Kingspan sandwich construction

Discussion in 'Boat Design' started by congellous, Sep 23, 2013.

  1. congellous
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    congellous Junior Member

    I'm getting the impression that sailors are crashing into walls and other boats a lot so you need to make it as reinforced as possible in case you sink ?
     
  2. Jeremy Harris
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    Jeremy Harris Senior Member

    There are big risks with using thick core sections in a structural foam sandwich using composites. Many insulation grade foams are friable and will breck up after being exposed to vibration and flexing. I saw this happen years ago with a PU core, we noticed what seemed to be de-lamination of the skin, drilled a hole to investigate and the core ran out as dust.

    PIR (which is what most Kingspan insulation boards use) is often more friable than some PU foams and would, in my view, be a poor choice.

    You could use XPS insulation as a core, as long as you are going to use epoxy resin. XPS has been used very successfully for years in Rutan aircraft, and works well in thick sections. It's probably my preferred way of making complex one-off composite parts without a mould, in fact. A high grade XPS foam should have enough shear strength. The only thing to watch is getting a good bond between the XPS and the skins. The preferred method (and one I've found works very well) is to work a coat of resin thickened with microballoons into the surface of the foam, as this then gives a good mechanical key for the subsequent resin/glass layers to bond to.

    There is a good reason that specialist core foams have been developed, though, and that is because they generally have the right mix of properties, like tolerance to styrene (for polyester/vinyl ester layups), shear and compressive strength and resistance to vibration/shock induced internal break down.
     
  3. FAST FRED
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    FAST FRED Senior Member

    A 100mm sandwich polyurethane and say 5mm either side is better then just 10mm single skin GRP, no ?

    Sure , but the core MUST stay intact.

    The only core material TESTED to stay intact is for boats , or some air craft.

    Aint cheap but it works .
     
  4. Willallison
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    Willallison Senior Member

    Congellous.... I think you must be missing the point..... none of what I wrote is 'beyond the scope' of your question - it is all information that must be garnered in order to make a calculation of the required core / skin properties.

    Yes - there are thousands of boats out there that have been built by trial and error, without any calculations whatsoever.... there are thousands more that have failed - sometimes disastrously - as a result. Nobody is trying to dodge answering your question...its simply that there is nowhere enough info available to do so.

    JH's advice re very thick cores is worth taking serious consideration of too.... without appropriate shear qualities you will end up with two skins that are operating independently of one another....
     
  5. congellous
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    congellous Junior Member

    The HD kingspan is polystyrene based and has a foil skin that I imagine would adhere quite well to epoxy. Shear is an issue, however I have heard of people drilling holes in a grid to let the resin connect from one side to the other that sounded like a good idea. Also, I'm not making an ocean going boat it's a canal cruiser and maybe fair weather coastal class at most. Mr Harris has basically trumped every other comment up to now which have ranged from the ides is impossible to whatever.....my conclusion is - The HD XPS based heavy duty kingspan insulation that I was intended to use is ideal as long as shear and vibration is taken into account (it's electric). Also everyone is crashing into walls and each other so I need puncture resistance, point taken I didn't realise it was that bad, so I may have a layer of plywood and 3 layers of diolen in addition to my build up on the outside of the hull and crash zones.
     
  6. Jeremy Harris
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    Jeremy Harris Senior Member

    Epoxy definitely won't bond well to the very thin aluminium foil on the insulation boards, buy the plain XPS without the foil on if this is what you're going to try and use (it's readily available). Make sure the polystyrene is definitely XPS and not EPS, EPS will not work as a structural core, as it's bead blown and lacks shear strength (XPS is extruded, EPS is bead blown).

    Make sure you have a good bonding regime to the exposed foam. XPS has a fine surface and may need to be lightly sanded to open the pores before bonding. Use the epoxy/microballoon slurry suggestion to get a better bond to the foam. If you read up on the Rutan method of homebuilt aircraft construction you'll get a good feel for how to work with XPS as a thick core.

    We're not talking power plant vibration when it comes to core friability and breakdown, but wave action. The type of power plant makes no significant difference. Even the small ripples in an inland waterway will cause the hull to flex slightly, stressing the foam. It's important that this stress is kept below the foam fatigue limit for the intended life of the boat, which in practice means aiming for a foam stress level that is below the fatigue threshold.

    The layup needs to be engineered to take the point loads, static and dynamic, at each location where they may be applied and spread them into the skin or to underlying strengthening members (you can embed hard points and bulkheads into the foam, again look at the way that Rutan wing spars are made as an example). You can fabricate Z spars inside the foam easily, as longitudinal stringers or as bulkheads, by laying up glass on the edge of a bit of foam then abutting another section of foam and folding the glass over, forming a good structural connection between the inner and outer skins that significantly increases the core shear strength, which in turn locally increases the stiffness and bending resistance of the whole laminate. Don't do the "drilling holes and pouring resin in" trick, it does nothing to enhance the shear strength of the core as the resin is far too brittle. What will happen is that the resin will take all the initial shear load (because it has a Young's Modulus that is far greater than that of the foam) and the resin columns will then promptly shear, leaving the remaining foam to take the shear loads for the rest of the life of the boat.

    The main structural issues for an inland waterways boat are resistance to sideways crushing forces (from contact with the side or other boats in locks) plus fore and aft impact resistance for the inevitable knocks such a boat gets. The underside of the hull fore and aft also needs beefing up to take the inevitable lock cill encounters.
     
  7. congellous
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    congellous Junior Member

    Great thanks for the tips I'll read up on that. I was just designing a diaphragm build up where the styrofoam is used as impact resistance and fully adhered but has folded grp sections connecting to the inner skin creating cells at c/c of the sheet, thanks again
     
  8. SamSam
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    SamSam Senior Member

    You have to design a canal boat to resist crushing by all the other steel canal boats, often rented to and driven by inexperienced people. From what I can tell, there is an awful lot of abrasion also, boats scraping against each other or against rough canal and lock walls.

    If you use a styrene based foam, contact with the liquid or confined fumes of gasoline, diesel and other solvents will dissolve it.

    The laminate might stick to the aluminum foil skin but the skin isn't stuck to the foam but with a light glue. The laminate and skin will peel off with very little effort. Try peeling the foil off some insulation to see how easy it is.

    I've posted a number of times about the way Carolina Skiffs were made.

    [​IMG]

    Their patent has expired and it's public domain so anybody can use the process now.

    http://pdfpiw.uspto.gov/.piw?PageNu...1=4495884.PN.%26OS=PN/4495884%26RS=PN/4495884

    The patent tells you how to build a boat like theirs and the materials used. The first paragraph of page 6 tells you what kind of foam is used. That foam is a construction type foam, used a lot in hot tar roofing and other types. It's available at construction prices vs. marine prices, usually at wholesale roofing supply companies.

    On the first page of the patent it shows the references cited. You can look up those patents to see other interesting ways of boat building.
     
  9. congellous
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    congellous Junior Member

    I bet people in yachts must be worried about their lovely hulls scraping into a little wall or bridge or a snub nosed narrowboat. I suppose some fenders would do it though eh ? I don't want it to look like a bumper car !
     
  10. Jeremy Harris
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    Jeremy Harris Senior Member

    Fenders don't really help with the potential crushing load, as the problem is the sheer mass of the boats that will bash into the side of your boat in a wide lock. Some of the big locks can get pretty rough, especially if the paddles are opened quickly. Your boat will have to withstand a side impact of a few tonnes from adjacent boats from time to time, which is one reason why steel narrow boats have such a big, reinforced rub rail running right around the boat.

    Yachts and small boats will avoid sharing locks with steel barges and narrow boats if they possibly can, for this very reason. It isn't always possible, though, especially during times when water restrictions are in place.
     
  11. congellous
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    congellous Junior Member

    Thanks Jeremy, I think I'll do the same on the grounds of it being a wide beam as well, then I don't have to have a steel hull. I suppose next danger is marinas with cruiser flying into your side while docking or is it a bit more pedestrian.
     

  12. FAST FRED
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    FAST FRED Senior Member

    Cruising,

    Some days you tie to the big tug boat,

    Some days the big tug ties up to YOU!
     
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