carbon spar cross beam augmentation

Discussion in 'Multihulls' started by barrymac, Nov 28, 2019.

?

Is adding a carbon fibre cross beam in the bridge under the mast of a catamaran design crazy?

  1. Can't work

    1 vote(s)
    100.0%
  2. Too expensive

    0 vote(s)
    0.0%
  3. Would add a lot of value and increase performance

    0 vote(s)
    0.0%
  1. barrymac
    Joined: Nov 2019
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    barrymac New Member

    I'm considering a grainger Raku project, but would like to push up the stiffness and strength of the bridge under the mast as it's the most hard working part of the structure. Any views on adding a carbon cross beam to the design?
     
  2. fastsailing
    Joined: Sep 2017
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    fastsailing Junior Member

    I don't know anything about Raku.

    How was the mast step supposed to be supported according to the plan?
    Alloy beam?
    GRP beam?
    Wood?
    Other than beam (plate?)?

    You should consider that carbon fiber is very stiff along its length direction, and thus takes up major part of the loads, if not all of them. If you use carbon beam make sure it's strong enough to take all the loads. Otherwise you need to calculate the load shearing, not just assume something. How about distribution of torque, is your beam supposed to take up all that too or not. If not make sure it's not going to break due to torque, if designed only for bending and shear due to mast step loads.
     
  3. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    In principle it is a bad idea. Stiffening a small part of a structure creates concentrated stresses and leads to failure. Also, your survey if biased to get the answer you want: people to agree with you. What do you mean by "can't work"? The other two are definitely wrong, so I am choosing the first as the best
     
  4. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    That does not have to be true, there would have to be a study of the structure to see how that extra structure varies the stresses distribution. It could happen that stress concentration occurs in some area but those stresses do not have to exceed those who experienced the points of the original structure. In any case, since we are adding structure, let's add what is necessary at those stress concentration points. But, imo, a pillar, if it is possible to place it or reinforce the existing one, will always be better than a transverse beam.
    Having said all this, I must also say that commenting on the best solution is not possible without knowing the existing structure and the current supports of the mast.
     
  5. redreuben
    Joined: Jan 2009
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    redreuben redreuben

    What is the evidence to say that the structure as designed needs reinforcement ?
     
  6. TANSL
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    TANSL Senior Member

    By giving you an easy answer, I will tell you that without knowing the design loads used by the designer, nothing can be said about it. It could also happen, of course, that the design loads used are incorrect. So, the evidence you ask for does not exist.
     
  7. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    As also noted above by redreuben, why?
    Why do you wish to increase the strength/stiffness...have you performed some calculations, or, is it just a "feeling"...?
     
  8. redreuben
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    redreuben redreuben

    Tansl,
    So your first assumption is the designer doesn’t know what he’s doing ?
     
  9. TANSL
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    TANSL Senior Member

    @redreuben, No, at all, I have no idea what the designer has designed, the assumptions he has made or the calculations made. How am I going to question that work? I did not work that way.
     
  10. barrymac
    Joined: Nov 2019
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    barrymac New Member

    My question was based on 5 years ownership of a Fontaine Pajot Bahia 46, now 20 years old, which shows signs of stress in this area on the surface. I've seen other Bahia's with similar marks. I presume this model was modelled well, from a well resourced company. As such I figured if there was one area I would overbuild a little it would be here, but not being qualified to evaluate the idea, thought I'd see what people here thought.

    The mast is supported by a pillar, and I guess this would be bearing only compression loads. It is the twisting of the bridge by the hulls combined with shock loads of wave slamming that I am most concerned about. I don't want to feel delicate about beating upwind in inclement conditions, as I do now in the Bahia. This results in increasing the tacking angle to something a bit depressing.
     
  11. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    The structure needs to have the stresses spread out rather than concentrated it is failing. However, the cracks may simply be on the gelcoat and not structural. Hav e you removed the gelcoat to inspect the damage?
     
  12. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    You have to study how the structure is at that junction of the foot of the mast with the pillar that supports it. At that point they can produce, as designed by the designer, not only a compression force that is transmitted to the keel by the pillar, but, perhaps, important bending moments and efforts transmitted to the deck. But it is not sensible to say anything concrete if we do not know the configuration of the mast and its union to the keel and deck.
    The structure does not need to have the stresse spread out..., it is the structure that must spread these stresses properly.
     
  13. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    I don't know about Raku's but Schionning Wildernesses use large layups of unis in their beams. Old Multi has the layup of a Schionning like cat of about 13 metres in his structure thread. I have seen a friend lay up a heap of unis into the top of a routed out balsa duflex panel. The unis can be put on top of the bulkhead (which is a shear web) and laminated on the underside of the deck. There are more unis on the top of the bulkhead than the bottom - because the glass does not like compression as much as tension.

    If you were looking to stiffen a Raku, then adding more glass or switching to a similar amount of carbon instead of this uni could be good. It would be best to ask Grainger. He would probably say don't change it and ask if you have sailed on a Raku and found it too flexible. I have a reasonably light main beam on my cat and have no stress cracks there. However it is made very differently to other similar cats - so there seems to be a wide variation in build technique of the main beams of cats. The only way I can notice deflection is by looking at the rigging going slack in heavy air, just before reefing, but that is probably mostly wire stretch rather than deformation.

    Designers are often happy for someone to take a punt and do something only a little out of the ordinary. The can then learn from it and incorporate it in the next design. It could be fine with Grainger for you to replace the standard E glass unis with carbon. I can't see a problem right off the top of my head but Grainger would know the structure best.

    cheers

    Phil
     

  14. barrymac
    Joined: Nov 2019
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    Location: Turkey

    barrymac New Member

    I have more details now and noticed the Raku does indeed recommend Carbon Uni strips around this area. I imagine this among other things will ensure it's a far stiffer boat than my current setup.
     
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