Rotating Dagger boards on a cruising catamaran

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

  1. Becaris
    Joined: Aug 2008
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    Becaris Junior Member

    Now that I better understand the ventilation issue as bubbles messing with the foil, can someone explain why this happens, and perhaps their is a solution? What if the top of the case was sealed so that the air could not flow? What if there were neoprene baffels near the water exit acting as seals, or both? What if the exit point was raised another foot above waterline? The board goes quite deep, just how far down can these bubbles be going, especially if methods to reduce them were used? Again, excuse my ignorance, I am just trying to get these two casings the size of a moose out of the center of my cabin ;)
     
  2. CatBuilder

    CatBuilder Previous Member

    No no no... that's not it.

    The air comes down at the water/air interface... the surface of the water.

    If your foil starts above the surface of the water (and is a normal NACA type foil), it causes air to get sucked down from the surface. That's ventilation.

    It has nothing to do with your exit point or your daggerboard trunk. It has to do with some of the foil being above the flowing water and some being below the flowing water while in use.

    You must submerge your exit point.
     
  3. truecougarblue
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    truecougarblue Junior Member

    If the boards are outside the hulls and rotated down wouldn't this also create ventilation? If it isn't a major problem for boards outside the hull I have a hard time believing it would be a major issue for boards projected from inside the hull.
     
  4. CatBuilder

    CatBuilder Previous Member

    1 person likes this.
  5. Gary Baigent
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    Gary Baigent Senior Member

    Becaris, there is an answer to having a board or foil exit above waterline and that is (and I should have mentioned it earlier, but didn't because it applies mostly/only to flying hydrofoil yachts, and confuses the issue) ... and that is to rake the board/foil forward 5 degrees or more. The angle forward stops (most of the time) air rushing down the foil leading edge, because it has to move forward against the water pressure. Also fences set down the foil will do the same thing - but you can't do that on your boat because you need to be able to lift boards. Conversely a sloping back board or foil with no fences, exit above water, would ventilate continuously.
     
  6. redreuben
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    redreuben redreuben

    And also tilting the board forward on a cruising boat means you will continuously pick up weed ropes and other crap around the board as it is going to be held in place by the water pressure.
    RR
     
  7. redreuben
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    redreuben redreuben

    I forgot that forward rake was a solution to ventilation, I still think the central board (between the hulls) is a good idea for a cruiser !
    RR
     
  8. truecougarblue
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    truecougarblue Junior Member

    With the idea that a forward raked board would be a 'crap catcher' it would make sense then that maybe the outside the hull swing down is the best viable configuration. To clear debris the board just gets rotated through aft, washing off the debris, them reset to rake angle.
     
  9. Gary Baigent
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    Gary Baigent Senior Member

    True, a raked forward board or foil will pick up weed ... but so does a bulb keel, swinging or fixed ... and the weed is really stuck there, whereas on a daggerboard boat .... you just pull the board up, then drop it down again. Clean. Simple.
     
  10. tspeer
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    tspeer Senior Member

    The top of the centerboard case has to be above water level to do this too. And it's best if the boat is designed so the board trunk is safely above water level in fresh water as well as salt water. Don't ask why I think this is so important! :)

    The original sketch showed a centerboard that was below the cabin sole, which I take it means it was below water level. The control lines WILL jam some day, and if the top of the trunk is below water level, this means hauling the boat to fix what should have been a minor problem.

    Another issue with a centerboard is how to seal the slot when the board is down. Rubber seals can be inverted by water pressure and then jam the board in the up position after it has been raised. If anyone has a satisfactory solution for sealing the slot, I'd love to hear about it so I can implement it on my boat.
     
  11. tspeer
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    tspeer Senior Member

    John Shuttleworth has a design in his catalog that has a forward raked daggerboard trunk, but the exposed part of the board is tapered so the leading edge is not raked forward on the exposed portion of the board. You can't do this with a high aspect ratio board, however.
     
  12. redreuben
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    redreuben redreuben

    Guys, the whole daggerboard centreboard thing is an open and shut case (was that a pun ?) Becaris just has to digest the facts and decide wether he wants a boat that goes to windward and is easy to operate and maintain or a marginally bigger interior and less than optimal performance on the wind and more maintenance/hassle.
    Given that he has said it is a slim hulled boat I would have thought the daggerboard was a no brainer. If loss of space is a problem add 2 feet !
    RR
    over and out.
     
  13. Becaris
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    Becaris Junior Member

    Unfortunately, the hulls are finished, so no widening them. I might consider raking the boards forward, or moving them in a bit to get the exits under the waterline. I will consult with the origional designer and builder and see which they recommend.

    Thanks for all the posts everyone, I learned a lot and it helped.

    Merry Xmas.
     
  14. Gary Baigent
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    Gary Baigent Senior Member

    One final word, Becaris, if you follow the informed comments here this Xmas day and sensibly have the board exits below water, then you don't have to rake the boards forward, just make them vertical. Cheers.
     

  15. CatBuilder

    CatBuilder Previous Member

    Yep, just like Gary said in the last post. You can also gain a little walking room by tipping the top of the board outward toward the lifelines. The shoulders are wider than the waist and feet, so it makes a little extra room. I'm working on a narrow hull boat with the same problems. I know where you are coming from.
     
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