question on daggerboards for catamaran

Discussion in 'Hydrodynamics and Aerodynamics' started by massandspace, Jun 25, 2018.

  1. massandspace
    Joined: Sep 2017
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    Location: Salt Chuck

    massandspace Junior Member

    Designing now either higher aspect boards or lower aspect fixed keels for a small custom sailing cat...weighing the pros and cons of each.

    My question:

    Does ONE board at a given size have the same foiling "energy" as 4 boards that are 1/4 the draft. In other words, imagine, for example, a daggerboard with an underwater size when extended of 12" fore aft by 48" tall. If you were to cut it in 4 pieces horizontally (12" by 12" each) and put those 4 back onto the bottom on different parts of the hull, would that give the same amount (or at least very similar) of upwind foiling ability to the boat as just one 12" by 48" board?

    I ask as I am thinking it might be a solution for me to get decent upwind performance while NOT eating up all my interior space as I would need to do if putting in full sized boards/cases nearer the center of the boat inside the hulls. 4 "mini foils" closer to the ends would fit the boat much better. Yes, I understand I would need to build four cases and 4 boards....

    Pic of rough sketch on CAD attached....the 6 proposed foils (4 boards and 2 rudders) in blue...
     

    Attached Files:

  2. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    The single taller board would be much better! You don't mean "foiling"-right? A vertical board could be good for lateral resistance(not foiling). And spreading vertical boards out like that would probably affect maneuverability negatively. Have you considered a single board under the bridgedeck in the center of the boat? There's a thread in "Multihulls" about that-pros and cons....
    Good Luck!

    Heres the thread-it's not just about why it won't work but also why it does work:
    Bridgedeck centreboard why don't they work??? https://www.boatdesign.net/threads/bridgedeck-centreboard-why-dont-they-work.57051/
     
  3. brian eiland
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    brian eiland Senior Member

    Doug Lord likes this.
  4. Doug Lord
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    Doug Lord Flight Ready

    Brian see the post before yours.........
     
  5. brian eiland
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    brian eiland Senior Member

    Sorry Doug, I must be going blind,...or just losing it....ha...ha
     
  6. Doug Lord
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    Doug Lord Flight Ready

    Nah! You actually improved the info with the extra detail-which I did't see at first!
     
  7. massandspace
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    massandspace Junior Member

    Thanks all but no one has come close to answering the question. And I am NOT interested in a single centerboard centered...?

    Again...if you cut a daggerboard in half horizontally and used the cut half for another board....how would those 2 boards compare to one longer board...
     
  8. Doug Lord
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    Doug Lord Flight Ready

    ----------------------------------
    I directly answered that question in the FIRST sentence of post #2!!
     
  9. massandspace
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    massandspace Junior Member

    Sorry but I do not see an answer there...you only mentioned a single taller board as better, and I agree, but that is not my question....
     
  10. SamSam
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    SamSam Senior Member

    Let me hypothesize here, as in guess a bit.

    I think daggerboards perform at least two functions. They resist leeward drift and provide a pivot point to work with the rudder for better maneuverability.
    If you put one on each end, I would think you would lose the pivot point ability and gain resistance to turning. You would have to gain more speed for tacking, you would lose a lot more tacks and have to re-attempt them, or have to jibe for a different tack or resort to backing the rudder once you've stalled and are drifting backwards. The backing the rudder maneuver would also be hampered by both the forward and aft dagger boards, so it's possible you would be reduced to using jibes as the main method of changing tacks.

    I wonder how it would work to eliminate the dagger boards and just put 4 rudders on the ends that would turn opposite each other

    It might also be the case if one daggerboard was 4' long, two of them might have to be 3' long each in order to reach enough undisturbed water. So that would add resistance to hamper speed and further hamper tacking ability.

    One more possibility is in rough water, with the boat bobbing fore and aft, the dagger boards grips on the water might be constantly changing, resistance to leeward drifting switching from end to end and the helm having to be constantly compensating.

    Center of lateral resistance and helm balance are two terms that come to mind, and you would be messing with them.

    Again, these are guesses, points to ponder and possibly be easily dismissed.
     
    Last edited: Aug 2, 2018
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  11. SamSam
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    SamSam Senior Member

    You might also place regular dagger boards inside the hulls but to the outside of the boat so as to keep them mostly out of the way in the interior.
     
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  12. upchurchmr
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    upchurchmr Senior Member

    Two shorter boards have more drag than one board.
    You need more length on each of the two short boards to get the same lift.
     
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