External Daggerboards

Discussion in 'Multihulls' started by Fanie, Mar 11, 2012.

  1. HydroNick
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    HydroNick Nick S

    I have not read the thread in detail, and trust this relevant. I always thought the option on the Nicolas Gruet designed Ekolo'Kat 33 and 34 was pretty good in terms of getting the daggerboards as far out of the living space as possible.

    http://www.ngyachtdesign.com/multicoques_voile.php?id=10
     
  2. sigurd
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    sigurd Pompuous Pangolin

    Here is a quote from Rick W. over at Rob's Harryproa Yahoogroup.

    "The difference in drag for a given lift between a ventilating board and end plated board is 4 times. The non ventilating, non end plated board is twice as good as one ventilating and half as good as one with an effective end plate."

    I am not sure how to interpret the constraints for that comparison. But the board being discussed was "1.5 wide and draws 5.5' when vertical" (Gardner Pomper) - a leeboard close to (or even extending down from) the side of a flat bottom proa hull.

    1) In a flat bottom hull one could stick a horizontal, lateral axle through the hullside right down at the bottom, and attach the leeboard to that. It would kick up, and it could be that it could be faired ok and get its endplate from the hull and fairing.

    2) In an oversize daggerboard trunk, it is possible to construct a four bar linkage that pulls up a high AR board as it is pushed backwards in collision. I saw this on the web with a Tornado (those trunks are gigantic), but I can't find it.

    3) I think that explosive bolts is an underused item. They could hold a keel, and be triggered by a forward looking sonar or a long forward facing prod. Alternately, make the hull up/down symmetrical so that if it gets ripped open by the board, one can flip it and go on sailing while the laminate dries for repair. This would add other possibilities, such as a different size of riblets for each side of the hull.
     
  3. sigurd
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    sigurd Pompuous Pangolin

    doug:
    "
    I'm not sure, anymore, how serious the penalty is. The Moth has two vertical foils with no "endplating" from the hull yet it is still the fastest sailboat under 20'?
    "

    The moth is heeling to windward, unloading the surface piercing strut and letting the submerged hydrofoil react the leeway force. If the strut doesn't lift then it has no induced drag.

    The hydrofoils haven't endplate but their span is pretty vast.
     
  4. Doug Lord
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    Doug Lord Flight Ready

    -------------------
    Sigurd you're right for the daggerboard upwind with Veel Heel but what about the rudder? Probably similar. There are many rudders used on dinghies that are surface piercing with no foil attached. I'm just not convinced(anymore) that surface piercing is the bad thing it is portrayed as being when so many surface piercing applications work well.
     
  5. sigurd
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    sigurd Pompuous Pangolin

    I just think rudders or the moth struts are not necessarily a good comparison to leeboards, since it is much more difficult with them to know the lift coefficient.

    But some people with a symmetrical daggerboard cat that could fly a hull might try whether it was faster to have the lee or the windward board down.

    I imagine some flying tris must have tried both vaka and ama daggerboard, and might have formed an opinion.
     
  6. sigurd
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    sigurd Pompuous Pangolin

    How big does that "endplate hull" have to be, anyway? I mean, if one has a daggerboard case hanging from a beam or something, does it make sense to extend its fairing to below the water, and make it into a) a horizontal endplate? b) an extension of the daggeboard chord?
     
  7. Avoid Rocks
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    Avoid Rocks Junior Member

    Resurrecting an old thread but I've been thinking about and researching leeboards and this thread was interesting and my question is related to what sigurd happened to end the thread with, without an answer.

    I was wondering was whether an endplate could work for improving a leeboard's (or external daggerboard's) performance. Similar to the ventilation/cavitation plate that outboard engines have. It wouldn't change the behavior where it pierces the surface but it might improve the flow over the foil below the plate, by restricting the turbulence to only above the plate? This would in (my armchair) theory increase the effective area of the foil.

    Edit: These might also be attached to some kind of buoyant part so that they'd always be in a specific spot in relation to the waterline.
     
  8. upchurchmr
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    upchurchmr Senior Member

    It would only work if you put the leeboard "vertical" each time.
    Leeboards are often spoken of being raked differently for different points of sail.
    Any geometry other than vertical you would have a plate at an angle to the water flow, catching lots of drag. As you hauled them up to reduce drag on the windward side, you would get a temporary heavy drag as they approached the surface.

    If you want to do this you, you might as well just make daggerboard cases attached to the side of the boat

    I didn't get your edit, would that also address the issue?
     
  9. Avoid Rocks
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    Avoid Rocks Junior Member

    Good point. Daggerboards would make a lot more sense.

    What I meant was that you could fix the plate to the correct position in relation to the water surface by making it able to slide up and down the board using buoyancy, for when the hull is immersed due to heeling. But then again, heeling would also have the same effect of tilting the plate in relation to the water surface, so it would probably have to be more complicated than what we're looking for here. The same effect with pitching. Back to the drawing board. :)
     

  10. upchurchmr
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    upchurchmr Senior Member

    Daggerboards have always taken on the attitude of the boat hull, and work quite well.
    If you want perfect then go ahead and add side to side tilting to get the most force for the area. But most boats are not adjusted to reach that "perfect" state, except by accident.

    KISS (not meant to insult you, just a good concept).
     
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