Design of 16' foot beach catamaran

Discussion in 'Multihulls' started by hashtag_laeuft, Aug 15, 2017.

  1. upchurchmr
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    upchurchmr Senior Member

    Trying to keep the bows from twisting apart from each other involves 3 things.
    1. Torsional resistance of the beams.
    2. Bending of the beams.
    3. Attachment stiffness.

    Imagine the front beam bending only in torsion. This forces the aft beam to bend (unless its attachment is really sloppy).
    In reality, if the attachment is stiff, both beams rotate torsionally and bend to share the load.
    For round beams, it is easy to make a mating "saddle" in the hull to make it tight.
    For square beams, it is harder to make a perfectly mating saddle.

    If you use metal straps for attachment, the end the bolts go thru needs to be very stiff so they do not deflect.
    Its hard to get the straps to fit perfectly on a square (leaving no gaps).
    For round you need to also bolt the strap to the beam in the middle of the strap, or be prepared to really torque down the bolts hard.
    If you use square, you don't need to bolt the strap to the beam, IF, they fit perfectly.

    Or you could just lash the beam to the hull, letting it all be sloppy and the hulls to flex a lot - which I don't like.
     
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  2. BobBill
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    BobBill Senior Member

    I think this is very useful info, and wish I knew earlier. Instead, I copied existing cats (NACRA) designs, which feature tube, on my updated Malibu Outrigger build, as noted in pic. Seems either would do job.

    1gnarly.jpg

    Forewardportakabolts.jpg
     
  3. upchurchmr
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    upchurchmr Senior Member

    Nacra's had a strap over the tube, if I remember right. And a closely fitted saddle area into the hull.
    [​IMG]
     
  4. BobBill
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    BobBill Senior Member

    Yep. I locked each tube down similarly, to limit movement, used orig Nacra parts to do as close as possible. Thought of rectangular akas, but I stayed with tubes to make it easier. Seems to do fine, so far. Stiff as can be, even on old Hobie 18 pockets I had to build up. But, have to admit, rectangles would have been easier to work with, at least on outrigger side...maybe.

    I planned to do foam ama with chines, but holding off as current tracks so well.
     
  5. waynemarlow
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    waynemarlow Senior Member

    Why try and reinvent the wheel guys, round tube properly bonded into mating recesses with structural silicon and pinned with 1/2 moon blocks and bolts has proved over and over to be the best way. Add a dolphin striker to take the mast loads on the front beam and quite ordinary cheap and easily available tube can be used.

    For a reference on most of the early F16's we used 3mm x 80mm OD on the front and 2.6mm x 80mm rear. We found that any flex in the hull was largely down to poor layup strategies and design in and around the beam landings. Also remember that the rig also contributes to quite a bit of resistance to twist.

    In my own boat with the rig taking some of twisting loads, lifting one bow 20mm would begin to lift the bow on the other side. On Catsailor.com and the F16 forum type in Bitsa and there's quite a lengthy F16 build and look up the poster Wouter who had quite a discussion on the pros and cons of front beams in a quite detailed thread.
     
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  6. hashtag_laeuft
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    hashtag_laeuft Junior Member

    Hey guys ... thank's for your answers.

    Wayne, I found your project on catsailor.com ... very impressive. I just saw one picture via google, may you put some interesting fotos here?

    The solution on Bob's last picture is nice and clean, which I would prefer. At my existing catamaran the bolts are going completly through the beam with some little spreaders in which prevent the beam from deformation. The saddles are made of cold moldet veneer sheets around the beam. It all fits very good as you can see in the attached picture. I just wanted to know if there is a better way to this but how it seems it is very proven.

    dsc00135t8kbo.jpg


    I also have another question regarding the structural design.

    On the next picture you can see my inner layout of the hulls. The frames are set where the beams will be located to transfere the loads and forces in the hull. The daggerboard trunk is also surrounded by frames. But maybe I'll clip the one more aft as these supporting the deck. The shrouds will be attached at the one infront of the daggerboard. As already mentioned the deck will be supported by deck-frames an stringers. The big brown one at the STB side will support the trampoline. The fore stay will be attached at the inclined frame in the front. To prevent the boat from sinking and to stiffen it out horizontal styrodur foam plates will be installed as showen.

    Seite-vorne.png

    Now my question is how I can calculate the approximately longitudinal and transverse strength of the hull. For big ships with a high L/B ration just the midship section is used to calculate the longitudinal strength. But when I want to calculate the transverse strength the section varies quite a lot from where the forestay is attached to the one in the middle. My task is to know the minimum angle of the bridle and thus the hight where the following forestay is attached. Is it possible to calculate this by hand or do I need numerical FE methods for this?

    I have the maximum rightingmoment and the tension in the shrouds but how do I get the tension in the forestay with consideration of the force by the mainsheet?

    Can anybody help me with that? Maybe Wayne, I think you did several calculations like this for your F16 project? Would be nice, thank's a lot!!
     
  7. BobBill
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    BobBill Senior Member

    Perusing this thread, it occurred to me this project might be term paper etc, which is more about theory and not pragmatic when it comes to actually dealing with launching and sailing. As most learn, it is one thing to design, but the doing is what makes success.

    Have to ask, is this rig to be dry sailed, as in all up storage on the hard or beach, or will it be rigged each time from trailer?
     
  8. hashtag_laeuft
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    hashtag_laeuft Junior Member

    Hey Bob ...

    the start of this thread was not realy the best :D And yeah it is very much theory here because I want to learn more about designing and constructing boats ... especially catamarans. This is no term paper or anything like that it is my hobby and I just wanne get better. Enveryone starts from zero ... so ... this is my startingpoint.

    My goal is to design (and to build) a beachcat according to the F16 class rules as any other F16 catamaran out there. So in my case the boat will be stored fully rigged at land with the possibility to fully disassemble it.
     
    Last edited: Oct 18, 2017
  9. BobBill
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    BobBill Senior Member

    Thanks. Had to ask.

    I bought and used original plans for old wood Malibu Outrigger, to update using glass, carbon and T6 metal, etc.

    Naive me!

    First was advised Hobie 18 hull not strong enough to handle un-stayed mast...and rightfully, so added stays...

    All works fine, but, when rigged it I learned how hard rigging is do do right and also found out the boat, as I configured, honestly is best sitting somewhere "rigged, without sail;" but I trailer it...

    Takes forty minutes to rig. Double the plan.

    "The plan" isn't reality, as I learned.

    Boat does it, though, even with the longer rig time.

    Very good, thanks!
     
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  10. BobBill
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    BobBill Senior Member

    Re the pics of the aka anchor point above...I wanted to "mold" to hull, but could not, really did not want to fool with it. So, had a T-6 plate (you can see edge) made up with rails along side to keep the the aka in place, and, used NACRA beam anchors and orig stainless straps to hold aka...may not be esthetic but all works...might even have been over-kill. After full season, is fine...
     

  11. waynemarlow
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    waynemarlow Senior Member

    Umm the loads are quite high inward if you have a conventional forestay as in highly angled outer connection stays to the forestay at the bottom of the jib. Older designs had a compression tube between the hulls to over come this. The F16's went a different way with outer hull connecting stays, connecting about 1/2 way up the fore stay with a solid tube extension below finishing about 600mm above the spinnaker pole. Diagonals are taken out to the hulls and one directly down to pull the spinny pole up into an arc to give it strength. The jib is pulled conventionally on a halyard up the upper half and then a zipped section is put over the solid tube. It just allows the outer hull connecting stay to be at a much reduced angle and thus limit the inward pull on the hulls.

    Use 4mm D12 Max on the upper section and 3mm D12 Max on the lower and you will be fine. Remember its the tension on the main sheet that keeps the forestay tight, not the rear side stays, they are only there to keep things upright until the mainsheet is on. We use 10:1 minimum on the mainsheet just to bend the mast and keep tension on the forestay.
     
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