Bouyancy Bag Alternatives

Discussion in 'Boat Design' started by Vantage475T, Aug 4, 2016.

  1. Vantage475T
    Joined: Aug 2016
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    Location: Lymington,UK

    Vantage475T Adventure Trimarans


    I wondered what people thought about any bouyancy bag alternatives. Looking in the local chandlery they are not cheap and I want to add a bit of fail safe into the hulls of my trimmaran project in case of serious damage.

    Each outrigger will displace around 200kg at 200mm with internal bulkheads every 50cm or so and I thought I could fill the voids the empty fizzy drink bottles as they are extremely tough and durable and basically free.

    Each 2 litre bottle weighs around 50g so I could get around 40kg of emergency bouyancy for 1 kg weight which seems pretty good and more importantly they will fit in the voids in the hulls nice and neatly and should require no checking.

    The bouyancy bags I can find are all the wrong shape and seem easily damaged on any sharp edges and I imagine will lose some air overtime and as I won't easily be able to get into each of the bulkheads to top them up once the decks are on it seems a bit pointless. The bulkheads have holes in them to ensure air can circulate internally.

    Some will have small hatches but really more for ventilation than anything else.

    Any point in doing this or any thoughts on any other alternatives.
  2. Squidly-Diddly
    Joined: Sep 2007
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    Location: SF bay

    Squidly-Diddly Senior Member

    Yeah, I'd go with plastic bottles for a few reasons.

    1) you can fill them full or 1/2 with water for E-rations or ballast as needed.

    2) not only are they tough but "quantity has a quality all its own" so even if a few somehow get holed its OK.

    3) you can always use airbags to take up irregular space in corners, or just to quiet the bottles. Maybe a large 44 gallon construction cleanup black bag then fill that with plastic bottles once its lining the space.

    4)larger jugs come with handles for easy looping and tying, in case you need to make a emergency device of some sort, or they could stay 'with the boat' even if the hull is ripped wide open.

    That plastic degrades fast in sunlight and isn't designed to last forever but should be good for a few years as long as not exposed to gasoline.
  3. Ike
    Joined: Apr 2006
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    Location: Washington

    Ike Senior Member

    Plastic bottles have been used many times for flotation. In fact there have been whole boats built out of plastic bottles. So it's a good alternative. Make sure they are all plastics. No metal caps. I have seen people use plastic milk jugs with metal caps and the caps rust and leak.
  4. rasorinc
    Joined: Nov 2007
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    Location: OREGON

    rasorinc Senior Member

    Yes, go with plastic bottles. It is best to seal the tops to the body with a couple of drops of epoxy and tie some 40 or 60 pound fishing line around the tops with lots of slack so in a breakup they don't all float away.
  5. waikikin
    Joined: Jan 2006
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    Location: Australia

    waikikin Senior Member

    You could use some 4L goonbags as well, makes a nice pillow as well.
  6. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    Buy a good inflatable liferaft.
    In the event of a catastrophy, you won't have time to run around tying bottles togather!
  7. stinkydog
    Joined: Jul 2016
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    Location: michigan

    stinkydog New Member

    Bottles work great, use them in many builds. Problem is that being round they 'lose' a lot of space when holed.
    Good thing is that you can shape the plastic pop bottles with hair dryers and compressed air to fit more snugly in different areas.
    Use a mold (wood or metal) that's a snug fit on the bottle, drill a hole in a cap to accept a bolt-in style tire valve, slip bottle in and apply light air pressure, then heat with a hot hair dryer.
    Hex shaped molds make best use of space without weakening the bottle, but square or even flat shapes are possible.

  8. Wavewacker
    Joined: Aug 2010
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    Location: Springfield, Mo.

    Wavewacker Senior Member

    I assume you have access holes. Go to Harbor Freight, buy small inner tubes for riding lawn mowers, stuff those in there and air them up, might put lock tight on the caps. Use some paracord to tie them together.

    A blunt hull hit can crush plastic bottles, blow the tops off or bust them open. The inner tube will give and absorb the blow, you'll bounce away before they burst. Probably cheaper than foam and they won't absorb water.
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