Thickness of a sandwich composite panel, and a few other related questions?

Discussion in 'Materials' started by Peter Griebel, Feb 23, 2022.

  1. AndrewK
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    AndrewK Senior Member

    Hello Peter, I like your upcoming project.

    RX gave you an accurate thickness for your laminates, I will give you a general rule for stitched fabrics and 1:1 glass:resin weight hand laminate.

    cloth kg.s.m X 1.2 = laminate thickness mm

    In your case 600gsm glass is 0.6 X 1.2 = 0.72mm

    I hope you can find some good quality plywood as it will be the least complicated and fastest way to build your boat.

    If you do proceed to build in PVC core I will add my comments.
    Your designer has given you the laminate schedule which is 600gsm Double Bias for both faces, why would you not stay with that????
    In Australia most boats built are not to Classification Code and much larger and more powerful boats are built with 600gsm laminates and are fit for purpose.
    I too would recommend building in female stations.

    Cheers
    Andrew
     
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  2. Peter Griebel
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    Peter Griebel Junior Member

    Thanks Andrew,

    The 600 GMS was not specified by the NA, but only his quick rule of thumb calculation. Fallguy used 600 GMS on some deck panels if I remember correctly, and even though the span was not that big, they still flexed when walking on them. I don't worry so much about the weight of the extra material, only the cost and extra work :)

    Regarding the female stations, I'm still not sure if I fully understand the reasoning for them. Fallguy mention that the hull would have a rigid structure before flipping it, due to the bulkheads already being placed. The other reason that was given is that the PVC foam could sack, but don't that depend on the number of stations, and not whether it is male or female stations.

    I'll try to explain my concern with the female stations:

    My plan is to cut the hull panels and bulkheads on a CNC router. If I build the hull on male stations, the critical measurements where the bulkheads meet the hull will hold, even if the hull panels end up being slightly thicker than calculated. Below is a 3D drawing of the male stations as suggested by the NA (for the original plywood build). The distance between the stations are 400 mm.


    Screen Shot 2022-03-07 at 15.07.41.png
    All the stations, except the transom, are temporary. To ensure the hull doesn't flex when flipping I could keep the stations attached till it was flipped. When flipped the hull will be placed in 3-4 female stations. At this point the hull panels will have been glassed on both sides already and the chines hull joins reenforcement. wouldn't that be enough stiff for me to be able to walk inside when installing the bulkheads???
     
  3. AndrewK
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    AndrewK Senior Member

    Hi Peter, If you are not concerned with weight then the ISO minimum thickness might be a good choice for you.
    When structural loads allow for thinner laminates it comes down to personal comfort factor how thin you want to go and still be happy that you have a serviceable boat. "Serviceable" is subjective.
    For example I am perfectly happy with 450gsm laminates over 80kg foam core for a tender, some are not happy with 1800gsm.
    But for my 12m catamaran decks I chose to have an unbalanced laminate and went for 400gsm double bias + 600gsm biaxial top laminate/16mm core/600gsm biaxial inner laminate. The outer 1000gsm was infused at full vacuum so still only 0.8mm thick.
    My comfort comes from having been involved with overhaul of a couple large catamarans with incredibly thin laminates.

    Building in female or male stations is also a personal choice, you will be right with either approach.
    An option for you is to first only laminate the individual hull panels on the inside and then glue and bend them around the permanent bulkheads like Niels did with his ply build.

    Andrew
     
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  4. fallguy
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    fallguy Senior Member

    If you build 3-4 female cradles and we assume a walking area of 20'; your spans are like 5'. And no, they are not stiff enough. If you ribband the cradles; then yes.

    The female is the way to fly. It is only counterintuitive. The idea you will cnc cut the bulkheads accurately is somewhat inaccurate. The biggest problem is they cannot be right sized and must be undersized or you will have a permanent hump where they are in contact with hull panels. I cannot emphasize timhis enough, the best way to install a bulkhead is with padding.

    The 400mm distance is fine for prelaminated panels, but not for glassing foam only. For foam only, you would need ribbands on the male jig.

    It is my opinion to use 600g biax and 200-400g woven for your hull panels. I recommend you buy a few materials locally and experiment with the layup and use the maximum hull spans for experiments. The wovens are easier to finish versus two layers of 600g biax.

    Keeping all those male stations in place is going to make a very heavy lift/flip. And if it is all for the bulkheads; then not wise because bulkheads need to be smaller than the opening to avoid hull humps and to allow for epoxy putties or padding.
     
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  5. The Wing Guy
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    The Wing Guy Junior Member

    I've built things in foam core and in plywood, and for a boat hull, plywood is much nicer and faster to work with. I built a 19lb 12 foot kayak in Okoume, and could not have built a lighter boat in foam core while retaining any puncture/impact resistance. There are some shapes that are easier to make with foam core, but if you already have a design drawn for plywood, then foam core with skins thick enough for impact and puncture resistance has no compelling advantages, and has some construction difficulties... unless the designer also has a foam core version drawn with all the details. If not, then you can get many opinions and guesses for how things might be done, but it is really the NA who should be supplying all the details.

    Compressive strength of plywood is apt to be several thousand psi, while that figure for foam core is about an order of magnitude lower, depending upon density. Thus, foam core is traditionally removed and replaced with plywood, G10, etc. where hardware is to be mounted. If there are compressive strength issues in other places in the boat (at bulkheads, etc) the issues might not show up until the boat is built with foam core. If the designer has not built a boat in foam core, then best to stick with plywood. You need more than rules of thumb to transition from one type of construction to something entirely different. A plywood panel with a thin skin of fiberglass in epoxy is an entirely different structure than a foam core composite panel.

    Bending single-sided foam core panels into place (and then applying the other skin) is not for the faint of heart if you are looking for fair surfaces.

    Bidirectional fabric is probably overkill for a plywood boat. Woven fabrics are much nicer to work with.

    A partially built plywood boat (for example with inside fillets, but no outside seam taping or overall covering) is easy to work with: you can often walk around inside, drop tools, etc. The boat can reach a point of being structurally sound for shop loads much sooner in the building process. A 2x4 falling on an unskinned foam panel can do a great deal of damage, but bounce off a plywood panel.

    Got a female mold? Then using foam core is the way to go.

    Granted this is all personal preference. Board shapers seem to thrive on all the dust of working with foam. I'd just like working with wood better than plastic.
     
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  6. Peter Griebel
    Joined: Feb 2022
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    Peter Griebel Junior Member

    Hi The Wing Guy,

    Thanks for your advise. Unfortunately foam core is not about personal preference, but my only option, as I can't get marine grade plywood here in Thailand (unless I make it myself). So I have to live with the extra construction difficulties, or abandon the project all together. I do appreciate the many advise I get here, even though they sometime points in different directions. I'm not sure if I would dare a project like this without the fact that your guys spend your time on answering my novice questions :)

    I have been working on the drawings for the female stations, just to start getting used to the idea. I'm also working on a 3D drawing of the bulkheads and other parts of the boat, as it is the best way for me to visualise the whole build and warp my head around the complexity.

    Screen Shot 2022-03-10 at 13.43.27.png
     
    Last edited: Mar 10, 2022
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  7. Rumars
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    Rumars Senior Member

    Henseval worked for VPLP, I'm sure he encountered foam sandwich in his career, and has plenty of friends if he needs help.
    For me, it's a simple thing, if you don't trust a designer, don't use him. Pay rx for a clean sheet design with all the features you like and take his advice on the best way to build it.

    Instead of fooling around with flat panels you can have a round bottom, maybe with a topside chine to preserve interior volume. Building can be on stringered stations or with a single use mold with thermoformed or contour cut foam.
     
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  8. rxcomposite
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    rxcomposite Senior Member

    Foam core is just a different material. And very very light. To import it, you will be like shipping air.

    Marine plywood and single skin laminate are almost on equal basis. If marine ply is available in your area, then it is cheaper build.
     
  9. fallguy
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    fallguy Senior Member

    On my female stations, I screwed spacers in between where I walked. Your bottom must be continuous underfoot based on the laminate weights proposed. This is as simple as laying uniform thickness timbers along the bottom. Ribbands also need to be considered, but need to be offset from chines for bondline work. You can either use ship tape or plastic on the bottom vee.

    Walking into the boat is really convenient, but you must not delam your panels under point loading. Consider a foot and then say 150# and a 3/4" wide landing area. The pressure on the laminate will be on the order of 50psi. And yes, this could delam your panels on every station.

    And the designer or someone needs to assist you with the deck because there are lots of places for higher density foams. That does not change for the mould method, but is important; otherwise local loads might cause problems. This is, for example, cleats, anywhere mast anything is involved, perhaps around dagger or boards, etc.

    You will not regret the female mould. I only regret that I shrunk the boat 1.5" width for my ribbands and wish I had notched them.

    Make sure you can lift the boat out of the forms. I had to make a split in my forms ahead of time because I did not have head clearance for the lift. You probably can remove your forms, but it is easier to lift the boat out and then teardown. I split one side at a chine and then put the stations back together with a thin kerf wedge and wood on each side with screws. Then we removed all that to lift. All depends on your build scenario, but look ahead.
     
  10. Rumars
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    Rumars Senior Member

    If foam is to expensive there is always wood as a core. Ceiba is plentiful and there are also balsa plantations in Thailand.
     
  11. AndrewK
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    AndrewK Senior Member

    Peter, I still find it hard to believe you can not get marine plywood.
    Plywood that you can get what standards does it meet? it does not have to be branded marine.

    Have you committed for the designer to rework the design for foam composite at this stage?

    In regards to the stiffness of the 15mm foam core and 600gsm laminates, this will be stiffer than the specified ply with 300gsm facing on one side.
    Perhaps RX can calculate the two for comparisons with his spreadsheets?

    Rumars, his Chinese PVC core is cheap so I understand the attraction. What I struggle with is the idea of thick laminates so the cored boat ends up being heavier than ply and not the other way around.
     
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  12. AndrewK
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    AndrewK Senior Member

    Peter, with the female batten mould there is also the option of only half hull. Gives you complete access without having to walk on anything.
    This is how I built my 12m hulls.
     
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  13. Peter Griebel
    Joined: Feb 2022
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    Peter Griebel Junior Member

    An update on my project.

    After our discussion here about building the boat in PVC foam and the many advise I got, I thought it would be worth spending some more time trying to finding some marine grade plywood. More about that below.

    Meanwhile the NA Eric Henseval has made the design changes that I wanted. The result is a catboat rig with a unstayed carbon mast, where the sail furl around the mast. Besides that she now has a twin keel, and I have redesigned the deck and cockpit quite radically. Here is a 3D render of how she looks. We call her "Souriceau et le Chat"

    Souriceau et ly Chat.png

    Finding marine grade plywood in Thailand is not easy, as non is produced here. Importing it myself is not an option as I need a special permission to import wood. I managed to find a small stock of MLH (mixed light hardwood) Plywood from Samling (Malaysia) on Phuket island and had them cut up a sheet so I could get a sample. When I finished the boil test a week later he had sold half of his stock.... He had been trying to get new supplies for a year, so that just illustrate how difficult it is. Left was only marine grade Keruing plywood with a density of 821 kg/m3.

    Then last week I found some Baltic Birch Plywood glued with Phenolic WBP Glue with a density of 720 kg/m3. After talking it over with the NA, I thought I would give it a try. The sample looked very promising and it was completely without any voids and with uniform ply thickness, much better than the MLH plywood.

    IMG_2091.jpeg


    When I found out that Francois Vivier build his own Pen-Hir in Birch Plywood I got even more exited. Here is what he write about it on his website:

    "The boatbuilder and I have decided that the first Pen-Hir will be built respecting sustainable development, as much as possible, and we have launched a research and development program with several laboratories. We chose to exclude all tropical woods and in particular okume or mahogany plywood. The boat is made of first-class Finnish birch plywood which is extremely resistant and stiff. The only drawback is that birch, as Okume, is not a durable wood. Therefore the hull and deck are epoxy sheathed and all edges properly protected."

    However after 40 hours of boil test the face veneer started bulging on 2 out of 4 samples, but the core remains solid as a rock. I talked with the supplier and he say that the face veneer probably is glued using a melamine E1 glue, because a Phenolic WBP glue would penetrate the face veneer and discolour it.

    IMG_2090.jpeg

    Back to square one, or am I?

    I'm now talking with the supplier about finding someone with a sander that can take a full sheet of plywood. All this only because you guys made a good point of telling me how much more difficult it would be to build the boat as a PVC foam sandwich construction :)
     
  14. fallguy
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    fallguy Senior Member

    I don't understand 40 hour boil test.

    A forty hour test will damage the wood, fully saturate it swell it unmercifully.

    If this happened in 40 minutes amd 40 hours a typo, I'd not use.

    And a question, do you plan to fiberglass sheath the hull in epoxy?

    Won't the sail be hard to wrap around a standing mast on the bow?
     
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  15. Peter Griebel
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    Peter Griebel Junior Member

    As far as I understand, the boil test for BS 1088 marine grade plywood is 72 hours, but I must admit that it is difficult to find a clear answer to this. I did managed to find this information on buildworld.co.uk:

    "The WBP rating is a glue standard developed in the United Kingdom and set down in British Standards Institution standard 1203:1963. Phenolic resin WBP can withstand being submerged in boiling water for one to three days; melanin WBP can do so for 10 to 20 hours. The exact length of time depends on the grade of the wood veneers and the glue being used."

    To be more precise the face veneer started bulging after about 6 hours. The hull and deck will be sheeted externally with 300 g/m2. fiber glass.
     
    Last edited: May 12, 2022
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