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

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

  1. fallguy
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    fallguy Senior Member

    Looks like he said wr.

    I will find the right one tomorrow. So late.
     
  2. rxcomposite
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    rxcomposite Senior Member

    In a bind, I use 0/90 WR for all jobs and just lay it up at +45 (which is +45-45) degree if I need biax.

    Most biax are heavy, stitched uni with mat on one side. It is designed for poly resins. With WR, i think it is available up tp 400 gr/m2.

    For a small boat, inside and outside skin can be the same. It is because fibers are poor in compression by about 15%. To compensate, more plies are added. Not practical if the calculations is showing you need to add 200 grams more. Round it off.
     
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  3. fallguy
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    fallguy Senior Member

    Do you have the data for adding 1200g glass and resin by hand at 100% resin to glass each side or 2400g glass and same resin to compare the 15mm core to the ply as spec'd?

    I'm nervous it will be almost as heavy as ply without vac for this much skin.

    I can run the numbers tomorrow as well.
     
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  4. rxcomposite
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    rxcomposite Senior Member

    Sleep well FG. I will have the numbers tomorrow.
     
  5. Peter Griebel
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    Peter Griebel Junior Member

    Your guys are incredible... Thanks

    Today I have been to "woodstreet" on the outskirts of Bangkok, and as expected I didn't had any luck finding marine grade plywood there either, so the foam core is still the only solution at hand.

    Got a quote from clarisonic.com, a Thai based PVC foam supplier who buy the foam in big blocks and slice it up themselves. Prices seems very expensive, at just shy of USD 200,- for a sheet of 15 mm x 1200 mm x 2400 mm with a density of 80 kg/m3 (asked before being advised that H60 would be enough). Clarasonic will be providing CNC routing service in 3 weeks time, so they would be able to cut everything for me saving me a lot of time. There are lots of other guys with CNC routers, but they don't have experience with PVC Foam.

    I found another dealer that can sell Divinycell at a better price, but they only have H80 12 mm thickness, but the price is almost half of Clarasonic at USD 104,- for a full sheet.

    I will also try to see if it's cheaper to import from UTEK in China. Another solution would be to import plywood, but if I remember correctly, Thailand have some very heavy import duties on wood.
     
  6. fallguy
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    fallguy Senior Member

    Important to note; foam takes longer to build the hull.

    Each foam component will take you two days of table time or floor time to laminate. Depending upon other factors like jig design or station molds; there may be more time for the panels to cure closer to fully on a flat surface.

    Plywood panels can all be assembled on the jig. Table time is time to cut them. So think like the entire boat can be formed and on the jig in two weeks. A week for jig and a week for panels; another week for stitching and gluing and another week for laminating. Say four weeks to exterior glass on stations. Flip and laminate and build the insides. About a year to build that wood hull.

    The foam will take you about two to four times longer. Using say 4 days per panel to cut and to laminate and flip and laminate and then screw to stations is probably a good estimate. So foam will take about 4 times as long to build. More work; probably about two-three years to build the boat is aggressive if you work hard.

    So, you must weigh your time against the money. Perhaps you can skip some import duties if you bought the panels cnc. I know a company in Florida and I'd say the cnc panels costs would be 8-12000 in ply. But the build would be mucj faster.

    No matter whether you buy cnc foam or cnc ply or either in sheet; the time savings for wood is real. And I am probably closer to right that it'll take you 3 years in foam and one year in plywood. Plywood does really demand epoxy. You could use esters for foam, but I would not for the nasty odors involved.

    If you get the weight down the foam may be a faster boat. Also worth considering, but the final hull weight is important and no reason to build in foam if no weight saved. So need to know the numbers well; even if the tabbing is more will add more weight in foam. And vacuum bagging or infusion of the panels at my 2400gm would drive quite a bit of weight out. To tive you some crude estimates...5 meters boat, say 8 meters glass or a total of 40sqm would be 40*(30% of 2400 or 800g) or say about 32000g or 32kg weight reduction or about 70 pounds. Frankly, infusion would be my recmendation. I built my boat wet bagging. So hard and my resin losses were higher than infusion because to wet bag well; epoxy use was up over 100% for me to wet out quicker.

    Anyhow, keep these in mind. I am drifting to sleep more..
     
    Last edited: Feb 24, 2022
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  7. fallguy
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    fallguy Senior Member

    So, quick calcs. Numbers are good, but they are like I suspected; very bad for foam.

    9mm 4x8 okume panel with 300g each side is 44 pounds

    15mm 60 core 4x8 is 6.3#
    Two layers of 18oz glass each side hand laid is 32 pounds

    Comparison is 44 pounds ply to 38.6 pounds foam h60 for 4x8.

    or converted to kg/sm I get

    plywood is 6.73
    foam is 5.85

    these are not good numbers Peter; the foam weights are way too high and the tabbing might be more for foam

    For hand laminating, I think you will need to go below ISO to something like 600g biax and a 300g wr. This will reduce the weight of the resin and glass by 25%.

    Alternatively; you could bag the job and infuse and save a lot of resin... something on the order of 70% of the resin or so would save 30% of 16 pounds or about 4.8 pounds against the original 4x8 panel. Still not as much as 900gsm per side.

    see what the designer and rx say

    I'd probably try for either a 850gsm triax like my boat or 600gsm biax and a 300gsm wr on top of it or a 300gsm uni, which is better, but harder to finish. This would drop the weights to 4.63kg/m2.

    Infusion at 65% of the resin would drop the last one to 3.44kg/m2.


    Give rx some time to respond, shoot holes in my numbers.

    If you extrapolate these numbers across say 30 square meters of hull; here are som kg comparisons for the panels only; not tabbing, no bulkheads. Not a final boat weight!

    plywood 9mm okume 300gsm hand is 202kg
    foam 900gsm infused is 103kg
    foam 900gsm hand laid is 139kg
    foam 1200gsm hand laid is 176kg
    foam 1200gsm infused is 128kg
     
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  8. fallguy
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    fallguy Senior Member

    You can order okume ply from these guys in Florida.

    also really good epoxy

    boat builder central
    772-742-8535

    might save you enough on money and time to pay the duties

    I spent maybe 45k there in the last 3 years. Great customer service.
     
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  9. fallguy
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    fallguy Senior Member

    Something to go slow on is a foam order. Probably the foam can be used for a dagger or centerboard, rudder even, but needs to be ordered at proper density; not h60

    a few other places would need a better foam as well; like mast foot base and other places where foam density encounters loading forces-perhaps where stays attach etc. rx might be able to advise a bit, but designer ought to as well if you ask
     
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  10. Peter Griebel
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    Peter Griebel Junior Member

    Thanks for the calculations.

    The theoretical hull area is 15.35 m2, and the deck and cockpit is 9.55 m2.

    Does the weight matter? As I understand it from the NA, any weight saved on the foam core will have to be added as ballast.

    Regarding the layup, I think aiming for something that can handle blue water is preferable, even though I probable never gain the courage to sail her back to Denmark :)

    Thanks for the suggestion of Boat Builder Central, I'll try to look into it. Not sure if there are any other ways to ship this kind of size and weight, other than buying a whole 20 foot container. The container shipping prices are trough the roof at the moment, so it will probably be way more expensive than foam.

    It would be cheaper to get the plywood shipped from China, so if there are anyone who know of a reputable Chinese supplier of marine grade plywood, I would love to get that contact.

    If plywood was easily available I would go for that right away. There seems to be a lot more work involved in making a foam core boat, but there are one thing to consider as well, and that is that even though materials are expensive in Thailand, wages are not. It would cost somewhere between USD 300-450,- a month to hire a full time helping hand (that is someone with no experience laying up glass, just like me haha). When you say it will take about tree years to build I assume that's because you think it will have to be done part time. My work is only part time, so I will probably be able to work on the boat most days.

    What density of foam should be used in high stress areas?

    I'm still open to the idea of making a vacuum infusion table, but do think the whole process is a bit complicate. I guess I could use the strong back as the base for a layup table or vacuum table.
     
  11. rxcomposite
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    rxcomposite Senior Member

    Using the boat drawings/particulars posted, I can estimate that your largest panel size is 0.9 x 0.6 m. ISO calculates 7.7 kN/m2 of bottom pressure for a sheltered water operation. Round of to 8.

    Using core sizing spreadsheet I posted here in the forum many times ago. With 9 mm Okoume plywood, the core shear required is 0.67, Okoume has 2. That leaves plenty of room with a safety factor of 2.

    Encapsulating the plywood with 300 g/m2 of WR, derived skin stress is 72 (N/mm2), allowable is 76. Close but still within the safety factor of 2.

    With a recommended 15 mm H80 core, I need only 0.134 N/mm2 of core shear stress. H80 has 1.15. Too much.

    Let us try H60, 15mm with 600 g/m2 of skin lamination. Derived skin stress is 20 (N/mm2), allowable is 76. The skin is barely stressed. Problem is the skin thickness is only 0.7 mm thick. The minimum allowed is 1.5 mm.

    With a 1.5 mm skin thickness of 600 gm WR + Biax, Skin stress went down to 11.3 N/mm2. Still strong but compliant.

    Further evaluation shows you can go down to H45 10 mm thick, 8 kN/m2 pressure and still pass the criteria. This has been validated by a software.

    There are many approaches.
     
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  12. fallguy
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    fallguy Senior Member

    Just keep in mind ballast can mean food rations, water cans, wine bottles. Any weight saved in a boat is earned.

    see what the na says about the foam core laminate schedule, you have expert, experienced advice from rx and a hack builder advice from me questioning the layup; it is really light at 600
     
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  13. fallguy
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    fallguy Senior Member

    I might have some 600g panels around here; if you know the bulkhead spacings midship largest spans, I can give you some idea of what 600 does under load

    my bulkheads are made with 600g
     
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  14. Peter Griebel
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    Peter Griebel Junior Member


    Haha, yes that is important. I think the max displacement is around 1000 - 1100 kg, so if build in ply that will only leave around 500 kg for wine and other provisions.
     

  15. Peter Griebel
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    Peter Griebel Junior Member

    I now got some prices from a few suppliers, and they are spanning very vide. The prices from UTEK are delivered in Bangkok, and include my estimate of import tax and VAT.

    The other two suppliers are both located in Thailand. It's a shame that Clara Sonic are so expensive, because they are slicing to any thickness and can do the CNC routing as well.

    Neotech have very few sizes and densities in stock, but are reasonable in price. Seems like I will end up with ordering from UTEK.

    Screen Shot 2022-02-25 at 13.37.22.png
     
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