Trimaran crossbeam calculations

Discussion in 'Multihulls' started by langdon2, Jun 5, 2011.

  1. cavalier mk2
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    cavalier mk2 Senior Member

    Walk (don't run;) to a machine shop and tryout a bolt in a hole with .001 tolerance.
     
  2. langdon2
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    langdon2 Junior Member

    Hi, Cavalier So what's the answer to bolting an aluminium crossbeam so the hole in the crossbeam is the same size in 2015 as it was in 2011? I didn't make up the idea of a tolerance of .001" . It comes from the Wolfson unit, no less. Cheers
     
    Last edited: Jun 9, 2011
  3. Ad Hoc
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    Ad Hoc Naval Architect

    langdon2

    If you're going to bolt, the main concerns would be bearing stress (to prevent tearing), available shear area in the alloy, the material of the bolt, the maximum allowable torque on the bolt (as this relates to its clamping force), and making sure the hole through the ally is smooth and not full of shavings or roughly cut.

    If you sort out the laod paths, from beam to bolt and from bolt to hull frame/structure, then it is straight forward.
     
  4. langdon2
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    langdon2 Junior Member

    Bolting aluminium beams

    Ad Hoc Thank you for the check list, very useful.
     
  5. cavalier mk2
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    cavalier mk2 Senior Member

    I'd think about using a tapered socket then the bolt becomes more of a lock instead of a load carrier, but then I wouldn't use aluminum anyway! Lashings with a socket would eliminate the bolts on the setup too. I don't like the fatigue curve in the loooong haul for aluminum but agree its easy to use. Robert Harris' old trimaran book shows one of his designs utilizing aluminum tubes and sockets which might be a good comparison for you.
     
  6. Gary Baigent
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    Gary Baigent Senior Member

    Langdon, a suggestion: why don't you forget about problematic, guaranteed to loosen or break, metal fastenings and just glass, then unidirectional carbon the alloy beams in solid. Far superior to point loaded nuts and bolts because with glass,carbon epoxy, the attachment area is spread widely and dissipates the loads into hull sides and bulkheads.
     
  7. cavalier mk2
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    cavalier mk2 Senior Member

    Brilliant Gary! Though isn't the usual attraction of tinker toys being able to take them apart? When I parted out the A'frame Searunner the first thing that came to notice was the mixed metals in a salt water environment causing the usual havoc. For that boat they make sense but I'd sure keep the metals coated/separated.
     
  8. daiquiri
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    daiquiri Engineering and Design

    That's indeed a very good idea Gary, but I agree with cavalier - without graphite it would be better.
    Aluminum and graphite are galvanically active in salt water, so a layer of coating is required between them. But even with the coating I would be wary of using the two materials together, because cyclic loads and movements of the crossbeam might wear the coating with time and set off the corrosion of aluminum.
     
  9. Doug Lord
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    Doug Lord Flight Ready

    ==================
    I believe Gary suggested putting glass next to the aluminum....
     
  10. Gary Baigent
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    Gary Baigent Senior Member

    Thanks Doug, you noticed. Cav and daiquiri, a couple of non-violent glass/epoxy layers against metal shields the materials from the later layers of carbon against metal fizzing problems, doesn't happen Of course laminating the beams to hulls is permanent for the life of the boat; you won't be demounting ... unless you use an axe.
     
  11. cavalier mk2
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    cavalier mk2 Senior Member

    Having cut up a boat before Gary I'd have to recommend a sawzall for most composites. Do you bond straight to the anodizing or rough it up first? It sounds like a good method for some boats though the replacing of what should be considered a wear item do to cycle fatigue sounds a bit imprecise. Reminds me of the Pink Floyd tune "Careful with that Axe Eugene"
     
  12. Richard Woods
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    Richard Woods Woods Designs

    Whenever I have epoxied glass to aluminium I have "wet sanded" it first, so that the aluminium doesn't oxidise. Seems sensible to me, but I may be wrong.

    I agree with Catsketcher, trimarans don't make much sense as cruising boats. However they do make more comfortable day sailers than open deck catamarans. So my new trimaran is a 22ft (currently) trailable coastal racer/basic cruiser. So yet another take on the Buccaneer 24 concept.

    Having said that, I always allow a long gestation period (9 months is typical, and, of course, appropriate) between initially sketching a design and actually producing builder's drawings. So don't expect anything until 2012.

    GB4 was a very fast boat and great fun to sail in flat water. But horrible in waves or strong winds. We were quite happy with the basic hull structure and cross beams. The weak areas were the beam fairings and some of the deck coamings (the latter because they were fitted at the end of a very rushed build - 3 months to launch from lofting for a 54ft one off)

    The other problem area was with the rig. Extra stays were fitted as it flexed too much. Of course these days it would have had a fractional rig and so the same mast section could have been used safely.

    So I don't think the boat actually gained much weight. Rob James was a very talented sailor and without him on board performance suffered. GB4 was designed and built for only one race so it was never really campaigned properly in major events after that.

    And another multihull history snippet. GB4 was just about the last major design John Shuttleworth did when he worked for Derek. I joined Derek in 1979, but before then both John and I had gone ocean cruising. In fact we met once in mid Atlantic, 700 miles from land. I was on a catamaran, he on his trimaran. In light winds we hove to for a chat (I have photos somewhere). You don't often see other yachts crossing an ocean, so for two multihull designers to meet at sea is unusual to say the least

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Richard

    Unless you bond the aluminium within a few seconds of 'wet sanding' it, it makes no difference. The film of oxide on aluminium forms very rapidly indeed. It starts in much less than a second when exposed to oxygen.

    However, you really need that oxide layer to enhance its corrosion properties too.
     
  14. Richard Woods
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    Richard Woods Woods Designs

    Maybe I need to clarify

    I meant wet sand with epoxy, not water. The epoxy cures and seals the sanded surface. Or is that still a waste of time?

    Richard Woods

    www.sailingcatamarans.com
     

  15. Gary Baigent
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    Gary Baigent Senior Member

    I use 40 grit and although it immediately oxidizes, the course grit cuts a mechanical bond that gives adhesion for the epoxy and first couple of glass layers for insulation - then you lay in and over the carbon tows spreading/fanning the loads away over ring frames and bulkheads. After curing, you can forget about the sleeved connections; unlike the paranoia of concern for metal to metal beam connecting nonsense.
     
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