Folding Trimaran Beam Design - Why not this? (see pic)

Discussion in 'Multihulls' started by Jetboy, Nov 20, 2014.

  1. Jetboy
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    Jetboy Senior Member

    Is there a great reason not to use this type of design? I need to work out the links to the correct lengths to get a more flat motion of the floats so they stay at water level when folding, but what about the concept of having a split beam. It sure seems to fold up nicer to a smaller package - ie lower total height.

    Ignore the sliding beam on the left side - I've drawn this with quite a few different options.

    Thoughts? Comments?
     

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  2. Jetboy
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    Jetboy Senior Member

    For comparison:
     

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  3. bruceb
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    bruceb Senior Member

    Another folder

    Jet, I have looked at various designs a lot- the problems are in the real world fitment and loading. What size tri are you considering?
    On that particular design, the mid beam hinge would carry a very high bending/tension load. The parts would get to be quite heavy on a tri that was very large. Also, float immersion has to be considered as well as deck/hatch access in the folded position. Nothing is "easy" about tris. :rolleyes:
    However, if you have some ideas on how to make it work, the basic geometry is ok. Just check your load numbers and material strength and see what you think.
    Besides looking at boats, I have spent quite a lot of time watching construction cranes and excavators to see if I could get a design inspiration- no luck so far, their solution always seems to be more steel than a boat could support :(
    At this time, the latest Farrier beam designs are still some of the better actually produced and tested folders, but certainly not the only good solution.
    B.
     
  4. Jetboy
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    Jetboy Senior Member

    Bruce -
    The second pic is a Farrier design and the design of this boat's original plans. Building them in fiberglass is daunting to me as a novice, and I fear extremely time consuming. So I've been looking at options for an aluminum version. I already have an aluminum welding setup and experience and I can replicate the arms from the plan in aluminum for around $1,000 depending on cutting costs and assemble and weld them together in a few hours vs my estimate of 40 hours of cutting and laying up fiberglass. For me it just seems to make sense to go aluminum.

    The alternative design I drew is very similar except that rather than having the upper linkage recess into the folding arm, it is the arm. My thought is that the upper link in a farrier design in practice is taking the lion's share of the compressive loading already, and it should be able to handle the torque applied as well. Unfortunately I don't have the software to do an FEA to test with more dynamic loads. All I can really do is basic calculations. It would be extremely easy to manufacture from aluminum square tube - but not very sexy looking. I'm leaning toward easy, but curious if there are significant reasons not to - like unusual loads on those points for some reason I'm not thinking of.

    My tri is only 18' long and maybe 1500lbs max weight, so it's light and simple. I suspect a couple pine 2x4's would even be strong enough.
     
  5. warwick
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    warwick Senior Member

    How would you go locking the arm in place when open?

    There is a european tri with a similar folding system, interms of two hinges in the cross beam. (one in the middle, and one by the float) I think it was 42 foot.
     
  6. Jetboy
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    Jetboy Senior Member

    This is a crude version - faster than drawing up a new one -, but something like this is what I was thinking. I believe once it's unfolded the shroud tension should provide most of the force to keep it locked.

    [​IMG]
     
  7. John Perry
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    John Perry Senior Member

    Yes, you have eliminated one of the links in each half beam of the Farrier system but, at risk of stating the obvious, the downside is that you have put a hinge joint in the beam very close to the position of maximum bending moment! You would need a very strongly locking hinge at that point to avoid the outer part of the beam folding upwards under sailing loads. No doubt this could be done, but I guess that it would add more weight than you could save by eliminating the upper one of the two Farrier beam support links, the upper one actually being the lighter of the two since it is only significantly loaded when the float is folded, not when under sail. The type of hinge in the photo you show is the kind of geometry you could use, but for a larger trimaran I think it would need to be massive, it might also tend to lead to thicker float beams (vertically) than would otherwise be necessary, so more windage and more water drag if/when the beams hit the wave tops. For an 18 foot boat, the type of hinge shown in your photo might be feasible, are there any figures for the bending moment it can carry? But even if it is feasible, what is the advantage - just a lower air draft when the boat is folded (and the mast lowered)??

    I wonder, do you really need a folding mechanism for a trimaran the size you are designing? It may not be large enough for marina berthing to become difficult and in any case that type of boat is probably more likely to be stored on shore than in a marina. If so, folding would only be necessary for getting it on a road trailer or launching trolley. So, how heavy is each float? Once the boat is on a trailer and hauled up a slipway could you just lift off the floats and put them on/in the main hull, then do likewise with the cross beams - probably simpler and lighter than a folding system.

    I agree that an aluminium fabrication could be worth considering for an economical trimaran beam construction. Earlier this year I was chatting to someone who had home built cross beams for a Farrier designed trimaran (from carbon fibre) and he was telling me what a lot of work it proved to be! I can well imagine that if you are skilled at working with aluminium alloy that could be a lot quicker, especially with CNC plate cutting, but perhaps not quite as light if you are looking for best performance?
     
  8. Jetboy
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    Jetboy Senior Member

    I don't really need a folding mechanism. It could easily be done with just sliding aluminum tubes as the beam of the main hull and floats combined is narrow enough for highway towing. It's the obvious solution for such a small boat. The only downside to sliding beams is that they encroach the cabin space. I'm not terribly concerned with this, but it is a consideration. It would likely be the cheapest and easiest, but not quite as convenient.

    I intend to trailer this boat every use - there are few marinas where I normally sail and monthly rates are very high. So my primary goal is quick assembly. The third goal is narrow folding because it will be parked along my driveway at my house, which is fenced on both sides - But it is about 20' wide, so it's easy enough to get around, however narrower is better. In the winter I'd like to put it inside the garage and for this it will need to disassemble.

    Formed Aluminum tube with a nice arc would be ideal, but I don't believe anyone in my area has the proper machinery to bend and then temper aluminum in this size.
     
  9. Fanie
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    Fanie Fanie

    I too think there may be structural problems with a hinge like that. At one stage I played with similar and it always came down to too much weight and not enough strength.

    For the same, you can just as well glass a stainless pipe top and bottom of the beam sides and make it an integral part of the beam. Afterwards you pull the SS pipes and slot the tube to form the hinge, the pipe on the one side then becomes the hinge and the other pipe the lock. You still have to add enough material to make up the strength since the hinge on each side is only half the material.
    Hinges does that, they half the width :D
     
  10. Fanie
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    Fanie Fanie

  11. Jetboy
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    Jetboy Senior Member

    I think I've come up with a simple elegant solution for a slider, that goes extra narrow without lifting the amas. I'll draw it up later and throw on some pictures.
     
  12. Fanie
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    Fanie Fanie

  13. bruceb
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    bruceb Senior Member

    Slide

    Jet, I have studied every folding system I could find, and for a boat the size of yours, aluminum or fiberglass sliding beams are probably one of the best choices. Since you are comfortable working with alloy, I would stick with what you know.
    On a tri as small as yours, sliding, swinging/pivoting, or simple one hinge folding (like a small Pivor) all are effective and simple to build.
    The trailer tongue hinge is certainly robust, and might be a pretty good solution if you used water stays. I also have wondered what one of those weighs?
    For what it is worth, the upper link on a Farrier system is barely loaded under sail, all the compression loads are taken by pads at the beam/hull inner end, which then puts a considerable bending moment on the beam mid section when the float is forced down under sail. The beams are complex to build, and have to use a lot of expensive carbon if they are going to be light and strong.
    B
     
  14. basil
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    basil Senior Member

    Is there any sort of issues that may need addressing with a Piver style hinge while trailering? Uneven road surfaces (bumps) etc are likely to load up localised areas - no?
     

  15. bruceb
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    bruceb Senior Member

    Bumps?

    Basil, how bad are your roads down there ;) and how fast do you tow?
    As long as the float hulls are supported by some padding when folded, I don't think the hinge should have very much loading in the folded position.
    B
     
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