beam/sleeve for demountable CAT

Discussion in 'Multihulls' started by sail, Oct 11, 2009.

  1. sail
    Joined: Oct 2009
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    sail Junior Member

    I would like to use the same mast extrusion for the beams OF A DEMOUNTABLE CAT. I will use sleeve that will inside the beam. I would like to know what clearance I should have between the outside wall of the sleeve and the inside wall of the beam so as to have an easy sliding effect without excess play when sailing hard upwind in the waves.
    Thank you for your help!
     
  2. gonzo
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    gonzo Senior Member

    It depends on the size, the material, the work it will have to do and the shape of the extrusion. If you are using mast extrusions they are probably castings already made for them.
     
  3. Squidly-Diddly
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    Squidly-Diddly Senior Member

    I would have a lot of play, but use screws or wedges to

    prevent excess play.

    I'm not sure what you mean.

    Do you want to make a two-piece mast using the same extrusion as the beams for the lower part of the mast?

    I would drill small holes in the the lower section of the mast and use screws to push the upper section tight against one side of the inside of the lower section.
     
  4. sail
    Joined: Oct 2009
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    sail Junior Member

    The mast extrusion will be used for the 2 cross beams of the demountable cat. If I want to demount, I need to have sleeves that will be fixed on the movable part and will slide inside the fixed extrusion part of the cross beam. The question is how much clearance should should I have between the external sleeve all and the internal extrusion part? My guess is between 0.25 to 0.5 mm(0.5 mm to 1 mm total diameter). Thank you all!
     
  5. bill broome
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    bill broome Senior Member

    i hope you are talking about using simple round section aluminum tubes. real mast sections have tracks cast in, so the interior shape is not even close to the outside.

    using sliding tubes is very difficult, requiring precise lining up of all the axes. and quite unnecessary- if you use straps to hold the tubes to the hull, you can release the straps at diagonal corners and pull the hulls together. pull out of the water and then release the remaining straps , to pack the tubes.
     
  6. BWD
    Joined: Aug 2006
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    BWD Senior Member

    That is a VERY BAD IDEA

    squid, I can say no more.
    speechless.....
     
  7. farjoe
    Joined: Oct 2003
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    farjoe Senior Member

    That method seems quite wide spread in Australia/New Zealand. It seems they build the outer sleeve from fibreglass. Research the 8.5M class for possibly more info.

    I use straps and I already have a lot of problems with setting up due to the unevenness of my launching area. With sleeves it would be a nightmare whatever the gap you build in. I have heard that the beams are put on with the hulls in the water. I bet one would get quite unpopular on a heavily used slipway.

    I would also be worried with corrosion caused by sea water trapped between the beam and the sleeve.
     
  8. Chris Ostlind

    Chris Ostlind Previous Member

    I have built my own beam ferrules before out of a pair of fiberglass shells cast from the inside surface of a section of identical tubing, foam filled and then joined with a marine plywood web glued into a slot cut in the foam.

    Once the joining epoxy cures, lightly sand the outside surface until the desired fit is achieved for a snug, insertable ferrule that is perfect for the internal shape of the mast section you are using for the beam.

    The ferrule needs to be built from sufficient layers of glass to handle the loads expected, or the cat beam will fold nicely at the point of the ferrule when put under load.
     
  9. gonzo
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    gonzo Senior Member

    Hobie Cats have been built like that for 45 years. The corners of the trampolin have a solid aluminum casting that fits into a section identical to the mast. They have pins to hold it in place. They do not fit very tight.
     
  10. themanshed
    Joined: Sep 2009
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    themanshed Senior Member


    Chris have you built composite beams? I'm looking at building demountable beams for my tri, but desire composite material
     
  11. sail
    Joined: Oct 2009
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    sail Junior Member

    Hi All
    I am in China where I can get things very cheap. I can have mast extrusion dies made (for Al 6061T-6) for LESS THAN 1/2 the price or importing one mast (let alone the boom !!)! Fiberglass, epoxy, PVC foam are also cheaper than in the West.
    I will probably leave a 2 mm clearance between the sleeve's outer wall and cross's beam, inside wall.5 layers of FG glass+ epoxy should bring it to fill in about 1 mm leaving the 1 mm play. The Inside of the cross beams and mast will be cold galvanized with Zn epoxy. I am open to any constructive suggestion. Thanks for your comments!
     
  12. gonzo
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    gonzo Senior Member

    Aluminum should be anodized not galvanized.
     
  13. BWD
    Joined: Aug 2006
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    BWD Senior Member

    I know he say mast in there somewhere but I thought it was clear (murkily clear?) he was speaking mostly of crossbeams.
    I meant I thought it was not a good idea for crossbeams, holding things that don't fit together with screws:!: . I think they should fit better ;)
     

  14. Chris Ostlind

    Chris Ostlind Previous Member

    It kinda depends on the type of engineered structure one is looking for.

    I have used masts as beams on demountable trimarans and they work remarkably well, as long as one does not ask them to do things for which they have insufficient strength.

    The last tri on which I used beams in this fashion had segmented beams to allow demounting and then remounting of the amas at a point much closer to the vaka hull. This feature allowed the sailing trimaran to become a really stable motor launch with the sailing rig removed.

    The mast sections were also good at providing a clean bolt-roped trampoline to be incorporated for a smooth surface that required no lashings on the fore, or aft, beams.

    Much like the SeaCart30 tri, my boat used waterstays. This transitioned the used mast/segmented aka beam to a compression structure, rather than a tensioned function and took all sorts of loading out of the custom ferrules.
     
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