Catamaran Seagull Striker Questions?

Discussion in 'Multihulls' started by UNCIVILIZED, Nov 14, 2016.

  1. UNCIVILIZED
    Joined: Jun 2014
    Posts: 166
    Likes: 1, Points: 18, Legacy Rep: 31
    Location: Land O' the Great Lakes

    UNCIVILIZED DIY Junkyard MadScientist

    Guys, I'm looking at a 13m catamaran, & noticed what (to me) is an oddity in her forward crossbeam's rigging. The vertical "spreader" that makes up her Seagull Striker is but a single, vertical piece. Meaning that on her forward crossbeam, instead of having 2 vertical spreaders coming together & meeting where the wire runs over them. There's but one vertical spreader (piece) coming up off of the forward crossbeam.

    Has anyone ever seen one like this before? Why might it be built like this? And why are such fittings commonly made from 2 spreaders set on angles close to being perpendicular to the forward crossbeam? "Spreaders" which then (usually) meet 2' about above the beam, where the wire between the hulls runs ovetop of them.
    Oh, & BTW, what's the proper name for that wire?

    I'd post pics, but both my computer, & photobucket account are giving me fits. So if my description of these rigging bits needs work, please let me know what part needs revising.

    Thanks
     
  2. catsketcher
    Joined: Mar 2006
    Posts: 1,203
    Likes: 88, Points: 48, Legacy Rep: 790
    Location: Australia

    catsketcher Senior Member

    My mum had a single element seagull striker but it seemed less strong than others. Eventually it was changed but there is no real reason to do so. Usually the double element seagull striker leaves the middle free for the forestay and maybe prodder, sometimes even an anchor roller.

    Using spreader section rather than tube allows extra fore and aft strength at the bottom attachment. Saying this I have tube that has lasted fine.
     
  3. Barra
    Joined: Feb 2014
    Posts: 94
    Likes: 4, Points: 8, Legacy Rep: 32
    Location: Perth

    Barra Junior Member

    the two spreader, upside down "V" arangement leaves room for the furling drum to be set low.

    Some of the early cats had a single spreader with the forestay attached to the bridle on top of the spreader (ie spreader and forestay aligned). Good for hanked sails as it keeps the bulk of the dropped sail out of the way of the muddy anchoring tackle. The tack of the hanked headsail is attached at the base of the spreader adjacent to the fore beam.

    I use this system on my boat. 450 ft^2 of #1 can be a struggle at times :)
     
  4. UNCIVILIZED
    Joined: Jun 2014
    Posts: 166
    Likes: 1, Points: 18, Legacy Rep: 31
    Location: Land O' the Great Lakes

    UNCIVILIZED DIY Junkyard MadScientist

    The furling/headstay thing makes sense. And I also wonder if having 2 spreaders better distributes the load on the beam. Since I'm not sure that crossbeams have compression tubes at the spreaders like masts do. Thoughts on this?
     
  5. Barra
    Joined: Feb 2014
    Posts: 94
    Likes: 4, Points: 8, Legacy Rep: 32
    Location: Perth

    Barra Junior Member

    Depends on the design.

    If you keep all the loads on top of the beam, then the beam is just locating the forestay and the loads are taken by the bridle wires (forestay upward pull) and the deck walkway (any aft load exerted by the sheet/sail load).
    [​IMG]
     
  6. upchurchmr
    Joined: Feb 2011
    Posts: 2,865
    Likes: 89, Points: 48, Legacy Rep: 579
    Location: Ft. Worth, Tx, USA

    upchurchmr Senior Member

    You seem to be ingnoring the compression load developed in the front beam by the shallow angle of the bridle.
     
  7. Barra
    Joined: Feb 2014
    Posts: 94
    Likes: 4, Points: 8, Legacy Rep: 32
    Location: Perth

    Barra Junior Member

    You seem to be ignoring what was asked.

    To help you. The OP was asking about load distribution from the spreaders.:confused:
     

  8. UNCIVILIZED
    Joined: Jun 2014
    Posts: 166
    Likes: 1, Points: 18, Legacy Rep: 31
    Location: Land O' the Great Lakes

    UNCIVILIZED DIY Junkyard MadScientist

    Do you know how to calculate such loads as they pertain to a cross beam's scantlings, when the other loads that it sees are also factored in? If so I'd be interested to know. As certainly such beams see some pretty severe racking, & torsional loads, in addition to the semi-static ones created by their rigging. About which the latter should be a bit easier to calculate. Especially when armed with a Loos gauge, & a copy of The Rigger's Apprentice. Non?

    Usually the loads on & created by rigging are reasonable if they're kept at n angle of more than about 12 deg. IIRC, which works out to 5:1 +/-. The latter figure about which I'm certain, the former is easy to ascertain by looking in Brion Toss's book. Which, it's not hard to measure such rigging on any catamaran in order to see what these angles are, since no mast climbing is involved.

    My thinking on this might be in need of a bit of fine tuning, with me being as yet uncaffeinated today ;)
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.