self righting

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

  1. aiix
    Joined: Aug 2009
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    aiix New Member

    hye evrybody..this is my first writing here.i would like to ask about self-righting boat.is there any formula to use ensure the boat have capability to have self righting? or the arrangement or it required special design? thank you. :)
     
  2. TeddyDiver
    Joined: Dec 2007
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    TeddyDiver Gollywobbler

    There's a bunch of formulas and specific graphs giving some insight of the subject... If interested of some reading then Larsson&Eliasson Principles of Yacht Design is the choice.. :)
     
  3. dskira

    dskira Previous Member

    Perhaps hiring a naval architect will be an idea.
    Don't take it the wrong way, I am not making fun of you, I am just trying to tell you that when you are sailing and the weather turn bad, you want to be sure to come home.
    The end use of a boat is to sail safely.
    Now if you just want to learn a little more about boats, TeddyDiver is right about his choice of book, and also take a look at Dave Guerr book "The nature of Boat" Very useful.
    Cheers
    Daniel
     
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  4. lewisboats
    Joined: Oct 2002
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    lewisboats Obsessed Member

    Self righting from what? 90 deg...150 deg...180 deg? 90 deg...easy, 150 deg...difficult but manageable...180 deg...there's a tuffy. You have to have the boat be more unstable at 180 than at lesser degrees which is a set of compromises that most wouldn't want to live with given the current design preferences. Google "Marchaj" and look up Seaworthiness: The Forgotten Factor. A search of "Yacht stability" and "Sailboat stability" wouldn't go amiss either.
     
  5. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    To make a boat self righting, you have to make it unstable when capsized to the degree you want it to self right from. During the old IOR days, 1970's, racing boats were required to self right from a 110 deg. capsize.

    This is achievable even with light Beamy hulls if you go with somewhat high sides, say 1/3 Beam in midship freeboard, a heavy ballast keel, and somewhat flaired sides. Such a boat could recover from a more extreme capsize, but only if partially flooded. This is because the water that came on board could shift to the low side, depriving the inverted boat of low side buoyancy shift. This could take some time, however, and when the boat righted, it would have a great deal of water that needed to be pumped out.

    To do much better, say a recovery from a 125 deg or more capsize, you need to go heavier. When I say heavier, I mean heavier in relation to Beam, not Length.

    A convention I use is dividing the boat's displacement in lbs by 64 then multiplying that by 20.0. Next, I divide that by the Length of the boat and by the Beam of the boat squared. I look for a a result of at least 1.0 or higher.

    Next, I look for the height of the widest section of the boat, not including the keel or the cabin. It should be around 60% of the Beam.

    Even with a shallow keel, such a boat should self right well, if it sails at all. This is because it is relyining on weight distribution as much as buoyancy shift to give it sail carrying power. When that is the case, it should be very unstable when upside down.

    The price for this is performance.

    This was made clear in the early BOC single handed races around the world. Narrow boats were tried with very deep keels (as deep as 20 ft or more), but were soon beaten by Very beamy boats with similar keel depths. These boats often ended up being being quite stable upside down. One was found six months after its skipper was last heard from, floating pacidly upside down in the boisterous southern ocean with its still attached bulb keel sticking high up in the air.

    So, the moral is, for good self righting ability, one must go either narrow and deep, or narrow and high sided, or a combination of both.
     
  6. Paul Kotzebue

    Paul Kotzebue Previous Member

    The only ways to know if a vessel is self righting is:

    1. Capsize the boat and see if it rights itself. This is actually done on commercial and military vessels that are required to self-right when upside down, and also on the Mini-Transat racing sailboats.

    2. Perform an accurate weight estimate or conduct an inclining experiment, then calculate righting arm curves for the vessel's anticiapted load conditions.

    Rule of thumb formulas can be inaccurate and misleading and should not be relied upon for safety issues such as stability.
     
  7. Submarine Tom

    Submarine Tom Previous Member

    sharpii2,

    I love your formula! I get 1.5 for my houseboat. Is this substantial or marginal?

    I might point out that my houseboat is anything but self-righting as you might imagine.

    Tom
     
  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Training wheels, really big training wheels.

    Actually you can get quite precise stability information with a few, fairly simple at dockside tests. Further refinement of these figures wouldn't be especially useful, unless conformity to a specific set of rules is required.

    To directly answer Aiix questions about stability, the question isn't sufficiently specific about what you need, in order to answer with little more than generalities. In short yes, there are special design considerations to make a particular boat self righting. No, there isn't one formula, but several, seemly conflicting at times, formulas, which determine self righting capabilities.

    In other words, do you have a design in mind and wish to know it it's self righting?
     
  9. Paul Kotzebue

    Paul Kotzebue Previous Member

    PAR,

    Aside from metacentric height, what precise stability information can you get from dockside tests?

    Just curious ...
     
  10. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Using roll timing to calculate the GM can offer a considerable amount of information about what your boat is going to do in a sea. I feel the original poster is more interested in motion, comfort and safety then "self righting" formulas.

    Lots of folks, usually inexperienced sailors insist on ridiculous design requirements, thinking they're saving their own butt, when in reality if they'd had some real deep water experiences, they'd understand that motion and comfort, in a well designed (and safe) package are much more important then burdening the yacht with design considerations that will likely never be tested in the life of a yacht.

    This isn't to say there isn't a place for self righting, but it's important to understand the needs of the off shore yacht's crew, which very rarely requires ultimately insuring self righting ability.

    So yes, calculating "Brewer's" Motion Comfort formula and roll/beam timing with a pocket calculator at dock side, is generally more valuable then a specific angle of maximum positive stability, which will never been seen by the owner, but the motion in a rough go, certainly will.
     
  11. Paul Kotzebue

    Paul Kotzebue Previous Member

    Fair enough. Many safe and seaworthy offshore yachts have been designed and built without determining the angle of vanishing stability. In the 1970's and 1980's I worked for several prominent sailboat designer's, and I don't remember anyone determining the angle of vanishing stability during that time.

    As far as dockside tests go, I think a simplified inclining experiment will give you a more accurate GM than timing the roll period. The formula commonly used to estimate GM knowing the roll period uses a roll coefficient that varies from boat to boat. You can determine the roll coefficient if you know the GM, but if you're trying to find GM you don't know the roll coefficient. You can get a reasonably precise estimate of GM from roll period if you know the boat's radius of gyration about the longitudinal axis through the center of gravity, but determining that is downright gnarly.

    Ted Brewer's motion comfort formula does not predict or evaluate a boat's stability or it's ability to be self righting. Knowing the roll period and upright GM is not enough to determine if the boat will right itself when capsized.

    Experienced designers and sailors can intuitively predict if a boat is self righting based on it's general charicteristics. However, the only way to know for sure if a boat is self righting, is to actually capsize the boat or determine if it has a positive righting arm when capsized by calculation.
     
  12. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Brewer's formula does tell you if the boat is more comfortable, compared to another, though it does tend to favor some boat shapes over others.

    The inclining test is the other thing I would have tossed up, but generally we're in agreement that knowing your boat has 127 degrees (as an example) of heel before you have to become terribly concerned, is moot when underway, if you have any real sea experience.
     
  13. Paul Kotzebue

    Paul Kotzebue Previous Member

    Well put. I do agree.
     
  14. sharpii2
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    sharpii2 Senior Member

    True. Very true.

    Just trying to give the guy a good starting off point.

    A lot of boats and ships have gone down because their cargo shifted.

    Same thing can happen with a yacht. Especially if it is laden with lots of cruising gear and stores.
     

  15. sharpii2
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    sharpii2 Senior Member

    Your houseboat is not designed to carry sails, is it?

    The 'rules' I gave were just general principles to screen for what might be self righting and probably would be in the parlorance of sailboat design.

    My main point being that a sail boat gets its righting ability by buoyance shift (the most efficient way to get sail carrying capability, weight wise) and by weight distribution.

    With a 'Heft Factor' of 1.5, it is difficult but not impossible to get adequate sail carrying ability by buoyancy shift. Two types of hulls that can do this come to mind: Multihulls and scows.

    My guess is that your houseboat is one of these. Or a very close approximation.

    If it is, I would not be surprised if it could carry sails.
     
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