Dagger Board with Kick-up Bottom Section

Discussion in 'Boat Design' started by bill price, Aug 22, 2017.

  1. bill price
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    bill price Junior Member

    Thinking about a Dagger Board with a bottom section that will rotate ( kick up) if you hit bottom, to reduce impact on trunk.
    I know it's complicated, but for sailing in real skinny waters , it may have some advantages.
    Any experience or designs that would be helpful?
    BP
     
  2. Blueknarr
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    Blueknarr Senior Member

    I played with this idea twenty or so years ago. Nothing I tried worked. Ended up reshaping the leading edge of bottom half of dagger board so that it would raise when contacting seafloor.

    Paul
     
  3. HJS
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    HJS Member

  4. bill price
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    bill price Junior Member

    Paul,
    How much angle on the foot? or Radius?
    Were you satisfied with it?
    Thanks. BP
     
  5. messabout
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    messabout Senior Member

    Why not use a conventional pivoting centerboard and thereby avoid reinventing the wheel.? Sure enough the centerboard case is larger than the dagger board case but centerboards have worked well for a few hundred years.

    Disclaimer: I do not favor centerboards. I like the dagger because it is smaller, lighter, and easily removable. On the other hand a centerboard has the advantage of fine tuning the helm feel and being able to easily reduce its area, and potential drag, by pulling a string, when on a run . It is almost always more practical if one is to encounter frequent grounding.
     
  6. bill price
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    bill price Junior Member

    I agree with you "dagger because it is smaller, lighter, and easily removable."
    Just thinking that a Dagger with a "rotatable" lower foot would have the same benefits, with the advantage of kick up on grounding.
    Should be easily manufactured.
     
  7. Blueknarr
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    Blueknarr Senior Member


    Bill

    It was many many many eons ago.
    I think I maxed out the radius. 8in radius on 6in wide board.
    Also remember sheathing top of board with very slick plastic. It worked wonderfully, even raising when crossing weeds.

    Thanks for reviving pleasant memmories of sailing a sunfish as a teen.

    Paul
     
  8. upchurchmr
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    upchurchmr Senior Member

    "Should be easily manufactured"??????
    Please show your concept.
     
  9. messabout
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    messabout Senior Member

    Yes please do a sketch of your proposed dagger articulation.

    Seems to me that if the hinge is below the bottom of the boat, the board can fold aft but the chord of the board will still be below the bottom. If you are grounding, the board will have to press on the bottom of the boat and it will still need the draft of the boat plus the chord of the dagger. Not to be a naysayer, just thinking about the situation as my feeble brain imagines it. If in the case of a run when the dagger is partially retracted, as is common practice, then the hinge will be somewhere inside the case and the board cannot hinge aft......No?
     
  10. bill price
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    bill price Junior Member

    Rough Sketch RD Board
     

    Attached Files:

  11. bill price
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    bill price Junior Member

    I tried to attach a PDF sketch of the Roating Daggerboard, but I can't get it to open. Can y'all see it? BP
     
  12. upchurchmr
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    upchurchmr Senior Member

    Bill,

    Your center board is not going to work well to produce side force due to all the circular slots.
    Then its going to be weak since you cut out 1/2 of the thickness on each part.
    The actual strength will be less than that.
    Looks like you will only be reducing the depth of the board by 1/2 or less - is that enough?

    Sorry, but I don't think this is easy at all.
    The board in post #3 looks like a lot better setup, IMHO.
     
  13. upchurchmr
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    upchurchmr Senior Member

    Actually, its worse than I said. I neglected the fact that a board is not flat.
    The upper board will not have a circular front edge, since the lower board will be the only thing with the fwd 1/3 of the aerofoil shape.
    So the parts of the upper board holding on to the pivoting bolt will be much smaller , in the fwd and aft direction.
    Even less strength.
    The aft 1/3 or more will be the same.
     
  14. bill price
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    bill price Junior Member

    Mr. Upchurch,
    1) Actually, there are no circular slots. The broken lines in the sketch are intended to simply show location and configuration of the lower section mounted within the upper frame, and the (shock chord) lanyard tube .
    2) Strength of Cheeks in the Upper Frame is dependent on material, width and section. Obviously, a RD Board of thin, narrow week material wouldn't work. Thicker and stronger materials might in some conditions . It's an engineering trade off.
    3) Depth of the lower rotatable section is variable. The idea is to have a section that will rotate and reduce effect of impact of a hard hit on bottom, on the DB trunk.
    4) Some Dagger-boards are flat ( Sun fish DB has no aerofoil section at all) , but certainly most larger, higher performance boards are of aerofoil sections. Interestingly, I have observed very thin Centerboards of Windsurfers to be surprisingly strong. One would think some such structural assemblies should be strong enough to function in this concept.
    Also note.. most bottom hits are on the bottom forefoot of the board , not up the face, so a deep lower section is not necessary for the concept to work as desired in the high percentage of circumstances.... And a more shallow lower section will have less lateral resistance pressures, thereby requiring lesser structure in the cheeks , than if the Lower section was deep. Again, that's an engineering ( experience) trade off.
    5) The rounded nature of the fittable area between the Lower Section within the cheeks of the Upper Section is provided so the leading and after edge of the lower section can form a relatively continuously ( Short time) smooth surface with cheeks of the upper section. While such fit will have some friction, it would be far less than the typical CenterBoard slot. ( Obviously an aerofoil section will diminish the width of the Cheeks, but such reduction of width should be formable as relevant.
    6) The concept would facilitate gybing DB's as well, which CB can't effectively do.
    Mr, Upchurch,, My question was , have there been any examples of this concept?
    Thinking about it, whereas traditional constructions have been of wood, it may be reasonable to assume that there has been little experience with a design concept that would require more modern materials.)
    Nonetheless, all this may be more trouble than it's worth ( unless you break a DB box : )
    Thanks for your insightful comments. BP
     

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  15. upchurchmr
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    upchurchmr Senior Member

    Good luck, as drawn the lower board won't rotate - given my assumptions.
    We could talk about assumptions for days.
    I wouldn't try this and I was a structural designer for aircraft for 35 years.
    That don't mean I'm not wrong. Been wrong before.

    My Coronado 15 has a centerboard that is rigged to gibe. Most of the competitive boats have the system.

    I'll be interested to see what you come up with , if you try.

    Marc
     
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