AC45 Dagger board control

Discussion in 'Multihulls' started by kiwi_bob, Sep 10, 2011.

  1. kiwi_bob
    Joined: Aug 2011
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    kiwi_bob Junior Member

    Been watching the AC45's they have long dagger boards (maybe 4m), relatively short towers (Maybe 1.8M) to haul them up. The height of the hull is approx 1m. I'm not sure of the exact dimensions here but seemingly the boards are able to be fully retracted by control ropes.

    The issue with these dimensions is that when the board is fully down the attachment point for the rope/strap must be fully submerged by a meter or so. Is there a slot in the board or some sort of bolt rope like arrangement that the control rope is slotted into? I suspect the latter but I don't know this and I'm worried I might be missing something obvious here.

    There must be something clever going on otherwise you'd have a very draggy rope vibrating in the water beside the board below the boat. Be interested to know the setup if anyone of you luminaries have got close enough to have a good look at it... I'm going to have similar towers on my boat for the same purpose and the lower I can keep them the better...

    Any Ideas?
     
  2. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    This rough sketch shows the board up and down and the control arm up and down relative to the hull. This is just a workable idea-I think.
    hull=3.4'
    board= 12'
    control arm(lift only) 6'
    --
    board retracts 7.75'
    control arm moves 2' (retract only)(line from deck to bottom of control arm,to top of control arm, to attachment point on board. When line is pulled, control arm extends then hits stop and board comes up. In retracted position control arm remains up 2.75'-max extension 5' above deck).
    --separate line deploys board(and also results in control arm retracting)
    --only two lines
    Note: in my version the control arm moves-it doesn't really have to and I don't think it does on the AC 45.

    click on image: pictures show 'control arm' up some when board is down...
     

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  3. kiwi_bob
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    kiwi_bob Junior Member

    I've stared at your diagram for a while and am none the wiser unfortunately. Do they actually use a control arm of some sort in the AC45's or are you guessing?

    Looking at the photo's of the AC45's you have posted the boards don't seem to over-top the towers by more than the hull height so maybe they haven't done anything clever at all... I was watching the Portsmouth AC coverage this morning and it did look like the boards were much higher than that reletive to the towers... Could be wrong.
     
  4. Doug Lord
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    Doug Lord Flight Ready

    ================
    Look at the center picture above-the thing next to the board I'm calling a "control arm"-I don't know what they call it.(You call it a 'tower'.) The whatchamacallit doesn't have to move. On my sketch the whatchamacallit moves and one line pulls the board up, one pulls it down. Lines are not shown.
     
  5. kiwi_bob
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    kiwi_bob Junior Member

    OK, what I'm calling a "tower" you are calling a "control arm"? (lets call it a control arm from now on) and it can go up and down by the height of the hull too? That kind of makes sense and maybe accounts for what I thought I was seeing. Are you 100% sure that that is what they are doing on the AC45?
     
  6. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    ===============
    I put the distances everything moves on my first post. The control arm doesn't have to move ,though. Moving only reduces its height above deck by about 2'. I have no idea how they actually do it but this would work......
    --
    On the sketch both the board and the control arm are represented by two lines-one for up and one for down. Their relationship to the hull is shown by a fine line representing the bottom and one representing the deck.
     
  7. lesburn1
    Joined: Oct 2007
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    lesburn1 Junior Member

    Length of "control arm"

    The length of the control arm only needs the length of the centerboard in the up position minus the amount of the centerboard in the case when it is up.
     
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  8. bruceb
    Joined: Nov 2008
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    bruceb Senior Member

    board control

    lesburn, that is how I have set up my boards, and it seems as if it will work fine. My boards extend 52 inches below the float bottom, but only require a 22" "tower" above the float deck. My boards float, so they also require a down haul. B
     
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