Sailing boats' Stability, STIX and Old Ratios

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

  1. Alfredoc
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    Alfredoc Junior Member

    the total height of the mast is 16.320 mt but to the deck the height of the mast is 14.930 mt

    C category :confused: no fear next time who i have 20 knots :D:D

    good dinner

    ciao

    Alfredo
     
  2. Guillermo
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    Guillermo Ingeniero Naval

    Alfredoc,
    Still revising my numbers, I have a doubt with the Wind Moment Factor. I'll be back to you later as I'm now leaving for some sailing (gorgeous sunny day!) :)
    Cheers.
     
  3. Alfredoc
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    Alfredoc Junior Member

    enjoy :):):)

    ....here in Milano are raining :(:(:(

    ciao

    A.
     
  4. Guillermo
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    Guillermo Ingeniero Naval

    Hi Alfredoc!
    Finally I've found some time to revise things.
    I get an estimated AVS of 116º and with this an STIX of around 32 (there was a bug in the FWM), for an Afl around 100º. This figure is right into the A category, but AVS is not enough to comply.

    As the formula to estimate AVS does not properly take into account volumes over deck, its value is probably higher (differences with the real thing can be as much as 15º). If we asume AVS as being around 120º (which complies) and Afl as 110º, which are probably more approximate figures, then STIX would be around 34-35.

    The precise STIX can only be calculated with the correct data and GZ curve.

    Cheers.
     
  5. Alfredoc
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    Alfredoc Junior Member

    Thanks a lot Guillermo that's a very good news!!!!

    Now i must take contact with JOUBERT NIVELT for a real A class certificate, i hope to find them because the e-mail adress who is in internet is old :(
    Otherwise i must go in la Rochelle :D:D:D:D

    Thanks too much again

    Ciao

    Alfredo
     
  6. Guillermo
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    Guillermo Ingeniero Naval

    Don't trust very much my STIX numbers, as they are a rough approximation only, with a lot of assumptions for the unknown data.

    Cheers.
     
  7. Guillermo
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    Guillermo Ingeniero Naval

  8. Guillermo
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    Guillermo Ingeniero Naval

    Hans Christian 48T

    I heve been discussing STIX matters lately at an spanish forum, so I have somewhat neglected this thread, the original one on the subject, because of my sparse time available.

    I originally opened this thread stating: "I post this thread to discuss Sailing boats' Stability, their STIX and the correlation with 'old' ratios and parameters."

    Well, through this thread and the spanish forum one I have had the opportunity of analyze quite a bit of boats' numbers, as well as getting some feedback from owners, designers and boatyards about their seaworthiness and behaviour. It has been, and it is yet, an interesting exercise and a learning opportunity.

    In this process I have found some nice examples of how STIX is a tricky number which may lead to incorrect assumptions about a boat's ability to cope with bad weather from the point of view of safety, if it is taken as the only information to make up our mind (which is what we do when we do not have not enough formation/information about a particular subject and a "magic" number is offered to us). We have discussed about this before in this thread, even passionately, as well as about some of such boats, usually light and fast ones.

    Today I want to bring here another example of how STIX alone can mislead our judgement, this time in a contrary case: A classic heavy and well proven seaworthy boat with a surprisingly low STIX, the Hans Christian 48T (http://www.hanschristianyachts.com/t48/48home.htm)

    [​IMG]

    Well, this pretty lady (to my taste at least), with a proven record of safe blue water sailing around the world, has an STIX of 32.5 (!!) which is barely over the limit to consider her in A Category under the RCD. Amazing! :eek:

    The reason for this is the FDS (Dynamic Stability Factor) in the STIX formula is very low (0.53 thus spoiling the rest of factors which show good figures), because the considered downflooding angle is not specially high (86º) due to her traditional low sheerline, and the GZ curve is relatively "flat" (but her RMs are pretty good because of her heavy displacement). AVS is 114º (All this in the MOC, minimum operating condition, as defined in ISO 12217. Ruled AVS for this boat is 100º)

    Here some of her excellent traditional ratios, which give us a clue of her nice behaviour at sea:

    Motion Comfort Ratio = 45,42
    Capsize Safety Factor = 1,61
    Roll acceleration = 0,05
    Stability Index = 1,17

    I want to heartly thank Jack Hall of Pantawee Marine & Hans Christian Yachts and Rick van Mourik of Rimo Yachting, their representative in Europe, for their kindness and honesty providing to me the detailed STIX calculation and stability curve. Not all boatyards are available to happily release such information, as I have checked myself.

    Cheers.
     
    Last edited: Jan 28, 2011
    2 people like this.
  9. TeddyDiver
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    TeddyDiver Gollywobbler

    That's really great of them :D
     
  10. Tad
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    Tad Boat Designer

    Guillermo.....

    I'm just curious.....is the stability curve provided for the Hans Christian based on a calculated VCG or on the results of an inclining experiment?

    I ask because I have a theory that almost all published stability curves of stock production boats are done with VCG as calculated from a base boat. Every cruising boat on earth is equipped with additional heavy items on the deck, above the deck, or in the rig. My interest is in the real VCG of "ready for sea" cruising boats.......any info is welcome.......

    Thanks
     
  11. Guillermo
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    Guillermo Ingeniero Naval

    A good question Tad. As per ISO 12217 stability for that boat has to be derived from an stability test (Class A & 12m+ length => Option 1 & Notified Body => stab test), and I asumed that. Anyway I will ask them.

    Cheers.
     
  12. fcfc
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    fcfc Senior Member

    Are you sure ?

    annex C 2.2 case c) from ISO12217-2:2002 explicitely allow for calculated VCG under some conditions.
     
  13. Guillermo
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    Guillermo Ingeniero Naval

    Yes, you're right. But in my opinion that's not the better practice and I always do the inclining experiment. That's why I asumed, perhaps naïvely, everybody does. As said, I'll ask HC Yachts.

    Best regards.
     
  14. Tad
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    Tad Boat Designer

    I don't think any naval architect will argue that an inclining is not the best way to establish actual "as built" VCG.......but the buying public do not know this.....and they do not understand the significance of load condition when the calculation or inclining is done.

    For instance on a steel cruising sailboat I recently ran stability calculations for the AVS at full load is 95 degrees and at arrival condition (10% tanks) only 83 degrees (VCG from inclining).......This is with the big rubber boat and outboard up in the davits, genoa on the furler, full batten mainsail on the boom (not hoisted), two anchors on the bow, big liferaft on the cabintop, dodger up, radar and solar panels and windmill and barbecue all in place........
     

  15. Guillermo
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    Guillermo Ingeniero Naval

    After telling the people of HC Yachts about this thread, they do not answer my e-mails any more....:(
     
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