PET Bottle Floatation

Discussion in 'Boat Design' started by Katoh, Aug 21, 2011.

  1. Katoh
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    Katoh Senior Member

    Colleagues
    While restoring my latest project, I have failed to come across any foam floatation in the hull of any significance. I must admit I have only being working along the keel line and lets say 400mm (1.2') either side of it. Being an Australian breed of boat, of 92 vintage, I assume by law it must have some sort of flotation in the hull. But sadly I am yet to find it and do not wish to go drilling willy nilly around the sealed deck everywhere to go looking for it.
    I have had to drill a 110mm (4.5") access hole through the fuel well to gain access to hull which I will simply will install an inspection hatch over.
    For my own peace of mind I had the idea that I could fill the hull through the inspection hatch full of empty 1.25liter (approx 0.3 Gal) PET Bottles, with lids on of course. I can't see how these bottles would be able to fail in the hull, unless I am missing something. I have mixed sample batches of biodiesel in these bottles stored fuel and other chemicals and found them to be virtually indestructible. I cant see them fowling or clogging the bilge , nor being able to hold water to cause corrosion. Just a Win Win situation
    Is this a good or bad idea?
    The boat is a 6m plate aluminium hull, I/O with sealed self draining deck.

    Katoh
     
  2. Submarine Tom

    Submarine Tom Previous Member

    What is the crush depth of these bottles, do you know?

    -Tom
     
  3. Jeremy Harris
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    Jeremy Harris Senior Member

    I took some PET pop bottles down to around 30ft many years ago and they started to crush, but then so does ordinary closed cell foam. My guess is they are OK for surface floatation on a small boat, where they won't get immersed to any significant depth.

    Their crush resistance comes pretty much wholly from the air pressure inside them though, so if you could pressurise them they'd go to a greater depth OK. I think that pop bottles have a working pressure of around 30 to 50psi, with a burst pressure that's somewhere around 150psi.
     
  4. Katoh
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    Katoh Senior Member

    Tom
    I have no idea!
    Ill go with Jeremy , what he says sounds about right.
    The other thing here is if the bottles go down more than a meter they have failed in my eyes. They are there to keep the hull afloat if it takes on water, one meter down and you can kiss that boat goodbye.
    Cheers
    Katoh
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Some production boats use these bottles, I read of a South African power cat that employed them, (glass hull), but be sure to remove them before doing any welding. :D
     
  6. Poida
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    Poida Senior Member

    As far as I know there is no law regarding floation in boats. With ali boats they use the sealed hull as floatation mainly for capsizing safety.

    I have a timber boat and there is no floatation and I have never seen a timber boat with flotaion.

    I also had a GRP boat, alright none of us have lived without some sin, and that only used the sealed hull as flotation.

    Now the plastic bottle. It is not so much how deep they can go, although is relative. It is how much weight they can withstand before they burst.

    Don't forget it has to hold the weight of the boat up if the hull is peirced.

    So you would need to test the bottle under a press and check the pressure at bursting.
     
  7. Submarine Tom

    Submarine Tom Previous Member

    The next question is: Is there enough buoyancy in the bottles to keep the vessel afloat when completely flooded?

    -Tom
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    "In general, buoyancy may be provided in the form of foam or bottles, although one would imagine that, for example, the type of inflatable buoyancy sometimes used in yachts would be acceptable. Whilst various other ‘way-out’ solutions have been proposed, the bottles or foam seem to be the most practical, as well as cost effective, answer and we can look at those in more detail. The specifications for both are fairly straightforward and require that the foam is a closed cell type and that bottles used are HDPE (high density polyethylene) grade2 with secure watertight caps-obviously not your average cool drink bottle."

    Taken from:http://www.leisuremarine.za.com/newsdisplay.asp?i={95D936BE-F52F-410B-AE29-B078381FAD40}
     
  9. Katoh
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    Katoh Senior Member

    Another interesting thought that was put to me today was to use PING PONG balls, apparently they were tested on that fabulous show Myth Busters if they could be used to raise a sunken boat, which apparently they can, and the crew proved this. The only concern there would be if ever there was a leak in the diesel tank will the balls withstand the diesel or simply turn to a gooey plastic mush.
    They would be very easily pumped into the hull with compressed air, might have to do some experimenting with them.
    The question here is to try and keep the vessel afloat if ever the hull was compromised.
    Cheers for all the comments
    Katoh
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Not sure what ping-pong balls are made of ? I guess if they go in easy, they could come out of a damaged hull as easily. Have you considered flexible polyethylene foam ? Thats the stuff they make those "pool noodles" out of. And obviously they don't absorb water.
     
  11. Katoh
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    Katoh Senior Member

    I have read somewhere on the forum that they are not recommended as floatation in hulls, from what I remember it had something to do with moister absorption over time, but I may be totally wrong.
    The other factor is the access hole is only 4" dia. I cant see me getting to many of these in the hull, without cutting more holes which means more patching.
    Still a good thought.
    Katoh
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    The evil of corrosion is never far away with alloy, so I guess you have to consider what effect any kind of material in contact with it might have, too.
     
  13. Katoh
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    Katoh Senior Member

    That is so true, this whole rebuild is based around corrosion repair in the hull.
    One thing I noticed was the amount of corrosion to the tank had were the foam was up against the sides. With balls there would be minimal surface contact being a sphere and always a path were water could drain easily.
    Never know maybe a win win situation.
    Katoh
     
  14. daiquiri
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    daiquiri Engineering and Design

    Ping-pong are made of celluloid plastic, a very inflammable material, moisture-sensitive and non-resistant to solvents. So you'd have to make sure they're protected from all of the above in order to use them as flotation material in a boat.
    Cheers
     

  15. Katoh
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    Katoh Senior Member

    Thanks that's the sort of information I was hoping some had, Puts a real No on using them, I wonder if there is an alternative made from a different material, maybe some sort of PVC ball.
    Cheers

    Katoh
     
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