Making this boat self righting.

Discussion in 'Stability' started by zurk, Apr 22, 2014.

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  1. zurk
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    zurk Junior Member

    I have the following boat design (attached) which I would like to make self righting (assume dry hull weight of 2000lbs only, no pax, fuel or anything else - need to make it self righting only when dry).
    Assuming there are two rib style 500lb lift capacity floats attached to the sides, one 500lb lift style float on the top, with 1000lbs flotation foam inside the hull, what is the height (X") i would need to place the top lift float at to ensure the boat is self righting (assume calm water with no waves and boat is placed upside down) i.e. it wont come to rest between the top float and side floats or sit stable upside down ?
    If someone has a formula or anything they normally use to calculate it - if you can share that would be great.
     

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  2. TANSL
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    TANSL Senior Member

    I would like some expert opine on the subject. I think that the boat is self righting must have a positive righting arm for a 180 ° tilt (which does not seem to be the case with this boat). In any case, the ability to right the ship depend only of hull forms. A float at the top of the cab can prevent the vessel is tilted more than 90 degrees but can not get it to be self righting.
    Another question: If the tub and the wheelhouse is not closed, the water will enter them and be totally impossible for the boat to regain its upright position.
    Of course there is not "a formula or anything they normally use to calculate it "
     
  3. SukiSolo
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    SukiSolo Senior Member

    Not forgetting that even with a sealed in buoyant 'floor' it would require a fair lever to get it upright, from even a 90 deg positon.

    It's all centre of gravity, centre of buoyancy and lever arms/ratios. The metacentric height and position too. Witness the recent Korean ferry...and even the 'unsinkable' Titanic wasn't...
     
  4. zurk
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    zurk Junior Member

    yep assume the tub and wheelhouse is not closed. i dont mind it floating level (which it would anyway since it has 1000lbs of foam + 1000 lbs of floats for a 2000lb weight hull) as long as it self rights.
    i would think hull shape would have no effect on the ability to self right ? in any case this is a deep V planing hull.
    if there is a formula to calculate it - let me know. it might depend on lots of things but there should be a simple way of calculating this, right ?
     
  5. TANSL
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    TANSL Senior Member

    Absolutely not.
    My advice is that before getting into these issues, you study the basic principles of naval architecture. They are not difficult, but you need to know.
    For many things that you put on the ship, if the forms are not right, never be self righting.
    A "V" shaped hull has everything you need to not be able to be self righting.
    Cheers.
     
  6. zurk
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    zurk Junior Member

    dude. saying i should be studying it is the same as saying you dont know. which is fine. i dont want to be a naval architect either.
    A V shaped 40+ foot coast guard boat is self righting. all you need is enough buoyancy and leverage. take enough buoyancy and anything can float. take enough HP and anything can fly. its just a matter of how much i need at what height. i dont know how to calculate that but hopefully a real naval architect can chime in to enlighten us.
     
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  7. TANSL
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    TANSL Senior Member

    You're right, I wanted to say is that since you do not know certain things we 're talking about, should study . I keep advising you that you study because , otherwise , you can say really crazy things , By using the example you have set , however much power you put in , there are things that can not fly. They need to have some special shape to fly.
    Do not confuse self righting and insubmersibility. They are very different things and to be able to explain them , and that you understand them , sorry to say , but you should study something . Buoyancy is one thing and another righting arm . You should read things to understand.
    How high you put a float, it will never self righting your boat. You should study some concepts to understand.
    V shaped 40 + foot coast guard boat is self righting . Okay but you have to think as if she is self righting by V hull forms or by forms of the superstructure. Is this open boat like yours ? .
    zurk, my advice is that if you do not want to believe what I say because you do not understand, get informed , to read , to study.
    Tell you to study , in my opinion , not to offend . In this situation I think, honestly, that's the best I can tell you.
     
  8. zurk
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    zurk Junior Member

    you can make anything fly with enough HP. it doesnt need to have a special shape. Look at rockets. No wings, a cylinder. you can make a brick fly if you have enough power.
    i can make a boat fly. all you do it attach a big enough helium balloon which displaces enough air.
    look - this problem is just physics. the boat is a brick of metal weighing 2000 lbs. what is the height and displacement of the top float i need to get it to a 90 degree angle from a 180 degree one ? i know it will self right at 90 degrees. naval architecture/hull forms/shapes are just BS to cloud the simple physics underneath.
    Torque is the product of the distance from the point of rotation to where the force is applied x the force x the sine of the angle between the line you measure distance along and the line of the force. I just need to know how much i need.
     
  9. TANSL
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    TANSL Senior Member

    Clearly, my English is not good and I give things a different meaning than you give them.
    To me what makes a rocket is not fly, what does a bullet is not flying, what does a balloon is not flying.
    For me self righting the boat means, after heeling over 90 degrees, for example, to recover its zero heel position. And that, with a float on the ceiling, you give up a lot, is not possible in your boat.
    By the way, thanks for your explanation of the force and moment of a force, including sine and cosine. You've opened a new horizon in front of my eyes. These deep knowledge of physics you have will help you to understand that I am right. And, again, study everything you can about those things you want to talk. You're welcome.
     
  10. zurk
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    zurk Junior Member

    well i think youre wrong - because these guys do it already. http://www.henshaw.co.uk/ribs/39-how-does-it-work
    anyway, from what it looks like 2 x 200 lb floats placed lengthwise on either end slightly offset would do it.
    For a 7.5 m long rib with 2.1 m float height 1.1 m above boat edge makes it self righting. or basically 10% float height above the lowest part of the boat to the top of the boat handrails. They use 1.1m diameter floats with 0.75m side floats. Assuming of course that their engineering drawings are correct. which they might not be.
     
  11. TANSL
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    TANSL Senior Member

    You are quite right, Zurk, these guys did it.
    Now tell them to do the same with your boat.
    It is impossible.
    You have a solution, change your boat and do the same as these guys.
    But I do not want to be negative but I want to help. As you know a lot of forces and moments, you can calculate everything immediately. If you need the curve of righting arms of your boat, I will calculate it (if you give me the lines drawing your boat). I will do this part, which is the most difficult. The rest I leave in your hands. Knowing the weight of the boat and the GZ curve, it will be easy to calculate the momento needed to counter it. That is, put the distance you like better and get the force (or float volumen) directly. Does this have anything to do with the formula you were looking for?
    No need to thank me.
     
  12. PAR
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    PAR Yacht Designer/Builder

    Zurk, your understanding of how floating objects might become self righting, is lacking some fundamental understanding. The RIB you've referenced was intentionally designed to have a highly level of buoyancy and initial stability, then an inflatable bag to make her, less stable when inverted. In fact, without these bags, she'd remain in the turtle position, with only a ship mounted crane as hope to right her.

    Your boat above, has inherent flaws in it's approach. The 1,000 pounds of foam in the bilge, coupled with what I'll assume is two 500 pound bags in the cabin will insure she will stay firmly inverted, without the assistance of a crane. Do a simple cantilever calculation to roughly figure out how much leverage you'll need, to invert your half flooded boat from over 90 degrees and you'll quickly see there's a few issues.

    Is it possible to make your boat self righting - yes. Is it practical - not so much and you'll lose a lot of internal volume to "devices" for events that will likely never be needed.

    What is the need for a self righting craft of this configuration? Simply put, if you take this craft into Cat. A conditions, you can exspect to get rolled, as it's not well suited for these conditions. This said, she appears to be a Cat. B vessel (maybe), if a bit small and she'd do well, if handled properly.

    In a nut shell, self righting craft have been designed many times and none of them have anything remotely resembling the configuration you've shown above. The RIB example has it's fixed floatation mounted above the LWL and can inflate bags to further improve it's inverted instability. The fishing boat you've shown has a significant percentage of floatation below it's CG which is the worst place for it, if inverted. It's probability of down flooding is high and though some bags may keep her on her side, major consideration and revision would be necessary, to make this type of boat self righting.

    Again, what is the perception of this self righting need? Your comments about the "simple physics" as BS, is clearly how much you don't understand. If it was simple, you wouldn't be here and some blog online would have posted all the necessary data and formulas you need decades ago. Stability is actually a fairly complex set of calculations, through 180 degrees. I perform them regularly and I do surely wish they weren't as complex. I can assure you they are fair more convoluted, than your seeming perceptions of them.
     
  13. zurk
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    zurk Junior Member

    assuming the 500 pound bags are on the side of the boat bolted in place - not in the cabin, does this make the boat any different than a rib ?
    ignore the usage - its not relevant to this calculation and will simply derail this thread.
    can you show me how to do the simple cantilever calculation ? Thats what i am looking for.
    im not looking for stability calc - simple self righting is enough regardless if the hull becomes a floating bathtub in the process. if it floats right side up then thats all i need.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    PAR and TANSL are right, of course, trying to turn any boat into a self-righting vessel as an afterthought is an uphill job. A bouyancy volume high up on the cabin roof will likely not return it to 90 degrees heeled, even. Getting it to that stage is the easy part, however, once the roof-top bouyancy is out of play, there must be a lever arm acting to rotate it the rest of the way, that means the displaced volumes must be further from the bottom of the boat than the centre of gravity, or all you will have is a boat laying pretty much on its side.
     

  15. zurk
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    zurk Junior Member

    which is what the side mounted 500 lb lifting bags should do though, correct ? once it is at a 90 degree angle the side bags should act to bring it level (but under water with a flooded hull - like i said i dont mind it with the water line to the side rails as long as it is right side up).
     
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