carbon fiber for a dinghy?

Discussion in 'Materials' started by rnlock, Aug 15, 2022.

  1. rnlock
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    rnlock Senior Member

    If you were making, say, a 7 1/2 foot dinghy, you didn't want to mess with foam, but you were crazy enough to use carbon fiber, what kind and how much carbon fiber? Or would it just be nuts to do this? This dinghy wouldn't be abused much, and the lighter the better if it's not flimsy.
     
  2. wet feet
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    wet feet Senior Member

    The big risk is puncturing such a light boat that would otherwise have sufficient strength.By the time you have arrived at a stiff enough hull,that will resist odd pebbles,the cost of carbon and the inevitable epoxy really mounts up.If you are wealthy enough and determined enough-go ahead.You may just have given the Guinness Book of Records another category to monitor,most expensive dinghy of it's size ever built..Probably best to use a twill weave .If you have plans to use anything other than a female mould,consider it a fail of the very lax sanity test.

    Or you could buy some gaboon ply and build something that wouldn't be such a wrench if the tide takes it away.For use this summer you might find a bargain on the usual sales sites.
     
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  3. fallguy
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    fallguy Senior Member

    Carbon fits well with plywood afa elongation goes. I think an ultralight ply-carbon dinghy is a great idea. Solid carbon, not so much.

    Curious about 4mm ply with carbon/aramid twill each side...
     
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  4. rnlock
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    rnlock Senior Member

    I think 4mm ply might almost be enough by itself. A balsa core would be lighter, and would justify the carbon. ;-)

    Hoping someone will have a specific answer as to the type and amount of carbon fiber, so I can decide for myself. Even if I'm crazy.
     
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  5. Tops
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    Tops Senior Member

    I would not do it as carbon fiber, just make something with plywood, tape and resin the seams with bias tape so it doesn't split as it rounds the rounded corners, paint, enjoy. I forgot the 'bias' and 'paint' part on mine (8 foot PDR) and am now looking at sanding off split tape and sun-baked epoxy used as varnish and checked plywood to repair it after 5 tough years in the weather.

    If you wanted to see how core and skin laminates interact, you could sign up for free at the Vectorply website and use the Vectorlam software online.

    I was originally told by a supplier to buy 2 layers of 1808 45/-45 w/ mat for my current repair on the 21-footer. By running comparisons, talking to folks here, and being coached by someone at Vectorply, I determined I needed 3 layers of the 1808 with 2 layers back at 0/90 degrees with new plywood to be comparable (+ or - a percent or two on key mechanical indicators) to the existing thickness of chopped mat and plywood, whose plywood was destroyed by water ingress. Being able to salvage the purchased (glass) and on-hand materials (fresh ply) is nice. Using Coosa would have required even more Coosa and more glass to be equitable. Overbuilding could have led to parts of the boat being stiffer than intended, creating stress elsewhere on the hull.

    I totally get wanting to use carbon fiber though. I added some Twaron 'vectornet' off-cuts when I tabbed the new side berths back to the hull, right where I intend to sit, thinking that it'd 'hold me up' better... :) I like to use small pieces of carbon fiber on wooden paddle blades and what not for show, the rest of the time I bury them in colored epoxy so they don't heat up from being black in the sunlight and cause other issues.

    @fallguy ,The aramid/carbon twill would look sharp, like a We-no-nah canoe, but one would have to keep it inside or cover it.
     
  6. fallguy
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    fallguy Senior Member

    4mm ply unsheathed is really not enough, a wear layer on each side would work and add stiffness and the pretty factor

    Despite our recent online spat, the 4mm ply or 3mm could get laminated inside with glass and then all tabbed together stitch n glue in a female jig and then you could lay the carbon on the exterior for looks in a double go down the middle to each side. You could spray it with a UV clear something or a few coats of epoxy and then the uv clear and it'd be gorgeous.

    Otherwise, you can also do the same thing with a lighter core if you want to push the weight down, something like M60, but the properties are misaligned..touche'? and I was warned by @rxcomposite when discussing a dinghy that stiffness was the goal and a single layer of glass wouldn't provide it on foam

    Here is my carbon/aramid console. The boat would be easier as this was sprayed with uncooperative sunshield in mould. Any clear UV spray would be much simpler on unwaxed substrates.

    It is a way cool idea. Wish I could spec layups. 3mm ply would be super light, but in order to radius the outside edges; you'd need some decent fillet inside.
    948458E6-A693-4C94-A313-28BD53847953.jpeg
     
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  7. rnlock
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    rnlock Senior Member

    I mentioned it above, but for this project, ply wouldn't really be right. Think miniature Cape Cod Cat. All curvy.

    If I used carbon fiber, it would be for lightness, not appearance, and it would be painted over.
     
  8. fallguy
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    fallguy Senior Member

    The problem of light is that it is relative.

    3mm okume ply is 11 pounds
    3 sheets is 33 pounds
    Also 96 sqft or 10 yards or so
    A layer of 5 ounce carbon and 6 ounce woven doubled is 22 ounces or 220 ounces of resin and glass or 14 pounds

    A 47 pound, call it 50 pound dinghy is light.

    Now do a layup for foam. It isn't lighter, unless you reduce resin content because you can't use that layup in foam. So, you end up infusing or wet bagging.
     
  9. ondarvr
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    ondarvr Senior Member

    There isn't a way for someone to give you a layup schedule that is "correct" for what you have asked.

    It depends on the exact shape of the design, the payload, how carefully you plan to handle it, how much you plan to use it, what are you willing to give up to achieve the lightest possible build, etc.

    I had a very light and thin boat about that size, but made with glass. I carried a roll of duct tape to make repairs when it hit a rock. I finally added multiple layers of glass to the most vulnerable areas on the hull. Carbon isn't as easy to fix after its been damaged.

    It doesn't take much of a laminate to allow a boat to float, with the right shape one layer of carbon would do it. Making it a usable craft requires much more effort on the designer/builder's part.
     
  10. rnlock
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    rnlock Senior Member

    I though I'd shrink something like this 1895 catboat until it displaced, say, 350 lbs. Intended for single sailor up to 250 lbs, which I think might be under 8 feet. Deadwood would be eliminated. It would have a dagger or centerboard, possibly a little longer and higher aspect ratio than typical for this type. The dagger or centerboard would be heavily weighted, maybe even 50 lbs, and could be carried to the boat and put in while it was floating.
    https://lh5.googleusercontent.com/-N0TqtGm_mgY/TtVi-2Po4PI/AAAAAAAAC0I/GGmIa2xrW18/s640/Top-6.jpg

    Or, maybe, start with something like this, with the same features:
    Atkin & Co. - Handy Andy https://atkin.mysticseaport.org/Dinks/HandyAndy.html
    My approximate model in Solidworks has 43 square feet of hull, though I might go for a little less freeboard.

    There would be a partial deck, with a coaming, and maybe a bit more freeboard. I'd hope to minimize dirt and water catching corners, though if the weight problem was a serious obstacle, I might consider a skin on frame deck. I'd still want to be able to cartop it, and the lighter it was, the longer I could keep doing that. Right now, I think I could cartop at least 75 lbs of boat, and maybe more. In a couple of decades, maybe not, so I'm hoping for less than 50 lbs. unrigged. Also, with a boat that light, it can be carried past the long line at the boat ramp and be dropped in the water next to the boat ramp. A very satisfying thing to do at, say, this boat ramp:
    Google Maps https://goo.gl/maps/1FR2g9dTM6FVS5B26
    If it's the one I remember, it was very busy that weekend.

    Use would be occasional, with moderate care. Not a boat I'd loan out to anyone I didn't think was a more careful sailor than me. Which definitely precludes one friend I can think of. "Are you sure you know where we are? There are a lot of rocks." "Yes." KLUNK BANG THUMP.

    Could you tell me what layup you used in that boat, before and after the reinforcements? I suspect that anything moderately stiff and puncture resistant will be plenty strong.
     
  11. rnlock
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    rnlock Senior Member

    50 lbs would be light if that was everything not removable from the hull, but it's not. OTOH, I have a 7 foot dinghy, originally built for a friend, that weighs that much despite being really squarish and being made from Southern Yellow Pine underlayment. It doesn't have a partial deck, coaming, etc.

    I think if I was going to use plywood, I'd just use something a little thicker and forget about the carbon. As far as a foam core, what's the point if you can't use less glass or carbon? Vacuum bagging is a real pain, especially if you don't do it often. I once spent a week working for a guy who vacuum bagged 18 foot boats, and even in that case, despite all his practice, it was kind of a pain. If I was going to build 10 of these, it might be worth it, but around boat 3 I'd get terribly bored. Maybe boat 2. So a core would only be worth it if it didn't require vacuum bagging, I think.

    Anyway, I'm still trying to figure out what a full carbon layup would require, though I might have to find cheaper carbon! Soller has 19 ounce twill for $34 per yard. If a single layer of that was enough, it might be one way to go, but multiple layers increase expense fast. I suppose I could just figure out how stiff 5 or 6 mm ply would be, and work from there.
     
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  12. fallguy
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    fallguy Senior Member

    19 oz carbon over what?
     
  13. rnlock
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    rnlock Senior Member

    Well, that's the question. Whether air would be enough.
     
  14. fallguy
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    fallguy Senior Member

    Well, do you have a mold?

    I mean, something has to provide the shape.

    I don't know for sure, but man 19 oz carbon ain't much.. pretty sure I can bust it with a foot stomp.
     

  15. ondarvr
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    ondarvr Senior Member

    A 50 pound center board, even if set down gently could damage a thin carbon laminate, drop it and you might be in for a total rebuild.

    Mine was a cat with a sail kit, it was one 1.5 oz mat and one 24 oz roving to start with. The keels on these are known to be thin and fragile. I put 3 or 4 layers of 1708 on the keels and 1 layer on the lower portion of the hull. This helped a great deal, no more duct tape needed. I wasn't using it as a sail boat at the time, I used it for fishing.
     
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