Optimal crossbeam placement for small cat?

Discussion in 'Multihulls' started by rayaldridge, Dec 29, 2010.

  1. rayaldridge
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    rayaldridge Senior Member

    Okay, here's the basics; I'm drawing a 20 foot cat to use a recycled Nacra 5.2 rig. This will be a folding cat, with a sailing beam of about 12 feet, and folding to legal trailering beam.

    My folding scheme requires a central spine running fore and aft. This means that the mast placement is not a factor in the crossbeam placement, since the mast is to be stepped between the beams on that spine.

    Because of the manner in which the boat folds, it is not possible to have conventional bow nets for handling headsails (the spine moves forward relative to the hulls when folding)

    The mast is placed almost 50 % aft, so that the jib tack is well aft as well. My question is: how far forward can I place the forebeam? This is a two crossbeam scheme. The forward cross beam need not carry the forestay-- it can be a foot or two aft of the forestay bridle, and still provide good safe access for foresail handling.

    How much is too much, when it comes to moving a crossbeam forward?

    The aft beam is well aft. My assumption is that as long as there is adequate span across the hull decks (the beam would not be very strong if attached at the bows) then the more you separate the two beams, the stronger the connection between the hulls. Naturally, I don't want to put the beam in the first couple feet of the boat, but if I set it, say 25% aft, will that be okay?
     
  2. dstgean
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    dstgean Senior Member

    Ray,

    Will you be using both the jib and main from the 5.2? If so the headstay can't really go further than the bows unless you intend to put a regular jib on a prodder. As such, you can only move the mast so far forward. My hobie 18--similar in some respects to your donor 5.2 has 2' of bow in front of the bow bridles, 7' or so of space from headstay to mast and another 7 or so so aft beam--with 2' of boom overhang making the main end nearly flush with the stern.

    With some of the 5.2 constraints in mind, copy the geometry that worked on the 5.2 and move the centerboard in relation to the new CE. If you keep your foil in the same place as the 5.2's, you'll automatically be in the ballpark. Additionally, your large rudder will make up the rest of the power you need.

    Dan
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    If I understand you correctly…you have beams as such:

    Cat Beams.jpg

    The location does not really matter where you put them. These beams must take out the two main dominate load cases, the transverse bending moment and the pitch connection moment. (There is a 3rd, but this if the L/B ratio is close to 1.0). Once you know the magnitude of these loads, you can calculate the scantling of the required beam at the given location.

    Thus, if you have already done this on your previous cat, it is just a simple matter of recalculating the required scantling, given the new location with the known loadings.
     
  4. Steve W
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    Steve W Senior Member

    I think you are on the right track, you only need enough width to make a good structural connection so i think your 25% aft should be fine, i dont know if you are aware that Lock Crowther did a few large offshore racing cats with the forebeam well foreward with a two beam settup. Shotover was one i believe at 60ft. He used very arched beams so the fwd one wouldnt bury and trip the boat if they stuffed the bows. Probably not an issue with you boat but something to consider.
    Steve.
     
  5. rayaldridge
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    rayaldridge Senior Member

    Thanks, all. I don't think my approach is particularly extreme, but I was concerned that there might be some good reason that structural beams are rarely seen well forward. Proportionally, the beam on the new boat is a good bit farther forward than Slider's main beam-- but the latter carries the mast step.

    [​IMG]

    I hope this will work. Because of the folding, conventional nets are not possible, so I have to have decks.
     
  6. Steve W
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    Steve W Senior Member

    Ray,whats your leaning at this point on the folding system?
    Steve.
     
  7. rayaldridge
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    rayaldridge Senior Member

    Steve, I'm leaning toward a system that folds horizontally at the armpits and spine.
     
  8. Steve W
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    Steve W Senior Member

    So it will be a similar configuration to Simpsons Takeaway but with a center spine instead of the Link all ?
    Steve.
     

  9. rayaldridge
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    rayaldridge Senior Member

    Yes, although since I'm pursuing a low-tech, low-cost approach, the idea is that the hinges don't have to carry the sailing forces-- the strength will come not from the hinges but from removable blocking that extends across the hinges. Thomas Firth Jones used a similar system with his Brine Shrimp, though the hinges opened upward.

    This boat is really the result of an analysis of the factors that make trailering a demountable catamaran such a major pain. My personal situation is that the slip that is available for our use (due to a generous neighbor) is only 10 feet wide. I want to be able to leave the mast up when folded to fit in the slip.
     
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