Carbon fiber davit design.

Discussion in 'Multihulls' started by svquintana, Jun 28, 2016.

  1. svquintana
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    svquintana Junior Member

    Hello. I'm building a Catamaran and would like to build carbon fiber davits for the dinghy. I've read "cantilevered davits" and would appreciate it if someone could explain to me where the main strength should come from on my davits.

    Assuming square tubular davits (they'll actually be somewhat tapered); do I want more carbon on the horizontal sections or on the vertical sections? It seems like the horizontal sections are supposed to be thicker/stronger, but for some reason that's counter intuitive to me.

    The davits are going to be holding a 150kg dinghy, possibly partly filled with water, let's say 650 kg total, (2mx1mx0.5m open space inside) with a 4:1 safety ratio. The davits will have the load at 1 metre from their mounting point.

    My plan, seat of the pants, (knowing nothing about design, but using other carbon fiber parts as a guide) was to use 5mm uni directional carbon fiber all around, plus 2mm of off axis fibers interspersed within the stack. I'm guessing this is overkill, which is what "seat of the pants" guys must do.

    My original plan was to also use core, but reading this forum makes me think that the core would be unnecessary.

    Comments?

    Thanks in advance for taking the time to read this thread.

    Cheers.
    Paul.
     
  2. UpOnStands
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    UpOnStands Senior Member

    2x1x0.5 m = 1 cubic meter = 1025 kg?
     
  3. CDK
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    CDK retired engineer

    In a way you have already answered your own question.
    With 1 cubic meter of water in it, your dinghy weighs 1150kg. With the load point 1 meter from the mounting point, each davit is subjected to a moment of 575kgf or 5750nM at the mounting point. With your suggested safety ratio of 4:1 that is more than 2 metric tons on each mounting point, an immense load!

    In the past I've made similar davits from stainless steel and found the weak point to be the mounting flange and the transom it is attached to.
     
  4. svquintana
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    svquintana Junior Member

    Thanks for your replies. I meant to imply that the dinghy would be only partly full, and used 50% as my example. I've never had a dinghy on davits that filled, but I have had some partly fill, hence my example of roughly half a cubic metre of water.

    I understand the load, but I don't understand which part of the davits should be thicker, the horizontal or the vertical parts. And why.

    Cheers.
    Paul.
     
  5. UpOnStands
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    UpOnStands Senior Member

    the horizontal faces are expected carry the load and the vertical web prevents the faces from collapsing.
    making the web the strength member is a waste of material
     
  6. svquintana
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    svquintana Junior Member

    Aha, thank you. A starting point. Would it make sense to have the horizontal faces cored?

    Paul.
     
  7. UpOnStands
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    UpOnStands Senior Member

    short answer, no.
    coring the horizontal beams is a waste of time and material.
    there are two basic beam types - I girder and box.
    building a box beam around a foam core is likely to be the easiest for the home builder.
    making sure that the lower flange stack is wrinkle-free is going to be tricky
    as CDK notes, passing the loads into the boat is critical. the transom must be designed/built with these loads in mind.
    These davits "SEEM" well socketed - tied into the transom.
    [​IMG]
    this is inflatable or rigid? cat size?
     
  8. TANSL
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    TANSL Senior Member

    Could we see a sketch of the davit and how do you fix it to the transom or deck?. Without this, it is difficult to advise you what to do. Besides, of course, to say that for both, beams working in bending or beams workin in compression, it is preferable to be hollow as the "core" does not contribute much to its resistance.
     
  9. svquintana
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    svquintana Junior Member

    Thank you for your responses.

    I couldn't get the link to work, so I couldn't see the davits. Well, the link worked, but no davit photo showed up.

    The boat is a 50 foot, resin infused, cat. The dinghy is a RIB; Caribe C-12 with Suzuki DF20 outboard motor.

    I'm afraid my computer skills are not up to posting/making drawings.

    The davit design hasn't been finalized, but I wanted to put a bit of an arch in it; partly for aesthetics, partly because I think it would make them a little stronger, and partly to get the dinghy a little higher off the water.

    Now, knowing that the horizontal pieces are the pieces that will carry the load, I can say that I'll likely build the horizontal pieces, then join them with the (lighter layup) vertical pieces, leaving 45cm at the root "unjoined". Then I'll cut horizontal slots in my aft beam (for the horizontal pieces), and shove them (45cm long) through the outer bulkhead and join them to the inner bulkhead, then install the vertical pieces (42cm long) to the horizontal pieces within the beam. I will also build a wedge, or gusset, on the outside of the beam and attached to the lower horizontal piece, to help reduce point loading on the davits, and beam.

    Any comments on the 7mm layup with 5mm of 0 degree uni carbon? Too much? Too little?

    I was planning for the horizontal pieces to be about 20cm wide, and 20 cm apart at the root, narrowing to 10cm at the outer ends.

    Thanks for your participation.

    Paul.
     
  10. TANSL
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    TANSL Senior Member

    I not fully understand how it will be your davit but I want to warn you that the union of the davit with the hull is one of the parts to be studied more carefully because of the efforts concentrated there. Not only the horizontal arm must withstand bending moments, union with the hull is very critical.
     
  11. svquintana
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    svquintana Junior Member

    Tansl, thank you for your post.

    I believe the aft beam to be sufficiently strong.

    It was designed to keep the hulls aligned, and support the mainsheet loads, and has a significantly thick layup of fiber glass. Two bulkheads of one metre tall and between 4.5m and 7.5m wide, joined by several bulkheads within the beam. Each of the, 5.5m and 7.5m, bulkheads has nearly 3mm of triaxial glass on each side, they are quite massive, and heavy.

    Cheers, and thank you.

    Paul.
     
  12. hump101
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    hump101 Senior Member

    Given the value of the cat, and the tender, I'm struggling to understand why the davits are not being properly designed. If you don't have the knowledge to do it properly, then get someone who can. Several points:

    1. The loading on the davits will be primarily vertical, but not only vertical, and the davits must resist horizontal loads similar in magnitude to the vertical mass, hence why most davits are box sections. You need to properly quantify the loading before you can consider any design.

    2. Trying to design composite structures using beam theory is inappropriate. Composite materials are anisotropic, so load paths that beam theory will ignore, such as through-thickness forces derived from distortion of the section, can prove to be the critical loads when the through thickness strength is orders of magnitude less than the in-plane strength. You need to use appropriate tools for the design, such as FEA, that can consider local as well as global buckling behaviour, dynamic response, etc.

    3. As TANSL has tried to warn you, the highest loads occur at the interface with the beam on which the davits are mounted, which is also a primary structure of the boat carrying its own set of loads separate from those of the davit. You need to ensure that this structure is able to carry ALL the loads that it will be subjected to, both locally and globally, and that the holes you cut in it do not compromise it in any way. The structure you describe sounds like you will be carrying the vertical load into the end of a 3mm thick glass skin, so that, and the inner skin, and the core, will require significant reinforcement before even considering the davit.

    I would make a single FE model of both the davits and the beam, and solve the model for a range of the worst case loadings. If you aren't happy to do this yourself, then the cost to pay someone else to do so would be around the cost of the outboard on the tender, less than the cost of the material and labour in the davits.
     
  13. svquintana
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    svquintana Junior Member

    Thanks for your reply. I may indeed get the boat's designer to do the design for the davits.
    But it doesn't hurt to educate myself before doing so.

    Thanks for your input.

    Paul.
     
  14. svquintana
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    svquintana Junior Member

    Thanks for posting that photo. It does look well socketed.
    With the angle off horizontal like that, I imagine they can be much lighter built than a horizontal set.

    Cheers.
    Paul.
     

  15. UpOnStands
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    UpOnStands Senior Member

    here's an rough test

    assuming half filled only with water total load = 650 kg
    4 x safety but two davits so 1300 kg at end of each davit 1 meter from
    aft face of transom
    transom is 45 cm thick, so reaction load on front face of transom = 1300*(1/0.45) = 2888 kgf
    assuming davit pivoting on aft face, vertical loading is 2888+1300 = 4188 kg.
    remember the old Greek "give me a fulcrum and I'll move the world."
    hope this analysis is correct.

    suggest that you sling the dingy so it is carried vertical across its width -- without fail -- to prevent water collection --- without fail. Water is just too damn heavy.
    long time cruisers agree?
     
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