F28R Daggerboard specs

Discussion in 'Multihulls' started by Brainzguy, Jul 29, 2019.

  1. Brainzguy
    Joined: Jul 2019
    Posts: 3
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    Location: Houston, TX

    Brainzguy New Member

    I damaged my Corsair F28R daggerboard sailing in Mexico in the Spring. Because replacements are pricey, am planning to repair it. Had a neat plan and was about to put it into action when I determined that I wasn't sure what the original foil shape was. The foil is similar to a NACA 10 foil but since it is swept back, and I presume the "foil" needed to be perpendicular to the water surface. I was able to simulate both sections in my Cad-Cam program.
    Anyway I took a number of measures and found that it wasn't a NACA 10 as the leading part of the foil was narrower than expected and this wasn't corrected when I rotated back 10 degrees.
    I know this foil provides some lift because I had to keep it retracted during my trip home and found I had a lee helm. This boat sails in the 0-25 kt range.
    What I don't know is: Do foil lift shapes change when placed in water (that is, would a slimmer entry be desirable in water compared to in air?) I wouldn't think so but ...
    Does anyone know what the design specs for the foil shape were for this daggerboard? I have twice written to Corsair Vietnam and have been unable to get this information.
     
  2. Doug Halsey
    Joined: Feb 2007
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    Location: California, USA

    Doug Halsey Senior Member

    Generally speaking, foils perform the same in air, water, or any other fluid, as long as comparisons are made at equal Reynolds numbers.

    The NACA 4-digit shapes are decent all-around performers, so you wouldn't go too-far wrong using the NACA 0010.
     
  3. Brainzguy
    Joined: Jul 2019
    Posts: 3
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    Location: Houston, TX

    Brainzguy New Member

    Thanks for your response. Helpful. I found Paul Zander had posted an article on Centerboards and rudders in 2010. Apparently, the correct NACA0010 shape would improve performance at wider angles of attack. I also learned he was experimenting with an parallel sided foil which moved the AOA back farther. But found n0thing on foils with a narrower than normal front of the wing config.
    Re Air vs Water there are two differences. 1. cavitation, which as near as I could tell is not important unless you achieve 50 kt, and 2. separation which may primarily be a very slow movement hazard. I will say that at speed (15-20Kt), I sometimes loose rudder attachment.
    The F28R is a rather unusual boat and I have learned to be very cautious about modifications.
     
  4. peterbike
    Joined: Dec 2017
    Posts: 32
    Likes: 5, Points: 8
    Location: melbourne

    peterbike Junior Member

    Seeing as you are already looking at your foil's ; re the rudder problem.
    is it the foil shape, wrong position on the boat, not enough leading edge, etc, etc, ?
    curious ....
     

  5. Brainzguy
    Joined: Jul 2019
    Posts: 3
    Likes: 0, Points: 1
    Location: Houston, TX

    Brainzguy New Member

    My concern is the shape of the foil. When comparing to standard NACA foils, it measures as a NACA 0010 foil, but the leading edge is narrower than the true foil shape by about 1/4 inch. The leading edge of a foil can be modeled by a series of overlapping circles, and the radius of these is quite a bit smaller than expected. Looking at pressure diagrams this suggest that the foil is underperforming at a number of angles. Not sure if it creates or reduces friction in a marine foil.
     
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