Foam Boats?

Discussion in 'Boat Design' started by ringer2410, Jan 29, 2008.

  1. ringer2410
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    ringer2410 New Member

    I am newly into the building of boats and have not made one myself but have helped a friend along his quest for the perfect homemade boat. I was wondering if a small boat (layout or Jon) could be made from polystyrene foam and be coated allover in glass cloth. Hey, it works for surfboards why not a little larger.
     
  2. Pericles
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    Pericles Senior Member

    Don't go there! Please search this forum for advice about polystyrene foam. It works for surfboards, however, it will let you down as a boat, not straight away, but when you are not looking. Silk purses are not made from sows' ears!!

    Pericles

    PS, I'm not being a killjoy, but the Darwin Awards exist because there are people who mistakenly believe they can buck the trend.

    http://www.darwinawards.com/
     
  3. kengrome
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    kengrome Senior Member

    I built an 8' long paddle trimaran out of 1/2 inch thick white polystyrene "bead foam" (expanded, not extruded) and it works great! I'm not convinced that it is any lighter than a similar boat made of 5mm plywood, and it was harder to build than plywood would have been. But since the hull is more than twice as thick it is absolutely unsinkable -- even if it were broken into several pieces.

    I do not leave it in the water, I store it upside down on saw horses when I'm not using it, because if it had a hole it might eventually get waterlogged if left in the water all the time (although I'm sure this would take months). But I used two layers of 18oz woven roving on the bottom for abrasion resistance so it will likely never get a hole in it anyways.

    As it is now the boat is light weight, easy to handle, and can haul two full-sized adults out onto the ocean with no problem whatsoever.
     
  4. Pericles
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    Pericles Senior Member

    Ken,

    Beware Darwin Awards!:( :(

    Pericles
     
  5. marshmat
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    marshmat Senior Member

    Ken,
    It sounds to me like the foam in your paddle tri served as a form, and now serves as flotation, but is not serving much of a structural purpose (as the 36 oz of glass would seem adequate for most of the structural conditions on an 8-footer). Is this more or less correct?
    Using extruded polystyrene (XPS) as flotation and formwork, not as a structure, can work. I would strongly advise against using any kind of polystyrene as a strucutre. The expanded (EPS) variety, better recognized as electronics-packing foam, is probably not such a hot idea here unless you're going to cut/dissolve it out later....
     
  6. kengrome
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    kengrome Senior Member

    The EPS serves the same structural purpose as any other foam core in a composite sandwich boat -- the foam separates the glass/epoxy skins so they can handle the structural loads.

    By the way, the 36oz glass is only located on the bottom center of the hull. I wanted one central 'wear strip' along the bottom so I used a vee bottom instead of a flat bottom. The rest of the boat is sheathed in 6 oz glass and epoxy and it is plenty strong enough for a boat this size -- and with no wood it will never rot.

    Well, I did make the amas out of plywood, but that's because wood is much faster easier and cheaper to work with. the main hull is all foam / epoxy / glass with water-based acrylic latex paint on top.

    So can expanded polystyrene as I have proven in this experiment. Maybe I would not build a 20 footer out of it, but for this boat it is really overkill in terms of the boat's actual strength.

    It's a composite sandwich, most of the structural strength is in the skins, not in the core.

    All theories aside, it's a better boat than if it were made of plywood without the glass and epoxy. I feel safer in this boat than in a 5mm plywood boat of the same size, so in my opinion it's a better boat than a plywood boat might be. Nevertheless it costs twice as much as a plywood boat if not more, which is why I would not do it again the same way.
     
  7. Pericles
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    Pericles Senior Member

    Ken,

    Because the shear strength of EPS (polystyrene) is so weak, you are not creating a composite as you claimed. Racking will cause the glass epoxy layers to tear away from the EPS. Sorry to have to point that out.

    The consequences are described below:

    "While the core keeps the skins an equal distance apart from each other thereby increasing the stiffness, it also bears most of the shear loading. In bending, the lower skin is in tension, while the upper (or inner) skin is in compression thereby putting the core in shear (See Figure 1). In order for the sandwich to function correctly the adhesive layers between the skins and the core must be able to transfer the loads, and thereby be as least as strong as the core material. Without a proper bond, the three entities work as separate beams/plates and the stiffness is lost. This is why proper core/skin bonding is so critical."

    The extract is from Page 1.

    http://www.boatdesign.net/articles/foam-core/index.htm

    You may remember my idea about using Foamglas as a foam core. It appeared to have all the boxes ticked, but as soon as I tried to bend the one sided, single laminate composite panel I had constructed, as per page 3 of Trevor Gundberg's paper, all the grid scored pieces I had so carefully measured and cut (Figure 8) parted company from the cured epoxy glass cloth, with an ominous tearing sound and fell to the floor. :( :( :( :(

    See my post 18 at

    http://boatdesign.net/forums/showthread.php?p=168383&highlight=Foamglas#post168383

    I now have rather a large pile of Foamglas that will serve as wall insulation when next I build an extension to a house. It'll work great for that. :D

    Pericles
     
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  8. kengrome
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    kengrome Senior Member

    Hi Perry,

    You haven't pointed out a thing that I did not already know well in advance of building this little boat ... but that's for trying!

    :)

    My opinion is that the boat is so small, stiff and compact that it would take a disastrous accident in order to make the core separate from the skins. Even then the boat won't come apart -- and it will hardly flex more than it already does given the hull shape. But even if it does come apart, the boat will still float for a very long time!

    You have to remember that I didn't build this as a huge boat. It's just a toy boat -- a hell of a lot stiffer and stronger toy boat than I ever expected it to be -- and definitely stiffer and stronger than a 5mm plywood boat would have been.

    Personally I couldn't care less if the skins break away from the core. The core has no place to go in such a small and tightly constructed hull anyways ... and it's not like this little paddle boat is ever going to be subject to the stresses imposted on a high speed planing hull.

    My guess is that it will outlast me and my son, and maybe his future kids as well, as long as it continues to be stored upside down when not in use.

    Don't worry, I "get it" ... but there comes a time and a place where the issues that may be so important in a larger boat simply have no bearing any more in such a small boat -- and this is a perfect example.
     
  9. ringer2410
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    ringer2410 New Member

    It was just an idea and I had seen a few made on the web but I think its a plywood boat for me. Thanks to all who responded so quickly.
     

  10. Pericles
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    Pericles Senior Member

    Ken,

    One way I considered using the Foamglas is as a sandwich filling (?) between 2 layers of 12 mm marine ply, as the construction would have greater mechanical stiffness. For a flat bottomed vessel like a narrowboat the hull thickness could be over 75 mm, if I used the 50 mm panels I have.

    Unsinkable and better insulated than the steel boats, I reckon there still could be some mileage in the concept even now.

    http://www.selway-fisher.com/Mcover30.htm

    Pericles
     
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