Air Tank for reserve of buoyancy

Discussion in 'Boat Design' started by Cacciatore, Nov 30, 2020.

  1. Cacciatore
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    Cacciatore Junior Member

    Hi Guys ,

    have you ever built Air Tanks to increase the reserve of buoyancy for a small boat (below 24 m.) . The ISO 12217-2 request that they shall be subject to a pressure test, carried out at an initial overpressure,
    with a permitted pressure drop within 30 s .
    Have you some pictures for the installation of air inlet valve in similar tanks?
    If you are asking to go ahead with PU foam...i don't like foam :D.

    Regards
     
  2. gonzo
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    gonzo Senior Member

    At 24 m it would not be generally considered a small boat. Is this designed to comply with a class?
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    He didn't specify the size, just what it was less than ! Probably clarifying that will help.
     
  4. Blueknarr
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    Blueknarr Senior Member

    I have used inflatable fenders as reserve bouyancy on dingies.

    Some racing classes have inflatable bags specially made to fit.
     
  5. Cacciatore
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    Cacciatore Junior Member

    Thanks for reply, the vessel is a small 32 ft. Catamaran certified CE for Eu matket. Anyway the rules are referred to Iso 12217-2 and the target is to ensure reserve of buoyancy without using foam but using air tank using fibercarbon sandwich.
     
  6. BlueBell
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    BlueBell "Whatever..."

    Closed cell foam negates the need for a pressure valve on the void space.
    Foam also insures the space doesn't fill with water when/if compromised.

    Why don't you like foam?
     
  7. Cacciatore
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    Cacciatore Junior Member

    Due water moisture absorption of PU and EPS foam after several years and the weight added of the foam. Have you a good brand for foam?
     
  8. BlueBell
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    BlueBell "Whatever..."

    How does the foam absorb water in a sealed compartment?

    Just make your sealed compartments smaller, then you won't need to pressure compensate.
    But you'll add more weight than if you simply used foam in the first place.
     
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  9. Blueknarr
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    Blueknarr Senior Member

    Because compartments are rarely truly sealed!! Somehow someway water will find a crack to invade thru. Wet foam in "sealed" compartments is extremely common.
     
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  10. Blueknarr
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    Blueknarr Senior Member

    Use any bouyancy material for filling that will pass thru an inspection port. Change out filling as needed. Empty water bottles with their lids on work well and are readily available for cheap.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    plastic drink bottles (capped, of course !) are widely used, and while they don't fill the void, they beat the water absorption issue, and as mentioned, relatively cheap.
     
  12. fredrosse
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    fredrosse USACE Steam

    If using screw cap plastic water bottles or similar, it is my understanding that the lids for the containers must be also plastic, with a good watertight seal. Metal lids will corrode over time, and this is why they are not allowed. I would think an economical way to use expanding foam is to mix sealed empty bottles with the foam before it sets up, thus saving much cost of the foam itself. Perhaps 60% - 70% of the buoyancy from sealed empty bottles.

    Using sealed plastic bottles without foam, held in a closed compartment would also work, each gallon bottle would provide about 8 pounds of buoyancy when submerged in sea water, alternately, a 1 liter bottle would provide about 1 kilogram buoyancy under submergence.

    This information is merely hearsay I have collected over the years, and I have no idea if there are specific rules about using sealed bottles, and if there is significant other criteria.
     
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  13. Mr Efficiency
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    Mr Efficiency Senior Member

    It has occurred to me that using bottles and foam together, would cut the costs. And weight too, I expect.
     
  14. Will Gilmore
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    Will Gilmore Senior Member

    How about a sealed compartment with a pump to remove any intruding water over time? Pump air in and water out.

    I have an idea for emergency cabin airbags. If a cabin floods, hidden airbags inflate like life jackets or life rafts to fill the cabin space and force water out. In certain strategic locations along the hull, the bags could potentially block the breach and stop water ingress. Of course, you don't want to be trapped in a cabin filling with airbags, but you also don't want to be trapped in a cabin sinking to the bottom of the ocean either.

    -Will (Dragonfly)
     

  15. Rumars
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    Rumars Senior Member

    Use XPS for flotation, even the 30kg/cum foam has less then 1% by volume water absorbtion, measured under total immersion over 48 months. That is less then 10kg of water for one cubic meter of foam, it's negligible.
    Long-term water absorption tests for frost insulation materials taking into account frost attack - ScienceDirect https://www.sciencedirect.com/science/article/pii/S2214509514000060 for a scientific test, there are similar articles out there since this is a common issue for the insulation industry.

    Using air chambers is not going to save much weight, the chambers need to be small enough to not affect the reserve buoyancy if flooded, and that means more structure. Unfortunately you cannot predict where the hull will be pierced and how, that's why one big chamber even filled with bottles or tennis balls will not do. If you do use them you need some form of pressure equalization, either via goretex vents or rubber diaphragm.

    Foam has the added advantage of beeing able to be distributed along the boat, if flooding occurs the boat will still maintain a level attitude. This has been done even with ballasted monohulls, the Etap's can sail with all seacocks open and the boat flooded.
    What is the boat going to weigh, fully loaded for cruising? 2-3t would require 60-90kg of foam, if the boat is not racing the weight is tolerable, it's the equivalent of one person.
     
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