self righting

Discussion in 'Sailboats' started by aiix, Aug 11, 2009.

  1. Brent Swain
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    Brent Swain Member

    Not including the cabin would make the calculations grossly inaccurate, as the buoyancy in a cabin can add thousands of foot pounds of righting moment to an inverted hull. The wheelhouse on my 31 footer, in inverted position, has enough buoyancy to add the equivalent righting moment of adding 3,000 lbs of ballast to her keels. Round the world racers have been drastically improving the self righting ability of their boats by simply adding a bit of extra camber to their decks.
    Of course, this depends on totally watertight hatches, which no modern boat should go to sea without.
    Brent
     
  2. lat 64
    Joined: Aug 2009
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    lat 64 Junior Member

    It seems a little drastic to careen my old bargain boat until it tips over just to see if will pop up again. So.. I've been wondering, if I built a model correct in hull shape, ballast ratio, and ballast placement, can I get useful information about capsize and recovery from it?

    Do any designers use tank tests for this?

    Just itching for a reason to build a model,
    Russ
     
  3. Paul Kotzebue

    Paul Kotzebue Previous Member

    You can get stability information from capsizing a scale model. However you need to consider the laws of mechanical similitude:

    Displacement will vary as the cube of the scale factor.
    Heeling moment will vary as the cube of the scale factor.
    Righting moment will vary as the fourth power of the scale factor.

    The angle of vanishing stability will be the same on the model as it is for the full size vessel, provided the model center of gravity is in the correct location. If you want to do an accurate capsizing test of the model you will need to know the exact location of the VCG of the full size boat .... which is done by inclining the boat a few degrees and doing calculations based on the hull form. If you can do that there is no point in building the model since you have all the information required to find the stability characteristics by calculation. Knowing the ballast ratio and ballast placement is not enough, you need to know where the center of gravity is.

    Model tests aren't used to predict static stability because it is much easier and probably more accurate to use a computer program to calculate righting arms. You do need an accurate "computer model" of the hull to run the calculations. Model tests are used to predict seakeeping behavior in waves.

    Another point I should make is that too much emphasis can be put on the angle of vanishing stability, or AVS. It is possible for a boat to have a relatively high AVS and still be tender and unseaworthy.
     
  4. Brent Swain
    Joined: Mar 2002
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    Location: British Columbia

    Brent Swain Member

    Build the model and try it out, keeping in mind Paul's point, that you have to keep the VCG the same as the full sized boat for capsize tests to be accurate. Given that almost all of the heavy stowage will be below the vcg, a full sized yacht that has been lived in for a while will have much greater ultimate stability , as long as the lockers stay full when the boat is capsized, and have adequate latches to keep them closed.
    A model gives a great demonstration of the huge improvement in ultimate stability that the buoyancy of a wheelhouse gives a boat.
    A model also gives a very accurate double check of other calculations, like LCG, displacement and aesthetics.
    Brent
     
  5. dskira

    dskira Previous Member

    This is the secret of you success.
    Nothing to add to that.
    Cheers
    Daniel
     
  6. lat 64
    Joined: Aug 2009
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    lat 64 Junior Member

    Thanks for the reply. I think I highjacked this thread.

    I was thinking that "correct in ballast placement" covered keeping VCG correct.

    I suppose if my model wheel house or deck were too thick and heavy this would put me off a bit. I guessed that the effects were scaleable, but my results would only be as accurate as my model.

    I think I could try to do the inclining on both the model and the real boat and that would let me know how far off my model is... maybe.

    "Righting moment will vary as the fourth power of the scale factor."
    Yes I will research this one.

    The cause of all this is that I have been reading Daniel Parrott's Tall Ships Down and have gotten excited to learn more.

    attached is my "inclining" from last January. It turns out the ice fould my data.:D

    Thank for tolerating my questions,

    Russ
     

    Attached Files:

    Last edited: Aug 26, 2009
  7. Brent Swain
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    Location: British Columbia

    Brent Swain Member

    Let us know how the model tests and the full sized boat tests compare.
    Brent
     
  8. lat 64
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    lat 64 Junior Member

    next year

    I live far from the boat. I will only find time to build a model this winter and work at the real boat next spring. Will report if I am successful in any of this.

    I will be reading this forum for all that I can about getting an accurate model.

    Cheers,

    Russ
     
  9. sharpii2
    Joined: May 2004
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    Location: Michigan, USA

    sharpii2 Senior Member

    True, very true.

    When I included the cabin house on my own design, the theoretical self righting ability went way past 141 degrees. In fact i doubt if it would float at all upside down.

    The reason I advise leaving the cabin house off is because of windows. Which tend to get larger as the house gets taller.

    So, just when you think you are getting a big boost from the cabin house, you may actually be merely courting delusion.

    On my own design, I have worked to improve the house design to harden it against blows from the sea.

    As far as the Open Whatever designs go, I don't see how adding a little deck camber can do much to hamper inverted stability. To do that to any degree you would need an almost semi circle deck section. Not good to walk on.

    I have thought of a sponson concept where the interior of a wide, shallow, low sided hull would be divided length wise by two length wise bulkheads into three compartments. The one in the middle would the accomadations. The two outer ones would be designed to flood or drain when the boat is inverted. This would reduce the effective Beam to the width of the center section, but only when the boat was upside down.

    Once righted, the flooded section would be drained by auto bailors, and or bilge pumps.

    The biggest problem with this concept is that you will not want to store anything in the outer sections (sponsons) that you don't want to get wet.

    It would also require a somewhat complicated system of valves which will either be designed to work on their own, the ideal set up, or could be worked by the skipper.

    The latter would probably be the simplest and most reliable, but would be useless if the skipper was not in the boat at the time or incapacitated.

    If it could work, I would call a boat with this system "self rescuing" instead of "self righting".
     

  10. Brent Swain
    Joined: Mar 2002
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    Location: British Columbia

    Brent Swain Member

    I keep my windows small, and half inch plexi, and my boats, including, cabins are welded steel , so have no worries about them giving out. My aluminium hatches are as airtight as the lid on a presure cooker.
    A good way to get a quick idea of the ultimate stability for a boat is to compare it's midship section to that of a beach ball. A beach ball needs very little ballast to make it 100% self righting. So midship sections that resemble a beach ball ( High cabintop camber, trunk cabin, wheelhouse, moderate beam, etc) have no problem with self righting. A raft however, can easily remain inverted with a very high ballast ratio, so midships sections that resemble a raft ( excessive beam, shallow hull, combined with flush decks and no cabins or wheelhouses) may have major problems with ultiimate stability.
    Brent
     
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