Figuring out a Carbon Rig, Starting with a Known Aluminum Rig?

Discussion in 'Multihulls' started by Chotu, Nov 4, 2020.

  1. Chotu
    Joined: Mar 2018
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    Location: Florida

    Chotu Junior Member

    I had a thought regarding rigging. I’m ordering a bunch of structural pultrusions shortly.

    Had me thinking I may price out carbon fiber/epoxy mast pultrusions at the same time.

    If I did this, I doubt they could do a pultrusion as complex as the attached pic without significant tooling cost.

    So what if they just did me an oval and I added a track for batten cars on the outside of it?

    1). Is that a good idea with a carbon fiber mast?

    2). Is it advisable at all? Will it work or will they get jammed and stick or some other undesirable effect? We are talking large roach, flat top, full batten main.

    3). How do I attach my tangs, spreaders, other hardware to a carbon fiber mast? Can I do this with fasteners somehow? Or am I stuck using epoxy, which I can’t use?

    4). If I know the 2nd moment of inertia as well as an aluminum section that works for the rig, how can I specify a carbon section, given the material properties are different?
    Moment of inertia only defines the shape of the mast. It does NOT define wall thickness. How do I know what wall thickness to use in carbon given I know which to use in Aluminum? The below wall thickness is for aluminum.


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  2. oldmulti
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    oldmulti Senior Member

  3. redreuben
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    redreuben redreuben

  4. fastsailing
    Joined: Sep 2017
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    fastsailing Senior Member

    You need to know E-modulus for both materials as well, and for carbon mast section, it is obviously different in different directions. Typically it is at least the same as for an allow mast section in longitudinal direction for a carbon mast section. For a carbon section not designed for a mast, it's your responsibility to work it out, and your risk as well.

    Not correct at all.
    2nd moment of area multiplied by E-modulus defines stiffness of the section in bending. 2nd moment of area very much depend on both wall thickness and shape, but does not define neither shape, nor wall thickness.

    By first finding out what E-modulus is in different directions, and that will depend on:
    Amount of fibers in directions used, the directions used, layout sequence, exact fiber type used, as well as details about the process used for lamination.
    Are all those given, or can you have influence on them? If given, you must ask info for E-modulus from the manufacturer in relevant directions.
    (either 0/ 90 / 45 degs, or in directions of fibers as being used in the product)
    After that you should calculate bending stiffness and bending strength for a panel in each direction, and make sure they are at least the same as for alloy used in comparison. That way you will end up with a section that is not too weak, but it won't be as light as possible. It will be either too strong or too stiff in some directions, just like the alloy mast section is. For an alloy section, only the longitudinal properties can be closely optimized. In other directions it is typically either too strong, too stiff, or both. But you can not determine by how much just by looking at properties of the correctly sized section.

    After the section is defined, the mast can of course still need local reinforcement. Interlaminar shear strength of carbon wall does not match properties of aluminium alloy. Thus alloy mast wall survives while carbon wall without specific area reinforcements does not.
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