Angle of Vanishing stability.

Discussion in 'Stability' started by Mychael, Aug 19, 2006.

  1. MikeJohns
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    MikeJohns Senior Member

    Great diatribe but you missed the point.

    I was talking about designers published stability curves and the effect that typical additions that you'll find on any cruiser have on that stability .

    A light boats COG very quickly moves up with typical additional gear and stores. In comparison heavy boats can take a much greater load before the COG shifts significantly. Just a simple relationship that anyone who has completed a few weights and moments calcs could tell you.

    Your character with his additions might just survive in a heavy displacement craft , but in a light weight the same character is doomed.......
     
  2. Crag Cay
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    Crag Cay Senior Member

    Thanks for the patronising comment. However, displacement and CoG are only part of the picture when it comes to Limits of Vanishing Stability. The breakthrough in lifeboat design in the 70's was when we fully grasped this.

    My 'diatribe' was purely a warning to those who may draw comfort from having heavy displacement - it may mask problems in stability, but it wil never correct them.

    Modelling only tells you so much. I think it would be very interesting to take some popular boats of typical design down to Poole, load them with weights to compensate for crew, half tanks, cabin sole awash and plenty of loose gear below, and use the crane to flip them upside down. If it's the only way to really prove designs for the RNLI, why not 'blue water' pleasure boats as well. It would make great publicity material. Ah..... if they came up.
     
  3. MikeJohns
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    MikeJohns Senior Member

    Where do you draw the line between patronising and educational?
    If you are not a novice then I guess it is patronising but by your post you did not appear that knowledgable sorry.

    Weights and moments calcs are often a real eye-opener for the novice designer and you get a very good feel for just how the COG and COB relate. With light boats it's alarming to see just how much the COG shifts with a bit of weight on deck ....then back into the design spiral.


    Be sure not to confuse dynamic stability with static vanishing stability. The mathematical models are very accurate at predicting statics.
     
  4. Guillermo
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    Guillermo Ingeniero Naval

    Well, they are. At least for professional ones.
     
  5. Guillermo
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    Guillermo Ingeniero Naval

    Selfrighting test is mandatory for life saving boats and it tests not only the selfrighting ability, which can be quite accurately demonstrated by calculus only, but also, and no less important, watertightness, structural integrity and the ability of engines and equipment to survive the inverted position. Very specifically the ability of engines going iddle when inverted, without stopping or over-revving.
     
  6. Mychael
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    Mychael Mychael

    It would be good if every yacht designer gave a claer and open report on the AVS of a particular design. The more information available for a buyer the better he is able to make and informed and (correct for him) choice.

    Mychael
     
  7. Guillermo
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    Guillermo Ingeniero Naval

    I'm not so sure. I deal with professional boat owners and can tell you most (And I mean MOST) of them ever look at the stability booklets.
    And from what I know about the recreational market, MOST owners don't even know what are you talking about.
     
  8. hiracer
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    hiracer Senior Member

    Agreed, but many of us are capable of some level of discernment, if only the information were available. It's not. And that's a problem in itself. Let's not confuse problems.

    If various AVSs depending on load conditions were available for each boat model, eventually the recreational market, or some segment of it, would get the point about COG. Cocktail discussions would be different. It's the chicken-and-the-egg thing. In this case, I think the information is needed first, before buyers will slowly acclimate themselves to it.
     
  9. Mychael
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    Mychael Mychael

    I agree with Hiracer. I learnt to ask questions about sailing as I went along. I also knew to ask a lot of questions before I ever even got started, I'm not any sort of expert but I know enough to know I need to know certain things.
    Some of the information I'd like to have is just simply not available and it should be.
    I also agree with Guillermo in that a lot of owners don't know what you are talking about. That's just the new "breed" of person and is the same with boats as it has been with motor cars like some drivers cannot even operate a manual car. But the education and information needs to be there for those that seek it.
    To highlight this "non-knowledge" boater type. I was in a discussion with a "smelly boat" (power boat ) owner at a course I was doing. He had just purchased a new power boat (probablly around 20odd foot) with all the latest gizmo's. He was not into fishing, just "getting out in the boat" . In any case I had to explain to him what I owned and his reply to me was "I'll probably get a sailboat when I'm older and want to take it easy". He had no concept of what is actually involved in sailing. He was much offended when I replied that "yachties" say the same thing about buying a power boat when they get a bit too old for hauling up sails and stuff.

    Mychael
     
  10. M&M Ovenden
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    M&M Ovenden Senior Member

    I'm not too sure what to think of that godliness of the AVS. It doesn't mean enough to me on it's own. A boat could have great capability to right itself up, if it's tender and keeps rolling, how good is it?
    If a boat is sold as having an AVS of 145 degrees, it doesn't seem enough to decide it will be a good ocean going boat as it also has to maintain a comfortable motion. Why are conversations on a boat's stability always led buy how good it is at coming back up and never by how good it is at staying up?
    I understand the importance to withstand a knockdown but I find good stability info should say more about a boat.

    Murielle
     
  11. lazeyjack

    lazeyjack Guest

    fair enough, stability can mean many things, at anchor, downwind, directional,
    After you,ve been involved for donkeys years you kinda get a feel for how"stable" a boat is
    How the lines are, how they will effect the boat upwind, downwind, in a quartering sea
    I think what people are just saying here is, that a stiff boat beats a tender one? Take a look at my gallery and guess the AVS ? LOOKS TENDER? LOOKS high wooded, well try get the rail down!!
     
  12. Mychael
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    Mychael Mychael

    And to go the full circle of this thread.Just saying that really a lot more information should be made available by the designers/builders of boats.
    Then those that want to know can know and those that can make sense of the data are able to make a good informed decision, the rest can make their choice from the glossy brochures.

    Mychael
     
  13. hiracer
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    hiracer Senior Member

    Too funny; too true.
     
  14. Guillermo
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    Guillermo Ingeniero Naval

    Of course I absolutely agree. But maybe some wouldn't be able to sell their boats.....:rolleyes:
     

  15. Mychael
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    Mychael Mychael

    Then it would be like cars, market competition. We'd benifit in the end as all designers would be forced to do it smarter/better to stay in the race.


    Mychael
     
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