Stiffening up my Cat

Discussion in 'Multihulls' started by Cat2Fold, Jun 20, 2013.

  1. Cat2Fold
    Joined: May 2012
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    Cat2Fold Junior Member

    Hello all you smart boat design type folks,
    I am the owner of Cat2Fold. A double beam cat that folds on the water via two scissoring, carbon fiber beams.
    The boat is 36' long with a 24' beam. Freestanding biplane rig.
    When I'm out in the rough stuff, I'm finding a fair bit of movement between the hulls. More than I'd like, but it may be that this boat is supposed to move and flex.
    There is currently no front crossbeam on the boat. The front of the net (trampoline) is soft.
    What I have been wondering is; could I add a third structural beam to the boat to help stiffen her up?
    About 5' back from the bows, there is an anchor locker bulkhead. The bulkhead is only in the upper 2-3' of the hull (the bulkhead is not the full height of the hull). The width is about 2'.
    I have been contemplating buying some 2-2.5' long carbon tubes (6" ID?), splitting them lengthwise, glassing them into the boat at the bulkhead area, then getting a (6" OD?) aluminum tube (can't afford carbon) threading it through the glassed in carbon tubes, and use some sort of clamps to hold everything together.
    Is this a good idea? Bad idea? Will one extra beam add any thing structurally to the boat? Is a 2' bury significant enough? Attaching it to a partial bulkhead?
    Is 6" a good size? Would 4" be OK? What wall thickness?
    What about clamps?
    I can attach pics if that would help.
    Any ideas would be greatly appreciated.
    Thanks,
    Brian
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Yes please :)
     
  3. Cat2Fold
    Joined: May 2012
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    Cat2Fold Junior Member

    Ok. Here are some pics...
     

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  4. Cat2Fold
    Joined: May 2012
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    Cat2Fold Junior Member

    And here is the anchor well...
    You can see some pretty good pics of the boat itself, here in my gallery.
     

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  5. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    Gday Brian

    Can you explain a little more about the type of flexing you are experiencing? There are a few load types that it may be
    - the hulls are both pivoting top inwards and outwards (when viewed from the stern) This could be because your pivot pins are getting looser. As your beams are not very deep even a small amount of clearance will allow a large rotation. Your rig will not impose a constant torque on the hulls (shrouds pull the outside of the hull up and impose a torque) then your hulls could wobble in and out.
    - torsion - in this case your stern or bow lifts before the hull lifts as a whole. Every cat will have some twist. The Bamboo Bombers of Malcolm Tennant even used ropes led to the masthead to reduce twist. You can't do this as your rig can't be used to make a truss. Again torsion requires a very stiff beam mount.
    -translation - in this one hull gets slightly ahead of the other and oscillates back and forth. This is easier to fix. Farriers tris use a set of X wires to stop this. I think your boat already has these.

    It seems as though your boat does need stiffening - if you are unhappy then the boat needs to make you feel safe and secure again. Probably the best way is to get the beam ends stiffer. Replacing bushes would be a good start but if these are fine I would consider some surgery but different from what you are thinking.

    I would consider adding bracing struts from near the waterline to a point about 1.2 metres in from the end of the beam. These would increase the apparent depth of your beams and increase the stiffness and torsion resistance of the boat. I would ring Kurt Hughes and ask for the laminate schedule of the beams and see how much beef there is in the beams. Usually bracing reduces stress but I would like to see a nice laminate of unidirectional glass feeding the load out into the beams at the attachment point. I don't know exactly how she folds but you could attach the braces after unfolding.

    Gotta get to work

    Phil
     
  6. upchurchmr
    Joined: Feb 2011
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    upchurchmr Senior Member

    A refresher on how the beams and folding setup works would help with where to improve stiffness.
     
  7. Petros
    Joined: Oct 2007
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    Petros Senior Member

    It seems to me the first place to look is all of the moving/pivioting parts for wear and excessive clearance. Correct that first, than consider adding some extra braces if necessary.
     
  8. catsketcher
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    catsketcher Senior Member

    more thoughts

    Cat-2-fold is a very interesting boat. The scissor beams run along a trench on the inside of each hull. I am interested in knowing whether the beams when extended in the full position rely on the pins at the outer end for support of whether the ends have a pad that pushes onto the hull side just as expansion finishes.

    The beams are about 250mm deep and the boat is about 2m deep (it has standing headroom). The centreboards will provide a significant torque that can only be resisted by the beam ends. These ends are under huge torque.

    The unstayed masts increase this load. I guess the sail area to be 300sqft each main and the CE to be about 8m above the CLR. This gives a moment in 25 knots of 4000N (sail load) x 8m = 32000Nm. Add this onto the moment from the centreboard CLR to CB about 1m (which has the same torque direction) and you get 36000Nm. This torque has to be resisted by a beam with top and bottom bearing pin 250mm apart (0.25m) Both torques are equal but opposite. What is the force on the top and bottom pin?

    t=Fd
    F=t/d
    F=36000/0.25
    F=144000N shear load at top and bottom extremity of pin

    That's about 14 tonnes of load on the pin. Just from sailing loads in 25knots without any stress from hull lifting - just rig and centreboard moments.

    If a brace is added then the stress reduces by the increase in depth. Adding a brace 1m below the beam will reduce the load by about 4 times. It is no wonder that typical cat design has beams with large bury, either in long tubes or with deep ends alingned with bulkheads (same idea but just oriented 90 degrees differently) Cat 2 Fold can't do this. She is a little like having a cat with two beams and each beam is in a very close fitting socket about 300mm long. That is not typical. But the boat works so she is obviously well designed and built.

    There is a good 1m overlap in the middle of the beams. Are these hinge pins okay or do they slop? IIRC these overlaps are bolted as well so there should be minimal slop here.

    To go back to the original question - would a forward telescopic beam help? I am not so sure it would. There would have to be play in the system to slide and the play would mean that the beam would not take up load before the current system. On top of that if torsion resistance is an issue (and I think it will be) then adding only one beam will not assist greatly. Two beams are needed to make a rigid torsion box with the hulls.

    I would like to see the pins in the folding system only undergo a single type of loading to reduce wear and fatigue. If the hulls are wobbling then I would like to see struts being used to pre-stress the hulls into one load condition - use compression struts as a brace at the beam ends. In the end there may need to be a well engineered brace system that can handle cycling from tension to compression regularly without any slop.

    Can anyone guess what I think about in my spare time when I am not working or building my own folding cat?

    cheers

    Phil
     
  9. Skyak
    Joined: Jul 2012
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    Skyak Senior Member

    The simplest way to stiffen up the boat would be to add a strut between the masts with a X stays running to the opposite hulls. A lesser partial improvement would be to take the horizontal traveler load off the hull and onto the beams.
     
  10. kvsgkvng
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    kvsgkvng Senior Member

    Brian,

    Looking at your catamaran shown in very small picture on your avatar, I could see that it has two masts. I am not sure about stays, but it appears to have none. Without reading in detail other replies it appears to me that the motion you don't like is caused by uneven loading of each sail.

    The easiest way to stiffen two parallel beams without much of additional hardware is to use cross steel cables with turnbuckles. It will stiffen any two parallel beams with internal stiffness. Bulkheads are good for this purpose. If you tie with a steel tie the middle intersection of these two cables, it would stiffen this structure even more. If you add a cross beam at the outer ends of stiffening cables it would help even more. I am not saying it is easy, at least I think I helped to identify the cause of this problem.

    My take is the problem is with two unsupported masts. I attached a picture for you to view.



    Best regards.
     

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  11. Cat2Fold
    Joined: May 2012
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    Cat2Fold Junior Member

    Hey guys, Thanks for all the responses. Sorry, I haven't been responding as fast as I should.
    The movement I'm referring to (not sure of the name) is when one bow is lifted on a wave before the other bow hits the same wave and also lifts up.
    My primitive brain could tell that there would be a lot of force applied to the hull attachment points between the sails and the daggerboards, just as catsketcher pointed out through some amazing analyzing and mathematical skills.
    However, this is not where the boat feels loose.
    Where the beams attach to the hulls seems rock solid still. It is where the two halves of each beam come together that I can detect some play and twist.
    In reality, the boats flexibility isn't even noticed while out day sailing. However, go out for an upwind, overnight sail, and all of a sudden the darkness reveals creaks, pops, and twists that just make it hard to relax.
    My desire to make sure this boat is absolutely bombproof stems from my goal to expand my sailing horizons. Last year I sailed over 3300 miles (from San Carlos, Mexico to Zihuatenejo, Mex. and back) in 6 months. Eventually I'd like to cross the Pacific.
    I know a 36' open brigedeck catamaran might not appeal to most as being a sufficient sized and appointed ocean cruiser, but I have everything I need in a boat and more with Cat2Fold, if I knew for certain that she is STRONG!
    The idea of adding an extra beam (or two) I thought might help a lot. The way I've been using Cat2Fold, opening her up once a year to use her for 4-6 months at a time, I don't necessarily need her to be able to open at the boat ramp in 4 minutes and 20 seconds, I don't care if it takes all day.
    For attaching the extra beams, I was imagining a clamping system that works something along the lines of the way a modern bicycle handlebar attaches to its stem. Half of a saddle securely bonded into the boat structure, lay down a tube (carbon or aluminum) with the same OD as the saddles ID, and clamp it down TIGHT!
    Anyhow, this is probably a mute point for now, I think I just need to use the boat as it is. I'm already struggling to make enough $ to go be in Mex for another 6 months.
    All these ideas I have of improving my boat with a new this or a new that (and I have many ideas) are being crushed by my economic reality.
    Anyhow, any other thoughts are greatly appreciated.
    Hope to see some of you sailing down in Pacific Mexico sometime soon.
    Thanks,
    Brian Charette
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    If you could post some pictures of this joint and some dimensions/figures too.

    Without knowing any more details about the boat, it could be, that the design of the joint is intended to be flexible under high loads. Since this will allow the loods to "pass through" rather than being "locked in", in a simplified sense. Deflection in itself is not necessaily a cause for concern if it is "designed" in and expected. Just look at an aircraft wing, as a simple example...it flexes, but so what?
     
  13. Richard Woods
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    Richard Woods Woods Designs

  14. Ad Hoc
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    Ad Hoc Naval Architect

    It is thanks Richard. Clearly those joints can only support shear forces, as it is ostensibly a "pin ended joint". So the bending moments and torsion on the beams can only be effectively carried by the main beams themselves, not the pin joint. So it is not unsurprising that there is movement under high load, as that is all the joint can do, to work, as such.

    The connection of the beams to the hull, is "built in", which is totally different scenario to that at the joint mid-span. Also the beams, at the joint, will work as a 2 separate composites, since there is no real attachment between the 2 beams other than via the pinned joint. Which cannot transfer the load paths between the 2 beams.
     

  15. catsketcher
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    catsketcher Senior Member

    If you look closely you can see that there is a bolt at the end of the beam ends. It is tightened with a winch handle so there are three connections with the top and bottom beam halves. Could use more bolts to reduce movement and make the central section more homogeneous and able to transmit shear better.
     
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