Making this boat self righting.

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

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  1. Mr Efficiency
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    Mr Efficiency Senior Member

    You will mind if it sinks to the side rails, because that means there is more weight than bouyancy to support it ! :rolleyes: The only way the side bag idea will work is if there is little or no bouyancy low down once the hull is flooded, if there is underfloor chambers or foam, fuel tanks even, they will counter the bag, and you will likely still be lying over in the water. You will be like a catamaran or proa, with two centres of bouyancy spaced apart. But if no bouyancy in the bottom of the boat, then the side bag will need sufficient bouyancy to support the boat, 500 lbs a side is way short of the mark. You may start to see this is not so easy as you thought ! Even if all this works to right the boat, with the boat now supported only by two side tubes high up, it will be floating right-way-up, and very stable too, but quite full of water, probably cold water ! Were it possible to pump the water or bail it out, which is doubtful, the boat will start to rise in the water, and become unstable as the bouyancy tubes leave the water, you could easily find it will flop over on its side again, as there will be a lot of free water still inside the boat.
     
  2. zurk
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    zurk Junior Member

    so it will basically self right until it reaches the side and essentially float at a 80-ish degree angle until the water in the boat is pumped out by the bilge pumps ? I can live with that. you get high performance fully submersible bilge pumps with built in batteries these days and the outlet can be at the side rails or slightly higher.
    the original question remains - what height do i need for the top foam to get it to a 80-ish degree angle from a 180 degree angle (upside down completely).
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    I would not like to say what it will do, it depends on two things, displaced volumes, and weight and where it is centred, if you leave out the complication of sea state, which presumably will not be calm if your boat has been flipped. As a minimum you will require very substantial bouyancy tubes that will likely push the width to beyond the legal towing width, considering the boat as-is will be close to that figure, presumably. And as I say, you really need the underfloor area to flood, with no bouyant spaces. But then, if you hole the boat, split the hull or whatever, you will be flooded and immobile quick smart, supported by those tubes only, if you are lucky and she doesn't tip before she settles.
     
  4. zurk
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    zurk Junior Member

    well assume we can ignore all considerations of towing width by making the side bags retractable and deployable pre-launch in a locked configuration. also assume the hull wont be holed or whatever and the sea state is calm. the boat is just dropped from a crane upside down onto a flat water surface. can we get her to a 80-ish degree angle and what is the height of the top foam to do so ?
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    The other option would be to lose about half the internal space in your boat by having the bouyancy built- in, inside, along the sides, high up, but I doubt that would appeal, but please yourself on that one !
     
  6. zurk
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    zurk Junior Member

    well the 500lb lifting on either side would achieve the same effect without losing internal space so i dont see how that would help ? besides i can put up to 2200lb lifting on either side outside the boat right now so there is no limitation on flotation on the high sides.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    How do you make a 2000-odd pound capacity tube "retract" ? Whichever way you turn, you are going to encounter problems. Have you worked out what a 2000 lb capacity tube would be sized at ? Huge, unless you are going to have a curved one from bow to stern. Maybe you should have bought the IRB !
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    How does 500 lbs of bouyancy support 2000 lbs of boat ? Answer: "not very well " :D
     
  9. zurk
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    zurk Junior Member

    it flips up and down and locks into place. like i said...its already been taken care of.
    all i need is the calculation of what is the minimum height of the foam (and if you think it needs more how much should there be - i have the option of 200lbs, 500lbs, 1000lbs or 2200lbs of lift capacity) to get it to an 80-ish degree angle. once the bilge pumps can work, i can get it vertical easy. vertical is not the issue. getting it correctly oriented to the 80-ish degree angle is.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    I see you are in love with this idea, as is usually the case when people fall in love with something, they are blind to contrary indications. If your boat is going to weigh 2000 lbs, it will need at least 2000 lbs of bouyancy, or it is ta-ta world. Bilge pumps, if you manage to operate them, will be useless applied to the boat lain over on its side, whatever you pump out will be replaced immediately by in-flooding. Forget this 500 lb side tube, it is completely inadequate, unless you want the boat to sink to the stage it is depending on the roof-top bouyancy bulb to take up the main task. Make it big enough and the whole crew can sit on it, a unique semi-submersible really !
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    How do you get to 80 degrees, your roof-top bouyancy will be clear of the water, you have 500 lbs of bouyancy high up on one side, there has to be 1500 lbs of bouyancy underwater somewhere else, to have it supported, at equilibrium. If it is under-floor, your boat will not be righting, unless the COG is similar to a ballasted keel sail boat. My advice is build a small scale model faithful to the shape of your boat, add and subtract bits and pieces of foam, and weights, if required, and see what the mechanics of it are,, you may get a better feel for it that way.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    You could have both external tubes, and internal bouyancy high up on the sides, but the centre of bouyancy needs to be high up, and able to support greater than the total weight of the boat, on each side, so say 5000 lbs total for both sides. About 80 cubic feet in all. And assuming the centre of gravity in the boat is 'south' of that centre of bouyancy, and by a margin, (you are not at 90 degrees) it might just work. But that leaves you with a deeply flooded, although stable boat, If everyone wants to climb on the roof, you might escape hypothermia for a while.

    I should probably correct what I say about needing that much bouyancy per side, but it would be advantageous for the purpose of the boat righting itself with less water inside, if it has to sink to the stage of the opposite side bouyancy being needed to support the boat, it will be full of water when it comes right way up.
     
  13. zurk
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    zurk Junior Member

    so 2200lbs at both sides and 2200 lbs on the roof ? sounds good.
    is there a calculation for this ? i dont want to overbuy, but obviously want a stable flooded configuration. can i go less on the roof ? whats the minimum i can go on the roof.
    not blind to contrary indications but the calculations dont seem to indicate it is impossible.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Not so fast ! You speak of being happy with a flooded boat, so long as it is upright, but if it is only being supported by bouyancy tubes high up along the sides, it will become unstable as the boat rises with water being pumped out, if you can get pumps to work, before you get it to the stage where internal water is reduced sufficiently. You really want as little water in it as possible when it goes right way up, I can see how that might be achieved, but it would require a lot of bouyancy each side, maybe more than is realistically attainable, and the COG would be more critical than another scenario where the boat rights, but full of water. The issue of the cabin-top bouyancy is another matter.
     

  15. zurk
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    zurk Junior Member

    will have dual baffles to reduce sloshing. more concerned about getting to that point. how much roof flotation is required and how high should it be ?
     
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