Folding crossbeam design for tri

Discussion in 'Multihulls' started by erikhaha, Nov 9, 2008.

  1. erikhaha
    Joined: Sep 2007
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    erikhaha Junior Member

    First I'd like to say that I searched this site for specific info about this design and I found very little.

    I am in the process of building a 20' tri & now I am having thoughts about making the crossbeams upward folding.

    I have done some calcs and I have determined that a 2"x2"x1/4" SQ t6 6061 Al Tube would provide sufficient strength for the crossbeams.

    I have thought of ways to design the crossbeams to fold and I have settled on one method that I think will work, as well as be structurally sound.

    Note that I am not a naval architect, and this is my 1st boat that I am building. I am a structural engineer by profession, and I deal with beam and connection design due to gravity and seismic loads on a regular basis, but simplifying the forces on the crossbeams due to windloading is much more difficult then the static conditions that I normally work with.

    Attached are a few cad pics of my initial design. Any helpfull comments would be greatly appreciated.

    Attached Files:

  2. ancient kayaker
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    ancient kayaker aka Terry Haines

    I'm not a NA either but it sounds a bit underspec'd to me. Assuming a safety factor of 2.5 and 1/8 thickness, if your cat is about 12 ft wide, then 2 of those beams would have trouble lifting the windward hull out of the water. The leeward beams are going to have much more stress because of mast and crew weight plus shock loads.
  3. Fanie
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    Fanie Fanie

  4. catsketcher
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    catsketcher Senior Member

    reverse engineer


    I can't say much about the idea - it has been around since the start of tris - Piver was lost on a folding 25ft Mariner and the telstar has been around for ages. To do the engineering I would try to find a similar sized boat and then reverse engineer it. A Windrider rave has alloy beams and they don't seem to fall apart, about similar size too. My personal opinion is that puttting hinges in the middle of a simply supported beam is asking for loads to concentrate on the hinges - why not a waterstay or strut to turn the beam into a column? Go from beam loading to column loading and reduce stress.

    To reverse engineer this look at the Twiggy, Val, or probably better the Dragonfly 25ft. By just looking at the size and angle of the waterstay you should be able to work out the load on the beam as a whole. although for a small tri I would go solid strut so that it also works in compression on the beach.

    As for getting specific calcs for beams - this is a really hard one. As the load conditions are hard to pin down there really isn't a rule of thumb that is freely given out. The guys who make their living engineering boats don't frequent this forum - apart from one or two like Eric Sponberg who throws in a gem every now and then. The hard work is up to you. Look around and see what works and then design your beams to the same load condition.


    Phil Thompson
  5. oldsailor7
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    oldsailor7 Senior Member

    Since you are a structural engineer, my advice to you is to calculate your stresses for expected static loading, then multiply by 6. Then design your structural components to complement that.

    I had a Piver Nugget 24' Tri which I sailed wirh my family in all weathers for four years. It had exactly the same folding layout you show----and it worked fine.
  6. erikhaha
    Joined: Sep 2007
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    erikhaha Junior Member

    Oldsailor thanks for the info, but I am just a little currious as to the FS of 6. I am assuming that you are trying to say that you can never account for every situation that is possible, or in this case mother nature.

    Yes I realize this but I am just trying to get a general consensus as to how much is enough. You can see that I have attached a spreadsheet with various tabs that calculate reactions and moments on a simple trimaran. I think that this model will get me in the ballpark, as far as beam sizes go, but designing a hinge mechanism that will be easy to operate and able to sustain the loading will be another task.

    Thanks for the info about your tri with similar folding beams.

    Attached Files:

  7. oldsailor7
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    oldsailor7 Senior Member

    Don't laugh, but the hinge on my Piver Nugget was a galvanised steel gate hinge, like you can buy in any hardware store.
    Since the hinge is on top it is almost always in compression. The mast shrouds hold the outboard hulls up. The lower bolt attachment was two pieces of galvanised angle iron with thru bolts to the upper plates of the hinge, and the tangs of the angle simply bolted together. In four years of sailing this simple system gave me no problems on a bigger and heavier boat than your design.

    The FS 6 rule of thumb came to me via the late Multihull designer Lock Crowther.

    He was a great believer in such sayings as:- "Simplicate and add less weight" -(Cessna), "Form follows function" and "If, on test, an item doesn't fail at it's designed limit---it's too heavy".

    Like Richard Woods, Lock travelled the world to sail on his own and other designers boats, to gain experience, which was then applied to his designs.
    Last edited: Nov 22, 2008
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