daggerboard shape: suggestions?

Discussion in 'Boat Design' started by peterchech, Jun 10, 2011.

  1. peterchech
    Joined: Aug 2010
    Posts: 241
    Likes: 9, Points: 0, Legacy Rep: 127
    Location: new jersey

    peterchech Senior Member

    I posted this in multihulls too but may get a wider audience here:

    I have just finished the foil shape on a daggerboard, naca 0012 I believe, 4 feet long by about 14 inches chord and 1 3/4 inch thickness. It is for an outrigger canoe, and I expect to sustain speeds around 10 knots.

    I have read that the bottom of the board (the end) should not be left flat. Right now it would be a 1 3/4 inch wide and 14 inch long vacuum in the water. But how should I shape it? The thinner it is at the end the better I believe. This gives less area to create a vacuum. I have also read that a curved profile, basically a circular tip (think a spitfire wing but backwards), will bring the point of vacuum even further back, to the tip, limiting further the area available for vacuum. Is this right?

    Any suggestions for the outline of my relatively low aspect ratio daggerboard?
     
  2. daiquiri
    Joined: May 2004
    Posts: 5,372
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    If you want to round the shape of the tips, do it on the leading-edge side and just as much as you think is necessary to let seaweed, plastic bags and ropes slip below it smoothly, and to minimize damage if you hit a ground below. From the hydrodynamic point of view, sharp square tips are better than rounded ones because they tend to increase the distance between the wing-tip vortices. Or, since we talk about a daggerboard, a sharp square tip will push the keel-tip vortex further down than a rounded-tip could do - and hence tend to increase the apparent aspect ratio of the daggerboard.
    Cheers
     
    1 person likes this.
  3. peterchech
    Joined: Aug 2010
    Posts: 241
    Likes: 9, Points: 0, Legacy Rep: 127
    Location: new jersey

    peterchech Senior Member

    Grazie daiquiri!

    I will keep it square then. Should i flatten width at the very bottom then? Or round it, or just leave it square?
     
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