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

    not possible for the boat to be stuck on its side as the buoyancy on its side is less than the weight of the boat. it would tend to sink until it achieved neutral buoyancy which is when its 3/4 sunk and therefore stable with the weight of the water above the flotation supported partially by the extra side floats and the weight of the boat neutral due to hull foam and partial side floats.
     
  2. PAR
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    PAR Yacht Designer/Builder

    The first image in the 6 position roll isn't accurate. The above the LWL floatation would need to be significantly more then the displacement of the vessel to achieve this trim state - frankly not possible, given the existing structure of a light weight, wide open deck house, even with reinforcement, such as some angle stock for a bag support structure. Starting with an imposable trim state, means the rest of the drawings are moot, but lets move on. The second and third image don't seem to be moving around the appropriate centers, of course a huge assumption, given we don't have any hydro data on the design. As to the rest of it, based on inaccurate centers and data, these too are simply hopeful, wishful thinking and little more than pretty pictures.

    Without doing the math, this discussion is a fishing expedition for information the OP may or may not accept as reality. Ignoring the realities of the physics involved, is a fool's errand. Zurk, no matter how you look at it, you only have three choices, try to wing it and hope for the best and given your understanding, stay within swimming distance of the shoreline - hire a professional and get the appropriate calculations performed - or third try to educate yourself and do them on your own. Knowing what you need to learn, the self education part will be quite time consuming, without some focused guidance.

    Finally accept the fact, that vessel performance is employment based. Without a clue of the general conditions she'll operate in or about why this need is perceived or understanding this requirement, is like designing something for every potential eventuality, which is imposable. Some things just have to be within an acceptable risk level, such as a reasonable AVS for a craft of this general configuration, with whatever intended mode of operation, in whatever expected sea state. Without these most basic elements for analysis, the rest of it is just banter for banter's sake, which too is a fools errand.

    You've been repeatedly give the basics, but you're ignoring the realities of these, by not offering the required and elementary specifications and requirements. Either provide the hydros and SOR or go swimming, it's your call. If you hire a professional, the first questions will be for the SOR and known hydro's. If you educate yourself, the first thing you'll have to calculate, will be the hydro's and then establish a reasonable SOR for yourself. If you elect for the crap shoot, I'm sure we'll read about your adventures in the paper someday. We do many times every year, hopefully you'll survive. Even professionals get it wrong from time to time, but more importantly, even if over engineered with a huge safety margin, somethings just can't be predicted and you end up screwed anyway, in spite of the best laid plans and contrivances. This is the reality of engineering things that place humans in unnatural environments. An elevator is an unnatural environment, so is flying at 35,000 feet, where any major issue means a sudden stop at the bottom of the ride down. Some of us engineer for these environments (and worse) and even with 5:1 safety margins, still can get folks killed, so pick you poison carefully.
     
  3. DCockey
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    DCockey Senior Member

    zurk, a few questions. Why are you interested in how to make the vessel self-righting? Is this an "academic" exercise, or are you seriously interested in building/modifying a boat as described? Do you anticipate operating in conditions where capsize is a significant risk? Is it to meet a specific requirement someone else has imposed?

    What happens to the crew if the boat capsizes? Would they be expected to make their way clear of the boat until it is righted as is the case for the RNLI Atlantic RIB crews?
     
  4. zurk
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    zurk Junior Member

    whats SOR ?
    the "professionals" can barely build a boat to standards. asking them to go out of their comfort zone isnt going to happen. ive asked a bunch. they are taking months to even build a boat to the published standards let alone go beyond that. cars are designed for any eventuality - i cant see why boats wouldnt be.
    first image just shows when the boat is dumped into the water by the crane. its not a stable state.
    all i need to do is raise the weight of water by about a foot - 62.428 lb/cu ft x boat length is all i need basically right ? lifting straps hold the flotation in place rated for 5000lb breaking strength and circle the boat so im not too worried about the straps breaking off. presumably the hull can support the weight of the boat and take the extra abuse.
    i'll wing it in shallow water. worst case is the boat sinks. i can retrieve it with a crane and try again.

    capsize is not a significant risk FYI but i would like a boat which can take it. crew will stay clear till the boat does its thing.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Thanks for the 6 pic sequence zurk, but I can't see you progressing beyond stage one, which appears to be a stable upturned position. I think TANSL was indicating as much. May I propose you go to the big side tubes I mentioned early on, each able to support the entire boat weight ? They would need to be around 2 feet in diameter. Your two orange floataion devices are replaced with inflatable units that are only activated when the roll-over has happened, but not simultaneously. I have illustrated the sequence in the (very) rough sketch attached. Provided the centre of gravity of the boat stays clearly 'south' of the centre of your side tubes, it should roll back to upright relatively dry. But you have a lot of calculations, structural issues, and expense to attend to, as well as a boat that looks very strange indeed ! And it is quite unlikely to capsize with such side tubes in the first place. But if you must have it......
     

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  6. TANSL
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    TANSL Senior Member

    zurk, It is a pleasure to see the security you have in your assessments. Not a calculation, or a solid base where support reasoning.
    Besides talking, why do not you do some calculation?, I'll advise you and wish that the fall is not very painful.
     
  7. zurk
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    zurk Junior Member

    first is only stable temporarily because of trapped air in the hull. probably stay stable for 10-20 seconds i guess.
    the reason i dont want full tubes which will support the boat weight on one side is because it may end up in a stable position on its side or at 180 degrees which would be bad.
    anyway, i'll start with half the weight then if needed i'll add the extra flotation.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    You have a diagram that shows the two yellow side chambers supporting the weight of the boat, along with the two small orange devices. If that is enough to support the boat, it is in a stable inverted position, and will not right. If they won't support the boat, and the two smaller yellow chambers need to come into play. it is still not exhibiting the instability needed to right. If you need more still, your boat just sunk. Unless the trapped air is still holding it up. Please note that if the boat somehow now manages to roll, the air will escape, and the boat will sink.
     
  9. Rastapop
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    Rastapop Naval Architect

    Your six images:

    1: The two orange floats away from the centreline are going to make it harder to roll. Put them together on the centreline.

    2. The two yellow floats in the water will be rolling in the opposite direction to the one you want. Only the orange ones are helping here, and even with the longer lever arm won't be enough.

    You won't get to step 3.

    Without trying to calculate numbers - the orange float needs to be on the centreline, and larger than the total yellow floats on one side.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Going that route, it wll have to be inflatable for the orange one. And big.
     
  11. zurk
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    zurk Junior Member

    ok i'll put a single 1100lb on the centerline. getting complicated enough so that i need to model it in a bathtub before i commit. will need a kitchen scale and foam for that looks like.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    I showed you yesterday, zurk, how having it offset did the job.
     
  13. Rastapop
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    Rastapop Naval Architect

    It'll have to be huge - it has to drag the yellow ones on the side and the hull floatation down through the water until they're beneath the CoG.

    [​IMG]
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Of course if your cabin bouyancy is well forward of the COG, all you might do is raise the bow of the upturned boat. Lots of things to consider here zurk.
     

  15. Rastapop
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    Rastapop Naval Architect

    Zurk, Mr E's dynamic 4 float solution is by far the best, of course.

    I wasn't trying to offer you useful advice, because I don't really think it's possible. Just pointing out why it doesn't work.
     
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