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

    Hey Luso, welcome (are you really a Luso descendent?)

    You don't give me all the data that it is needed to make an approximate calculation of the AVS or LPS . The ballast is needed.
    But I will give you the means to perform an approximated calculation.

    Read this page for definitions:

    http://dan.pfeiffer.net/boat/ratios.htm

    "The limit of Positive stability (LPS) is the roll angle at which a boat will no longer right itself and become inverted (capsized)".

    and this one for understanding the calculation and error margins:

    http://dan.pfeiffer.net/10m/avs_calculation.htm

    And here you can perform directly the calculations, using a calculator:

    http://www.sailingusa.info/cal__avs.htm


    Cheers
     
  2. Guillermo
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    Guillermo Ingeniero Naval

    luso:
    If you have a serious concern about her stability, the best thing you can do is to hire the services of an NA to perform a proper stabiility experience and then, armed with body lines (do you have them?), general arrangement plan, sails plan and tanks capacities tables, do all calculations needed. All approximate methods are that: approximate; and some times grossly inacurate.
    Anyhow if you provide me with the following:
    Lwl, Bwl (if known), medium load displacement, ballast, hull body draught, total draught, sail area (only the triangles, not the full size of the sails) and the heeling arm, this is the vertical distance between sails center of effort and the lateral center of resistance of the hull (your best estimative), I can provide you with the following estimated ratios, parameters and stability data:

    Length/Beam Ratio
    Ballast/Disp Ratio
    Displacement/Length Ratio
    Sail Area/Disp. Ratio
    Hull speed
    Potential Maximum Speed
    Velocity Ratio
    Capsize Safety Factor
    Motion Comfort Ratio
    Heft Ratio
    Angle of Vanishing Stability
    Roll Period (You can easily check this by yourself)
    Roll Acceleration
    Stability Index
    Upright Heeling Moment
    Heeling Moment at 1º
    Dellenbaugh Angle
    Initial Metacentric height
    Righting Arm 10º
    Righting Arm 20º
    Righting Arm 30º
    Vertical COG position

    As said before, results may or may be not be accurate, but at least will give you a clue.

    Also photos and drawings from her would be of great help to better estimate some measurements.
    Cheers.
    Guillermo
     
  3. Guillermo
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    Guillermo Ingeniero Naval

    Paulo,
    From my point of view (I've stated this before), comfort is an important component of seaworthiness for cruising boats. A tired crew may come into dangerous situations, and we are talking here about the RCD, which is intended for the recreational market, not the racing one.
    A proper globetrotter cruiser under category "A", should look after her crew, in my opinion, being the quality of motion (comfort) an important issue. In fact, as quality of motion depends greatly on beam (not too big) and displacement (the higher the better, generally speaking), those greatly influence the STIX, making it higher if properly dimensioned.

    I'd appreciate very much if you could provide me with the stability curves of the boats you listed in your last post (Do those curves indicate downflooding angle?). Do you have them? Could I get them from you? The rest of info needed to perform a full approximated STIX calculation for those boats (to analize the single factors), I hope I can get it from internet, as well as I can estimate Hce and Hlp, from the comercial drawings (But if you can provide also those, the better!).
    Thanks in advance!
    Cheers.
     
  4. luso
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    luso New Member

    Hello Guys .
    Thank you for your interest in our delema.
    I've been to those sites Vega suggested and by those calculations I did I came with the stability value of 224.59 and the angle of vanishing stability of 111.86. What do does those values mean [ if they are correct]?
    Just in case you guys have the time to help me I have some mesurements of the boat.
    LOA= 14,93 MTS
    LWL= 13,12 MTS.
    B= 4,60 "
    BWL= 4,06 "
    DWL= 2,00 "
    MAX GROSS WEIGTH = 23,000 KG
    BALLAST= 7,000 KG.
    HULL SPEED = 8 TO 9 KNOTS
    L/B = 3,24 MTS.
    LWL/BW = 3,23 MTS.
    SAIL AREA+ 1,200FT SQ. [ I = 23 MTS. J=5.5MTS. P=16 MTS. E= 4MTS.]
    HULL DRAFT AT BEAM OF THE HULL IS = 1 METER
    RUB RAIL IS = 1,30 MTS.
    I will try to insert some photos .
    thank you
     

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  5. Guillermo
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    Guillermo Ingeniero Naval

    luso:
    I'm not sure about that 224.59 figure meaning. The +/- 112 AVS is, of course, an estimated one and seems low for an oceangoing cruiser. As your boat does not have big superstructures, probably the real AVS will not be far away from that. Anyhow it's important to know what you mean with 'Max Gross Weight'. Is it the estimated full load displacement? Or is it the weight of the finished construction, but without fuel, water, storages, crew, etc? (What is called the lightship condition). Please clarify this.
    Also: In the sail area you include both mast's sails? If yes, Could you provide the areas separately?
    Tonight I'll work at home on the numbers you provide and be back to you.
    Cheers.
     
  6. Guillermo
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    Guillermo Ingeniero Naval

  7. Vega
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    Vega Senior Member

    Luso, why don't you ask the correct numbers to Mr Caroff?

    After 30 years in business, he is still producing some nice work, like the Atlantis.

    http://caroffduflos.free.fr/

    Your hull seems to be a nice one.

    About those numbers, like Guillermo, I don't know how you have got what you call "stability value" and therefore I don't know what it means.

    The 112º AVS means that the boat could lie down on its side (at a 90º angle) and still flip back. The boat will not come back only when it roll further than 112º.

    Of course, it will not be exactly like that, because in the calculation you have made, the weight of the masts and sails is not taken into account, and that calculation is only an approximated one anyway.

    If I was you I would e-mail Mr. Caroff.:)

    cheers
     
  8. luso
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    luso New Member

    The calculations I came up with are from the site Vega suggested
    http://www.sailingusa.info/cal__avs.htm.
    In regards to the sails, the plan is 325 square feet per mast + 1200 for the genoa and 110 for the yankee. Today a wrote to Carof to confirm the displacement number. Hopefully he will respond soon. I'll let you know what he says. Thanks, Luso
     
  9. Guillermo
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    Guillermo Ingeniero Naval

    Here again the STIX calculator with an slight improvement. I'll also upload it at the Library pages.
    Cheers.
     

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  10. Guillermo
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    Guillermo Ingeniero Naval

    luso,
    With the data you provide and assuming a 127 sqm area (main mast triangles plus a mizzen 32 sqm), I get the following numbers:

    Ballast/Disp Ratio W/Disp = 0,3
    Displacement/Length Ratio D/L = 284,06
    Sail Area/Disp. Ratio SA/D = 15,99 (nice)
    Hull speed HSPD = 8,79 Kn
    Potential Maximum Speed PMS = 9,1 Kn
    Velocity Ratio VR = 1,04
    Capsize Safety Factor CSF = 1,63 (nice!)
    Motion Comfort Ratio MCR = 46,7 (nice!)
    Heft Ratio HF = 1,42 (nice!)
    Angle of Vanishing Stability AVS = 118 º (low, but better than 112º)
    Roll Period T = 4,62 Sec (nice!)
    Roll Acceleration Acc = 0,06 G's (nice!)
    Stability Index SI = 1 (nice!)

    So it seems to be a safe, fast and with comfortable movements boat, pretty able for ocean passages with short crews. The only concern is the low AVS number, although probably the real number may be higher. Interesting to know numbers from the designer.
    Cheers.
     
  11. Vega
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    Vega Senior Member

    Guillermo, probably the deck-saloon configuration will increase the AVS to something like 130º, and that is not a bad value.

    About the STIX calculator, are you using the Formula that was given by Larson and Eliason or the one given by Miller?

    I know, the Formula is just one, but both sources are credible and the first expression (for boats with > 10m) is different:

    (7+2.25XLbs) and (8+2.2XLbs)

    I don't know what is the right one (probably one was an initial experiment and the other the definitive version and the one in use). I will need sometime to find out the correct one. Perhaps you can check it out for me on the official papers that define Iso 12217 ?
     
  12. Guillermo
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    Guillermo Ingeniero Naval

    Downflooding angle influence. Case study: Bavaria 37

    I've taken some time to study the influence that the downflooding angle (which's info is almost always neglected from manufactures) has in the STIX number, by means of a 'case study': The new Bavaria 37.
    I've taken data from PBO magazine, October 2006 issue and worked on the stability curve there (I asume it's drawn for the stated displacement of 6900 kg, probably Mmsc, although this last is not clarified at the article).
    Downflooding angle is not stated in the article.
    I've also digitalized the profile and estimated the body draught, as well as Hce (6.635 m)and Hlp (0.528 m) heights over and under floatation, respectively.
    Armed with those numbers I get:

    Downflooding angle ---- STIX --- Dynamic Stability Factor ---Downflooding Factor

    85º ----- 31,372 --- 0,873 --- 0,944
    90º ----- 33,143 --- 0,920 --- 1,000
    95º ----- 34,788 --- 0,960 --- 1,056
    100º ---- 36,446 --- 1,002 --- 1,111
    105º ---- 37,763 --- 1,024 --- 1,167
    110º ---- 39,046 --- 1,045 --- 1,222

    So, going from 85º to 115º (extrapolating), the boat happens to come from not even being categorizable under "A" category, to an STIX most probably over 40, what Eliasson understands as a more desirable lower limit to STIX. This brings again some worries to me about the real meaning of STIX alone to categorize the seaworthiness of a boat. It may indicate that a boat with no great seaworthiness at all, could be well brought into the A category just adding up watertight volumes upwards.

    As we can see the downflooding angle has a major influence on STIX, but manufactures neglect this information. A pity. And maybe not only a pity, because as we saw some posts ago, there are three angles to be considered to determine which one is the downflooding angle, being not necessarily one of the companionway's corners.

    Note: ISO 12217 asks for a minimum AVS of 116,2º (Not very high) for this boat to be categorizable under A category. Her stated AVS is 123º.

    And now the 'Old' ratios and parameters for her:

    Ballast/Disp Ratio W/Disp = 0,29
    Displacement/Length Ratio D/L = 203,23
    Sail Area/Disp. Ratio SA/D = 16,67
    Hull speed HSPD = 7,61 Kn
    Potential Maximum Speed PMS = 8,26 Kn
    Velocity Ratio VR = 1,09
    Capsize Safety Factor CSF = 2,01 (Slightly over 2)
    Motion Comfort Ratio MCR = 24,13
    Heft Ratio HF = 0,83 (being values over 1 desirable)
    Angle of Vanishing Stability AVS = 115 º (Real: 123º)
    Roll Period T = 2,41 Sec (Low for a beam of 3.8 m)
    Roll Acceleration Acc = 0,17 G's (0.06 being considered as ideal for ocean-goers)
    Stability Index SI = 0,64 (1 to 1.1 being desirable)

    Because of this AVS and the minimum one asked for by ISO 12217, it seems like if the AVS of 123º is reached by the added volumes of the deck (most probably). Once again: Adding watertight volumes upwards (generally speaking) with no doubt increases the probability of surviving a capsize, certainly, but should we consider a boat as seaworthy only because of this? Not at all, in my opinion.

    It's interesting to reproduce some of David Harding's (PBO tester) statements on this boat:
    "...In these far-from-testing conditions (shifty 14 knots) we stablished that the boat would work her way up to about 6.5 knots on the wind provided she was sailed fairly free: trying to point rather than foot saw the speed plummet, despite the totally flat water."
    "...the 37's beamy, high-volume hull makes the steering highly sensitive to the angle of heel. We needed about half the lock on the wheel to keep her on track in the gusts...."
    "The new 37 does what you expect a Bavaria to do, only more so. She's big and remarkably inexpensive....though only time will tell how the current generation of Bavarias et al stand up to the effects of anno Domini."
     

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    Last edited: Oct 26, 2006
  13. Guillermo
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    Guillermo Ingeniero Naval

    Paulo,
    At ISO 12217-2:2002 currently in force it only appears that factor as being 7+2.25*LBS. Probably the other factor, conceived for boats with LBS under 10, was abandoned in the ISO 12217 validation process.
     
  14. luso
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    luso New Member

    Guilhermo thank you so much for the calculations. Does the AVS mean the angle that the boat can heel and right itself up? What happens if the boat passes that angle? will it remain inverted? If you don't mind me asking also what does capsize safety factor and motion confort mean? I hope I am not picking your brain to much but your knowledge is greatly appreciated. Thank you!
     

  15. luso
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    luso New Member

    I was reading one of your messages where you mention the size of the sails. We have two equal size masts. (50ft each). The measurements are as followed:eek:ne of the booms is 12 1/2ft and the other is 12ft. Both boom tracks go from 48 to 62 inches above deck (we were thinking to raise both boom tracks to 6ft so we can have a clearance under the boom of six feet. Is that a good idea?) We also have a hack on jib that we where thinking about replacing it with a roller furling and on the front inner stay we have a yankee hank on that we are thinking of also replacing it with a furling. Does that change anything regarding stability? Thanks!
     
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