Recommendations for new dinghy daggerboard

Discussion in 'Boat Design' started by RCC3, Jul 16, 2018.

  1. RCC3
    Joined: Jul 2018
    Posts: 5
    Likes: 0, Points: 1
    Location: Kirkland,WA

    RCC3 Junior Member

    Hi,
    Recently suggested a new daggerboard might improve my dinghy’s performance by extending the length. Current daggerboard—chord =12.25”, thickness =1.32”,10 degree sweep aft, lenght=37.25 along trailing edge, 8.5” in the daggerboard trunk. Will make the bottom elliptical for reduced drag. Dinghy(14ft) was designed for a single main. Considering adding 16” to the lenght.
    -is 16” too much lenght to add
    -is there any other NACA cross sections I should consider
    -will this length addition change the center of effort too much
    Thanks in advance.
     
  2. Heinrich Poigner
    Joined: Jul 2018
    Posts: 18
    Likes: 1, Points: 3
    Location: Austria

    Heinrich Poigner (He likes boats(and maths))

    Question: is the trunk designd in a way that one would be able to change the length of the daggerboard under way?
    would make for some fun time comparing it :D (please post results if you where to do so) (?my aswer to the first one?)
    As for the crossection ... I've read (in a rather clever book) that a daggerboard or kell profile should have a rather small radius up front in comparrison to a rudder because it is only expekted to have an angle of attack of 3-5° (other than that comparing crossections for the designed speed at Airfoil database search http://airfoiltools.com/search/index Maby?)

    uhh ... hard one ... need more nummbers I guess ??

    sounds like a fun projekt! Wana see numbers I you make the test!!! (maby even compare it for different windspeeds, waves, cours, .... ANOVA? :D ... probbably way to much to ask)
     
  3. tom28571
    Joined: Dec 2001
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    Location: Oriental, NC

    tom28571 Senior Member

    If a CB or DB foil is too small it should create a stall condition in beating to windward in moderate to higher wind when sails are trimmed in tightly and you are hiking. Indication of that is the boat not going where it is pointed but more to leeward than is acceptable. If you are unable to detect that condition, its not likely that you will be able to determine whether an improvement has been made. The details of shape, etc., are less important than the area of the DB.

    That said, a DB a bit too large or long is better than one too small.
     
  4. Richard Woods
    Joined: Jun 2006
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    Location: UK, USA and Canada

    Richard Woods Woods Designs

    It would be helpful to know what boat you have. Photo/dimensions?

    The foils have to balance the rig, so increasing the foil size could mean you can increase rig size. If you add length the loads will increase but you cannot make it thicker so will have to make it stronger

    If you have too high an AR it will stall out easily. You will find that happens coming out of a tack or sailing in a chop

    In general area first, then AR, then section shape. Also be aware that many sections have to be made very exactly and literally have a mirror finish to work properly, if not they won't have any benefit

    Richard Woods of Woods Designs
    www.sailingcatamarans.com
     
  5. RCC3
    Joined: Jul 2018
    Posts: 5
    Likes: 0, Points: 1
    Location: Kirkland,WA

    RCC3 Junior Member

    Thanks for the response and the link for the airfoil database. Yes, I can adjust the DB in the trunk. Hoping to have the foil in CAD later this week.
    Cheers!
     
  6. RCC3
    Joined: Jul 2018
    Posts: 5
    Likes: 0, Points: 1
    Location: Kirkland,WA

    RCC3 Junior Member

    Thank you for the response. Next time I have the boat out, I will pay special attention as to how well it maintains its course.
    Thanks again!
     
  7. RCC3
    Joined: Jul 2018
    Posts: 5
    Likes: 0, Points: 1
    Location: Kirkland,WA

    RCC3 Junior Member

    Thank you for the response. In the evaluation process now. Have noticed it doesn’t seem to come about in clean manner. Thought it might just be me due to learning curve with the boat. If I do make another DB, would consider having it machined instead doing the construction and shaping myself. Thanks!
     

  8. RCC3
    Joined: Jul 2018
    Posts: 5
    Likes: 0, Points: 1
    Location: Kirkland,WA

    RCC3 Junior Member

    Hi,
    Included some pictures. Starting to lean towards option 2 since it will be easier to make. Like option 1, but realized I will need make adjustments to the foil and blend at the bottom for damage tolerance/robustness. Concerned the trailing edge will be to thin.
     

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