Rotating Dagger boards on a cruising catamaran

Discussion in 'Multihulls' started by Becaris, Jun 11, 2011.

  1. Becaris
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    Becaris Junior Member

    Good point, but, for the moment, I have decided to go with vertical boards, but pushed far toward the outside of the hull. This will bring the exit point a few inches above waterline. Now, wait a moment before you all get on me, I do understand the 'gurgle' issue. I plan to use neoprene gaskets to block water from easily going up inside, and because the actual boxes will be above waterline, I should have little sealife growth to clean, have less than average drag when one board is up (no exposed empty slot below waterline), and it gets that casing out of the middle of my living space. Even though this is a 60' cat, it has fairly narrow hulls, so any loss of space really hurt that area. I will post a picture of the design if anyone is interested. OK, now let me have it :rolleyes:
     
  2. oldsailor7
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    oldsailor7 Senior Member

    The "gurgle issue" is not about the water going up inside the box----it's about the "air entrainment issue", because the board is surface piercing at a point where the water level is continually fluctuating.:eek:
     
  3. CatBuilder

    CatBuilder Previous Member

    Yup. Just as Oldsailor says. If you can nudge that board just a few inches inboard, you can get it below the waterline where it will not ventilate.

    Might be worth it. On my boat, I kept them nearly to centerline and will use the space for a locker. You need lockers here and there, so maybe you could use one by the board?

    Just an idea.
     
  4. Gary Baigent
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    Gary Baigent Senior Member

    Don't do that above water exit, Becaris, listen to what OS7 and Catbuilder are saying. Ventilation is a roaring, noisy *******, not only because you lose board "bite" and end up going sideways, it also is like heaving on a brake.
    .
     
  5. oldsailor7
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    oldsailor7 Senior Member

    For a particular deep ocean long distance race I originally built my Buc 24 with a non ballasted fin keel and it worked fine for that purpose.
    However it was too vulnerable for Harbour racing, so the following season I replaced the keel with a vertical daggerboard. The case top was planked in to the hull stringers and frame 5.
    This made a comfortable seat for the cook at the galley or a nice seat for two in the middle of the cabin.The dagger was capped so that when it was fully down it kept the cabin dry in even the most choppy weather. Since the plan form of the dagger had parallel edges, a hard fibreglass seal with a slot closely matching the foil shape of the dagger, was epoxied on to the bottom of the case. This eliminated turbulence and surging within the case and helped keep the boat dry. This worked well for me, but it did mean that you had to go down into the cabin to adjust the depth of the board. In practice we found we sailed with the board down most of the time. The only time you want the board fully up is when you are sailing dead down wind and a fast multihull is better off tacking down wind anyway, for which you need the board down.
    After all a properly shaped dagger is much more efficient at reducing leeway than the rounded shape of the hull(s).
     
  6. Becaris
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    Becaris Junior Member

    I am completely confused by what you said. How can I lose board bite when I have completely retracted the board so that there is no bite? And if you are talking about the board that is down, it is then just as deep as any other board. The two foot difference in exit hole height does not change my board depth. (I only draft 23 inches).

    As for the ventilation as mentioned by OS7 and CatB., I assume they are talking about water impacting the opening and forcing air to ventilate out the top due to increased pressure form the water entering.
     
  7. Becaris
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    Becaris Junior Member

    The space is needed for an additional cabin, so I can't do that.
     
  8. Becaris
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    Becaris Junior Member

    I can angle them in and get below waterline. It is just too bad there is no solution for the ventilation issue, having less drag, more space and less cleaning are a lot of plusses.
     
  9. redreuben
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    redreuben redreuben

    Given the issues you are having, maybe you should look at the whole picture rather than say boards can't go there because .....
    You are still at the design stage so be prepared to change EVERYTHING until you get the right combination.
    Into the mix I would suggest looking at just one daggerboard hard up against an inboard hull side. John Shuttleworth uses this in many of his cats down to 28ft so have a look. See also http://www.boatdesign.net/forums/multihulls/single-dagger-board-vs-double-boards-37787-6.html post #80
    If you compromise the efficiency of the boards for some accommodation priority you will regret it constantly, the boards have to work well so you can enjoy everything else.
    Merry Xmas and Seasons Greetings to all,
    RR
     
  10. Gary Baigent
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    Gary Baigent Senior Member

    Sorry Becaris, didn't explain myself clearly. "Bite" is when a daggerboard is working correctly, like when you have it right down, beating to windward, it's holding you on course and stopping you from skidding sideways. Get ventilation and this destroys the "grip" and the daggerboard or centreboard or keel or foil, becomes enveloped in bubbles, destroys the lift provided by the foil shape and creates drag. You don't want it. And if you had the board exit above waterline, as you suggested, then this would occur, air would rip down the leading edge of your board ALL the time. The exit has to be underwater.
    And like OS7 and Redreuben say, mostly your board will be right or near right down or set halfway when sailing offwind. The only time you'll have the board retracted is when in very shallow water.
     
  11. CatBuilder

    CatBuilder Previous Member

    No, the foil itself will not work (boat will not sail to windward well) if the foil ventilates. Ventilation is air getting down to the foil, stalling it.
     
  12. redreuben
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    redreuben redreuben

    Becaris,
    i was just looking back at post 7.
    What figures have you used to size the board ? Or are you making it to fit where you want it to go ?
     
  13. Becaris
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    Becaris Junior Member

    I'm just using the size, depth and position of the boards from the original plan drawing, so I am not having to calculate those figures. So far I am only messing with offset vs centerline position.
     
  14. Becaris
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    Becaris Junior Member


    Thank you for the clarification, and excuse an amateur designers ignorance, I am exploring different option and learning from all of you why and why not to do certain things.
     

  15. Becaris
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    Becaris Junior Member

    In one of my earlier concepts for this I considered putting both boards up along side the inner hulls and then rotating them down when needed. I was just concerned with the impact and drag of water hitting them in rough seas.
     
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