Fiberglass and foam sandwich construction, a few questions.

Discussion in 'Fiberglass and Composite Boat Building' started by CaptainObvious, Jun 29, 2018.

  1. CaptainObvious
    Joined: Jun 2018
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    Location: Thailand

    CaptainObvious Junior Member

    Hello there, just joined up.

    My GF is going to be the death of me I tell you... I say to her "I'd like to build a boat" and instead of talking some sense into me she says "Great idea!".

    I've been thinking of making a small catamaran, trailerable about 4.2/4.5 meters length, 1.75 beam for 4/6 people, no more than 600/700 kilograms with everyone in and a cruise speed of at least 10 knots; mostly for short trips out to the islands.

    I have experience with composites, I've designed and built a few composite model airplanes, mold, foam, vacuum bagging, all that sort of fun so I'm OK in the making things department, same same just bigger, I hope.

    Questions now, according to the rough size of the boat I have in mind, what would be a recommended fiberglass layup? As the CAD work I've been doing stands now the total area of skin for the hull and deck is aboutish 30 square meters, the sides of hull use (preliminary) a 20mm foam core and the bottom has a 15cm thick foam core to add reserve flotation and to raise the bottom up to a level where it will be wide enough to walk on it. I'm thinking of a 2mm thick fiberglass (2kg/sqm, right) layup for the whole thing but I don't know if I'm over or under doing it, thoughts on that?
    Of course the hull bottom would be thicker, 3mm perhaps?
    As for the type of fiberglass to use, I'm thinking of one layer of light FG roving over the gelcoat, and then chopped mat on top up to thickness using vinyl ester resin and hand layup.

    Next, the foam core, there are some parts were it would be difficult to carve the foam to fit snugly between the hull and the deck, so I was thinking of using expanding urethane foam to fill up difficult areas, is that a reasonable approach?
    What would be the best way to adhere the foam core to the inside of the hull and deck?

    I'm attaching a quick render of the hull design so you can get an idea of what it looks like; for now it's a rough sketch, after I settle down on the overall shape I'll do the detailed CAD design and then I'm sure a thousand and one other questions will present themselves.
    In the meantime another one, do the railings look too thin? as of now they are 5cm wide for approximately 2m between the front and rear posts.

    Render2.jpg
     
  2. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Looks good, but I would go with a proven design, especially one you have to pay for.

    Why polyester instead of epoxy?
    Cost to strength is about the same but lighter and better strength characteristics.
    But you'd have to use polystyrene foam.
    Is that available there?
     
  3. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    This is almost the same as Jacques Mertens Cat22. For $325 bucks; you'd get a complete set of plans and it is spec'd in ply, but buildable in foam. And if you want it smaller, you can size it down 10% easily afaik.

    gains for some of your fancy curves are close to zero, so I'd rethink them a bit; they will add hours and hours to the build time; especially when you try to get each side to match... doing the first one is easy; the second one matching is much harder, unless you mould both of them..however, if you insist then...

    get away from the urethane idea and learn how to make moulds; you can use the urethane or other foams to carve/sand/form the complex shapes you desire; then coat them with duratec and wax and laminate the shape; take it off the mould and laminate it into the boat with seam tapes - since you need two of everything; this would be the better way to fly

    most of the shapes on the cat22 and even your drawing would be developable or infusion would work on developable panels

    I'd recommend learning infusion and using epoxy.
     
  4. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    What is intended as the motive power for this ?
     
  5. CaptainObvious
    Joined: Jun 2018
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    CaptainObvious Junior Member

    Not Polyester, vinyl ester. Although my first choice would be to use epoxy I've read about and vinyl ester seems to have some advantages. But maybe I'm wrong.

    By polystyrene foam which type you mean? Expanded Polystyrene is readily available, but obviously not a good choice, I haven't found a supplier of extruded polystyrene though. However if push comes to shove I could get that from China and trucked over to Thailand. I've already checked for supplier for foam, PVC foam that is, places like this: Pvc Foam Core For Boat Manufacturing,Sandwich Pvc Foam Sheet - Buy Pvc Foam Core Sheet,Resin Infusion Foam,Vacuum Infusion Process Product on Alibaba.com https://www.alibaba.com/product-detail/PVC-foam-core-for-boat-manufacturing_60423358892.html?spm=0.galleryofferlist.2017203.5.777e27bbHsttn1&s=p
     
    Last edited: Jun 29, 2018
  6. CaptainObvious
    Joined: Jun 2018
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    Location: Thailand

    CaptainObvious Junior Member

    I checked that Cat22, similar yes but is seems much taller since it has a flat floor for people to sit/stand on instead of using the depth of the hulls; I'm trying to keep the height down as much as possible, the idea is to use a fold down top for storage and transportation.

    Yes I'm planning to make molds, I can use a large CNC machine to create the templates and formers for the mold, even some of the curved parts so that should simplify things. In any case, half of the fun is in building the thing. =D
    I'm not too concerned about complex shapes, I've done much more complicated plugs for model airplanes were I had to be precise to less than a millimeter, this would be bigger but I expect easier in that regard. On top of that one of my lines of work involves 3D scanning so I can use that to check the shape quite precisely if I need to.
    Also once I've finished the CAD work I plan to 3D print a 1:10th model for testing and using it as a reference.

    For the mold I was planning to do something along the lines of what you mentioned, I'd CNC formers, assemble them and fill the spaces with foam then fiberglass on top.
     

    Attached Files:

  7. CaptainObvious
    Joined: Jun 2018
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    CaptainObvious Junior Member

    Electric, with solar panels on top. For the moment I'm thinking of using a 3kw motor and LiFePo4 batteries, at least 5KWh with a 1600/1800 pW/h solar array. Since I don't expect to use the boat to motor around for more than a couple hours on nice sunny days (and it does get very sunny around here) I think it should work fine.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    What is the waterline length/ beam ratio of the individual demihulls ?
     
  9. CaptainObvious
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    CaptainObvious Junior Member

    By the way, yes, I've considered using infusion, that's one of the reasons I was asking about adhering the foam to the laminate.
    What I have in mind would be to do the hull and deck laminates, then add the foam cores to the sides and bottom of the hulls where the shapes are flat and consistent, lower the deck into the hull to rest over and between the foam core and then fill any remining voids with expanding urethane.
    Half of the hull, the middle, is an almost constant cross section, making the core for that should be easy, I could hot-wire it. The urethane foam would be mostly for the bow and aft sections.
     
  10. CaptainObvious
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    CaptainObvious Junior Member

    Last time I checked it was 8.4, I'm trying to keep it as slim as possible while having a shallow draft.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    I can't see you going much faster than walking pace here, Your only chance to go faster with minimal power in a small cat is super-slim hulls, and that goes with a minimum of weight.
     
  12. fallguy
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    fallguy Senior Member

    There would be little to gain using urethane foam. Typically the unusable spaces are just air filled compartments.

    What is the freeboard?
    Bdeck clearance?

    Something seems off a bit to me.
     
  13. CaptainObvious
    Joined: Jun 2018
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    CaptainObvious Junior Member

    3kW is 4HP, and from what I've read electric drives have higher thrust than an HP equivalent ICE, I think it should do more than a walking pace and if not it can always be upgraded.
     
  14. CaptainObvious
    Joined: Jun 2018
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    CaptainObvious Junior Member

    Freeboard is 50cm with full load, the hull bridge clearance is 30 cm.
    The reason I want to fill in those spaces is to increase rigidity and for safety.
     

  15. fallguy
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    fallguy Senior Member

    Keep in mind the foam core boat already floats. With nearly no engines and life jackets; air chambers forward would provide ample flotation. The urethane is not really a structural element to any boat I have seen.

    If anyone has built a urethane reinforced hull; I am not familiar with the practice.

    The seats then. Are those the two little benches on the stern? And the hulls serve as footwells?

    I also see little value in making the fordeck round on top. The foredeck, if flat, would be a usable space for say a cooler or luggage or even sunning. If it rains; the footwells will fill quickly; so you'd need two bilge pumps which is fine, but a bit unusual for a 4 meter rig.

    Perhaps I have a scaling problem, but it seems like a much nicer boat if you raise the footwells and aft freeboard and lower the aft bdeck. Of course this completely changes the design, but it looks like a paddleboat to me and I can't get past it.
     
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