angle of attack on daggerboards

Discussion in 'Multihulls' started by rapscallion, Oct 25, 2011.

  1. rapscallion
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    rapscallion Senior Member

    Does anyone have any idea how someone could measure the angle of attack on a daggerboard?
     
  2. DGreenwood
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    DGreenwood Senior Member

    rap
    Getting the angles on rudders and boards can be done on a smooth shop floor with the boat leveled up. I use lasers or strings or both depending on the job. An easy way is to transfer the leading edge and trailing edge points, of the board and the hull, to the floor and extend those angles out with a snap line. Measure the angle.
     
  3. Doug Lord
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    Doug Lord Flight Ready

    D Greenwoods system seems excellent to measure the angle of incidence relative to the boat-not the actual angle of attack of the foils-if thats what you meant.
    I read recently about a guy that used something like a weather vane in the water viewed thru a clear panel to measure the angle of the water flow relative to the direction of the hull(leeway). I'll try to find it. You need to measure the actual leeway the boat is making and apply that to the results you have for incidence to come up with angle of attack of the foils.
    Seems like it would be fairly simple to make up a device that you could lower between the hulls calibrated exactly to the centerline of the boat that could read out the exact angle of leeway.
    Angle of attack is the same as leeway if the boards are exactly parallel to the centerline of the boat. If the board was toed in 3 degrees(on the daggerboard and/or lee foil on a trimaran) and leeway was three degrees the angle of attack is 6 degrees. On a cat you need to do like Mr. Greenwood said
    and determine if the hulls and/or boards are toed in and what that angle is and then measure leeway to establish angle of attack.

    Illustrations-
    Top- Angle of Attack is relative to the flow,
    Bottom-Angle of Incidence is relative to the boat.
     

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  4. rapscallion
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    rapscallion Senior Member

    That's an interesting idea Doug, and you are right, I was talking about the angle of incidence.. the G32 has NACA 10 foil sections, and Jan mention that 3degrees is the magic number, and it occurred to me I have no idea how to measure that...
     
  5. cavalier mk2
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    cavalier mk2 Senior Member

    Or you could get a decent idea of the leeway using your compass and gps presuming they are calibrated. See where the nose is pointed and look at the actual heading on the gps. Measure for different speeds etc... If you use a lake you can skip allowing for current. Your rudder angle can be checked by noting helm position to degrees when out of the water so you can measure when sailing. It will probably be a little different than the daggers do to weather helm.
     
  6. rapscallion
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    rapscallion Senior Member

    That isn't a bad way to go! I could snap a line and draw

    A reference line in the cockpit...
     
  7. rapscallion
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    rapscallion Senior Member

    wow! To make things a wee bit more interesting.. the daggerboards actually pivot a bit to windward while underway! This G-32 is just full of tricks!
     
  8. bruceb
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    bruceb Senior Member

    2+ degrees?

    I set my float trunks at about 2 degrees with boards that have about a degree of attack built in. They seem about correct and very effective:cool: Having a nice flat floor (with a roof?) would be very nice, but my "shop" is my 12' wide driveway so my boat is never assembled until it is on the launch ramp. I have used a combination of sticks, rulers and construction lasers, with reasonable results. On my next set of trunks/boards, I intend to leave some room for adjustments. B
     
  9. DGreenwood
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    DGreenwood Senior Member

    Sorry rap I presumed that when you said A of A that your were talking about angle of incidence as that is what most people mean when they are trying to adjust rudders and boards. Rarely does anyone ask about Angle of Attack in the literal sense unless they have a rotating dagger board. Good luck.
     

  10. rapscallion
    Joined: Oct 2006
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    rapscallion Senior Member

    You were right.... I was incorrectly calling angle of incidence the angle of attack.... I discovered the boards "jibe" after I started the thread :) Thank you for the suggestions!
     
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