# Sailing boats' Stability, STIX and Old Ratios

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

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### VegaSenior Member

Guillermo, the ROC paper you have posted is exactly the same that I quoted, the one with the “Typo or the like”, the one that says : STIX is a development of SSS, but uses more detailed stability data, including some from a righting moment curve.

If you take a look at the formulas to calculate the STIX that you have posted, taken from "Principles of Yacht Design" (by Lars Larsson and Rolf Eliasson), you will see that both GZ and RM values are used and those values can be calculated or taken from the respective curves.

A RM value is needed for the Calculation of the knockdown recovery factor (FR). The RM value is used, as part of the calculation. This is a RM value - (GZ 90º* mMsc).

You have stated correctly that: “Righting moment is …”GZ * Disp” …and because mMsc is only the displacement (kg) in minimum sailing condition, You have a GZ*Disp, therefore a RM value that can be taken directly from the RM curve.

You need also a RM value to the calculations of the Wind Moment Factor (FWM). To calculate the Vaw, (that is needed to calculate the FWM), it is used, as part of the formula, the symbol GZd and that, according to the definitions contained in the paper, is the RM at the downflooding heel angle, another value that you can take from the RM curve.

The Gz curve is also needed to calculate the FDS factor.

(I have posted and zoomed the relevant parts of the STIX formula.)

So I guess that it can be said that both are needed and that the more correct statement is:

“STIX is arguably the most sophisticated stability screening tool yet available. ….
Since June 1998 all new recreational boats sold in the EU have been required by law (the RCD*) to have undergone a stability assessment with the preferred method being the application of ISO 12217. This means that all but a very few new monohull ballasted sailing boats sold in the UK (including all of those imported) should have had a GZ / RM curve generated, their displacement and AVS determined and a STIX calculated. “

This comes from a very interesting RYA paper and is an introduction to an even more interesting book, published by them about Stability ref (G23).

http://www.rya.org.uk/NR/rdonlyres/...tabilityIntro.pdf#search="stabilityintro pdf"

But this is of little importance; the main disagreement with you is related with this statement of yours:

And this statement is not completely correct. Length or the size of the boat has nothing to do with it. Displacement has.

For two boats with a similar GZ curve, the boat with more displacement is the one with a bigger RM, the one that is able to better resist heeling, or a capsize. That’s why smaller sailboats with a heavy displacement are many times more seaworthy than much bigger lightweight displacement boats.

For a similar GZ, the lightweight boat can have the double of the LOA of the heavyweight, but if the heavy one has more displacement, it will have a bigger RM and the force needed to capsize it will be bigger.

That’s why I can not agree with this:

And that’s why I have said:

Putting it in other words and quoting that RYA paper:

“For ocean-going and offshore yachts one of the most easily seen and meaningful aspects of a GZ curve is the AVS.

But a GZ curve and its AVS are by no means the whole transverse static stability story. A boats mass (displacement) is also very important.

A lever, when multiplied by the force pushing it, becomes a moment. With a boat the lever is the GZ and the force is the boat’s mass. So by multiplying GZ by the boat’s mass gives a righting moment (RM) curve.

As the area under this curve represents the energy needed to heel the boat, then for the same GZ curve, a boat of double the mass will need twice the energy to capsize (and twice the energy to re-right after capsize).
"

http://www.rya.org.uk/NR/rdonlyres/...tabilityIntro.pdf#search="stabilityintro pdf"

Or in the words of Angermark Marin, the Architect of Malo yachts:

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

….A much better assesment 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.
….

Putting in their words, or mine, we all are saying the same thing and that is:

GZ values are only the size of ARMS at different heeling positions, and its value is given in Europe, in meters, and in the USA in feet.

A buyer, while comparing boats, will not certainly be interested in the sizes of arms (GZ values), but in the resulting moment (the force) that comes from applying a mass (displacement) to the size of that arm and those are RM values (forces), expressed in force units.

Furthermore, Guillermo, may I point out that in the parameters that you have posted for the Red and Blue boats referred by you as the 'OLD' RATIOS AND PARAMETERS, the values that are given for accessing the boat’s stability are RM values, and not GZ values.

So I don’t real understand why you have replied in this disagreeable way:

After all, it is not only in the “Old Ratio Parameters” that RM values are used to access the stability of a boat.

You can see that Dudley Dix uses RM values and RM curves in his site for giving to costumers the stability data of his designs, the same with Malo and with the Globetrotter 45 and so on.

Unfortunately not many like to publish in the internet the stability data of their boats, but that’s another story.

http://dixdesign.com/dix43sta.htm
http://dixdesign.com/cr33sta.htm
http://dixdesign.com/dix61sta.htm
http://www.maloyachts.se/Portals/0/STAB-40.PDF
http://www.sponbergyachtdesign.com/Globetrotter45.htm

I don’t understand also why you say this:

A good way of accessing the knowledge of a person on a subject, is reading what he posts about it. Everything I have said was backed up with quotations from BIG and respected names in the Sailboat industry and you have not contradicted them. Those quotations practically say the same I have said before in other words and very close words.

It seems to me that if you disagree, you have to explain why and contradict the quotations that I have posted for they mean exactly the same I have said; you haven’t done that….So I really don’t understand.

I guess that the other occasion that you say I have been “irrespetuous” to you was the one when I have strongly disagreed with you about an old boat being faster in light winds than a modern boat.

We were comparing two boats regarding their speed in lightwind situations. The old boat had a superior displacement, a bigger wetted surface, a smaller RM and a smaller sail area, than the new boat, and all these factors by a very substantial margin and you insisted that the old boat was faster.

I think that you consider "irrespetuous" someone that disagrees with you, unless he is a professional and has a formal education, no matter if you are wrong or right.

Respectfully

Vega (Paulo)

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### terhohalmeBEng Boat Technology

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### GuillermoIngeniero Naval

Not so. I consider irrespetuous people who when somebody disagrees with them (like you've done) say things as: “You don't know what you're talking about”, “you are not being honest”, "you really don’t know much about sailingboats", "you utilize fallacious arguments", "you need to be always right" and the like.....Quite respetuous indeed!

About the GZ vs RM discussion, let's see if I'm able to make myself understand, because this is becoming ridiculous: When calculating a vessel's stability what we directly get from calculus is the GZ curve and not the GM one, which may be easily drawn afterwards based on the GZ curve. So, even of course RM enter some of the STIX formulas, we do not enter RM itself as input data, but separated in its two factors, GZ and displacement. Read again carefully my posts, if you're so kind, and think twice before asuming I'm stupid, please.

I would appreciate very much you being more polite when discussing, and more careful with your interpretation of what you read. If in doubt please ask, instead of scorning and asuming a patronizing tone. And please, It will be also very nice if you could avoid trying to teach me the very basics of boat's stability in the future. I don't recognize you with that ability. Neither about STIX. Only a few posts ago you even didn't know what FL, FB, FR and Vaw were.

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### GuillermoIngeniero Naval

Thanks a lot, termohalme.
It surprises me, but I recognize I've never worked with MINI CLASS rules.
Anyhow reading carefully what is stated there: "...showing that the boat answers the requirements of the category of conception B" it may be very well that this does not imply boats have to be categorized as B, but only fullfilling its requirements (for sure except the displacement one). In fact racing only boats do not need to be categorizerd under the RCD. Could it be?

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### terhohalmeBEng Boat Technology

In mini class prototypes yes, but if you read further:

"Production boats :
Production boats launched after June 16th, 1998 must be stamped "CE"
and classified in category of conception B."

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### GuillermoIngeniero Naval

I have to find out how this can be. Do you have a clue?

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### terhohalmeBEng Boat Technology

Guillermo,
In EN ISO 12217-2 in item 6.3.1 there is a normal requirement of m>1500 kg in category B, but in item 6.3.2 is an alternative requirement without mass. So, the boat should be heavy enough with large varnish stability or a watertight floating one.

Terho

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### Claus RiepeJunior Member

Terho:
I too am just looking at 6.3.2 .
Boats do not need to be heavy to get Cat. B.

But, I am unsure about the downflooding issues that go into STIX, especially when boats have quick draining recesses only. Can there be 'downflooding' into a quick draining cockpit at all? Downflooding itself is when water floods into a hull or bilge itself through gravity. So when you have a completely sealed hull cavity with just a quick (self) draining cockpit on top of it, i.e. above the waterline level, how can 'downflooding' occur at all, as the water would never enter the hull or bilge itself?

Likewise the downflooding angle. OK, in a seriously heeling boat some water may at some angle seep in into a small part of the cockpit which may at that time and heeling angle be -temporarily- below water surface level, but that water will automatically and quickly be drained out again as the boat uprights again.

To me, 'downflooding' means water getting right into a hull structure from where it cannot drain back out -quickly- automatically through own gravity. Boats with a completely sealed hull cavity and buoyancy to spare and with just a quick draining cockpit on top should not be subject to downflooding issues at all. Correct? -Or what do you make of it?

Claus

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### GuillermoIngeniero Naval

Yes, absolutely, thanks. I didn't realize, although I posted on 6.3.2 at this same thread. See http://boatdesign.net/forums/showpost.php?p=105071&postcount=36
How stupid I am! (Getting old... )

What is surprising here is that 6.3.2 requires a lower AVS (Only 75º for Cat. B!). Basically this point is applicable to boats able to float when flooded.

From MINI Class rules:

A watertight bulkhead must be located between 5% to 15% of LWL aft of the perpendicular of the bow.
J-7 - CONSTRUCTION
J-7-a The boats must be seaworthy and watertight.
......
JJ-8 - HULL AND DECK OPENINGS
.....
J-8-b The hatch boards for the companionway and deck openings must
be watertight.

I understand the forward watertight compartment provides enough bouyancy to keep the boat afloat. Anyhow it will be a very life-threatening position to wait for rescue, if the boat is floating bows up. I mean, you should abandon boat and go into the liferaft, so what's the needing of a such a floatation for? ISO rules for swamped tests for recreational boats require a more or less level floatation when flooded. Do MINIS have extra floatation compartments to allow for this? Could you tell us more on the matter?
Cheers

Last edited: Sep 23, 2006
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### GuillermoIngeniero Naval

Claus,
From ISO 12217-2:
Definitions.
.....
3.3.1 Downflooding opening: any opening (including the edge of a recess) that may admit water into the interior or bilge of a boat, or a recess, apart from those excluded in 6.2.1
.....
6.2.1.1 The requirements given below, and in 6.2.2 and 6.2.3, shall apply to all downflooding openings, except:
a) watertight recesses with a combined volume less than LhBhFm/40, or quick draining recesses.
.....

Cheers.

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### terhohalmeBEng Boat Technology

Guillermo,

Perhaps the half sinked mini is a powerfull sea anchor for liferaft?

Can't tell, we need a mini sailor to tell us more.

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### GuillermoIngeniero Naval

Yeap. Any MINI sailor or designer out there able to help?

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### Claus RiepeJunior Member

Thank you, but that is not the tricky bit.:

You may have missed 6.4.8, last sentence: There is reference to -large- 'openings' in general, not just to the clearly defined 'downflooding openings'.

Quick draining cockpits ARE clearly excepted from being 'downflooding openings'.
But it could be argued that they are still 'openings' in the sense of 6.4.8, so that in effect they WOULD return into the calculation of Downflooding factor and STIX, from which they were -at first glance- excluded through 3.3.1 icw 6.2.2.1 a).

This could well be just an editorial lapse of the ISO, or of its translation, but I am not sure, maybe it is intentional and relevant.

What would you say?

Claus

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### GuillermoIngeniero Naval

Claus,
ISO 12217-2:2002(E) point 6.4.8, is about the calculation of the downflooding factor (FDF). Last paragraph from there:

"Φda1 is the angle of heel at which openings which are not capable of closure to tightness degree 3 of ISO 12216 having a combined total area, expressed in square millimiters (mm2), greater than the number represented by (50Lh^2) first become inmersed."

So a for a boat of Lh=12 m, all openings greater than 7200 mm2 (0.0072 m2), which's closuring device (If any) doesn't comply with required tightness degree, shall be checked for this.

Those are small openings, not big. Let's say a vent with a diameter of 96 mm, or two of 68 mm diameter flooding at the same angle, i.e. (So the word "combined" in the rule)

I'm not sure what you mean, sorry...

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### Claus RiepeJunior Member

Guillermo:
No need to be confused.
Just take it that 'Greater than' means exactly that, 'greater than'.
Greater than -say- 7.200 sq mmtrs. means any opening GREATER than 7.200 sq mmtrs..
Not -as you surmise-SMALLER than 7.200.

Why do you think 6.4.8 applies for 'small openings' only? The opposite is true.

Claus

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