Foam sandwich dinghy, laminate schedule?

Discussion in 'Fiberglass and Composite Boat Building' started by RMA, Feb 16, 2020.

  1. Jolly Mon
    Joined: Jun 2019
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    Likes: 5, Points: 8
    Location: East Coast, USA

    Jolly Mon Junior Member

    I think you’re suggesting silk. The stuff I’m using is polyester.
     
  2. RMA
    Joined: Sep 2019
    Posts: 8
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    Location: Middletown, CT

    RMA Junior Member

    It's 100% polyester, "silk". Cheap enough. I'll test it out and report back.
     
    Jolly Mon likes this.
  3. Niclas Vestman
    Joined: Aug 2017
    Posts: 128
    Likes: 15, Points: 18
    Location: Malmoe, Sweden

    Niclas Vestman Senior Member

    I think it's low weight comes from the fact that it's basically a supersized windsurfer. Thin laminates not for banging around (guessing), and very little surface area, (asside from low freeboard, it's mostly semi parallel deck/bottom). Giving a circumference of laminate at the mid section of less than 3m/10', wheras the discussed dinghy has about 5,4m/18'.

    Frankly, your Duo weighs around 22,5 kg/50 pounds inc brackets and trim. Even if slightly narrower, it is probably both more resiststant to wear as well as easier, faster and cheaper to build.

    Just a thought. For a true ultralight and tough dinghy, I believe maybe a hybrid design would be possible to build even lighter. Maybe 1.5 feet longer and slightly narrower (10' x 4'-4,5' to get the same loadcarrying capacity while gaining some form stability for small outboard power. And maybe 4mm/ 1/6" ply floor 1,8oz glas inside+ 4oz outside with 5 1"x 1/4" underneeth sacrificial rubrails, for in and out ruggedness. But (fake)lapstrake style carbon foam sides, tilted about 12deg outwards. 3 x 8cm/3 1/4" "foam-planks" overlaping about 1" and doubled laminate over these 1" overlaping edges, giving built in stringers/"rubrails" (maybe about 9mm/ 3/8" H60 foam with 200gsm carbon doubled 1" over protruding outside edges (should ideally be about 60% of the weight of 4mm sheeted ply sides). And all topped of with 30cm/1' (small)diameter light weight inflatable tubes 900gsm/22oz pvc. And the "half size" tubes provide cushioning to the fragile sides, as well as flotation, stability and seating area. Most importantly giving a lot of thought to reduce surface area as well as using reducing weight where possible. That might be a true light weight option. .. (Just an amateurs train of thought/ wishful thinking).

    E.g. building basically an almost rectangular drawer with wide all around decks gives almost the largest possible surface area, with little gain in form stability.
     
  4. Niclas Vestman
    Joined: Aug 2017
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    Location: Malmoe, Sweden

    Niclas Vestman Senior Member

    Actually the idea is similar to the Woods Duo but even more so to the Winboat F-Rib 430. Just smaller in size. Can't upload a picture. Easy to Google.
     
  5. MVinter
    Joined: Feb 2020
    Posts: 2
    Likes: 0, Points: 1
    Location: Norway

    MVinter New Member

    I’m new to this forum, but have similar ideas as you have. I love the idea of the OC tender, but the price is just to high. I also would like to include self bailing, if possible without adding to much weight? Any ideas?

    I’m also wondering if it’s possible to ad rig and sail without adding to much weight. Some kind of windsurfer rig maybe? but the weight of center board case and mast foot will ad weight and how fun will it be to sail?
    But love the idea about electric outboard and sail as propulsion.

    I have fairly good skills and experience repairing fiberglass and working with epoxy, but absolutely no experience with building new things. Hoping to start with a similar dinghy as you are. I understand that ply is probably easiest, but i would guess a carbon or glass foam laminate is lighter and stiffer. Is it possible to cut “mold-frames”, and put pvc foam on the frames using sheets, stripplanking with foam and “squarecut foam” (sorry for not having English terms for everything). Then lay one layer of FG or CF outside, flip the hull, and repeat internally?

    sorry for hijacking your post, but seems we are “in the same boat”. Please tell me, and I will delete and repost new topic.
     
  6. waikikin
    Joined: Jan 2006
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    Location: Australia

    waikikin Senior Member

    If I was doing this on foam I think I'd be using light woven cloth on it maybe 2 x 6 oz on the outside & 1 x 10 oz on the inside, the laps at chines and centreline would add some margin for chafe and dragging on the beach & some extra layers easily added where needed and heaps easier to fair and lay down than stitched fabrics- a quick screed of filler to fill the weave is fast to fair & sand & if you get a bit of print later the woven is a nicer look .
    All the best from Jeff.
     
  7. RMA
    Joined: Sep 2019
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    Location: Middletown, CT

    RMA Junior Member

    MVinter, it seems like a perfect continuation of the thread to me.

    Yes, self-bailing would be nice. I haven't worked that out yet either.

    I also considered a sail rig and centerboard, but this would result in a totally different boat. In order to achieve even 70 lbs, this will be a fairly "tender" tender. To handle the extra loads associated with a sail, everything would need to be much stronger and much heavier. My guess is a sail rig and the added glass to stiffen up the hull would easily top 110-120 lbs. Also, the design of the OC tender could likely be replicated with plywood but there is no way you'd achieve similar weights.

    This will be my first boat build as well, and the method you describe is what I'm planning on. It's similar to the one-off foam/GFP construction seen here. Fortunately, my design has many large flat areas with the upper part of the hull having a curve in one direction. Only the bow will need some complex foam cuts.
     
  8. RMA
    Joined: Sep 2019
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    Location: Middletown, CT

    RMA Junior Member

    Good points, Jeff.

    My only concern with woven cloth is that it tends to require more resin to wet out (more weight) and isn't as stiff as biaxial.
     
  9. MVinter
    Joined: Feb 2020
    Posts: 2
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    Location: Norway

    MVinter New Member

    good to hear, then we are in the same boat.

    Definitely share your concerns regarding weight when reinforcing for mast and centerboard. The centerboard is ok for me as I think it’s a good idea to move the forward bench further aft, even if no senterboard. Don’t need the big open space, and who would sit in the bow rowing??? The bench will then stiffen the center board case.

    self bailing; the only fairly good solution I have thought of is a higher deck with skin (CF/FG) only upper side and fill void with expander epoxy foam. For now I have to concerns, water in foam and weight...

    Do you have self design, got plans from other or just freestyle in workshop? Which size are you thinking of? I have not decided yet.
     

  10. waikikin
    Joined: Jan 2006
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    Location: Australia

    waikikin Senior Member

    Hi RMA,

    You need to look at the structure and geometry/architecture of the various fabrics, the stitched fabrics on larger vessels are very effective and there's the non crimp as well which has advantages - on smaller boats I feel that the stitching as especially raised to one face & the size of the fibre bundles also really take some material/resin or filler to get to a smooth finish, the peel ply bridges the peaks with resin - the lighter woven drape and wet out like a dream and take just the lightest screed of lightweight filler to smooth, extra reinforcement can be added locally easily for anticipated loads. If you're doing a foam sandwich vessel the stiffness come from the separation of the skins either side of the core. I'd shy away from any vacuum assisted process and just lay the skins & screed some filler while still green. Your weight target seems very high for such a small vessel- best looka t how small racing sailboats are built then add a margin for durability- just my random thoughts:)
    Hope you enjoy the process.
    Jeff.
     
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