Carbon on Ply on Polystyrene

Discussion in 'Multihulls' started by cutawaycafe, May 9, 2013.

  1. cutawaycafe
    Joined: May 2013
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    cutawaycafe CutawayCafe

    Hi. This is a first post.

    I've designed a 44' hull (actually two of em, its a cat) that is to be a hollow polystyrene core, layered in three sheets of 3mm Gaboon ply on the outside and finished off with a single layer of carbon fiber. On the inside, two layers of 3mm Gaboon with a bamboo veneer on top. Polystyrene to be various thicknesses across the hull, but no less than 100mm. Frames set up every 600mm, being ply/foam sandwich. Stringers embedded into the polystyrene (hot-wire grooves cut). All glued together with epoxy prior to skinning with ply. Possibly the ply being cut into strips and double-diagonal planked, to get the compound curves.

    The hull shape is inspired by the Swedish proa seen elsewhere in this forum. The one called Akka. I see it as ideally two main hulls, each hull ketch-rigged (a four-master).

    I'm going for a construction method that lets me build and store components, then in a two-week period assemble the hulls, plank and carbon-cover. The inside work to be done afloat. The method used for the 30' catamaran - polystyrene covered with carbon fiber - was inspirational

    Anyone interested enough to offer advice or follow this work online?

    Thanks, Dan Bishop, Wellington New Zealand.
     
  2. Corley
    Joined: Oct 2009
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    Corley epoxy coated

    My personal view is strip plank composite with e-glass built over a mold form would be a better approach than your proposed scantling as it is quite lightweight and very strong, good for a custom round bilge multihull shape.

    The designers and builders of the box boat which had the solid foam core have changed their construction method after building the prototype it offers few benefits and quite considerable potential downsides.
     
  3. gonzo
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    gonzo Senior Member

    What is the purpose of the polysterene? Using a cheap foam will only lessen the value of the boat. Also, the percentage difference of the total cost of the boat is minimal. If you are using heavy veneers on either side, there is no need for any core.
     
  4. teamvmg
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    teamvmg Senior Member

    Does all polystyrene burn like I've seen some of it burn?
     
  5. gonzo
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    gonzo Senior Member

    Yes, but so does wood and epoxy.
     
  6. catsketcher
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    catsketcher Senior Member

    Gday Dan

    The usual idea of a core is to get it to hold a thin laminate in column. To do this it must have a high shear modulus and it must grab onto a laminate really well. Otherwise the core will let go and will be dead weight - its called delamination.

    It looks like your cat will be triple diagonal over stingers and frames which is pretty normal. Why you would put polystyrene in there after you have a perfectly strong structure as that is redundant engineering. The typical polystyrene core is very low density and will deflect before it gives any real support to the skins.

    Even the idea of putting carbon on the skins is tricky. Carbon and timber have very different Youngs moduli. This means that one is much stretchier than the other. If you don't get it just right you could get the carbon taking all of the load and then breaking leaving the now understrength timber to take the load and fail too. You have to ensure that both laminates can flex at the same rate - multiplying the E by the I is a quick way to do this.

    I will be the first to say it here - you are dreaming if you think you can plank and carbon sheath a 44ft cat in two weeks unless you are in a great shed with about 5 people. Most boats of that size would take 1000 hours to build as a simple hull only project.

    Talk to more builders and keep getting advice. It's free and far less troublesome than a half built boat.

    cheers

    Phil
     
  7. cutawaycafe
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    cutawaycafe CutawayCafe

    Not keen on strip-planking

    Thanks. All points taken. I am looking to avoid strip-planking because of the amount of fairing, also looking for better weight saving.

    As per the box boat: I heard the issues were the amount of epoxy soaked up by the poly, plus the lack of class controls when shaping. I did speak with Paul Barron, who builds surfboards using high-density polystyrene cores. I know surfboards are a far cry from 44' huls of course. Will look into the box boat issues more - thanks for that. Dan
     
  8. cutawaycafe
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    cutawaycafe CutawayCafe

    Thank you Phil - valuable stuff. I will keep talking to builders - it's the reason I'm here.

    Experiments with high-density polystyrene, epoxy and marine Gaboon ply have shown so far that the epoxy really bonds deep into the poly and grabs strongly to the ply. I appreciate the sheer factor though.

    The poly may well be redundant. This is becoming apparent.

    So too the carbon. Please tell me, the same issue must be true for GRP, yes?

    I actually am planning on a great shed and a small team, by the way. Having a two-week (100 man-hours multiplied by, say six people) is the plan. That is to get me from a pre-made kit of parts, to outer skins on and bonded/sealed, and cross-beams bonded on. This assumes the cross-beams will have been made earlier.

    The big work I'm trying to avoid is a lot of fairing of the outer skins. Hoping that a combination of wide ply strips plus some torturing will give the compound curves. The amount of curve is about the same as the AKka example. I've worked with steam when building clinkers, so thinking of fabricating a steel steam box for the 3mm ply veneers. Have to test that to make sure delamination does not occur.

    Thanks for the feedback, Phil.
     
  9. cutawaycafe
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    cutawaycafe CutawayCafe

    Thanks. The original plan was to cut the poly sections to approximate the hull shape, epoxy them to the frames and then shape. Gradually the need for the core has died away. The only thing I now think it could do is provide insulation, as I would like to sail in cold climes. I could just inject two-pack foam in afterward if I needed. The two-part foam is this weekends experiment, by the way, to see how well it grabs the timber.

    I'm also going to test the sheer strength of 100mm of polystyrene foam. I was planning to use very-high-density polystyrene. My test this weekend is with high-density polystyrene and with polyurethane foam.

    But overall, it does seem that a foam core is redundant.

    Regards, Dan
     
  10. cutawaycafe
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    cutawaycafe CutawayCafe

    This is gold. Meantime I'll move to the next part - the hull shape. I liked the Akka in it's roundedness, but thought it might bounce in a sea. Looking at the portholes, it seemed to have a very thick wall - perhaps 150mm to 200mm. It also floats very high.

    I took the roundedness, and looked at the Mosquito bomber/fighter (plywood monocoque construction) and kept the idea of a rounded coachroof that flowed seamlessly into the hull. Being from Wellington and the home of Hobbits, the round section did appeal.

    Have drawn a hull that is essentially 8' in diameter across, drawn down vertically to 10' to provide more of a 'bite' into the water. The bows have a finer entry than Akka, more of a stem and angled down to the water more - so the waterline length is greater.

    Moves to a wide soft bilge section amidships, flattening out even more to the transom.

    I sail a Tornado catamaran, and take inspiration from those tortured-ply hulls.

    The bridgedeck is to be open. This is essentially two distinct hulls.

    I'll get some drawings up this weekend.
     
  11. cutawaycafe
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    cutawaycafe CutawayCafe

    TeamVMG, Paul Barron (polystyrene surfboard cores) is in Chch also. Just mentioning.
     
  12. oldsailor7
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    oldsailor7 Senior Member

    In 1979 we built a 40ft Ocean racing Catamaran in Melbourne. It was called Bagatelle. It was designed by Lock Crowther.
    It was urethane foam core with polyester resin impregnated f/glass cloth on either side. It was strong and light and cleaned up the competition in Aus East coast races-----until it was lost on that reef. :eek:
     
  13. Corley
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    Corley epoxy coated

    cold molding is a legitimate way to build the boat and well proven for multihulls it's very labour intensive though and for me building a 25' hull shell in triple diagonal veneers was slow work. It's true I'm not able to work on it full time but nevertheless there you have it a 44' hull with the extra freeboard and length would be a much bigger job. I'd prefer to build in foam sandwich or composite strip plank if I was starting from scratch on a non sentimental project, you can heat form the vertical strip the pvc foam or strip plank it to achieve the required curves.

    If your interested in using ply I'd look at Kurt Hughes cylinder molding method it will give you a round bilge (well more parabolic really) and can cater for the quite full hull shapes found on cruising multihulls. Kurt has the numbers on making it work and a good back catalogue of builds to refer to as far as the possibility of achieving a certain hull shape. You can also vac bag additional core material onto the hull shell to achieve panel stiffness whilst replacing stringers.

    I don't really see the method of construction your proposing as being inherently lighter than a strip plank construction it's really just a matter of working out what kind of target weight and displacement your looking for and how much you are willing to spend on materials.
     
  14. cutawaycafe
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    cutawaycafe CutawayCafe

    Thanks Corley. I happened to chat to Kurt a few years ago when he was looking at cats with four and even six masts. I'll look at his cylinder molding method, and also at foam sandwich. Not keen on strip planking, again because of what I've heard about the fairing afterwards.

    Anyone have any view on sandwich with CF? Looks like CF has come down in price, especially on bulk quantities.

    I should mention: My rig is still just a twinkle, but after talking to Bernard Rhodes about his biplane rig, and Kurt a few years back, I was thinking of each hull being a ketch rig. Four masts. I'm putting this out there :) for comment.

    Great thing about this approach with the forum and other discussions is - the hull can match the most efficient build method chosen, as opposed to the build method being adapted into unknown territory. I want to sail more than I want to build.
     

  15. groper
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    groper Senior Member

    Your original idea sounds like more work, rather than less, to me... it also makes no sense from an engineering standpoint. If you put carbon skins on your ply, the carbon will take all the stress because its modulus is much higher, so the ply will just be dead weight... better to put the carbon on a foam core and let the carbon do its thing all by itself - thats how most of the race boats are built.

    Carbon has come down in price, however it is still dear as poison... i just got a quote for some carbon to make a small forebeam on my cat, $1700 just for the carbon alone, id hate to imagine how much it would cost to wrap a pair of 40+ft hulls and the main beams etc...

    Otherwise, you back to plywood alone, or e-glass sandwich etc to keep costs reasonable...

    I dont like your 4 masts idea either, thats 4 peices you have to build, step and rig instead of 1. It also means more windage, more booms, and most likely more weight and cost. KISS works well on boats, a single rotating wing mast and cutter rig would be my choice if performance is high on your list (assuming it is if your looking at carbon etc)

    Your manufacturing method is a great consideration, the hours involved vary considerably just with varying build methods... ive not found a quicker build method than dereck kelsalls swiftbuild sandwich system, or KSS... you would have to have him design the plans for you if you go that route however. There are variations of this method, and building with flat sandwich panels is popular here in australia, the only limitation being that the shape of the boat can only use developable panels (able to be bent or twisted from flat sheets) so no compound curvature can be achieved.
     
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