIX numbers became 38,838 for RED and 51,756 for BLUE.
Sorry.

Excellent post, Crag Cay.

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VegaSenior Member

Yes, I agree but I have some doubts about this:

When a seller sells a 29ft boat with three tons, advertising that it is a certified Class A boat he is not saying that in his opinion it is an oceangoing boat, he is saying that the boat has passed the official independent RCD tests that certify the boat as capable to withstand “Wave height max 7 metres (significative) Windspeed max. Force 10”.

So, if this is not true and proves to be grossly and tragically misleading, it is not the seller that should be object of lawsuit, but the RCD. In my opinion it is not the seller that is miss-selling a product, but the RCA that is miss-certifying a product.

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SeafraSailing Nerd

Are the "old" design parameters considered with the same weight and significance as the newer STIX factors?

How is GZ calculated at various angles?

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Crag CaySenior Member

I'm not claiming the RCD is beyond reproach. In fact ABS got out of plans appraisal and the small boat market just about the same time the EU was keen to jump in. It entirely possible that the requirements of each of the Categories will have to be revised if they prove to be inadequate.

My caution was to distinguish however, between the true aims of the Directive and the way compliance is used and projected by parts of the boat building industry. I still believe there is a greater danger of people being mis-sold a boat that proves unsuitable for them, than there is to find their dissatisfaction was due to a fundamental flaw in the RCD.

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SeafraSailing Nerd

The flaw comes from people designing a boat to meet an isolated design spec without regard to the larger picture of true stability.

People buy spec sheets. And not detailed spec sheets, rather select few numbers with exagerrated importance to be "understood" by the simple and the lazy.

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