offset beams on catamaran genius or deathtrap?

Discussion in 'Boat Design' started by sailor182, Jun 1, 2013.

  1. sailor182
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    sailor182 Junior Member

    Hello, I'm messing around with a system that would allow an open deck catamaran to slide in and out to reduce setup time off the trailer. My concern forgetting about the sliding aspect for now is how will the offset effect structural integrity.....:confused:

    I added an attachment to show you what I'm talking about note it is not to scale. :p
     

    Attached Files:

  2. rwatson
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    rwatson Senior Member

    Properly engineered, there is no reason that it couldnt work.

    Telescoping tubes have been used in the past, but that creates problems with stuff in the sleeve, making it awkward.

    Also, tubes are also not the optimal crossbeam shape. Parallel beams could solve some rigging problems.
     
  3. tomas
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    tomas Senior Member

    Kurt Hughes seems to have worked through the issues in his sliding designs

    [​IMG]

    [​IMG]
     
  4. sailor182
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    sailor182 Junior Member

    I'd be lying if I said this wasn't my inspiration.
     
  5. tomas
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    tomas Senior Member

    Well then, you can potentially spend years working on the structural/construction details yourself (with no guarantee of success) or pay someone to help you.

    Is your plan to design and build this yourself?

    Did you look at the Cat2Fold build?
    That's functionally close to what you're describing.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    You need to address the length of "overlap" that remains, and whether this is effective enough to transmit and shirk the bending moments from one to the other effectively.

    Only calculations will tell you if your current arrangement is feasible.

    The images shown above, clearly the "overlap" is significantly more than yours. Ergo...yours upon casual inspection is highly suspect and without any hard data, I would be very concerned.
     
  7. sailor182
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    sailor182 Junior Member

    On paper the only rigging issue I foresee is the traveler..... probably going to have to stow it when the cat is folded. As for the beams themselves tubes would be the easiest to fabricate. Carbon fiber is too expensive and I'm not sure if fiberglass is strong enough for the job.
     
  8. sailor182
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    sailor182 Junior Member

    After doing a little math I determined that the hull beam of the tri is a little over 8' cut that in half you get a 4' overlap give or take a few inches. I'm looking at about 1' overlap on that cat. It's not hard data but at least I know where I stand in comparison.
     
  9. eyschulman
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    eyschulman Senior Member

    A little problem. You really can't compare the tri with water stays to absorb the rig forces and convert them to compression with the cat arrangement where all the rig forces will be transmitted to the overlap. With out any math I would guess with only one foot of overlap you are going to need NASA's resources to hold a cat with any kind of rig together.
     
  10. groper
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    groper Senior Member

    On a positive note, with your setup and the sleeve in the middle of the beam, its in the lowest most loaded point on the beam. The highest loaded points are where the beam meets the hull - which is the beginning of the cantilever. This point is moot however, if you go and place a mast or similar in the center of it, in which case your really going to struggle to find a material strong enough which wont cripple you in terms of weight...
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    That's not correct. The location for the bending moment, in a classic transverse bending moment case is mid-span. When looking at a torsional longitudinal moment the max bending moment is mid-span.

    The fixing to the hull sides, is just a reaction load, i.e. shear.

    The central beam is not doing anything really for either of these 2 load cases. It is so close to the centre of the load cases, that its effectiveness in resisting the TBM or TLM is negligible. It is far better to put the structural stiffness where you need it, the 2 ends and delete the middle one.
     
  12. groper
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    groper Senior Member

    Interesting... what is it that im reading into from this except from the NR classification;

    [​IMG]

    My interpretation could be wrong, but id like to know why?
    Full details here; http://erules.veristar.com/dy/app/bootstrap.html

    Goto; NR500 classification of Yachts, Hull in composite materials and plywood, section 2 article 5, "global strength of catamaran (longitudinal and transverse)"
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Groper

    Take a hull that is experiencing a classic transverse bending moment (TBM). The hulls are being pulled together or pushed apart by the vertical load from each hull.

    Now, you could assume that the ends are fixed or built in, in which that analysis you show is correct if we treat the beam being “built in” as such (but the shear force line is wrong, assuming its a UDL). But is it?....what is holding the hull to stay fixed or built-in, in the structural sense, of the cross beams? The hulls are independently moving from each other and the along the length of the hull the structure stiffness is not the same, that is the moment of inertia of the section at the bow is different from that at midships and is different from that at the stern. Therefore, the conditions along the length of the hull could not be reasonably considered as “built in”.

    The fixing of the cross beams to the hull, they prevent translations, but no rotations; which is the basic definition of simply supported structure. Whereas built-in requires the ends to have zero deflection and zero slope , which is clearly not the case either espeicaly with a varying stiffness along its length. Thus see the basic loading below:

    typ TBM.jpg
     
  14. groper
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    groper Senior Member

    Well the context of the original posters question, is for a small trailerable catamaran, presumably a beach cat type of recreational boat or perhaps a small outboard motor type of catamaran, we dont know, but we do know its small. If a sailing beach cat type, these have to be designed with enough strength to "fly a hull", so the opposing hull needs to support the entire boat minus the lee hull, all in catilever fashion from the lee hull.

    Would i be correct in saying that in that case, the cantilever loads are higher than the simple TBM load case you described? And if so, then the beam to hull connection has to be the strongest point of the beams?
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    It depends upon the weight of each hull (total) and the lever, ie the hull spacing. Since it is moment (force x distance) ...thus you need to calculate for both your scenarios to establish which is the most onerous. But then of course you still have 2 other cross beam loading scenarios to consider too, not just the TBM.
     
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