Small cat crossbeam questions

Discussion in 'Multihulls' started by Nstray11, Apr 29, 2024.

  1. Nstray11
    Joined: Apr 2024
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    Location: South Carolina

    Nstray11 Junior Member

    Hi all,

    I have a Tiki 21 double canoe style cat - 1800lb, 12ft beam, very conservative rig + sail area. The 3 original wood / epoxy H-beam style crossbeams have problems so I’m going to replace them. I need the replacements to be very strong and lighter than the original 55lb beams.

    Several other owners replaced theirs with 6061 aluminum 4x4” .125” wall square beams. They had an aeronautical engineering run a computer program that calculated the forces on the middle beam where the mast is stepped in a 40kt full knockdown / capsize. It determined the aforementioned 4x4 beam would deflect .3” and that that was sufficiently strong. There is a dolphin stay for compression.

    I’d like to swap to aluminum. I’ve seen lots of alum beams on cats but never square, always circular or oval. I read circular tubes are stronger in ways but i also read a 4” square beam with the same wall thickness as a 4” circular beam will have a stronger moment of inertia. Are circular or oval or square the strongest for a small cat like this?

    6061 beams at 4 x 4” x .125” wall thickness are 28lb a piece and I find that shockingly light (half the originals weight) and it scares me, computer program aside.

    I’d like your thoughts and feedback on the strongest and lightest possibilities for these crossbeams. I have a broken in half carbon mast from an Azzura 310 I thought I’d make beams out of but I was told it can’t handle torsional loads like cats create since it’s a mast originally. Other than that carbon is too $$. I do have access to an aluminum welder. 70353390454__DC86752B-D314-4D33-8F3B-174A0A26D6DC.jpeg

    Thanks!
     
    Last edited: Apr 29, 2024
  2. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    A square extrusion will give you less weight than a circular one equivalent in inertia or resistant modulus. Perhaps an elliptical section would be best, for your boat.
    On the other hand, a deflection of 3" in the beam seems too high, but I have not done any calculations. It would be advisable to place additional reinforcement in the central part of the beam that supports the mast.
    I think that thinking about carbon for the beams of a boat like yours would be exaggerating the thing.
     
  3. Nstray11
    Joined: Apr 2024
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    Location: South Carolina

    Nstray11 Junior Member

    Hi,

    Thanks for the reply! Elliptical is an interesting suggestion. The deflection is a third of an inch not 3 inches. I was thinking of reinforcing the part of the beam where the mast is stepped as well. The problem is, how do I fit some thing 6 feet into the middle of the inside of the beam.

    I had an engineering friend suggest I weld a plate on top of where the mask goes to spread the load out. I like that idea.
     
  4. TANSL
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    TANSL Senior Member

    Sorry, my mistake, 0.3" is also a big figure for let's say 3 feet.
    Weld the plate but not on top but below the beam.
     
  5. Nstray11
    Joined: Apr 2024
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    Location: South Carolina

    Nstray11 Junior Member

    Good idea on welding it on the bottom. I believe it was .376” deflection over the full 12ft of the beam.
     
  6. TANSL
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    TANSL Senior Member

  7. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    Attached Files:

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  8. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Welcome to the forum Nstray11,

    The box beam you cite that of 4x4x1/8 will be sufficient.
    I've run a few back of a fag packet calc's, and the max stress will be circa 77MPa and a deflection of 5mm.
    This is assuming only the end box beams work Since the closer to the vessel's midships you place the 3rd beam (carrying the mast) it is less effective in resisting the torsional quartering seas load.
    It does of course contribute to the simple transverse bending loads.

    So, i have assumed only 2 beams, one at each end working, for both simple load cases.
    You do however, have them at some distance in, from the extreme ends, so my values would increase a tad.
    So my calculated 5mm deflection for example would increase pro-rata the distance less, from the ends, so perhaps up to around 6-8mm. This is consistent with your other 0.3" value.

    But it does appear upon first glance to be satisfactory.
     
    Last edited: Apr 30, 2024
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  9. Nstray11
    Joined: Apr 2024
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    Location: South Carolina

    Nstray11 Junior Member

    Hi,

    Love that aluminum beam tiki out in the Bay Area. I studied that blog pretty well and I’m copying that mast base almost exactly for my boat.

    Thanks for the link to the crossbeam question on the Wharram forums. I skimmed over that post previously, here’s my take: the Wharram community is wonderful. Like any niche community there are a lot of purists and naturally there can be resistance to change especially from wood to alum. Ironically the tiki 8m and several 21s and 26s have alum beams. As for as middle alum beams I know of there are 2 that have been sailing a while (Little Cat in the jpegs and a Norwegian 21). Then there are 3 that recently converted that race with larger than normal rigs consistently.

    What was considering for the middle beam was 4x4x.1875” wall instead of .125” wall.
     

  10. Nstray11
    Joined: Apr 2024
    Posts: 5
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    Location: South Carolina

    Nstray11 Junior Member

    Wow this is amazing that you did this. Very thoughtful, thank you!

    Here’s the analysis attached that was done if anyone’s interested. It’s based off a Tiki 21 with a Hobie 16 rig which which is taller and with more sail area so I’m assuming more force on the boat.
     

    Attached Files:

    Ad Hoc and bajansailor like this.
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