foam for sealed bouyancy

Discussion in 'Boatbuilding' started by Anatol, Jan 11, 2016.

  1. Anatol
    Joined: Feb 2015
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    Anatol Senior Member

    Building a ply/glass multi. I've used 2 part polyurethane (2lb, 4lb..) But recently was advised it can absorb water. Not good. Are there viable impermeable alternatives - styro, or whatever they make swimming noodles out of? Or should I simply seal up the box? I like the possible added rigidity of foam but do not want to have to open her up and dig out soggy foam :(
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    A sealed floatation chamber is just as effective providing buoyancy as a foam filled floatation chamber. Unless you're sailing in an area where evil, boat impaling swordfish like to congregate, you'll always be better off with a sealed chamber, than a foam filled one, in regard to a wooden boat. Even if the foam doesn't absorb moisture, it will trap condensation between it and the wood, which eventually leads to mold, mildew and rot. If you do sail in an area where suicidal swordfish, like to break their snouts off in people's hulls, consider using milk jugs with glued on tops, stuffed into the compartments you want sealed. You might get one or two punctured, but the rest will still offer some capacity, limiting how much that particular compartment will take up.
  3. Ike
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    Ike Senior Member

    As PAR said, you can just use sealed air chambers and actually gain about 2 lbs of buoyancy over foam. But your question was are there alternatives to 2 part polyurethane. Simple answer, yes. Pool noodles are made out of polyethylene. But it is hard to obtain even in block form. But it doesn't absorb water and is not affected by oils, cleaners, gasoline, etc. Styrofoam (a brand name) is a bad choice because it is so easily damaged by shock and vibration, and falls apart into little pieces. It is also easily dissolved by gas, oils, cleaners and just about any kind of solvent. However there are other polystyrene products (Styrofoam is a polystyrene) available, that are cheap and easy to use. You can get them in 2" thick boards, 2 ft by 8 ft at any home improvement store, sold as insulation foam. Unfortunately they also are affected by solvents, but they do not absorb water. So if you bag them in sealed plastic bags or coat them in epoxy they work just fine. If you really want to fill that space with something just in case water gets in, I have seen everything from ping pong balls to plastic bottles used. But the simplest solution is just a sealed air chamber.
  4. Rurudyne
    Joined: Mar 2014
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    Rurudyne Senior Member

    Haven't folks used ping pong balls to raise boats before?
  5. Ike
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    Ike Senior Member

    yes, they have
  6. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    You can coat with epoxy as you say Ike, but do not use polyester - it dissolves expanded polystyrene. However you may be able to use latex too. This does seal it reasonably though I am unsure about longevity. However it is a good way of using EPS as a former and then using a polyester resin and glass over the top without dissolving the foam!. You can buy expanded polystyrene up to 2' thick by 8' X 4' reasonably easily. It hotwires to size.

    IMHO you should only use the additional buoyancy in a design with one flotation chamber in the hull. Also need to ensure flotation does not escape from any hole!. A better way is to subdivide the buoyancy chambers so if one is pierced the others still work, that way they can be empty.

  7. sdowney717
    Joined: Nov 2010
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    sdowney717 Senior Member
    polyisocyanate closed cell rigid foam is also gasoline proof.
    And 2 in thick 4x8 foot sheets about $60.
    I looked into this some time ago for using in my boat. I was thinking of cutting it up and stuffing it into the bilge, mostly behind the semi watertight bulkhead in the rear of the boat.

    In case the rear of the boat flooded, maybe this could displace enough water so it would not sink. There was a boat kept at the marine where the ransom tore loose from the hull and the boat sunk in the York river in some heavy waves. So I thought how to prevent that for my own boat. And I have read of shaft struts torn from hulls sinking boats.

    What I did was seal the entire aft bulkhead to the hull going up to the cabin sole, which is about 2.5 to 3 feet measured form center to sole. Then I opened the seacock and tried to flood the hull, and the boat dropped maybe 2 inches and the water coming in slowed to a trickle. Took a long time to pump out the sea water. So maybe even if holed in the rear, my 1970 37 foot EggHarbor wooden sedan cruiser will stay afloat. This bulkhead sits right under the cabin doors and is a reinforced 3/4 inch plywood. It is screwed and sealed with a polyurethane sealant. Bulkhead sits about 6 feet forward of transom.
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