tub tri dillemas

Discussion in 'Multihulls' started by Vincent DePilli, Mar 14, 2012.

  1. Vincent DePilli
    Joined: Apr 2006
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    Vincent DePilli Junior Member

    tube tri dilemmas

    I would like to what people think the best resolution is for the design of a tube tri’s mast base area. By “tube tri”, I mean a trimaran that uses a full length prefabricated tube as a cross beam, with sea stays.

    I like the tube tri concept, because of the availability of very light and strong, aerodynamically shaped carbon tubes, and the structural efficiency of the sea stay. Ideally you put the cross beam on top of the main ring frame, and attach the sea stay to a chain plate mounted to that same ring frame. (Imagine using Hall’s SCR aerofoil shapped monolithic carbon for the sea stays—how cool would that be…)

    Assume further that you want a rotating mast, with most of the running rigging run back to the cockpit, and a deep, well shaped dagger board which is removable without removing the mast.

    The design issue with this program is that so much stuff comes together at the same place:

    The Beam wants to be right in front of the mast base.
    The dagger board wants to come out of the deck immediately in front of the mast (in which case it interferes with the beam) or behind the mast (in which case it interferes with the boom, or just to the side of the mast (in which case it interferes with the lines coming out of the mast base) .

    What do you think the best resolution to these conflicts would be, assuming that you have to retain (i) tube/sea stay structure, (ii) dagger board removable without removing the mast, (iii) rotating mast, and (iv) halyards led aft?

    Or do you think that one of those 4 requirements would have to go, and if so which one(s)?
     
  2. Corley
    Joined: Oct 2009
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    Location: Melbourne, Australia

    Corley epoxy coated

    Angle your daggerboard so the COE remains in the correct position and exit it forward of the front crossbeam. Carefully designed the daggerboard case structure can help with the compression loads of the mast and reinforcing the area under your full width crossbeam.

    Another structural approach is to end the crossbeam at a strengthened base at the gunwhale in line with the main bulkhead with the seastay attached to the same internal bulkhead structure the beam is a compression strut. That way the daggerboard can exit in the area otherwise occupied by the crossbeam.

    Another thought is to have the daggerboards in the floats and abandon the idea of a central daggerboard case its quite tedious though lifting and dropping daggerboards in the floats one of our club members has a boat with this format and when short tacking its a chore.
     
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  3. Vincent DePilli
    Joined: Apr 2006
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    Vincent DePilli Junior Member

    The angled dagger board does seem to be the most common solution. The board in float approach means more structure and expense in the floats, more load on the beam float connections etc. I think I would rather deal with the angled board.

    I really like the idea of a vertical board (lighter, more efficient hydrodynamically and structurally, so the board immediately aft of, or to the side of the mast is attractive. I believe that Newick offset board, but at the cost of having to locate winches on the mast. I don't know how one would manage the conflict between boom and board if the board were immediately aft of the mast.

    Anyone know how Kurt Hughs' tube tris are arranged?
     
  4. Corley
    Joined: Oct 2009
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    Location: Melbourne, Australia

    Corley epoxy coated

    The daggers on Kurts boats are set vertical and aft of the mast on the port or starboard of the centreline.

    On his formula 40 design its offset to the port side although on my version I'm going to use the shorter board option and the half height case with a gaiter around the area between the top of the case and the deck to allow me to adjust the amount the board that is down to a degree. You still need a slot in the deck to get the board right out though.
     

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  5. Vincent DePilli
    Joined: Apr 2006
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    Location: Seattle

    Vincent DePilli Junior Member

    Well duh. That takes care of boom interference. And you could still sneak the halyards by the front of the slot to a line organizer (whcnce they would head back to the winches.

    That seems an ideal solution with no downsides to me. (I am assuming the offset board is not a material dteriment relative to the centerline board)
     

  6. Corley
    Joined: Oct 2009
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    Location: Melbourne, Australia

    Corley epoxy coated

    There have been a lot of trimarans over the years with offset (from centreline) boards as long as they are straight relative to the boats centreline there doesn't seem to be any performance loss.
     
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