Homemade fiberglass mast strength calculations

Discussion in 'Boat Design' started by laukejas, Jan 24, 2021.

  1. rxcomposite
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    rxcomposite Senior Member

    What you are trying to do is filament winding in principle. Buy or borrow a book from a library Filament Winding Composite Structure by ST Peters et al. The calculations for thickness and strength at different angles (hoop/axial) of winding. The book itself is an engineers bible and can be daunting if you try design. It deals mainly with pressure vessel but the load model you are doing is a thin tapered cylinder, cantelever or partially supported and involves Eulers principles. If not a cylinder, can be oval or D shape but still, the thickness along the length will vary.

    Winding itself is easy. The madrel is rotated slowly and the filament is laid in tension. Excess resin is removed by squeege.
     
    Last edited: Jan 25, 2021
  2. kerosene
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    kerosene Senior Member

    no help with calcs here but ice sailors have used fiber over wooden mast successfully. I have read in many places that wooden mast will only get heavier with not enough gains from fiber but I trust that the ice boat folks wouldn't do it just for fun - and often seem more open minded than boating folks. not meant as a judgment.
     
  3. kerosene
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    kerosene Senior Member

  4. Robert Biegler
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    Robert Biegler Junior Member

  5. Olav
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    Olav naval architect

    The UK Cherub guys have set up some nice instructions on how to make a carbon mast: *click*

    The general process would be the same for any other kind of fibre if you do not want to use carbon.

    The trick with cutting the mast lengthwise if it happens to get stuck on the mandrel is very good. When I made some carbon tubes several years ago I tried a different way to release the tube after curing: I made a sleeve out of paper on the mandrel that could slide up and down, wrapped this with brown parcel tape, and eventually laminated my carbon tube. After the resin had cured, it was very easy to slide the tube (together with the paper sleeve) off the mandrel and then remove the paper/parcel tape stuff from inside the tube. Just use a long stick to poke between the sleeve and the tube to release and then pull out the (now somewhat crumpled) paper.

    I used to source my composite materials from R&G in Germany who also ship their stuff to other European countries. In addition they have a sales partner company in Lithuania.
     
  6. laukejas
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    laukejas Senior Member

    Thank you for such an extensive info. I now see that this is probably not a very feasible project. Some of the other answer suggested cutting a straight slot in the laminate to release it from the mold, and then glue it back together with a few more wraps of cloth on top, but I suppose aligning the cut back together would be quite unwieldy and further complicate the project.

    I have built wooden masts in the past when quality lumber was available, now it's not anymore. To get anything near decent quality would require shipping from Sweden or Norway, which would cost a ridiculous amount of money just for a few spars. The lumber available in Lithuania is of truly terrible quality, full of knots, defects, with crooked, loose grain, requiring scarphing every few feet of length.

    Ad Hoc, Rxcomposite, kerosene, Robert, Olav, thank you for your input as well, you gave some really good info and links. If I do go for composite mast, it will be most useful.


    Okay, then another idea. What if I build a birdsmouth wooden mast with a VERY thin wall (1/4" or less, just enough to hold the shape), and use it as a permanent core for fiberglass winding? That way, I wouldn't have to worry about removing the core or the poor quality of lumber, because the majority of structural strength (ideally 90% or more) would come from fiberglass laminate. This way, I would still end up with reasonably lighter mast than a purely wooden one (which is a nightmare to build in Lithuania due to afore-mentioned quality lumber shortage), and it should be much easier, right? Is this doable?

    And if I do go this way, would it still be possible to build a 2 piece mast with a slide-in joint, like commonly done in windsurfer masts? I suspect the joint would have to be fiberglassed as well on both pieces (male and female), and then wrapped with additional fiberglass or sanded until a perfect fit is achieved. Can this be done? I need 2 piece mast because I don't have storage space for 18 feet mast, and my workshop is much too short to build a single piece mast in the first place.
     
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  7. rxcomposite
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    rxcomposite Senior Member

    Try to get a copy of the book. It shows several way of making a plug, collapsible mandrel. One is salt that can be dissolved. High density styrofoam then dissolve in acetone.

    Or you can make it out of solid wood, designed so can split in in several pieces to remove.

    The challenge is you have unstayed mast and the section shape based on load distribution, will be moderate diameter at base, large at the point where boom is attached, tapering to the ends. You can make it just a simple taper, just build up thickness where it is needed but it won't look so good.

    Do you really want to build it out of carbon fiber? It is going to be a very expensive project.

    Uni glass + epoxy, maybe I can help.
     
  8. ondarvr
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    ondarvr Senior Member

    I used to make a lot of pipe and tubing.

    Most mandrels were made to collapse, but smaller ones may not be. We wrapped them with corrugated cardboard, this allowed the part to shrink and crush the cardboard and not be locked in place.

    Mylar was wrapped over the cardboard to create a mold surface that needed no prep. It's also slick.

    Both can be purchased in rolls at various widths.
     
    Last edited: Jan 28, 2021
  9. rxcomposite
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    rxcomposite Senior Member

    Greatly exaggerated, profile will be this based on load diagram. Unstayed Mast.png
     
  10. laukejas
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    laukejas Senior Member

    I didn't say I'm going to use carbon, I believe (unless I made some type somewhere!), I always planned to do this with fiberglass and epoxy. Carbon is waaaay too expensive and risky. I will try to get that book. Thanks for that illustration on taper too, that's exactly how I build all my masts. Collapsible core still sounds quite complicated, but then again, I haven't read that book yet. What about that idea with very thin walled wooden core that would remain as part of the structure?

    Could you tell me more about how you made these mandrels?
     
  11. rxcomposite
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    rxcomposite Senior Member

    The requirement for cylinders is that the inner layer has to be radially wound to increase hoop strength then axial wound (lomgitudinal) which takes up the bulk of the bending stress. Wood is longitudinally grained, like bamboo. It is finished with a +45/-45 layer to hold the uni together and to resist torsion.

    The hoop strength is needed to prevent the walls from collapsing. This can only be mitigated if you have nodes or bulkheads inside but it is not possible.

    So maybe you can layup 0 degree lamination first then glue the wood. Or maybe you can layup the wood on plywood patterns that defines the sections.
     
  12. rxcomposite
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    rxcomposite Senior Member

    It's me. Got mixed up on post #17.
     
  13. rxcomposite
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    rxcomposite Senior Member

    Think of it as bamboo. It has soft inner core with nodes. The core toughens and more mature as it reaches the outside. Covering is a dense calcium like surface. Problem with bamboo is it is dense and too heavy to use. Rattan is good. Soft inner core, very tough outer skin. Very light for its strength. Problem is it does not grow that big.

    Composite will be made with the same principle.
     
  14. laukejas
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    laukejas Senior Member

    So you're saying that the grain direction of the wood would make more sense to be on the outside? That makes sense to me, but then again, unidirectional fiberglass, even if laid at +45/-45 degrees would still have a higher elastic modulus than wood at comparable weight, right? Even if the grain direction of wood isn't ideal if it is left on the inside, I am simply considering this method because it should be very easy to make (compared to collapsible and removable cores)... Or am I wrong?
     

  15. ondarvr
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    ondarvr Senior Member

    For the most part the mandrels were metal and 20' long. But could be wood, and sometimes we used cardboard Sonotubes.

    Fishing poles are made in the same way, small diameter steel mold wrapped with carbon prepreg. They tightly wrap the outside with mylar to consolidate the laminate before curing.
     
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