Stiffening up my Cat

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

  1. Cat2Fold
    Joined: May 2012
    Posts: 56
    Likes: 1, Points: 0, Legacy Rep: 13
    Location: Felt, Idaho

    Cat2Fold Junior Member

    What about using some heavy duty ratchet straps and strapping the the two beam halves together somewhere near the outer bolts?
     
  2. John Perry
    Joined: Nov 2003
    Posts: 275
    Likes: 28, Points: 28, Legacy Rep: 129
    Location: UK

    John Perry Senior Member

    good to hear that the four main pivot joints between the crossbeams and hulls are 'rock solid' because the strength of this design depends on that being the case. As others have pointed out, these joints transfer considerable bending moment between the cross beams and the hulls. In rough conditions this bending moment is due to wave action as well as due to bending moments transferred to the hulls from the masts, centreboards and rudders.

    Vertical movement between the top and bottom 'leaves' of each beam, which I think is what you are now saying is the problem, sounds like a less serious problem and also one that I would expect to be easier to solve. Your suggested ratchet straps (or just a lashing?) sounds like a good solution, at least in the short term. It has the advantage that it could allow for small rotational movement of one leaf relative to the other, by this I mean rotation about an axis along the lengths of the 'leaves'. You either have to clamp the leaves together so tightly that this rotation cannot occur, or you have to clamp them together in such a way that the rotation can happen but does no harm. If you go for the former option you have to be sure that the beam leaves are sufficiently flexible in torsion that the twist that will develop along the length of the leaves will not overstress any material.

    But, if Catsketcher is correct about these top and bottom leaves being tied together with bolts, why are those bolts not adequately restraining the movement? My first thought about how such a connection might be designed would be to have a single bolt at each end of each beam assembly, these clamping the beam 'leaves' vertically together but with a stiff elastomeric washer between the leaves so as to allow just a tiny bit of 'rocking' movement. Without more information is hard to comment further. btw, does the boat designer have anything to say about this?
     
  3. Cat2Fold
    Joined: May 2012
    Posts: 56
    Likes: 1, Points: 0, Legacy Rep: 13
    Location: Felt, Idaho

    Cat2Fold Junior Member

    Kurt Hughes has said it would be hard for him to tell what's causing the slop without seeing it in person. I haven't discussed it very thoroughly with him.
    The boat is in storage on its trailer down in Mexico, so I can't get any direct measurements.

    You are correct, in that the beam leaves have some sort of stiff black elastomeric "gasket" around the perimeter. Probably there for the reason you mention- to allow some movement between the leaves.
    For some reason, the approximately 3" diameter stainless "bolts" bottom out on the "nut" which is adhered to the bottom of the lower leaf before the beam leaves compress together very tightly. Maybe this is by design. Maybe it's due to wear over time? Maybe I should fix these bolts so I can tighten the two leaves together tighter?
    Maybe, there is absolutely no more movement on the boat then when it was launched and I am just noticing it more over time spent on the boat in rough conditions?
     
  4. catsketcher
    Joined: Mar 2006
    Posts: 1,276
    Likes: 126, Points: 63, Legacy Rep: 790
    Location: Australia

    catsketcher Senior Member

    Cats it

    I would cast the beam ends rigidly in place with epoxy. Put lots of wax on the beam ends and epoxy glue in the socket of the other beam. Then extend the boat so the beams are fully in place. The epoxy will squeeze out and totally fill any voids. This will absolutely ensure there is no slop and if the distances are only small cab o sil and epoxy should do a fine job. I use this approach often and really like its flexibility and high precision. Plus its super cheap compared to any alternatives.

    cheers

    Phil
     
  5. Cat2Fold
    Joined: May 2012
    Posts: 56
    Likes: 1, Points: 0, Legacy Rep: 13
    Location: Felt, Idaho

    Cat2Fold Junior Member

    I'm not sure I understand what you are saying.
     
  6. catsketcher
    Joined: Mar 2006
    Posts: 1,276
    Likes: 126, Points: 63, Legacy Rep: 790
    Location: Australia

    catsketcher Senior Member

    If you get a tub full of epoxy glue and push a well waxed shape into it it will be a mould. When you remove the shape there will be an exact fit for your part. I would look at waxing one end of the beam and putting glue in the recess of the other. When the beam is straightened the waxed end willpush into the other end and squeeze out any glue in the way. When hard you pull the thing apart and you have an exact fit for the beam end. Ensure you wax the heck out of all of the beam end so that you can remove any drips of epoxy that fall out of the join.

    cheers

    Phil
     
  7. dstgean
    Joined: Aug 2006
    Posts: 142
    Likes: 5, Points: 0, Legacy Rep: 68
    Location: Chicago Area

    dstgean Senior Member

    beams

    Have you thought about going with large aluminum tubes for beams? If you use the boat as a once a year assembly/disassembly, you could go with a different system entirely--at the cost of redesigning the boat. Maybe go with what catsketcher says 1st...

    Dan
     
  8. Cat2Fold
    Joined: May 2012
    Posts: 56
    Likes: 1, Points: 0, Legacy Rep: 13
    Location: Felt, Idaho

    Cat2Fold Junior Member

    Yes, I have thought of that. I love the way I can launch and retrieve the boat by myself without the need of a crane or other lift.
    That's why I thought about adding a third structural beam to the boat after it has been launched and opened on the water.
    The Macgregor 36 uses a triple tube system.
    I'm still not sure it's not a viable way to go about this, but by the responses I'm receiving from this forum, I'm starting to doubt it.
    I'm not too worried about it for now...
    I may need to just keep using the boat the way I have been, then if/when I decide to cross an ocean or two, I'll reconsider my options then.
     
  9. pipeline
    Joined: Apr 2013
    Posts: 24
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Kailua, Oahu, Hawaii

    pipeline Junior Member

    I raced Hobie 18's and the fast Hobie 20 Fox successfully for a long time...

    We ALWAYS made sure that the tramps were as tightly laced as possible. I do the same for my big tri also. Really helps to stiffen the boat.
     

  10. Petros
    Joined: Oct 2007
    Posts: 2,936
    Likes: 139, Points: 63, Legacy Rep: 1593
    Location: Arlington, WA-USA

    Petros Senior Member

    Adding a few extra easy to remove struts, or the third beam as you suggest would likely help. But you first want to make sure that everything is correct about your current set-up. If there is a worn or missing part, or something is not fitting correctly, adding more struts and braces may not fix the problem.

    If indeed everything is as it should be, and the amount of movement you are seeing is normal to all of these kind of boats, than adding the extra struts might be the next step to make it stronger/stiffer.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.