Data on time required to self-right

Discussion in 'Stability' started by fatcat1111, Jun 17, 2014.

  1. fatcat1111
    Joined: Jun 2014
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    fatcat1111 New Member

    I'm searching for data - experimental or otherwise observed - on time required for capsized or turtled sailing vessels to self-right. Has anybody seen data like this floating around? Optimally the vessel's STIX index and detailed weather conditions would be included.

    TIA! :)
     
  2. Stumble
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    Stumble Senior Member

    On any boat or a specific? There are plenty of videos of canting keel open boats self righting in flat water(takes about 30 seconds). Most boats are non-self righting and need some wave action to flip them back over, how much depends on the design, in these cases the time it takes varies from immediatly to never depending on when they get a push.
     
  3. fatcat1111
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    fatcat1111 New Member

    I should have specified; I meant those sail boats whose time to self-right is less than infinity :). Multihulls and dinghies are out, so essentially I'm looking for data on keelboats.
     
  4. Stumble
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    Stumble Senior Member

    That's pretty much my point. Other than canting keel boats I don't know of any that are completely self righting. You would need a boat with no AVS, and while it might be possible to design one I doubt that one has been built. The compromises in order to do so would be pretty severe.
     
  5. UNCIVILIZED
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    UNCIVILIZED DIY Junkyard MadScientist

    The biggest aid in figuring out the answer to your question, is to get a copy of the righting moment curve of the boat. Or in a pinch, a Naval Architect can extrapolate a curve for your vessel, if you do an inclining moment test, & have the specifications for the boat. Such as displacement, amount of ballast, ballast placement & or vertical center of gravity (VCG), waterline beam, total beam, & a few other numbers... honestly, it's not rocket science, I swear :)

    PS: Of course, given the make & model of a vessel, it's usually not to hard to contact the builder & or designer, & they'll get you a copy of said curve (numbers). Or for that matter, set a broker to chasing after said info. Explaining to him why you want the numbers, & that your buying the boat is heavily contingent on said data.
    Sometimes Vessel Class Associations are a good resource too.

    When you look at the stability curve, the smaller the area for which the boat's stable inverted, the less of a "push" from a wave it'll take for her to come back up to floating the correct way again.

    Like if you take a look at the stability curve for say Dudley Dix's 38, were she to go inverted, it wouldn't take much for her to come back to being right side up again. As there's little area, relatively speaking, on the part of the curve where she's inverted. Particularly when compared to the area under the curve when she's upright.
    http://dixdesign.com/did38sta.htm
    http://dixdesign.com/38didi.htm
    Meaning were she to go turtle, odds are she wouldn't be real stable that way. And it wouldn't take much of a wave to right her if she somehow did get stuck in the inverted position.

    There is of course a catch. If you do go inverted (turtle), a LOT of boats will lose (read break) their rigs. And this:
    - Leaves you with a horror show mess, on deck in the WORST possible sea conditions. One where the spar pieces will try & bash holes in your hull. Not to mention lots of trailing lines to foul your propeller, assuming your engine will start.
    - Also due to the change in metacentric height, aka Vertical Center of Gravity, the boat will now both roll a LOT more severely, as well as be more prone to rolling over again. Given the loss of weight up high.
    - Plus, such an occurrence will SEVERELY compromise, if not down right eliminate, your ability to have directional control of the vessel. Which will of course make her more prone to follow on, repeated, roll overs. And a horrendous motion in the seaway.

    Do a little reading on the '79 Fastnet race, & you'll get some idea of what I mean.

    All of that said, knock on wood, there aren't a lot of boats which are very stable when inverted. Read for, say, 30-90sec. And IMO, the bigger concern is making certain that anything inside of your vessel which weighs more than 10lbs is securely fixed in place, as even a hard 50-60degree knock (down) can cause a lot of heavy things to start flying around down below.
    Creating both a mess (including coating everything in a nice slippery, nauseaous coating of diesel, as well as lots of missle hazards.

    And in truly severe weather (which most folks will never see, unless they're racing [read carrying too much sail for conditions - common], & or weather forecasting idiots), taking knock downs to 90 degrees isn't exactly what you'd call rare. But even engines have been known to come loose from their footings in weather where vessels take repeated 60 degree knocks.

    Call me a simpleton, but I don't want to try & stop a 75lb flying deep-cycle battery with my head. Neither that, nor a high speed, 6lb, glass bottle of rum either.

    Hope that's not TMI. It's just what comes to mind when folks ask me the question(s) akin to yours.
     
  6. TANSL
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    TANSL Senior Member

    In traditional working boats there is a formula to find the balance period:
    T = f * B / SQR (GM)
    f is a factor defined by practice, depending on the type of boat.
    This does not answer the question at all but it suggests me that there may be something similar to answer you.
     
  7. Eric Sponberg
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    Eric Sponberg Senior Member

    FatCat, Welcome to the forum. As you can see by these early replies so far, there is probably not a lot of data around to answer your question. You cannot determine the time to self-right, really, without doing at least a lot of full capsize tests to get a statistical average which you could then compare with each vessel's full stability curve. Every boat is different, and every survival capsize incident is different. And as indicated, a lot of things change whether the rig is intact or not during capsize.

    I recommend that you read a couple of reports and one very good book:

    "1979 Fastnet Race Inquiry", published by the Royal Yachting Association and the Royal Ocean Racing Club. This talks at length about the characteristics of storms, waves, capsizing, and human factors during the 1979 Fastnet Race.

    "Safety From Capsizing--Final Report of the Directors", published by the United States Yacht Racing Union and The Society of Naval Architects and Marine Engineers, June 1985. This is a follow-up study of capsizing done in the US as a result of the 1979 Fastnet Race Disaster. The US Yacht Racing Union is now known as the US Sailing Association.

    "Seaworthiness--The Forgotten Factor", by C. A. Marchaj, published by International Marine, 1986. This is Tony Marchaj's treatise on capsizing and the 1979 Fastnet Race disaster that looks scientifically at the dynamics of capsizing. This is a "must read" for anyone who ventures offshore.

    Start there with those three studies, and that will answer a lot of your questions. As for the time to right after capsize, I don't believe we can say definitively for any given boat design and capsize circumstance how long it would take any given boat to self-right. We can tell you what good design features are versus bad design features with regard to capsize, and we can say that some boats will right more quickly than other boats, generally, but to give a specific time or window of time, I don't think that can be done reliably since we don't have enough empirical data available on which to judge. That would require taking a lot of different boats out into the ocean and physically capsizing them and timing them for self righting. And some are not going to self-right; they will probably sink. That would be a horrendously expensive, risky, and time-consuming undertaking.

    You may also want to contact the engineers at The Wolfon Unit at the University of Southampton. They would have a lot more information about the science and nature of capsizing.

    I hope that helps.

    Eric
     

  8. JosephT
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    JosephT Senior Member

    Eric's references are good. The only technical term I've heard for this is "capsize ratio". The Boater's Pocket Reference by Thomas McEwen refers to it as both capsize ratio or capsize screening value (CSV). The formula for CSV does not factor in time.

    Time is obviously important especially if someone is trapped under the boat. Anything greater than 30 seconds could drown younger/older passangers. There are many other variables to consider as well (water temp, sea state, passenger fitness, individual lung capacity, etc.).
     
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