Daggerboard aerodynamic advice

Discussion in 'Sunfish Talk' started by mixmkr, Jul 4, 2017.

  1. mixmkr

    mixmkr Active Member

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    Old 69 original board....chatters when all the way down. Pull it up about 5" and it stops. Any suggesstions where to fair? Would it be the area just showing under the hull when all the way down? Its basically the same shape from top to bottom ..or just get the new one from intensity?...and save my time and effort
     
  2. beldar boathead

    beldar boathead Well-Known Member

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    Get one from Intensity - boat will perform much better. BTW, you need hydrodynamic advice, not aerodynamic.
     
  3. mixmkr

    mixmkr Active Member

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    oh...I've got a foiling fish!! Daggerboard is out of the water!! ha!
    Good point.
    I've had 10 other threads about the Intensity board. Might as well spend more $$ on the free fish. Parts add up. I've probably got $400 in this old boat by now. But...I'm off the streets and half way behaving.
     
  4. JohnCT

    JohnCT Active Member

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    A boat, is a hole in the water, you throw money into.

    All of them
     
  5. Webfoot1

    Webfoot1 Active Member

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    The factory made it good enough for recreational use and that was all. They beveled
    the edges but had no intention of using a airfoil shape. Intensity board is about as
    close as you'll ever get to the correct airfoil. I'm told the daggerboard slot in not
    wide enough to get the correct NACA profile. Once you get a Intensity board and
    racing sail the original rudder will go BRATTTT..... if it has not already. I guess
    you could think of it as just putting a playing card on you bicycle spokes, the motor
    sound makes you go faster!
     
  6. mixmkr

    mixmkr Active Member

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    So....save shipping and get a new rudder blade too? Not sure what "BRATTTT" means!

    I'm not looking for super hydrodynamic...just stop the chattering which is pretty annoying actually. I thought it odd that it stops by just pulling up a couple inches. That's a lot cheaper than a new board. (Maybe just cut the top 5 inches off!!)

    I have scads of fiberglass, filler, resin and plenty of things to sand with, so reshaping to STOP the chatter could be done...IF I knew which direction to go.

    What makes the new Intensity boards not chatter? The fact that it's not mahogany or is it the shape?....or both?

    My current wood board as you can see IS smaller than what is in the slot and wondering if it had the "slot" shape ALL the way down would make the difference. But when the board is up, the smaller section is partially in the daggerboard trunk...
    who knows....????
     

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  7. Webfoot1

    Webfoot1 Active Member

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    If you want to spend a little time, you can sand the wooden board into more of
    a airfoil shape. Pick a NACA profile that looks like it will work with the width
    of the board. If you want to try something a little quicker, square off the back
    of board. Doesn't have to be much, just a small flat area. Do both and you'll probably
    fix the problem. When you pull up the board you are changing the hydrodynamic resonance
    of the board and it stops fluttering.
     
  8. mixmkr

    mixmkr Active Member

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    I understand I'm changing something by pulling up the board...but am I decreasing resonance because of shorter length, less distance front to back, actual shape...or all??
    That's the question. What's so different.

    Squaring off the back would be a piece of cake. I guess I could try it and nothing lost if it doesn't work. Just not into testing and testing....so to speak.
    Do you think just a quick sanding to square it off...or add material and keep the width all the way to the trailing edge? True airfoil shape isn't a goal. Stopping chatter is.

    Like I said... Intensity is $170 plus shipping...so I'm guessing around $200. I can fair it for free.
     
  9. mixmkr

    mixmkr Active Member

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    Ok.... found this and wonder about what it's saying..............
    Of course my daggerboard is like #2. Making it into #3 would take a couple minutes of sanding..
    thoughts?



    [​IMG]



    Shaping your board
    If your foils (fins, keel, daggerboard etc) is not shaped, it will produce drag, which slows your boat down.
    Shape 1 is just a plain board. This will produce a lot of drag in the water, and you can feel it.
    Shape 2 is a plain board with the edges rounded off. This won't be as bad as #1, but it is still pretty bad. Once you start moving at speed, the board will start to vibrate, causing even more drag.
    Shape 3 is a board that has a rounded leading edge, and the aft edge is sharpened like a knife blade. This shape is far superior to shapes 1 and 2, and is easy to make.
    Shape 4 is a full foil shape (sorry for the poor drawing) and does perform better than shape 3. If you google NACA foils, you will find various formulas and dimensions of what various people think are optimum.
     
  10. Kevin Mc

    Kevin Mc Member

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    I always assumed the chatter is due to a non-symmetrical shape (or asymmetrical drag, or too much play between the board and the slot). Pulling the board up shortens the amount in the water so there's less of the board exposed to whichever's causing it , plus the shorter length is less likely to vibrate (but might start up again if speed were increased). Shaping the board to be symmetrical and sanding/finishing/waxing it evenly certainly won't hurt.
     
  11. Webfoot1

    Webfoot1 Active Member

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    Your board is acting like a flag in the wind. After the water hits the leading edge
    and travels 2/3's of the way down each side the water is separating from the surface of the
    board and creating turbulence. It look's like the water on the lee side of a rock in a stream.
    The turbulence is driving the rudder vibration. If you pull the board up the turbulence is
    reduced to the point were it can't drive the board into vibration. Sanding a flat on the
    trailing edge creates a vortex that reduces flow separation. At least it might with a proper
    foil shape. I don't think there is a easy fix without some effort. If you go the wood shaving
    route you'll probably learn much more but have at least $200 worth of labor into the board.
    The only down side to the Intensity board is the pain you'll feel if you hit a rock or something.
    "Six of one - A half dozen of the other." as Patrick McGoohan used to say.
     
  12. Sailflow

    Sailflow Member

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    Chatter is usually the board is warped. Buy a new racing board and you will glad you did. Longer, foiled shaped, when you roll tack it does not slide slip.
     
  13. Light and Variable Winds

    Light and Variable Winds Well-Known Member

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    I have the same board, was out today, and experienced the same vibes. :oops:

    I'm thinking that the board is vibrating because it is 'insufficiently loaded".

    Raising the board removes the excess board length that isn't "at work", which stops the fluttering.

    Thinning a wooden board that must take your full weight probably isn't a good idea. :confused:
     
  14. mixmkr

    mixmkr Active Member

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    Here's what I'll think I will do first. The original on the left...I'll add surface area and the extra 4 inches in length (to match the new composite boards or slightly longer), thinking that might load the board up some more, and maybe enough force to stop the chatter. I'll flatten the trailing edge as well. But you can see the lower end of the board isn't as "wide...front to back" as it is at the top. The new glass boards seem to be that way as well.
    Will see how that works first. I'll also try to mate the board up with the slot in the boat for a nice tight fit as well or add some Velcro strips or whatever.
    Should be able to knock it out in a 1/2 day and will take pics as I progress.
    Cross yer fingers for me!!

    If a couple quick attempts along these lines don't cut it, then I'll feel better about spending another $200 on this old fish and order the Intensity blade. ....Maybe more $$ with a new rudder blade too, eh? It's only $$$...right?
     

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  15. Alan S. Glos

    Alan S. Glos Active Member

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    Glue some thin indoor/outdoor carpet strips in the top of the daggerboard opening about an inch below the deck. This will eliminate the chatter or hum that you report. Use contact cement as directed to glue the strips in. One side is usually enough. If they are too thick and you can't get the board in, use sandpaper to thin out the carpet strips.

    Alan Glos
    Cazenovia, NY
     
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