Sailing boats' Stability, STIX and Old Ratios

[Guillermo]

Another day we could tell to forum my decision of buying a Beneteau Oceanis 473.

Antonio, may I post my findings on the STIX and ratios for the 473, as to discuss them here?

Cheers.

 

[Guillermo]

Paulo,
The better stability curve of the Sun Fast vs the Sun Odissey's one is because of the lower VCG induced by the deeper and bulbed keel. This increases STIX (Seeming to be 44 for the Odissey) although we should know what the effect of the increased sail area and higher CE (both bring down the STIX when their values increase). I stiil had no time to go through the Swans comparison, but that will be most interesting, as STIX seems to be clearly favouring long waterlined boats with deep, narrow bulbed keels.

Cheers.

 

[Vega]

Why don't you ask him and let us know?
.

I don't need.

If that boat was a Class A boat and approved as such we would say so.
What he says is not that the Multichine 28 IS a Class A boat but:

"The Multichine 28, differently from most boats of this size, CAN FIT at Category A ...

According to the stability index ( STIX) ...our design Multichine 28 belongs to category A...”

Next we show a résumé of the calculation and the STIX number for the MC28.

This means that according to those calculations, the boat has a STIX of 34.3 and therefore can fit category A....if those calculations are right

 

[Guillermo]

I tend to rely on the word of reputed designers as Roberto Barros. But if you do not, well, it's up to you.
Anyhow I will ask him and bring his attention towards this thread.

Cheers.

 

[Vega]

Paulo,
The better stability curve of the Sun Fast vs the Sun Odissey's one is because of the lower VCG induced by the deeper and bulbed keel. This increases STIX (Seeming to be 44 for the Odissey) although we should know what the effect of the increased sail area and higher CE (both bring down the STIX when their values increase) I stiil had no time to go through the Swans comparison, but that will be most interesting, as STIX seems to be clearly favouring long waterlined boats with deep, narrow bulbed keels.

Guillermo, as I have said, STIX number gives you just an idea, but if I have the boat stability curve and I know the boat, I don't use STIX to make an evaluation of a boat stability or his seaworthiness.

What I have said was: "the Sun-Fast has a better stability curve comparing with the cruising versions".

And you are right, that has to do with bigger draft and bulb weight.

About: "Stix seems to be favoring ....deep...bulbed keels"

I think that's the way it should be, because they lower the CG of the boat increasing RM and stability.

About:

"STIX seems to be clearly favoring long waterlined boats "

I will agree with you, even more if you add : “in detriment of displacement”.

What you are going to find with the Swans STIX is that the much bigger displacement (and RM curve) of the 46 is not sufficiently “favored” by the STIX formula, giving a completely false idea of the real stability of both boats.

 

[Vega]

I tend to rely on the word of reputed designers as Roberto Barros. But if you do not, well, it's up to you.
Anyhow I will ask him and bring his attention towards this thread.

Cheers.

You do what you want but you know very well that a boat is only a CLASS A boat after being approved by a notified body or Classification Societies or whatever is the legal body that has the power to do so in the EC.

I was just pointing out that he didn’t say that his boat is a CLASS A boat, but that according with his calculations, the boat can be approved as a CLASS A boat.

As he says that one of those are being built in Europe (Portugal) we will soon know if the boat will be a CLASS A boat (or not).

regards

 

[Guillermo]

Contessa 32 survided the Fastnet '79 . Here some estimated numbers for her:
(based on basic data available at: http://www.jeremyrogers.co.uk/index01.html and asuming Tc as 0.65 m and heeling arm as 6.0 m)

Length/Beam Ratio L/B = 2,7 being L=(0,7*Lwl +0,3* Lh) / Bmax
Ballast/Disp Ratio W/Disp = 0,48
Displacement/Length Ratio D/L = 307,04
Sail Area/Disp. Ratio SA/D = 20,06
Sail Area/Wetted surface SA/WS = 1,85
Power/ Disp. Ratio HP/D = 2,11 HP/ton
Hull speed HSPD = 6,56 Kn
Potential Maximum Speed PMS = 7,68 Kn
Velocity Ratio VR = 1,17
Capsize Safety Factor CSF = 1,85
Motion Comfort Ratio MCR = 26,46
Heft Ratio HF = 0,97

Angle of Vanishing Stability AVS = 137 º (Real: 157º)
Roll Period T = 2,67 Sec
Roll Acceleration Acc = 0,1 G's
Stability Index SI = 0,9
Dellenbaugh Angle DA = 28,77

Initial Metacentric height GMo = 0,71 m
Righting Arm 10º RA10 = 0,12 m
Righting Arm 20º RA20 = 0,22 m
Righting Arm 30º RA30 = 0,3 m

At Jeremy Roger's site STIX is not available and I've been informed by themselves they are unable to help about it (Curious, as they still produce this boat under order)

 

[Vega]

Guillermo, the CONTESSA 32 STIX is 33 and the AVS 155.

Cheers

 

[Guillermo]

Guillermo, the CONTESSA 32 STIX is 33

It is said to be 33. I'm not so sure now. See my post #132
Cheers.

 

[Vega]

If you have a look at Contessa 32 and Etap 32S at RYA list, you'll find their STIX numbers as being 33 and 36. The 33 value for Contessa 32 is not trustable enough (maybe it is not even 32) and the Etap 32S STIX is in fact 35.7 not 36, so an additional reason not to trust the RYA list.
.

I don’t understand what you mean. If you consider only full digits (as in the RYA list) 35.7 will correctly be rounded out to 36.

The STIX published in the RYA website are not calculated by the RYA.

In the RYA list they tell you always the origin of the data, and many times in older boats it is only a RORC STIX. Not in this case, they say it is an ISO 12217 STIX and that they have got the data from the IRC.

As the IRC does not calculate ISO 12217 STIX, that means, with all probability, that they have got the data from the manufacturer. As you have said the boat is still produced and that means that it is a certified boat and all certified boats have an ISO 12217 STIX.

Cheers

 

[Guillermo]

But I asked Contessa manufactures in written and got their answer saying they are not able to help....

 

[Vega]

Some manufacturers are very reluctant to make public the stability data of their boats. With some I can understand the reason, but with others I can not understand why. It happened to me with C-yachts, regarding the 12,5M. I like the boat and I have talked personally with them. They have said that they were sorry but their policy is not to disclose those data. Quite stupid, in my opinion, because I am sure that it is a boat with a good stability curve.

It is possible that the same thing applies to the guys from Contessa. The published STIX data (RYA) come from the IRC. If someone wants to race the Contessa those data have to be delivered to them, (or have a RORC STIX calculated). I think that is the way they have got it.

 

[Guillermo]

Also posted at "Famous sinkings" thread, it deserves some comentaries here:

"The probe into the mystery of what sank the Moquini with a highly-experienced crew has uncovered a litany of procedural irregularities as well as alleged problems with construction in the keel area of the yacht."
More info at:
http://abyc.org.za/index.php/2006/09...hat_went_wrong
http://www.sundaytribune.co.za/index...icleId=3451851

The problem when adopting a narrow and deep keel for cruising is that something like this may well happen at any time. Even with a sound hull and proper keel-hull joint arrangement, if a container or the like is hit at relatively high speeds, keel may get lost, with all its consequences.

Should a boat like this be A categorized, even asuming good workmanship? (For an A category assessement, not only STIX is taken into consideration, but also structure -ISO 12215- as well as many other things).

Is it not seaworthiness... "The fitness or safety of a vessel for its intended use"? May we consider a long and narrow keel with a heavy bulb in its end, as being seaworthy from the point of view of this definition? (asuming the intended use is the RCD's one, not racing)

 

[Vega]

The problem when adopting a narrow and deep keel for cruising is that something like this may well happen at any time.

Even with a sound hull and proper keel-hull joint arrangement, if a container or the like is hit at relatively high speeds, keel may get lost, with all its consequences.

¿Should a boat like this be A categorized..?

…May we consider a long and narrow keel with a heavy bulb in its end, as being seaworthy from the point of view of this definition?

Do you really think that fin bulbed boats should not be classified as class A boats? Or that a boat like that is not safe offshore?

Following that line of thinking, GRP or fiberglass boats, even if long keeled, should not be considered fit to go offshore and should not be classified as Class A boats. If they hit a container at full speed, they would not lose the keel, but the damage on the hull would be enough to sink them, anyway. Do you think that only metal long keeled boats are fit to go offshore?

I understand your personal preference regarding sailing boats but I think that it has an unrealistic influence in your global judgement about the seaworthiness of a sailboat. But this time it is far too much .

Besides, from what you say, we are led to think, that the Technical investigators have concluded that the design of the boat (Fin bulbed Keel) was responsible for the loss of the keel, or that the boat had hit a submerged object.

They say quite the opposite. The report says :

Several "possible contributing factors" were identified, including:

• sub-standard lamination of the hull near the keel;
• insufficient inspection of the hull condition;
• insufficient torque procedure to ensure the keel was secure;
• non-conformity to design specifications, and "poor workmanship".

The investigators asked the then Durban Institute of Technology to survey the hull and this revealed alleged discrepancies between the design of the yacht and how it was built.

The DIT reported that visual inspection and wall thickness testing showed, in the area of the keel, that the yacht did not conform to design specification and "the workmanship is appallingly substandard".

".. It is unlikely the yacht hit a container . . . because there would be damage to the hull elsewhere," the DIT study found.

" The investigators said a collision with a submerged object would have caused more damage to the hull than shown by the Moquini .”

It seems clear to me that the report points the blame not to the boat design, but to the non conformity of the boat to the Design specifications and to a workmanship that they consider: “appallingly substandard".

Guillermo, given the conclusions of the report I can not see how this example could serve the purpose of proving the inadequacy of fin keeled bulb designs for offshore sailing.

 

[Guillermo]

Do you really think that fin bulbed boats should not be classified as class A boats?

I'm just asking and you are again judging my intentions, as usual.

Or that a boat like that is not safe offshore?

But here I throw you here some fresh flesh: Definitely not for an all around globetrotter.

Cheers.