Keel Section

Discussion in 'Hydrodynamics and Aerodynamics' started by motorbike, Feb 21, 2015.

  1. motorbike
    Joined: Mar 2011
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    motorbike Senior Member

    Well I finally managed to get the boat out of the water and had a look at the keel, not a pretty sight! It has been worked on at some stage, smooth but not fair. The foil at the bottom seems to be a 0012 section and the root seems to be closer to a 63-008 section. Should I continue down that path and make it properly fair or is it worth altering the root section to an 00xx section. I clean the keel before every race but it will still be antifouled.
     
  2. Jamie Kennedy
    Joined: Jun 2015
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    Jamie Kennedy Senior Member

    Most important is the leading edge. Also the bottom should be well squared or veed, and the bottom front corner should not be rounded in profile so as to compromise the leading edge. The rest of the keel, fair, with a flat squared off trailing edge, but I would not worry too much about matching a NACA perfectly.
     
  3. Joakim
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    Joakim Senior Member

    Trailing edge is also very important. For optimum performance it needs to be thin (a few mm or less). Quite often they are 10-20 mm or even 30 mm thick, which adds 20-100% to the drag of the keel.
     
  4. motorbike
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    motorbike Senior Member

    The trailing edge is about 5mm, but the last 100 mm of the 2000 mm section has on average a 10mm convex shape, some work to be done there.

    Having a long chord, I presume that the best course of action is to keep it as thin as possible, that said it does stall out of the tacks when it windy and lumpy so it would seem that some careful attention is needed to the nose shape.
     
  5. Joakim
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    Joakim Senior Member

    Convex is very bad for the last 1/3 of the section. All good sections have straight or concave last 1/3. Many boats with 6-series keel sections use modification with letter A, which means that the last 1/3 is straight instead of being concave, which is would be more difficult to manufacture and not as solid structurally.
     
  6. motorbike
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    motorbike Senior Member

    I managed to take templates and the lower 1/3 of the keel is an 0010 section while near the root is very close to 66-007 gradually tapering into the lower part of the keel and the chord is 1.9m at the root. I was reading in the principles of yacht design that a 63 section is preferred but overlaying that template at the recommended width of 12% means a lot of filler, and even going to a 0007 section is slathering it on! in order to keep any unnecessary work to a minimum and upwind performance, is there any advantage to be had from altering the root section?
     
    Last edited: Jun 21, 2015
  7. motorbike
    Joined: Mar 2011
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    motorbike Senior Member

    Whats the best way to practically detail the trailing edge, there seems to be a general consensus that square and sharp is the best but the problem is that antifoul does not stick to corners. Marchaj suggests cutting off 5% of chord length gives no noticeable increase in drag, but on a 2 metre chord that's 100 mm or a 20mm truncation.

    [​IMG]
     

  8. Jamie Kennedy
    Joined: Jun 2015
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    Jamie Kennedy Senior Member

    Number 1, as close as you can get to squared off.
    I would risk the fouling there until I experienced it first hand on each specific boat and conditions.

    I would try to truncate at less than 5% though. Over time as you barrier coat and fair and so forth the run and trailing edge can really suffer.
    It is easier to fair to and square off a trailing edge that trails further to its theoretical point and you can always square it off more if you have some extra to play with.
     
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