Bolting Travelers to Carbon Fiber Beams

Discussion in 'Multihulls' started by UNCIVILIZED, Jul 5, 2017.

  1. UNCIVILIZED
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    UNCIVILIZED DIY Junkyard MadScientist

    On a lot of bigger multi's, especially tri's, it's common to bolt the traveler track to one of the rear main beams. Which, I would think that having so many bolts in a carbon fiber structure would weaken it a good bit. Since as a general rule it's not smart to bolt fittings onto carbon spars, or machine exit holes for lines into them, etc. So how do you get around this with travelers?

    Is there enough extra reinforcement built in to account for the bolts forming a "tear here" line? And or, might you build in a slab of G-10 to bond the bolts into? Speaking of which, are nuts & washers used on the fasteners, or are they simply epoxy bonded into the beam's structure? How about backing plates?
     
  2. UpOnStands
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    UpOnStands Senior Member

    A line of holes that runs parallel to the stress for which the beam was designed is generally not a significant issue. I assume we are talking about 10-12mm dia holes on 15-20 cm pitch.
    Drilling a series of holes across the stress line is definitely not recommended.
    Best of course is to form the holes when forming the beam so the fibers are not cut.
    As to how much/type of reinforcement, it depends on traveler loads and design of the beam.
     
  3. John Perry
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    John Perry Senior Member

    Although it was a slight deviation from the rest of my career, for a couple of years I was involved in the design and building of large sailing yachts, including some months working at a specialist company that made carbon and aluminium yacht spars. This was some years back, so I may not be right up to date with the most modern methods.

    For bolting heavily loaded fittings onto a carbon fibre mast we would never just tap a hole in the carbon fibre, that would only be done for very lightly loaded fasteners, perhaps to hold a block for a flag halyard or suchlike. Heavily loaded fasteners always screwed into some kind of backing plate. Sometimes the backing plates were stainless steel plate, either with tapped holes or with stainless steel nut(s) welded to the plate on the inside. Since in our case we were adding fittings to a complete mast tube we had some real fun and games inserting backing plates half way up a 100 foot plus mast, then getting a fastener to pick up the threads from outside the mast! Lots of time consuming work fiddling with drain rods and the like! This is probably one reason that carbon fibre masts are now made in two halves that are then glued together.

    If a long row of fasteners was needed to hold down a track then the backing 'plate' would be a long strip with a series of tapped holes through it, this avoided having to line up each hole individually. Sometimes we would use a strip of stainless steel with nuts welded to it, sometimes a bar of 'Tuffnel' with tapped holes and sometimes aluminium extrusion. If the later, a layer of GRP had to be laminated between the carbon and the aluminium to avoid electrolytic action. I guess G10 has similar properties to Tuffnel when it comes to holding screw fasteners, although I havent looked into that, so I guess you could consider using that for backing plates.
     
  4. UpOnStands
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    UpOnStands Senior Member

    Good to have your input. "This is probably one reason that carbon fibre masts are now made in two halves that are then glued together."
    What is the current thinking?
    Finish the two halves completely then glue? Or wind on carbon tow after gluing?
     
  5. redreuben
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    redreuben redreuben

    Probably not helpful but the best metal to use around carbon is titanium. $$$ but light.
    Retro fitting to carbon you should add extra laminate where possible.
    I have seen carbon nuts and bolts online, bicycles use them a lot. Not sure if the thread strength is up to the job or not.
     
  6. tspeer
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    tspeer Senior Member

    The traveler may not be bolted directly to the beam. There may be a flange that takes the bolts, either molded as part of the beam or bonded as a separate part.
     
  7. rob denney
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    rob denney Senior Member

    No secondary bonds (one piece mast) is a better option. It is now possible to build one piece masts with sheer webs/bulkheads and integral carbon track into which slugs are inserted for the track to bolt to. see attached This is far preferable to drilling holes. We are currently putting the finishing touches to a 3d printed mould for a mast with integral rcb track. In the past, these have ben made separately and glued on see attached. One of these has been on a 50'ter for the last 10 years with no sign of damage.
    Winding on carbon under load is not a problem, laying it on at anything other than 0 degrees (along the mast) then vacuuming or compressing it is, as the fibres have to conform to a smaller diameter and crinkle.
     

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  8. UpOnStands
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    UpOnStands Senior Member

    Sure is, unless you are a home builder.
     
  9. John Perry
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    John Perry Senior Member

    That looks very good Rob, how did you make it, or is that a secret? Maybe two female moulds joined together with hinges all along either the leading or trailing edge then fold together with a channel section in the middle before the resin has cured? - I did that once to make a small Tee shaped hydrofoil with a hollow strut.

    Carbon mast manufacture must have advanced a bit since I was involved in it.

    The firm I worked for (which is no longer in existance) used a very large filament winding machine to make mast tubes. It was a special machine that could wind a 'slow' spiral round the mandrel, so we could get fibres something like 10 degrees to the long axis. AFAIK the fibres did not crinkle, they were prepreg tow wound on under tension and vacuum was not considered necessary for consolidation although we did wind on a throw away top layer of 'shrink tape'. I think the main limitation of the process was that it is not possible to wind fibres truely paralel to the long axis. For stiffness it is necessary to have most of fibers truely longitudinal or very close to it, 10 degrees off axis is not really good enough. You also need some fibres at much larger off axis angles to keep the cross section shape stable against buckling and resist torsion, but that is easy to achieve with filament winding - but I expect you know all about that.
     
  10. rob denney
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    rob denney Senior Member

    The beams and telescoping mast sections I built for Bucket List were built to develop a technique for home built round tubes that did not need moulds or mandrels. Worked well, and we have since modified it to include bulkheads. Non round sections are more challenging to do in one piece without a pair of moulds, but can be built without recourse to gluing the halves together, which is always a worry for highly loaded 'blind' joins where pressure cannot be applied.

    John,
    The wing mast in the photos was infused in one piece. 2 moulds, extensive knowledge of laminate thickness and behaviour and two vac bags. Definitely not for the timid!

    10 degrees off axis is a major loss of stiffness. We apply the zero fibres by laying uni on between winding wraps. The filament winding applies the pressure to the unis and no further compaction is required. Filament winding is amazing for jobs that suit it and operators who know what they are doing. Attached is a curved, 7m long forebeam which we filament wound.
     

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  11. UpOnStands
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    UpOnStands Senior Member

    Looks good. Do you have a weight for that beam?
     
  12. rob denney
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    rob denney Senior Member

    It has a 420mm x 280mm elliptical section, 6.24m long, arched, with a lug for attachment of the mast forestay fitting, rated to 10,000kg and is designed to be strong enough to take the forestay loads without a striker. It weighs about 60kg, not including the prodder. It was filament wound dry, then vacuum infused.
     
  13. UpOnStands
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    UpOnStands Senior Member

    All good details and very impressive.
    "filament wound dry" sounds as easy as brain surgery. I'll leave that fun to the experts.
     

  14. UNCIVILIZED
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    UNCIVILIZED DIY Junkyard MadScientist

     
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