G10 for a tubular bracket?

Discussion in 'Fiberglass and Composite Boat Building' started by BWD, Jan 26, 2009.

  1. BWD
    Joined: Aug 2006
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    BWD Senior Member

    I need to make the crossbeam brackets for an 18' sailing outrigger canoe of my own "design." The boat is detailed here under the title "outrigger canoe plan."

    BOA is 6' and sailing weight to be under 500#.
    The ama is a low buoyancy design, about 200#.
    Crossbeams are to be round 4"x 0.125" 6061 Al tubes.

    As indicated by the title of the thread, I am thinking of using G10 tubes to fabricate the brackets to mount the tubes to the main hull.

    the max load I come up with on a bracket would be ~600 lbs,
    considering the case of a 200lbs boat monkey standing at the outboard end of the crossbeam, while the ama is momentarily out of the water.

    The idea is to make brackets that are strong but have just a little flex.
    So the tubular beam will slide in, and be secured athwartships by a bolt per bracket
    (This should be enough I think since the float volume is only ~200lbs, ie it won't fly the main hull).

    I thought this might be better than fabricating aluminum tube brackets, which would be subject to corrosion, and fatigue, as well as harder for me to work with or pay someone else to make.

    I know lashings are the convention instead of brackets but I want brackets to cut the set-up time down and reduce the PITA.

    So, to anyone with experience using these tubes, do you think it's workable?

    Specifically, by comparison to aluminum, I would think 3/16" wall g10 tube would do it, if not be total overkill,
    but it is a material I have not worked with before. The other question is width. I had in mind 4 inches (= 1 diameter)....
    As always, grateful for any feedback.
     

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  2. rasorinc
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    rasorinc Senior Member

    All this for an outrigger on a 18' hull seems to me to be way overbuilt.
    Just me 2 bits.
     
  3. marshmat
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    marshmat Senior Member

    I don't think I've ever seen NEMA G10 laminate used structurally like this. I thought it was mainly used as an electrical insulator. Can the stuff hold a bolt without cracking/fatigue problems? (Many composites cannot.)
    I would be tempted, if going for a bracket, to laminate one out of multiple layers of wood (it looks like you're using mahogany?) I think with tubular composite brackets, you might find it hard to get everything aligned just right- if the fit is tight the tubes will bind on the brackets when you try to slide the outrigger on, but if they're too loose you'll have the rattling of the bolts to annoy you. A big wooden saddle, with either lashings or through-bolts to the crossbeam, seems like it might be a bit easier to assemble at the launch ramp.
     
  4. BWD
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    BWD Senior Member

    Thanks for the replies.
    I have never seen anything of G-10 just like this either, hence the query.

    G-10 has good physical properties, and I know it mainly from G-10 windsurfing fins I have used for 10 years or so. These thin tapered foils are up to 1.5 feet long and handle good loads, maybe up to 100# or so, on a fin that's < 1/2 inch thick at the base, secured to the board by 1 or 2 screws tapped straight into the material, or into brass inserts. The first one I got with the threads cut in the G10 itself I thought would break, but after 7 years it has not, standing up to high cyclic loads, slamming over chop, jumping, etc. It's strong.

    Other applications include prop and rudder tubes on big boats, as well as skiff and dinghy parts.

    The current Wooden Boat mag has an article about using G10 for hardware backing and the like, which started the old brain gears turning. I'd previously considered winding up some brackets of uni FG and CF, but decided they would eventually crack when I hit them with something, given the difficulty of making really good thick laminates at home.

    G-10 Physical Properties are given in the mag. article as:

    Tensile 38-45ksi (=Al)
    Mod. of Elast. 2.4Msi (=1/4 of Al)
    Flex. st. 65-75ksi (compare Al yield 28ksi, shear of 24ksi)
    Compressive 65ksi
    Density 112lbs/ft^3 (Al =166)

    Anyone wonder, why I'd bother? it's a boat, that's why! :D
    I just want to build a boat that's easy to build and use, strong, light, fairly cheap, and mostly glued together.
    The G10 is not really cheap, but it needs no welding or real machining, and is flexible, noncorrosive and easily glue-able. So it may add strength while saving time and machining or welding cost.

    Marshmat you make very good points. It might be prudent to run the bolts through the beam but not the G10, straight into the beam landing/deck beam below.
    I especially respect the binding potential. An alternate plan for the brackets would have them as ring-clamps, slit at the top, possibly tensioned with big-league hose clamps, which would mitigate binding, but be uglier.
    But it's part of the plan that one could build brackets like the sketch above, then convert to a clamp in situ if it started to bind up too much -or convert it to a saddle, for that matter! Maybe I should just fast forward to the saddles, but saddle plus lashing = slow rigging, and saddle plus bolts = no flex....

    It looks like maybe nobody has tried this, but let me know if there are any more thoughts, and thanks for listening.
     
  5. BWD
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    BWD Senior Member

    Thinking about this some more, here's another alternative:

    Slit the G10 tubes at the top, forming something you could describe as a ring clamp or a bushing, and run a big T-bolt type stainless hose clamp around it, like the ones used in big diesel or racing turbo plumbing.
    These clamps seem to be formed of the same size strapping used on the main beams of nacra beach cats, so I think they would be strong enough unless the welds are bad. the better ones look to have 4 spot welds on each termination....
    This would give a strong, rigid connection between the crossbeam and the G10 tube, while allowing the G10 to flex a bit between its mounting saddles, mitigating binding potential and hopefully reducing fatigue and stress.
     

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  6. marshmat
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    marshmat Senior Member

    With the mechanical properties you describe above, I don't think you'll see significant flexing between the mounting saddles. Most of the flex will probably be in the long aluminum beam itself.
    Your last sketch looks like it could work. I'd be tempted to greatly increase the bonding area between the G10 piece and the wooden saddles- ie, make the saddles much thicker. There are big, beefy stainless clamps out there that ought to work- you might want to put a quick-release lever (like on a fancy bike's front wheel) on the clamps.
     
  7. BWD
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    BWD Senior Member

    I have seen pictures of clamps with a built in QR lever, looks nice, and cheaper than I would have thought.
    Re: bond area, maybe it needs more or maybe the the sketch doesn't make it clear?
    With 2 saddles covering 40% of the circumference, there would be 16in^2 bonding area.
    Epoxy supposedly gives 800-1000lbs/in^2.
    But in the real world, does 16in^2 sound like enough?
     

  8. phillysailor
    Joined: Nov 2012
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    phillysailor Junior Member

    An oldie but goodie thread

    Bump!

    How did this effort go? Did you go with the split G10 bracket with a quick release strap, or another method?

    I ask because I am considering various ways to mount a CF bowsprit to the deck of a narrow outrigger canoe. Leading contenders are a T-Track I've made of G10, with half bolted to the deck and half epoxied to the underside of the pole and some version of a mounting bracket like that described above.

    The drawback to the T-track are the off-axis forces (twisting) that might rip the track off the tube. The mounting bracket will probably be the winner, and I think I will lash a half tube as a clamp above the pole, pulling it down into a depression (or the mating half tube mounted) on the deck. Farther aft on the deck would be a cup bonded to the deck which receives the inboard end of the tube.
     
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