ISAF OSR Cat_0 Self-righting sailboat?

Discussion in 'Stability' started by Joakem, Feb 11, 2016.

  1. Joakem
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    Joakem Junior Member

    Hi,

    The ISAF sailing organisation specifies regulations for sailboats at sea.
    These are a.o. given in the Offshore Special Regulations.
    See: http://www.sailing.org/documents/offshorespecialregs/index.php

    For the most severe class (category 0) it specifies in 3.04.3 that it must be:
    "Capable of self-righting from an inverted position with or without reasonable intervention from the crew and independend of the condition of the rig"

    As I read this, a sailboat with a fixed (non-canting) keel must have an angle of vanishing stability of 180°. This seams very harsh to me and in contradiction of 3.04.2 AVS>100.

    Can someone help me out on the ISAF-accepted interpretation of this requirement?

    Thanks!
     
  2. TANSL
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    TANSL Senior Member

    I guess there is no contradiction. Some ships have to meet certain requirements and, for others, the requirements are higher.
     
  3. Joakem
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    Joakem Junior Member

    TANSL wrote:

    I guess there is no contradiction. Some ships have to meet certain requirements and, for others, the requirements are higher.

    That may be so.
    But than all world-racers with a fixed keel have an angle of vanishing stability of 180°.

    I'm doubtfull....
    Does anyone have actual knowledge about this?

    Much apreciated.
     
  4. gonzo
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    gonzo Senior Member

    It means that if it capsizes completely (the mast pointing straight down), the boat must not be stable. Some boats with extreme large beams will float upside down and won't recover. The rule is to make sure that when a boat capsizes it will be statically unstable and tend to right itself.
     
  5. TANSL
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    TANSL Senior Member

    What the rule says, nothing more and clearly explained, is that the righting arm to 180 degrees, must be positive. that does not mean at all that is self righting the ship since the hull rotated 180 degrees, with the sail secured to the mast, not easy to get it self-righting.
    In addition, the righting arm, as we all know, not only depends on the shapes of the ship.
     
  6. Joakem
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    Joakem Junior Member

    Plank-on-edge lead-sled?

    Thanks Gonzo, thanks TANSL,

    So you both effectively say that the GZ-curve must remain positive from 0 to 180 degrees heel?
    That is dificult for modernly proportioned racers.
    Which would mean that the new rule effectively prescribes either canting keels, or a return to the old-fashioned plank-on-edge lead-sleds. :confused:
    A curious developpement, does anyone know the rationale behind it?
    Regards
     
  7. TANSL
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    TANSL Senior Member

    Gonzo seems to know the rule, and so he explains what the rule is intended, but I do not know the rule. Therefore, I do not understand or justify it. I just wanted to clarify that a sailboat with a vanising stability angle of 180° does not have to be self-righting.
    I guess in this case, as in many others, security is preferred to other parameters.
     
  8. Stumble
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    Stumble Senior Member

    No the boat doesn't have to have an AVS of 180, it just requires the boat to have zero stability at 180.

    Chanting keels have special rules, but they have a secret weapon, they can can't the keel over to one side and almost all of them will immediatly self right in any conditions, even if they flip with the keel perfectly verticle.
     
  9. TANSL
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    TANSL Senior Member

    Forgive me but I think that "have zero stability at 180 degrees" does not mean anything. Usually when we talk about this issue, one refers to the value of the righting arm, which should be positive at 180 degrees. The angle at which the righting arm vanishes is called "AVS".
     
  10. gonzo
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    gonzo Senior Member

    Zero stability = AVS. That is when the righting arm vanishes.
     
  11. TANSL
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    TANSL Senior Member

    No Gonzo, zero stability is not equal to AVS. Zero stability may mean that the righting arm is zero. The righting arm is a measure of length while the AVS is an angle. Therefore, your equation is inhomogeneous in terms of units of both members. You're saying "pears = apples" and that you and I know that is not possible.
     
  12. gonzo
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    gonzo Senior Member

    How much stability do you claim there is at the AVS?
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    You may wish to have a read of the post here, that i explained on the basics of stability. It may assist you.

    AVS = as noted angle of vanishing stability. All this means is that the intersection of your GZ curve with the x-axis, or simply put the GZ = 0.00. This becomes the range of 'positive' stability. Beyond this it is known as negative stability.

    As Gonzo correct notes, the vessel must self right. Thus if the mast is pointing down, 180 degrees, any disturbance shall create a self right moment so the mast is no longer pointing down. Thus the vessel is not statically stable at 180 degree as shown in the GZ cruve 3rd image of my link, and to be more like the GZ curve first image in the link.

    Trust this helps.
     
  14. Stumble
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    Stumble Senior Member

    To the extent this is a real issue I would also point to 3.04.4 that indicates if a vessel is not capable of self righting from any position, then a minimum AVS of 130-.002*m islso acceptable.

    In effect the wording is easier to explain as requiring a minimum AVS of 130, unless the vessel is capable of self righting from a capsize. So a casting keel vessel that has a AVS of 100 degrees with the keel pinned in the center is allowed to race if she can self right by canting the keel and righting that way instead.
     

  15. PAR
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    PAR Yacht Designer/Builder

    WSOSR 3.04.1 -3.04.3 pretty much acknowledges the limitations of a reasonable AVS at OCR 120 degrees, STIX 130 degrees, or IRC SSS Base of 35 degrees or more for Cat. A (or better) racers. As a general rule, craft designed to meet these regulations don't go full turtle (reality), unless forcibly rolled, being too unstable from 140+ degrees or so, to naturally remain in this position.

    Simply put, in the typical sea state condisions that can cause a severe capsize/roll, the next few wave sets often can upset them enough, so they lie on their beam ends, typically at less than 120 degrees, so they self right, because of the sea state motion. This coupled with what really happens in a severe rolling, means the boat will have lost much, if not all of it's rig, which can improve it's ability to stand back up. Of course, they're not counting on losing the rig to permit self righting, but reality and rules committees do generally come to understanding in this regard, which is why the 120 degree AVS is now required (relatively new change).
     
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