Folding arm materials - Any reason not to use Aluminum?

Discussion in 'Multihulls' started by Jetboy, Feb 7, 2012.

  1. Jetboy
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    Jetboy Senior Member

    I'm uncertain what you mean by the main hull beam section. Are you referring to the original plans for the scarab 18? If so, my thought was to slightly modify the original plans to have twin parallel bulkheads at each beam location separated by approximately 4" and extending through the shell of the hull so that they are one piece with the internal structure rather than a single internal beam/brace. This seems a stronger solution to put the beam joints in double sheer rather than single sheer as the original plan anticipates. I believe it will also be faster and less complex for a backyard builder, possibly at the expense of a slight weight penalty.

    I'm probably going to be building the hull in marine ply as I cannot seem to find a reasonable way to buy foam core. Living in Utah, USA where sailing is not popular there are no marine supply outlets that offer foam cores and to ship 25 sheets is prohibitively expensive. I'm still looking for options, but I'm not coming up with much. The only kinda similar product i've been able to find is something called Sintra used for sign making. Since it's not intended for structural use I haven't found and good information on its strength properties. I can buy it locally, it is lighter than plywood, and it is said to bond well with resins. I don't think it's a great alternative, but I may buy a piece and put some fiberglass on it, then test it to see how it holds up.
     
  2. warwick
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    warwick Senior Member

    By the main hull center section.
    It appears that Ray Kendrick uses a beam structure, with in the main hull that the swing arms for the cross beams swing from, that end up inside the cross beam when unfolded.
     
  3. Jetboy
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    Jetboy Senior Member

    Yes. The original design is for a beam built separately and glassed into the hull. I'm sure it's more than strong enough. It just seems like I can make a few changes to make building easier for a backyard builder. Two parallel bulkheads seem like a simpler solution. I think the original design worked really hard to maximize cabin space, but for a daysailer like I'm going to use it, I'm ok with giving up a couple inches of cabin space to a few 3" bulkhead rings.
     
  4. old dog
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    old dog Junior Member

    float support beams

    Why don't you make open trusses o/o high tensile ally tube?
     
  5. Jetboy
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    Jetboy Senior Member

    I like the idea. The only thing is that they would be really time consuming to make. It is one of the ideas I've been kicking around. My thought was something like three 1" by .125 tubes in a triangle shaped truss. One big advantage of that is that the smaller tubing size is easier to bend and I could build the triangle "pointing down" if you will, so a single tube that might be straight on the bottom and a pair of parallel curved tubes on top. Then if I sheeted it with a single layer of cloth/epoxy it would have a nice profile to re-direct any wave contact downward rather than up. Might make for a dryer ride.

    It's just so easy to simply buy a couple sticks of 3" tubing that it's hard to justify the extra work to build trusses.


    At this point I might have to build the whole boat out of aluminum if I can't find something to build the hull out of. I'm have a really hard time finding any marine ply, foam core, or pp honeycomb in Utah USA. There basically is no boat building anywhere within about 800 miles of here, and the local suppliers of lumber want over $100/sheet for 6mm aquatek. It's also really expensive per piece to ship a small order of foam core from somewhere.
     
  6. ancient kayaker
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    ancient kayaker aka Terry Haines

  7. Jetboy
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    Jetboy Senior Member

    Thanks A K.

    I actually got an email from 3m back this morning. It sounds like I might be able to work through one of its local warehouse distributors and possibly wait for truckload that my stuff will fit on with and pick it up there. It might not be super fast, but I think I can get it a lot more reasonably that way. Composites One has a sales office here. They distribute for a number of composite brands. I doubt they stock anything, but I'm optimistic that they might be able to get it.
     
  8. Jetboy
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    Jetboy Senior Member

    As for the aluminum arms, I found a sort of design idea that kinda mirrors what I would probably build: (mine will be a lot less complex, but same idea)

    [​IMG]
     
  9. ancient kayaker
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    ancient kayaker aka Terry Haines

    Looks like something from my misspent youth in Aerospace!
     
  10. Petros
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    Petros Senior Member

    The beams do not just keep the amas from bending upward, but backward as well. The drag of the ama can be quite large when a wave swamps it if you are at speed (or from colliding with a dock). So you also have to consider that you need to brace the amas for fore and aft loads on them. That picture of the double rib would not have a lot of strength with the loads in the fore and aft direction. You can either wire brace them with crossed wires for and aft, or make sure the beams themselves can take the fore and aft loads.

    There are also some torsion loads as well, as when the hull is lifted by a wave at the bow and the ama is lifted at the stern. The torsion loads are not as large, but it can fail the beam if not considered in the design. A square or round tube can take loads in torsion and in all directions laterally, but a built-up member you have to design for all possible load conditions.
     
  11. old dog
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    old dog Junior Member

    ally truss beams

    The beauty of an open truss is just that. It doesn't need to be faired as the resistance, if it should hit green water, is a 1/4 of 5/5 of bugger all.
     
  12. old dog
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    old dog Junior Member

    quite right


    The pic is of a rocker arm assembly designed to move up and down only.Also, each part has been machined o/o billet and that is very expensive.
     
  13. ancient kayaker
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    ancient kayaker aka Terry Haines

    Sure costs a bundle to do it like that; the last real job I was on had a pile of special parts all machined out of Ally billets that went together something like this -
     

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  14. old dog
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    old dog Junior Member

    Hi A.K. Looks like a ***** to sail upwind! Like the colour scheme though.
     

  15. oldsailor7
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    oldsailor7 Senior Member

    I don't know but it seems we are talking about a lot of complication here.
    After all this is only an 18ft boat. My Piver Nugget was a 24 footer and had simple wooden crossbeams hinged outboard of the cabin with steel hinges and 3/8" bolts. The floats simply folded up onto the cabin top for storage or trailing.
    Worked a treat for four years of sailing and trailing with no problems.
    As Clyde Cessna famously said, "Simplicate and add less weight". :cool:
     
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