Range of positive stability

Discussion in 'Sailboats' started by Fred T, Jun 12, 2018.

  1. Fred T
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    Fred T New Member

    It seems that today’s sailboat industry, in an effort to sell boats which in plan view mimic current thinking in offshore racing boats (very broad beam carried right aft like an ironing board) has succeeded in creating a whole generation of vessels dangerously lacking seaworthiness. Am I wrong in this conclusion? Manufacturers like to talk about form stability obviating the need for heavy ballast. But doesn’t this result in boats with very limited range of positive stability, not to mention being very stable upside down?
     
  2. TANSL
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    TANSL Senior Member

    What do you mean by positive stability?
    Is that something to be avoided?. What problem does that represent?. It seems to me that in some ships it is obligatory to have positive GZ at 180 degrees
     
  3. Fred T
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    Fred T New Member

    Admittedly an amateur, so please feel free to take my observations with a grain of salt. As I understand it the range of positive stability is the distance along the GZ curve at which the curve intersects the zero line. Beyond this point the vessel has negative stability, thus will not right itself. A seaworthy sailing yacht might have a range of stability on the order of 120 degrees. I fear that the typical modern beamy sailboat, despite its “form stability” or initial resistance to heel, will have a dangerously small range of stability, though nobody seems to publish stability numbers any more.
     
  4. TANSL
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    TANSL Senior Member

  5. jehardiman
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    jehardiman Senior Member

    It is not as bad as you think (i.e. they are better than multi-hulls), basically because the boats are light. I think the bigger issue is the slope of the righting curve at the angle of vanishing stability (AVS); this is the same issue with multi-hulls. While the AVS is just one measure, it is very hard to recover from a precipitous righting curve. Today there is more emphasis on energy to capsize than righting arm, AVS, or angle of maximum stability.
     
  6. Fred T
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    Fred T New Member

    Thanks for your response. While I’m sure you’re right that the problem is worse with catamarans, it scares me to think about a “modern” monohull at sea rolling much past 50 degrees or so. The image of an ocean racer turtled sticks in my mind...
     
    Last edited: Jun 13, 2018
  7. BlueBell
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    BlueBell Ahhhhh...

    Fred T,

    I think you would really appreciate this book:

    Seaworthiness: The Forgotten Factor
    Written by: Czesław Marchaj
    in 1986
    9781888671094

    Fair winds
     
  8. Fred T
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    Fred T New Member

    Thanks FW, I really enjoy reading about boat design.
     
  9. BlueBell
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    BlueBell Ahhhhh...

  10. Dolfiman
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    Dolfiman Senior Member

    New 2018 rules for the Imoca class stability, in short :
    AVS "normal" (keel at 0°, no filed water ballast) : 127,5°
    AVS worst case (keel angle, ballast filled). : 110°
    The positive area under the stability curve shall be at least 5 times greater than the negative area.
    More details in pages 27-28 of the rules :
    http://www.sailing.org/tools/documents/ClassRulesIMOCA2018V1.0-[23695].pdf
    + this Jean Sans article in French, see the figure page 5 :
    http://sans.jean.free.fr/stab_60_FRA_JS-Bean.pdf
     
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  11. CT249
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    CT249 Senior Member

    I haven't read the Marchaj book in eons but arguably, its points can be disputed in significant ways. For example, I think it claims that the small IOR boats in the '79 Fastnet (which inspired the book) were dangerous. However, the deaths in those boats were basically caused by poor securing of equipment. The two such boats in which people were killed were both abandoned and both floated by themselves all through the storm, with hatches open. They were both recovered later. If the boats were so unsafe, they would not have floated by themselves with hatches open through a storm. The same occurred with the Carter 3/4 tonner Ariadne - she was abandoned by crew who them died and the boat was recovered later, having survived the storm with its hatches left open.

    If we are going to blame the boats for the failure of equipment associated with them, then we must also blame the more conservative, older, non-IOR boat (Ohlson 35 Flashlight) that lost two crew (IIRC) after their harnesses failed in a roll.

    I have never been able to find any numbers derived from real world experience (ie not a designer's board or a test tank) that indicates that the light, beamy boats are actually more dangerous. On the main occasions in which large numbers (ie 70-300) yachts have encountered conditions that have caused significant loss of life, the small lightweights have actually come out pretty much as well as the medium size and older boats, in terms of loss of life and of boats.

    All else being roughly equal, a lighter boat will go faster and also be at least risk of encountering rough conditions. Conditions rough enough to roll or sink a small boat are rare. Even if there is a chance that they may be more dangerous in rough conditions, this risk could easily be offset by the fact that they spend less time doing an equivalent passage and therefore have less chance of encountering such conditions.
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    What is often overlooked in these debates is the obvious elephant in the room. That being the seamanship and skill of the capt at the helm.
    This is highly subjective and thus not so easy to define, as a metric.

    I've read many accident reports where human error has been the initiate for such events, rather than the vessel and/or its ability in heavy weather.
     
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  13. jehardiman
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    jehardiman Senior Member

    Come on, I thought we went over this back in 2005 with the "mono vs multi" thread. This concept of "outrunning the weather" is patently incorrect and has been proved as such many times in the Route de Rhum. It only works with infinite sea room to lee and no time constraints. Sometime, either mono or multi, you are going to have to bash to weather to keep off a lee shore. That is what seaworthiness is about.
     
  14. CT249
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    CT249 Senior Member

    Errrr, I never said anything about "outrunning the weather".

    The point is that a boat that can complete a passage later is less exposed to bad weather, all else being equal. To use an extreme example - if one boat took 5 years to cross the Atlantic and another took 5 minutes to cross the Atlantic, the former boat would be exposed to more extreme weather.

    There seems to be no reason at all why the same principle does not apply to the difference between something like the much-maligned "Grimalkin" and the much-lauded Contessa 32. The former is faster and therefore over a season or two is surely spending less time on the ocean exposed to all the risks that entails. Whether that reduces risk more than the (allegedly) inferior inversion characteristics increases the risk is probably unknowable.

    If one looks at the database of some 500 yachts that have been in major races that have been subject to statistical analysis, one can find no real indication that the boats that Marchaj criticised are involved in more deaths and sinkings than the ones he lauded. Yes, there are many factors involved and the database is not huge, however if there is no reliable effect to be found then it appears that there is not a greatly increased risk posed by lighter boats.

    Not a single thing I said related to bashing to weather off a lee shore. However, practical experience also indicates that good light boats can bash off a lee shore very easily. As one example, in the 1998 Sydney-Hobart the 35 ft Midnight Rambler went upwind (at about 60 degrees to the true wind) at a time and place where the Bureau of Meteorology officially assessed the wind as averaging up to 50 to 60 knots and gusting 70 to 80 knots for hours on end, with extreme wave heights from 13 to 15 metres. She did it averaging several knots for about 20 hours, crossing Bass Strait.

    As Ad Hoc says, the quality of the boat is normally far less important than the quality of the sailor. As one example, how would one assess how "seaworthy" (or perhaps "bayworthy" or "harbourworthy") is a Laser? If you give it to someone who has not sailed dinghies much, it may be "unseaworthy" in 14 knots of breeze. If you give it to an experienced dinghy racer, it may be "unseaworthy" in 20 knots. If you hand it to Tom Slingsby or someone of national level, they can just go out and play in 25 knots offshore and never be at any real risk. So which set of skills does one use to assess the boat?
     

  15. BlueBell
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    BlueBell Ahhhhh...

    Sure, don't blame the equipment.
    It's always up to the Capt.
    But don't design/buy poor equipment.
     
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