Structural foam?

Discussion in 'Boat Design' started by skypoke, Jan 23, 2003.

  1. skypoke
    Joined: Dec 2002
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    skypoke Junior Member

    Working on my last boat rehab project we used a fair amount of two part pourable foam material. I was impressed with it's bond to surfaces (including skin) and the way it tightened up cavities, soles. I'm wondering if it might be a sensible way to reduce some frames and longitudnals in an aluminum boat. Seems that poured to completely fill the void from bilge to deck would make a tremendous "I-beam effect." (outboard powered catamaran)

    The stuff is available in varying densities depending on your needs for flotation vs strength. I wonder what the long term durability would be, if it would retain its bond with surfaces over time? I know gasoline would dissolve it but with watertight bulkheads and welded deck that shouldn't be an issue. Heaven help you if you ever needed to weld on it again, though!

  2. DavidG
    Joined: Jan 2002
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    Location: Chichester, England

    DavidG Junior Member

    Some thoughts;

    Some yachts are built with structural foam between the hull shell and the inner liner, some of these hold up well, but there have been cases were the foam becomes detached from the hull skin in the slamming regions, particularly if the yacht is used hard, at this point the sandwich effect is lost.

    Secondly, all though the void space is nominally water tight, and the foam is closed cell, any moisture inevitably gets in, this tends to puddle at the lowest point, it is very hard to get rid of. At which point if you are building in aluminium, how do you check for corrosion?

    Foam is often used as a former for top hat stiffeners, I understand that this often is found to crumble or disolve out.

    I am not suggesting that foam is unsuitable in the application that you are suggesting, but these might be problems you need to engineer out.

  3. skypoke
    Joined: Dec 2002
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    skypoke Junior Member


    Good points. I guess success would be dependant upon 100% filling of the spaces which is hard to achieve, the way the stuff expands means that it's easy to end up with some small voids around structural items. Boston Whaler seems to have it down to an art with some type of specialized injection system.

    I can see the possibility of putting in a drain at lowest point, probably a perforated tubing running fore and aft the length of the hulls. Wrap it with paper to prevent foam intrusion, water would degrade the paper and allow water to drain through. The loss of bond you mention would be disastrous in this case, if the foam loses contact; there goes your rigidity.

    I'm especially interested in this since it would work well with a "folded plate" style construction, the skins basically providing the shape of the craft as opposed to wrapping frames with plating. Relatively quick way of building the first prototype anyway.

  4. nero
    Joined: Aug 2003
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    Location: Marseille, France / Illinois, US

    nero Senior Member

    stringer/AB foam idea

    Working on a design that would use high-density, laminated, wood stringers (osage orange) set up on half-hull female frames.

    The stringers would have 4 to 10 cm space between them. I am thinking of wrapping the outside in plastic held up with strips of wood and then pouring the foam inbetween the voids.

    After it all expands, the foam could be faired down on the inside and epoxy/glass vacume bagged on. The outside would have tri-axel glass/epoxy.

    What this idea might give is a ridge, limited denting (problem of foam cores), high impact resistance (stringers placed closer in hull bottom and sides) , cost effective core. The core can be 3 cm thick for the same weight as WRC core. This may help reduce noise also?

    Related to your question of AB foam sticking to stuff. Perhaps a co-cure would work. Using epoxy(or other adhesive PU glue) to wet out surfaces before pouring in the foam.???
  5. Guest

    Guest Guest

    Foam Construction

    Foam has to be treated a little like concrete, it needs reinforcement and need a structure to bind it to other members.

    Build a framework, even a light one out of Aluminum like cage. Then seal it in foam. That way attaches to sides, framework etc... THe sum of the two is very greater in multple directions and will avoid separation. Becareful with foam and any UV. Also foam will release gases that can build up.

    Make sure that moisture can run down and leave sufficient space for water to collect or you will have either corrosion, rot or osmosis problem with whatever it touches. Good ventilation is important all the way to bottom.

    Otherwise it is great. I am thinking of build a dingy that is 100 foam with only a very light shell.

    I used to build sandwiched foam parts for fullsize race car modeling and always wondered of building a foam car.
  6. riverboathank
    Joined: Oct 2003
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    Location: Oshawa, Ontario

    riverboathank Junior Member

    Would 2 part foam be feasible to use as support under a diesel fuel tanks? They are of composite construction, 6 1/2' long, and access to the underside is very restricted (1/2-1"), being enclosed by 2 bulkheads, hull side & bottom, and partially in front by a hat stringer (F/G). The tanks will be cleated down around the top perimeter and I would just need ocassional support underneath to stop any flexing or drumming. Any ideas?

    Thank You, Regards, Hank
  7. Corpus Skipper
    Joined: Oct 2003
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    Corpus Skipper Hopeless Boataholic


    What are your tanks made of? If they're aluminum you definitely don't want to put any foam under them. If air circulation around the tank is cut off, moisture will collect and the tank will corrode. If you need additional support, with the limited space you described, I would bed starboard of appropriate thickness in 3M 5200 and glue it to the bottom of the tank, like a shim between the tank and it's foundation. Add these every 18" or so, and you'll have no problem. Hope this helps:D

  8. Corpus Skipper
    Joined: Oct 2003
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    Location: Corpus Christi TX

    Corpus Skipper Hopeless Boataholic

    Duh, now that I read your post again, I saw the tanks are composite. What I described should still work tho:D
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