Ballast/Displacement Ratio-minimum for self-righting

Discussion in 'Sailboats' started by Doug Lord, Sep 17, 2009.

  1. tspeer
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    tspeer Senior Member

    I'm not sure you're asking the right question. The condition you cite is for maximum righting moment. But there are two other cases that are equally important, if not more so.

    The first is the slope of the righting moment curve at zero heel angle - the initial stability. Alternatively, you may use the righting moment at some modest heel angle, say, 15 - 20 deg. This area is important because it determines how much sail area you can carry under the modest conditions when the boat will be most efficient.

    Then there's the maximum righting moment, which largely determines how much energy is required to capsize the boat for wind-induced capsize. It also determines how much heeling moment can be tolerated for a wind-driven capsize, of course.

    But I think the point you're really looking for is the heel angle at which the righting moment vanishes. The larger the heel angle, the smaller the region of upside-down stability and the more likely the boat will flip right-side up when placed upside-down in a wave-induced capsize. A wide boat, even with a heavy bulb keel, can have almost as large a range of stability upside down as right-side up - much like a multihull in that regard. Hence the Open 60's that have capsized in the Southern Ocean and stayed upside down. I think a lot of designers shoot for something like 140 degrees as the point of vanishing stability.

    So if you've calculated the maximum heeling moment that can be applied, it's not so much a matter of making the bulb some multiple of that moment, but rather one of looking at the range of positive stability. That could be achieved with a heavier bulb for a given hull form, or it may be achieved by the shape of the hull, freeboard and deck house.
     
  2. Doug Lord
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    Doug Lord Flight Ready

    Thanks Tom and Jehardiman-and everybody who has tried to help!
     
  3. BeauVrolyk
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    BeauVrolyk Sailor

    How long is a piece of string....

    Doug,

    My Father used to answer questions like the one you asked, when I was very young, by saying: "How long is a piece of string?" His point being, you haven't provided enough information in the question to elicit a useful answer.

    I think you need to decide how quickly you want this mythic boat to right herself, how much she'll resist heeling to begin with (so she can actually sail), and what sorts of seas your boat will be sailing in.

    In the process of answering these additional pieces of your question, you'll solve your initial query. This is because I believe (I don't know, but I'm pretty confident) that whatever righting moment you'd want from the keel, for any reasonable boat that a normal sailor would actually want to sail on, would completely swamp the forces you're discussing.

    For what it's worth, most folks start with a certain desired stability, either for speed or comfort, and then work backwards to get the ballast needed to provide that stability. Also, as others have said, the designer includes the form stability as well as the stability from ballast. While it may be intellectually interesting to you to know the balance point, and you might be tempted to just add a little extra weight to get a boat that is quite light and will eventually self-right, almost no one actually wants a boat like that.

    I am reminded of the time my friends took a picture of me and two of my friends standing on the side of the keel of a Santana-20 in the San Francisco Bay. It had broached badly and wasn't coming back up again (with its chute flogging sails full of water) even with nearly 450 extra pounds standing upon the keel. A great example of not enough ballast stability.
     
  4. Paul B

    Paul B Previous Member


    What you and other like Tom are missing is Doug's mythical boat has the hull flying above the water. He is thinking "giant foiling Moth". So he is not concerned about form stability, there is virtually none.


    What he is looking for is a simple number or very simplistic (linear scaling) formula that he can use in his on-line arguments when people point out the need for lightness in a foiler and how the idea of ballast works against this.

    He wants to be able to throw out a reply to anyone mentioning this fact by stating something like, "Tom Speer at Boatdesign.net calculated I only need a bulb weighing 37, so by my other formulas this PROVES my theory will be the greatest thing ever on the water!"

    So any mention of form stability or other actual design considerations are meaningless to his quest.
     
  5. masalai
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    masalai masalai

    On the original topic of the thread (for which I continually get "activity alerts") - - - - If you cannot keep the boat reasonably "upright" then you should not be operating it...
    Competence is apparently not one of your things so be content to sail a dingy that you can get "upright" when you tip it over... AFTER you have successfully completed some training... and assessment... then, if your shrink and trainer will decline to go sailing, then find something else like flying piece of lead...
     
  6. jehardiman
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    jehardiman Senior Member

    Actualy, I think you don't him enough credit and have missed the thrust of his concept development.

    Back in the late '60's there was concept put forward for high speed (100+ knots) sailing, a non-lifting pod foiler. This foiler used a lifting wingsail (some concepts had them Lighter Than Air) , a streamlined pod and a submerged foil as a depressor. Think of a kite sailer on a board with only a deeply immersed foil holding him down. He can get more speed because the foil can generate much more down force than the boarder, who only has his weight. And righting moment for sail carrying ability is not needed because instead of pushing the vehicle over, it is pulling the "boat" up.

    However, when the pod is resting at zero speed in the water, there is the need for stability righting moment. I think Doug's question is how much weight is neccessary in the foil to return a cylinderical pod upright in a reasonable amout of time.

    Here is a modern take on the concept using multiple hull instead of just one. From an aerodynamic, structural weight, and "crash and burn" impact point of view a single "hull" pod would be perferable.

    http://www.gizmag.com/go/3272/

    At least that is where I saw in the question going.
     
    Last edited: Sep 25, 2009
  7. Paul B

    Paul B Previous Member


    While the concept you put forward could be interesting, it is not what Doug has been on about for the past 8 years or so, and currently.

    You need to go read all the wackiness in his posts of the past few years before you start in on who has missed the thrust of his concept.
     
  8. Doug Lord
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    Doug Lord Flight Ready

    ==============================
    Jehardiman, "B" has no clue regarding the question I asked. Your suggestion is much closer. I wanted to look at this in a very limited way.

    -----------------
    Thanks, Masalai.
     
  9. sharpii2
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    sharpii2 Senior Member

    Oops!

    Hi again, Doug.

    With great embarrassment I have to admit a few mistakes I made on my post.

    1.) The Plank-on-Edge heeled over 60 deg would still have about 86% of its righting moment left, not 50%. Heeled over 30 deg, then it would have 50% of its righting moment left

    2.) The formula I gave you is not quite right. You need the square root of that formula to get your time to return to vertical.

    Bob
     
  10. Doug Lord
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    Doug Lord Flight Ready

    ===================
    Thanks, Bob-haven't had a chance to spend any time with it yet-but I will.
     
  11. sharpii2
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    sharpii2 Senior Member

    'V' foil

    Hi, Doug.

    I was just thinking.

    Have you considered a 'V' foil?

    What I was thinking of is a very narrow monohull which sat inside of a large 'V' foil which would have a 60 deg dihedral. This way, as the boat heels, the leeward arm of the 'V' would be providing most of the lift. The windward portion would be mostly out of the water. The keel bulb would, at this time, be about half its depth to windward, further enlarging the righting arm. The tips of this 'V' could be joined wing beam like structure which could double as a hiking board.

    The beauty of this system is that most of the weight as well as most of buoyancy would be reasonably close to the water surface. The goal here would be to reduce the whetted surface of the hull (it would probably never be completely free of the surface) and to increase the righting arm of the system.

    This system would be quite resistant to capsize due to the enormous dampening power of the arms of the 'V' foil as well as the long cross beam.
     
  12. Doug Lord
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    Doug Lord Flight Ready

    Bob, interesting idea-can you draw a rough sketch?
    ---------------------
    The type of problem I brought up earlier could occur on at least a couple of different types of boat that I'm interested in to one degree or another. The foiler concept Mr. "B" brought up doesn't fit the bill because it uses a very wide beam with buoyancy built in and would be self-righting with very little ballast because of the "assist" in righting from a pitchpole or capsize of that buoyancy.
    The concept I'm working on that inspired this thread doesn't have that built in buoyancy so would sometimes only be wholly dependent on the ballast in the bulb to right from a knockdown to at least 45-50 degree angle of heel where the primary means of RM would become effective again. It would be 100% dependent on the bulb to right from a pitchpole.
    The problem is that the sailing RM is not generated at all or very little by the bulb weight.
    I was trying to find a way to think this out clearly by separating the functions intially but it seems I've just made it more complicated for some to envision. Oh, well...
     
  13. sharpii2
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    sharpii2 Senior Member

    Ballasted monofoiler

    Hi, Doug

    You asked me for a sketch of my idea.

    Here it is:

    I know about as much about hydrofoils as you know about leprichans, so I have no idea if the proportions are anywhere near right. The all up displacement is around 1500 lbs.
     

    Attached Files:

  14. Doug Lord
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    Doug Lord Flight Ready

    Bob, looks like it has potential. If it was me I might make the beam wider, foils shorter and consider a canting keel. I'm convinced a self-righting foiler is possible though I think I would lean toward just two foils. Thanks for the sketch and the effort!
     

  15. Perm Stress
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    Perm Stress Senior Member

    Extra bulb weight

    Calculating how much bulb (or any other ballast to that end) is necessary to counterbalance the rig is valid and fast way to get first approximation of ballast weight necessary for SAFETY, and have nothing to do with any PERFORMANCE.
    For example, :
    * For Micro class (5.5m long) sailboats there is a requirement to remain in equilibrium or right themselves from mast horizontal position, with sail set + 18 kg of additional weight on the spinnaker hallyard.
    * There was similar requirement in old IOR rule: if certain stability parameter in formula is not satisfied, it was necessary to make actual stability test with mast horizontal, and additional weight on mast top; weight calculated according to dimensions of yacht
    * There is something similar in most modern handicap or development class rules

    This way, actual residual stability at 90 degrees of heel is measured (not, by the way, MAXIMUM righting moment, it could happen anywhere from ~15 degrees to 90 or even more depending on general architecture of boat).

    Now down to actual question of "What multiple of bulb weight, which just balance the rig, is actually necessary?".
    The real answer is "Nobody really know".

    However, some starting information could be found in Kurt Reinke book "Yachtbau". I believe it is also published in English. in this book, he explain this kind of calculation and give a graph for required residual righting moment, depending on hull length and area of operation, for monohull yachts.
    However, he also point out, that this calculation is only preliminary, and is not be compared with results of real detailed stability calcuation.
     
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