Foam vs Plywood

Discussion in 'Materials' started by rasorinc, Oct 29, 2012.

  1. rasorinc
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    rasorinc Senior Member

    The only foam I have ever used in boat building is to cut up Stryofoam for flotation. Im working on the roof design for the boat now and will use 1.5" x 1.75" lumber ( ripping an 8'- 2 x 4 in half ) to use as rafters on either 16" centers or 12" centers. I was going to lay either 1/4" ply or 3/8" ply over these rafters and then a layer of fiberglass to waterproof it. There will be some bend in the rafters for drainage.
    I am getting a little concerned about weight up high. The cabin roof is 9' long plus another 8' for covered rear deck. I will be about 7'-6" wide. What if I took 1" thick sheets of stryofoam and fiberglassed them down and then installed them on the roof with another light layer of fiberglas over everthing for waterproofing. Only thing to be placed on the roof is a 100 lbs. blow up inflatable raft over the rear deck.
    Any and all opinions will be appreciated. It would add to the the boat's floation factor.
     
  2. PAR
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    PAR Yacht Designer/Builder

    You could employ a foam sandwich cabin roof, several of my designs use them. You do have to engineer the composite construction, for the loads you'll expect and use appropriate techniques. This type of build method only works if there's sufficient laminate and most importantly, that it's completely encasing the foam, not to mention the foam needs to have the physical properties it'll need to preform it's tasks.

    You can use beams if you like, though they're not entirely necessary, assuming a well thought through composite roof. A typical route for your roof would be a 1/4" plywood skin, epoxied to both sides of the foam with light beams on edge, fairly widely spaced and a perimeter frame to dog it down to the cabin sides with. The beams would likely be 1x2's on edge, with the appropriate crown of course, epoxied between the foam sheets and offering something for the plywood to fasten into. You'll still need the perimeter frame and weight savings isn't substantial, as it's still a 1/2" worth of plywood over beams. Next up the ladder would be a fully composite roof with foam shaped over a mold, then skinned with sufficient laminate. You'll need some hard points to fasten it down, but these could be just hunks of plywood or solid timber where you need them. This is considerably lighter, but the goo factor rises exponentially.
     
  3. upchurchmr
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    upchurchmr Senior Member

    Par,

    Would you be recommending styrofoam for the fully composite roof?
    I believe styrofoam is not viable for such an application due to the low stiffness and strength of styrofoam.
     
  4. PAR
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    PAR Yacht Designer/Builder

    No Styrofoam isn't a structural foam and is only good for insulation and hold fake flowers upright in a vase.
     
  5. michael pierzga
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    michael pierzga Senior Member

    Contact BD net contributor TUNNELS and ask him about making a simple cabin roof by taking a quick mold off the top of a Ford Transit van or Police Cruiser when the officers slip inside for donuts and coffee. The old thread is somewhere on Boat design net.

    One design suggestion when using foam core is to use a thick core. It provides insulation to keep the top of your head from burning up and provides enough space...thickness ..to route electrical wires
     
  6. PAR
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    PAR Yacht Designer/Builder

    A mold is a pretty simple thing to make from a couple of pieces of MDF or even cheap solid stock and some furring strip stringers. Finding an automotive roof to mold from is ridiculous at best, particularly a 15' long by 7' 6" wide one. This said an school bus might have this width and certainly the length with a constant camber. Of course it will not be sheered, which might look odd.

    A thick core can offer some insulation qualities, but the core should be sized as should the laminate. At 35 degrees north (Knoxville, Tennessee), his insulation needs may be more for heat retention. Wires handily enough aren't very big, even in a chase, so a 3/4" - 1" core will be fine for the size he seems to want, unless he needs it to be especially stiff or strong. Without more information about the boat and the roofs role, anything more is just speculation.

    Can you post a picture Stan? Will this be a dance floor on July 4th? Fly bridge in the future? What will it attach to?
     
  7. tom28571
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    tom28571 Senior Member

    Here is a set of photos showing a foam cored pilothouse top from one of my boats. The foam is typical 3/4" styrofoam insulation. The internal beams are laminated 3/8" by 1" and the top is 6mm ply with 4mm on the bottom. Edges and other mounting points are blocked. Insulation value is significant in summer.

    http://www.flickr.com/photos/ed_boat/sets/72157625207792682/

    Plenty strong enough to walk on. First one was built with 4mm ply on top and is still strong enough but shows ripples in the surface.
     
  8. PAR
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    PAR Yacht Designer/Builder

    A lot of material in that roof Tom. It appears the foam is insulation value only. Nice work as is typically from you.
     
  9. tom28571
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    tom28571 Senior Member

    Paul,

    The first one with 4mm ply on top is now 12 years old and has withstood my walking around on it many times, so the simple styrofoam does contribute a significant amount of compressive strength. You are right though, that it is mainly for its insulation value. If structural foam were used and heat formed in place, the beams could be eliminated. It is a trade off between availability, cost and a bit more work as the weight is about the same. I've been watching some very large glass/foam/glass structural panels being vacuum formed lately and they are very nice but, for the home builder, I am sticking with the method shown as simple and effective.
     
  10. PAR
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    PAR Yacht Designer/Builder

    Agreed, good foam core structure are typically outside the realm of a back yard build or repair.
     
  11. Red Dwarf
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    Red Dwarf Senior Member

    Attached Files:

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  12. groper
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    groper Senior Member

    I was going to add that just because its styrofoam, doesnt mean it cannot be used structurally. Plenty of structural insulated panels with styrofoam cores being used everywhere from truck bodies to domestic homes to aircraft. I live in the tropics where we have to build houses to withstand cyclones (hurricanes) and structural engineering standards are required by law for these cyclone ratings. We are allowed to use styrofoam cored panels to build in these areas, usually a sandwich panel will be made from 100mm styrofoam and faced with 1mm sheet metal skins. The panels are so stiff we are allowed under the engineering standards, to span this panel roofing over 7m (21ft) between supports and is rated for wind conditions upto 50m/s. In reality, we often get more wind than this, but due to safety factors built in, the structures rarely fail. The only failures ive seen, occur when a flying object impacts it or a large tree falls on it etc...

    High density XPS (extruded polystyrene, not expanded polystyrene) can have sufficient mechanical properties to be used in boats for light structural purposes. For me, the main concern is the compressive strength of it with regard to impact or localized point loading such as people jumping on it etc which could leave dings in the laminate. A light plywood skin would seem ideal in this regard as it handles the localized compressive loads and essentially the foam inside only has to handle the shear which should be more than adequate in light structural areas especially when the core thickness is large - which is ideal for spanning a roof over the full beam of an average pleasure boat without needing central supports. Its cheap, reduces weight, provides good insulation, doesnt absorb water or rot, and should be more than adequate in most roofing circumstances where there is no trafficked area above it. I will be using a mixture of divinycell and XPS with multiaxial e-glass skins in the roof of my catamaran build...
     
  13. upchurchmr
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    upchurchmr Senior Member

    Groper,

    Are we talking the same thing? A 4" thick cabin top (100mm)? I think you are talking about a building where people will not be walking on it.

    If you are talking about using divinycell it will take all the shear load in your cabin top, due to its better stiffness. The XPS will be essentially unloaded, if I assume correctly what the design will be. Want to show us your design?

    My only reason for questioning styrofoam as structure was to keep people from expecting 1" (25mm) of foam between two thin skins would have any significant structure value. Divinycell clearly is completely different and clearly a useful structural element of a design.
     
  14. tom28571
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    tom28571 Senior Member

    I think we are all understanding the issues, if stating them a little differently. I used the small laminated wood beams inside to avoid subjecting the styrofoam to high sheer loads from walking on the structure. The internal beams also prevent skin/foam separation from whatever source. I do not think the local compressive loads are a problem with the ply skins preventing point loading.
     

  15. rasorinc
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    rasorinc Senior Member

    I cannot post a picture. New computer and printer in my future. For the Cabin area:
    Wall studs are 1"1/4 x 1"1/4 with a single top plate of the same size gluded and screwed.
    Rafters will rest on top of plate. Rafters are 1"1/4 x 1"3/4 spaced either 12" OC or 16" OC. A second side plate will be added to the inside pushed up to the bottom of rafter and screwed and glued. I will then install outside plywood up to the top surface of the rafter. This will be 3/8". An outside wall header will then be installed over the plywood, studs, and plate 1"1/4 x 3"1/4. This will end flush with the top of the rafter and be sanded to continue the crown and will serve as a small overhang. Roof plywood will then be installed on top
    either 1/4" or 3/8" flush to the outside edge of the exterior wall header. Roof and exterior wall header will be fiberglassed for water proofing. This will give me room in the rafter area for wiring and 1.5" of insulation. Then some sort of thin panaling to finish off the inside ceiling. I hope I'm clear here. After reading all comments to date this may still be the easiest, cheapest and lightest way to go. Stan
     
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