daggerboard location on tri

Discussion in 'Multihulls' started by al loomis2, Aug 7, 2012.

  1. al loomis2
    Joined: Jul 2012
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    al loomis2 New Member

    i'm converting a 7m cat to 2 tris, and it occurred to me that location of the dagger board might be radically different, due to the unequal shapes of amas compared with with the main hull, and this tri will heel a great deal more, as well. so, forward 10%? back 20%?.

    any one with experience in this area? i'm hoping it's not critical, but an ounce of experience is worth a tonne of hope...
     
  2. oldsailor7
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    oldsailor7 Senior Member

    Generally speaking the centre of effort of the board is aligned with the centre of effort of the sails, not the immersed side area of the hulls.
    The ideal hull has no side resistance at all, leaving the board/fin/dagger to provide the major resistance to leeway.
    However that ideal hull is not possible.
    Nevertheless the sail area is so very much greater than the immersed side area of the hull(s), that it doesn't really make that much difference.
    The nearest to the ideal hull shape for least resistance to leeway is half an oval lying on it's side. The least resistance to forward motion is a semi circle, because it has the least surface area.
    These, and other conflicting reasons, are why the design of suitable hulls for any particular boat, (and particularly a trimaran), is such a compromise. :eek:
     
  3. al loomis2
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    al loomis2 New Member

    thx for reply- i'm going to line up clr and csa as much as possible. but mulihulls develop an off-center drag as they heel which creates a turn to lee moment. hard to say how much effect there is, but i have seen a few tris with dboard in front of the mast, which i thought was at least partly in response to this drag.

    i'll just take a guess, not hard to move a dboard case if necessary, small ply and glass boats are good that way.
     
  4. cavalier mk2
    Joined: Mar 2010
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    cavalier mk2 Senior Member

    I'm going to elaborate a bit on what Paddy said and point out the improvements in sail carrying power and pointing in some Newick boats like the Tremelino which offered a "new moon" asymmetric ama for leeway resistance. Many other tris have varying degrees of asymmetry for this compromise reason. New foil assisted boats don't need to do this since the foil is taking the side load letting the amas have a least resistance shape.
    The board should be near the csa. your amas as they immerse should move the clr forward to compensate for the drag ( which isn't as much as you think) you are describing.
     

  5. al loomis2
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    al loomis2 New Member

    it's true there's not much drag on the ama, but it is on a long arm.

    lifting amas make a lot of sense on a tri, worth consideration. thx
     
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