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Re: [Michalak] Leeboards

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  • Martyn Long
    Hi Anders,   It shows you how to do this in that link I posted. See page 4 for the diagram. http://www.gurit.com/files/documents/Foil%20Construction.pdf  
    Message 1 of 81 , Jun 21, 2013
      Hi Anders,
       
      It shows you how to do this in that link I posted. See page 4 for the diagram. http://www.gurit.com/files/documents/Foil%20Construction.pdf
       
      Divide the leading edge into 4, then take the middle 2 parts (i.e. the middle half).
      Then you cut back to one seventh of the front to back distance.
       
      If you then just smooth the edges you get an elipse on the leading edge, and you fair back into the rest of the blade.
       
      As I said, I have done this a few times and it does work with relatively little effort. Last week I made a foam core in this way for my friend's cruising yacht. We then draped it with fibreglass and he then went on a trip using it a few days later - very successful apparently.
       
      The leading elipse is probably important to prevent eddies and stalling, and the final long taper also prevents eddies at the trailing edge, whilst giving the necessary leverage. A squared off trailing edge stops vibration (I get this by slicing along the just-cured fibreglass using a ruler, then filling in any gaps between the two layers with thickened epoxy).
      Unless you go damn fast and have a very thin and deep blade, the actual curve along the sides is probably not too critical.
       
       

      ________________________________
      From: Anders Bjorklund <andersbjorklund5@...>
      To: Michalak@yahoogroups.com
      Sent: Friday, 21 June 2013, 17:20
      Subject: Re: [Michalak] Leeboards

       

      "Make the leading edge a nice 30 deg ellipse about 50% of the plate
      thickness, bring that to full thickness at about 20% chord"

      Trying to figure out how to go about that. I get the 20% chord, but not how
      to draw a "30 degree ellipse about 50% of plate thickness". Anybody know
      offhand? Otherwise, I'll keep Googling ...

      Anders

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    • Samantha Roberts
      Yes, you are using just the end of the ellipse to shape the leading edge. ________________________________ From: Anders Bjorklund
      Message 81 of 81 , Jun 21, 2013
        Yes, you are using just the end of the ellipse to shape the leading edge.



        ________________________________
        From: Anders Bjorklund <andersbjorklund5@...>
        To: Michalak@yahoogroups.com
        Sent: Friday, June 21, 2013 8:08 PM
        Subject: Re: [Michalak] Leeboards



         
        I think I got it. Thanks. It looks like the tangent line back to the 20%
        chord would meet the ellipse VERY near its forward tip then. Interesting.

        Anders

        On Fri, Jun 21, 2013 at 2:10 PM, Samantha Roberts <
        samanthaeroberts@...> wrote:

        > A 30 deg ellipse is a circle viewed at 30 deg from the plane in which it
        > lies (if viewed at 90 deg, it appears as a full circle). This makes a 30
        > deg ellipse twice as long as it is wide. So if your plate thickness is 1",
        > the ellipse you need to draw is 1" long and 1/2" wide (50% of plate
        > thickness).
        >
        > Now draw a smooth curve that lies tangent to the ellipse and reaches full
        > thickness at 20% chord. Since John does not specify either this curve or
        > the one from 60% chord to the T.E., I assume he means that those shapes are
        > not critical. On the general principle of not trying to make water flow
        > around sharp corners, I would make those two curves come smoothly into the
        > flat section from 20% to 60% chord, but I am not sure how important that
        > might be.
        >

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