Stitch N Glue Foam Core Composite Hybrid, Plausible?

Discussion in 'Boatbuilding' started by CloudDiver, Jun 2, 2015.

  1. CloudDiver
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    CloudDiver Senior Member

    I've been bouncing around an idea in my head for years of a build method that is a hybrid between stitch & glue and foam core composite.

    The short version is to take a supply of foam core sheets in 4x 8 feet and laminate them on one or both sides with the desired cloth and vac bag with epoxy. Once a given quantity of sheet material has been laminated for the desired design the patterns are cut in a CNC Router. The objective is to create foam core composite sheets that 'behave' somewhat like plywood so a stich and glue build method will be successful.

    Honestly, I doubt I'm the only one who has ever though of this, so if anyone knows where it has already been done I'd be happy to see it posted here.

    This method seems like it could do well with many small boat designs, sail or power. For sailboats it could be good for hard chine designs, I doubt there is much, if any, benefit to stich & glue method for round bilge hulls.

    Will the epoxy, foam core sheets, glass fiber, and consumable vac bagging supplies yield a lower cost product than marine plywood? I doubt that, but I believe the advantages to the concept would be self evident.

    Looking forward to other's thoughts, and as mentioned before, this has probably been attempted or at least thought of before so lets hear it!
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

  3. SamSam
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    SamSam Senior Member

  4. CloudDiver
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    CloudDiver Senior Member

    Thanks for the links, always interesting. The flexibility of the foam is critical to getting it to 'behave' like plywood once infused, which is why I mentioned glass fabric only on one side. If that were case I would 'stitch' the CNC cut panels so that the glassed side is inside and tape from inside as usual with that method, then cut the stainless wire stitches and cold mold the outside.
    Interesting thing about the Powercat thread posted above, he still used traditional method of foam-core building, laying up his hulls in building frames. The thing about stitch and glue is that would not be required. The stiff panels would be spread with temporary spreaders and/or combination of any permanent ribs/bulkheads that the design calls for.

    I'm planning a visit to my local glass-fiber and structural foam supplier. I'm going to get samples of the materials to test flexibility per material thickness. I have a cost analysis sheet going and the tech sheets for the various sheets that include the foam density, compressive & sheer data, etc. I will probably end up making some test panels for destructive testing, but that may prove expensive when the published numbers should satisfy the information needed for calculating scantlings.
     
  5. SamSam
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    SamSam Senior Member

    I assume you mean hand laminate the outside, not cold mold.

    If you did that, then you'd lose the advantages of infusing and have a whole hull that would have to be manually faired.
     
  6. CloudDiver
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    CloudDiver Senior Member

    You're right, I wasn't specific enough and I was trying to get to another point regarding the flexibility if the foam core vs plywood. Let's say the scantlings of the design calls for a .5" foam core thickness with whatever X number of glass cloth laminates. You find that the .5" foam is too rigid and the CNC cut panels are not making the fair curves needed. So the pattern is cut from .25" foam core sheets and they have only been laminated on one side. Stitched together these thinner, more flexible sheets behave similar to .25" plywood and make nice sweeping lines when 'un-folding' and bracing the stitched panels into the hull forms, the laminated side being on the inside of the hull. Once taped on the inside and seams filled from the outside you address the outer-hull, that ultimately needs to have a .5" core plus laminates...
    So if you were to add your additional laminates directly, yes that is hand laminated. But to get your core thickness, you have an additional set of patterns CNC cut (no lamination either side) that is glued to the outside, in that case would be more aptly called 'cold molding'.

    There are a number of ways that process can go. You can go ahead and put a single or more laminated down and then the second layer of foam, so the end game would be a foam sandwich that has glass/from/glass/foam/glass, a triple decker if you will. I would still be bagging these steps for resin infusion. Of course all tis has to be taken into consideration in labor and consumable materials. The example I just walked through adds several steps!

     
  7. tom28571
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    tom28571 Senior Member

    Probably the first thing to know is, how flexible will the various laminates and thicknesses be? You may find that material glassed both sides will flex enough for most designs intended for S&G construction. Of course that will greatly depend on the glass used as whether it is woven (very flexible) or biax/triax (very stiff). Until you know the answer to that question, all other conjecture is just that.

    These answers are probably available since the method is used quite often. I watched a large cat go together where some of the panels were vacuum laminated just like you describe and they were done on a level table and then bent into the desired shape. Don't know the details though.
     
  8. philSweet
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    philSweet Senior Member

    One reasonable excuse for using this method is if you have access to a really nice layup table that produces flawless, full length planks much longer than you can make from standard core materials. But this is usually done at a size that requires plenty of framing. Some decent catamarans have been gotten out this way.
     
  9. CloudDiver
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    CloudDiver Senior Member

    Full length planks? Probably not, even on smaller designs. The core material I can buy locally comes in 40.2 x 85.8 inches, and some in 39.5 x 96 inches. The cost per sheet is half of what marine plywood costs (when you buy in full case quantity, 1/4' and 3/8" sheets), so when add the cost of S2 Glass and epoxy plus infusion consumables it is plausible the cost per completed sheet will still exceed Marine ply, but cost/benefit will be greater IMO.
    Anyway, I digress. I have access to a CNC router that will take 4' x 8' sheets, so at the end of the day I will still need to scarf planks... I'm prepared for that.

     
  10. tom28571
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    tom28571 Senior Member

    Don't understand the problem with making planks any length you want, limited only by the length of your laminate table and/or ingenuity. Scarfing finished panels seems to defeat a main advantage of the system.
     
  11. groper
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    groper Senior Member

    What your proposing is exactly what ive done - and many others before me.

    You may not have read my thread carefully enough, but only the forward hull "shoes" were infused in a frame and batten mold - the reason is this area has compound curvature and cannot me made from flat panels. You could strip plank it and reglass over it - which is another method which can also be used with composite planks instead of timber etc or simply hold the compound shape and glass 1 side at a time.

    Everything else, was made from composite infused foam core flat panels. Where i needed lots of curvature, the panels are either kerf cut and bent, or 1 side of the laminate is left out. The panel is then bent to desired curve, and the laminate is reinstated on that side. alot Less work is involved with infusing both sides of a panel at once, which is why designs that dont use compound curvature are more time efficient with this method. Gradual curves can be bent into desired shape with both laminates already in place - just like ply. Careful thought needs to go into the laminates tho, the stiffness of the panel can be tailored in different directions based on the fibre orientation of the laminates. Then the fibre orientation needs to agree with the global strength requirements of the boat.

    The panels can be any size you like, limited only by the size of your laminating table. I found for best time efficiency, i would make the largest size panels possible, and if i needed smaller panels than this, i would cut them out of the large panels with a circular saw- just like plywood. So i would draw out a pattern of which panels i needed in my CAD program, and nest them onto a large panel to work out what size panel i was actually going to infuse - then go do it.

    Your idea of using smaller, thinner foam cores and laminating together lacks practical thought - its frought with problems and inefficiency, which may not be apparent until you actually start doing it. Better methods are already in practice. For example you can use double cut foam core which will bend to pretty extreme compound curves if you need it to despite being pretty thick core - ive seen this used to infuse a perfect round bilge hull in a mold.
     
  12. CloudDiver
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    CloudDiver Senior Member

    I agree with everything you said below, the constructive criticism here is what helps to do a sanity check on this process. You are very right about the last part, laminating additional layers of foam and then glass after the first thin layer... Too much additional labor and really defeats the purpose of the proposed method. If it is going to work as intended the laminated panels must be the total thickness that is required for the design an as flexible as similar plywood would be. I believe this is still possible, I just have to generate some samples as proof of concept.

    I'm putting together a cost sheet with current pricing from my local suppliers to include the mechanical properties of the core foam, epoxy resin, and S2 glass I have available. I may build a small dinghy this winter to develop this build concept. I am considering a scow bow International Moth (non foiling, older design from the 60's). Something small I can knock out pretty quick, probably lofted the old fashioned way, and small/light enough to go in the bed of my truck... but the fun factor is still there.

     
  13. groper
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    groper Senior Member

    you dont need to scarf your core material either - simply butt glue the sheets together (or let the infusion resin do it for you -be careful of race tracking tho) and the laminate can go continuously over the panel/plank...
     
  14. CloudDiver
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    CloudDiver Senior Member

    The intent of the method is to infuse the panels before they are CNC cut. So in that case the would still need to be scarfed if the plank needs to be longer than the maximum length of the core material.
    Its completely possible to do it the way suggested, CNC cut your panels from the core alone and then bag/infuse after. There are pros and cons to this approach.
    Pro - as you stated there would be no need for a complex scarf and clamping with adhesive. Infusing in one piece results in a stronger panel or plank.
    Con - My intent for this method was start with all the core material in its full dimension sheets on a given table surface using a heavy duty reusable vac bag, infusing all the sheets assembly line style. They layout of the cut plan will maximize spacing panels on each sheet to minimize waste. By taking the longer panel or plank sections of core foam after CNC cutting and individually infusing them to the longer dimension you will need a greater supply of vac bagging consumables in various dimensions and additional labor time.

    The trick is this, what is your gain/loss ratio of glass, epoxy, and labor between the two proposed methods, infusing long panels individually vs scarfing already infused panel components?

    I think that would depend on the given design... an argument can be made for both concepts I think.

     

  15. groper
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    groper Senior Member

    Many boats are built in the method you describe - a company here in australia (ATL composites) makes the sheet panels in 2400x1200mm sizes and CNC routs them after they are cured so that everything required for the boat arrives as a flat pack to your door. Each routed piece is held in place in the panel with small tags (routing omitted) to hold them in place so everything doesnt move before you get the panels z-joins glued together. They came up with a neat method of joining the panels which is called a Z-joint, and basically the joint just glues together without the need for glass tapes over the seam and doesnt result in a significant change of stiffness over the joint area so they still bend reasonably fair. There is a 20% loss in strength across the joint - but this is rarely a problem in 99% of circumstances. The 2400x1200mm panels are called DUFLEX and the strip plank version is called DURACORE - look them up theres plenty of info.

    The point is - many other people /companies can also manufacture flat panels for you. So you can outsource this part of the build if you want to. If i was to do it again, i would have outsourced it as i didnt save as much money as i thought i would by making the panels myself - the big manufacturers get the materials at much cheaper prices so they make their margins there and it saves you heaps of time which is the biggest enemy of building a large boat.

    I also wouldnt stop there either - if it was a large boat - id take this idea and go supervise the build overseas where cheaper labour can be employed...
     
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