Sailing boats' Stability, STIX and Old Ratios

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

  1. Vega
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    Vega Senior Member

    Finally I was able to find the proper magazines and had time to scan the data on the Swan boats.

    I have suggested these two boats to you, because I believe that the Swan46 is a more seaworthy and stable boat than the swan 45 (even if this one is a very seaworthy one).

    The GZ curves of both boats are great, among the best in their categories, but the much superior displacement of the Swan 46 will give it a bigger RM curve. And that doesn’t show on the STIX number, being the one from the 45 slightly bigger.

    Yes, I think that it should be that way, but apparently it is not. The area considered is not the one under the RM curve, but the one under the GS curve.:(

    This way, displacement is not considered and what enters the calculations instead is the Length of the boat. If I am looking at it correctly this will benefit light boats and won’t take into account the role that displacement has in boat’s stability.:mad:

    About the GZ curves, the one in French is from the 45.

    cheers
     

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  2. Guillermo
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    Guillermo Ingeniero Naval

    Thanks a lot for the curves, Paulo, although I thought we were talking about the 44 and the 45, not 45 and 46. Anyhow I'll work on those and post results with commentaries.

    About the FDS thing:
    (By the way, I said 'righting arm', so GZ curve. Read my quote again)
    As I told before, to calculate some of the STIX factors the stability curves input info are taken from the GZ curve, not the RM one. But then displacement is introduced separately, so the multiplying of this last times righting arm gives the RM. This is done this way to calculate the FKR (knowdown recovery factor) and the Vaw (steady apparent widspeed to heel the vessel to Φd when carrying full sail) necessary to calculate the FWM (wind moment factor).

    As you correctly point out, it's surprising the so-called Dynamic Stability factor seems to take not into consideration the righting moments, as dynamic stability is related to the amount of energy available, thus the area under the righting moments curve. But they use only Agz (Area under GZ curve) and LH. As the STIX fathers were knowledgeable people, there is probably a reason for that. Let's follow Eliasson's explanation on FDS:
    "The reason for not using the righting-moment curve is that size is already accounted for in LBS. In the FDS, the righting arm curve is better compared to a righting-moment curve, so as not to give too much credit to the larger yacht, which is obviously already done with the LBS, FDL and FBD factors."
    I would like to add that Working Group 22 (STIX parent group) analized a list of Category 1 sailing boats models, known for not having had stability problems through their lives, and they developed a limiting RM-Loa data curve, so the symplistic FDS formula probably was derived from there or the like. I attach again Van Oossanen paper from the times of the STIX formulation developing.

    Cheers.
     

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  3. Guillermo
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    Guillermo Ingeniero Naval

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  4. Vega
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    Vega Senior Member

    Sorry about that. You are right about that quote, but you are not right about the swan 44. I have said that I had data on the Swan 45 and 46, not on the 44, (that’s an older boat; see post 101) and the comparison that I had proposed was between the 45 and the 46, precisely because the 46 is a much heavier boat and surprisingly, has a smaller STIX than the 45.

    About those explanations about the FDS, I have my doubts, and if you compare the RM stability curves of the Swan 45 and 46 and compare them with the STIX of those boats, you are going to see what I mean.

    About the boats you have posted, the Huzar 30 seems to be a very nice boat, with an Impressive Gz curve. The Multichine is another story. Its GZ curve is average and shows a significant inverted stability and a an AVS of only 117º, that I consider too low for a 30ft.

    About the AVS and the size of the boat I consider that the MCA guys have the right policies: Smaller the boat, the bigger should be the AVS (because small boats withstand more risk of capsizing than bigger boats).

    Take a look at this MCA “Code of practice” :
     

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  5. Guillermo
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    Guillermo Ingeniero Naval

    Yes, I agree. That's why probably it would be good, following Eliasson, to introduce a GZ area Ratio Factor measuring the relation among the positive and negative areas.
     
  6. Guillermo
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    Guillermo Ingeniero Naval

    Classic boat STIX

    I have had the opportunity to do some works for "Zara of Arran", a classic sailing boat from boatbuilder (and designer?) William Boag in Largs, Ayrshire, Scotland. William Boag had a boat yard in Largs and between 1955 and 1965 he built 50+ Loch Longs yachts. There are still quite large fleets of these (you may search under Cove Sailing Club on the Clyde) and, strangely, the largest fleet down at Aldburgh in Suffolk (Thanks from here to my scottish friend Colin Whimster, who provided this info on the boatbuilder, as I've found nothing in internet).

    Data for "Zara of Arran"

    Overall Hull Length = 9 m
    Length Waterline = 6,8 m
    Flooded Buoyancy (Y/N) = N
    Beam Waterline = 2,25 m
    Beam = 2,43 m
    Total draught = 1,29 m
    Body draught = 0,64 m
    Displacement MSC = 3943 kg
    Displacement Max = 4274 kg
    Height of CE above DWL = 3,64 m
    Height of CLR below DWL = 0,57 m
    Angle of vanishing stability = 152 deg
    Downflooding angle = 92 deg
    GZ at downflooding angle = 0,556 m
    GZ at 90 degrees = 0,564 m
    Sail Area = 38,7 sq.m
    Area to flooding (Agz) = 31,19 m.deg
    Area to AVS = 46,6 m.deg


    And now, let's see her ratios and STIX based on those numbers (Measurements have been taken directly from the boat. Displacement, GZ curves and downflooding angle calculated at my office after performing an inclining experiment):

    Ratios and parameters:

    Length/Beam Ratio L/B = 3,25 (Lwl + Lh)/2*Bmax
    Displacement/Length Ratio D/L = 379,13
    Sail Area/Disp. Ratio SA/D = 14,93
    Power/ Disp. Ratio HP/D = 1,91 HP/ton
    Hull speed HSPD = 6,33 Kn
    Potential Maximum Speed PMS = 6,72 Kn
    Velocity Ratio VR = 1,06
    Capsize Safety Factor CSF = 1,51
    Motion Comfort Ratio MCR = 37,22
    Heft Ratio HF = 1,57
    Angle of Vanishing Stability AVS = 150 º (Real: 152º)
    Roll Period T = 3,58 Sec
    Roll Acceleration Acc = 0,04 G's
    Stability Index SI = 1,47 (Tender)
    Dellenbaugh Angle DA = 38,53 (Very high)

    STIX calculation:

    Base Length Factor (LBS) = 7,533
    FL = 0,927
    Displacement Length Factor (FDL) = 1,035
    FB = 1,634
    Beam Displacement Factor (FBD) = 1,076
    FR 7,893
    Knockdown Recovery Factor (FKR) = 1,572
    Inversion Recovery Factor (FIR) = 1,240
    Dynamic Stability Factor (FDS) = 0,658 (Low because of the Dfl angle. Main cause for the low STIX)
    Vaw = 0,000 (Not applicable)
    Wind Moment Factor (FWM) = 1,000
    Downflooding Factor (FDF) = 1,022
    Delta = 0

    STIX 28,928

    DESIGN CATEGORY B
    Wave height max 4 metres
    Windspeed max. Force 8

    (Note: I am not asuming hatches, portlights, windows, etc., fulfill the requirements of ISO 12216 and 12217 for category A or even B, as they most probably don't. I'm not pretending, either, to say this is a going anywhere boat. This is only an exercise.)

    The basic problem to fulfill the A category requirements, from the hull forms point of view, is that the very narrow aft sections and relatively low freeboard of the hull (compared to modern ones) provide relatively little buoyancy when heeled, so the downflooding angle (to the upper corner of the companionway) remains low. If the boat would have somewhat wider sections and an slightly higher freeboard, as to provide a downflooding angle just over 100º, as well as a longer Lwl (shorter overhangs), it would have probably come to an STIX over 32, so fulfilling Category A requirements.

    So, in line with Crag Cay's concerns, here we have an example of forms that, although having proved their seaworthiness for decades, are penalized under the STIX (Compare with the Multichine 28 posted before)

    P.S.: Interesting to note the great influence on STIX of surviving ability when flooded. If we could provide "Zara of Arran" with means to float when flooded, delta factor would be equal to 5 and so STIX would come up to 33,928 so neatly coming into Category A zone.
     

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    Last edited: Nov 15, 2006
  7. Guillermo
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    Guillermo Ingeniero Naval

    Let me quote here some thoughts from a couple of knowledgeable guys about STIX and RCD safety in general:

    "...there's a feeling within the industry that there's room for a more demanding 'blue water' design category.
    There is little to differentiate between a true water cruiser, built to look after you in a storm, and a 9.7 m (32ft) coastal cruiser which has been numerically engineered to meet the minimum requirements for Design Category A"
    (David Greening, David Greening Yachting Design, Ltd, Chichester)

    "The requirements of the RCD can be seen as a base-line on which to develop seaworthy boats. It does not completely assess overall seaworthiness. For example it does not consider in detail:
    - the strength of the masts, sails or rigging
    - the seakindliness of the hull
    - the existence, size or reliability of the engine
    - the standard to which a boat is equipped or maintained"
    (Andrew Blyth)

    From my point of view, the seakindliness should not only consider keeping low all six possible accelerations induced by the sea, as to increase comfort of the crew and avoid damage to the equipment, but also, and most important, the damping of rolling movements and the minimizing of broaching tendency, back from the design stage, which are vital for seaworthiness in rough seas.

    Andrew Blyth adds one more quality also very important: Weatherliness (Effectiveness sailing to winward)

    Additionally, on top of the exigency of a large AVS (More than 130º for preference), Andrew Blyth suggests (for category A) it should be used 1/5 as a maximum value for the GZ area Ratio Factor (Negative/Positive), as well as having a maximum inverted moment less than half of the maximum positive one.
     
    Last edited: Nov 15, 2006
  8. Guillermo
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    Guillermo Ingeniero Naval

  9. Antonio Alcalá
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    Antonio Alcalá Ocean Yachtmaster

    Here I am

    Thanks Guillermo. Yes, I visited this url since 1 year ago comparing differents monohulls and stix numbers. Another day we could tell to forum my decision of buying a Beneteau Oceanis 473.

    Best regards
     
  10. SuperPiper
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    SuperPiper Men With Little Boats . .

    Excellent Document

    About page 20 of the document is data for the etap 21i. It is a boat similar to my own micro-cruiser. The etap 21i is listed 4 times: 2x for its shallow keel and 2x for its deep keel. But the STIX rating for the deep keel version is 'C' with a heavier keel but using the Weight FL = 1816 kg as the ISO12217 Weight and again for the deep keel version is 'B' with the lighter keel but using the Weight MC = 1296 kg as the ISO12217 Weight.

    I haven't studied the document. Guillermo, can you clarify this for me? Why would the STIX calculation use the Weight MC for one calc and Weight FL for the other?
     
  11. Guillermo
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    Guillermo Ingeniero Naval

    SuperPiper:
    To say which one is the definitive STIX for a boat, calculations have to be done for the Minimum Operating Condition (MOC) and Full Load Condition (FLC), at least, and choose the lowest of the two values. And it may happen that a boat has less STIX when fully loaded than in the MOC condition.

    But I think there is a mess with this Etap 21i four posts thing. On top of what you mention, there is also something curious: there are versions reaching B category with an STIX of 26, but one of them, with the same value only reaches category C (?)
    And there is a gross mistake: that 120 STIX....

    Because of this kind of misleading info, some contradictory data and the lacking of a lot of basic info for many boats, I think this RYA list has to be taken with great care.
    Cheers.
     
  12. Guillermo
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    Guillermo Ingeniero Naval

    Another interesting comparison

    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.

    Contessa 32's stability curve presents a maximum GZ of 0,7 m at 78º and an AVS of 157º, with an irrisory negative area. D/L ratio is 310.

    "Assent" a Contessa 32 skippered by Alan Ker came out pretty well of the '79 Fastnet race, being the smallest yacht to complete the race unscathed.

    Etap 32S's curve presents a a maximum GZ of 0,6 m at 55º and the AVS is 122,5º with a significative negative area. Negative/positive areas ratio is around 0,2. D/L ratio is 185.

    If we took from the Etap 32's STIX the effect of the 5 points given by its delta parameter (because of the floatation chambers I asume it has), figure comes down to a mere 30.7, so not reaching category A requirements.

    Another cumulative proof of the weaknesses of STIX as a clue to seaworthiness.

    Here an interesting comparison between the static stability of a Contessa 32 and a Half Tonner (Probably Nicholson designed "Grimalkin"), both participating at the '79 Fastnet Race: http://www.co32.org/BOAT_SECTION/STORIES/FASTNET_STUFF/fastnet.htm
    You may find deeper comparisons between this two boats at Marchaj's book "Seaworthiness, the forgotten factor"

    Cheers.
     
    Last edited: Nov 16, 2006
  13. Vega
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    Vega Senior Member

    Guillermo, the Multichine 28 is not a Class A boat. The data provided was not verified and approved. The designer can say what he wants, but I seriously doubt that the boat has that STIX. You have verified the Data and the calculations?

    STIX favors boats that are insubmergible, and that looks right to me. You can have a boat with a great stability but if you hit a container at full speed, unless you have a metal boat, or an insubmergible one, probably you will go down (of course, by the same criteria metal boats should benefit from the same criteria and I don’t think it is the case). But you give the impression that most modern boats use that criteria for the STIX ( I mean that they are insubmergible) and that is not the case.

    Regards
     
  14. Guillermo
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    Guillermo Ingeniero Naval

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

    That's only your very active imagination...;)

    On the flotability issue, I agree with you that floatation ability is an important safety one, but I would not use it to influence the STIX number in itself. I'd rather use an special notation for boats including this feature. Something like "Category A-F" or B-F or whatever. That would be more fair to compare the STIX numbers among different boats.

    Cheers.
     

  15. Vega
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    Vega Senior Member

    Most people would think that by buying a more voluminous and apparently heavier cruising boat will have a safer boat than the sleek cruiser-racer from the same manufacturer.

    That’s not the case, at least in what regards large production sail boats. I have said that referring the Bavaria Ocean line, comparing with the normal line of Bavarias, but here we have the same case with the Jeanneau 43 line. We can see that the cruiser-racer, the Sun-Fast has a better stability curve comparing with the cruising versions. The difference in displacement is minimal and don’t have any real influence on the stability of the different versions.

    Cheers
     

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