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This arrangement has one big advantages with few disadvantages over a solid board. The most obvious is that the angle between the two halves of the board can be varied by simply placing thinner or thicker blocks under the two centre ribs. This allows for a single board to replace the multiple balsa boards suggested in other sites. 1,Create a Building Board: Make a new board which incorporates a few improvements. Instead of using the NACA airfoil,we need to use a section of a parabola with a maximum depth at 43% of the chord. See the spreadsheet for details. Make the chord 500 mm to enable most type of sails to be produced. In addition you also need a pair of hinges made from 8 mm threaded rod which are placed about half way down the curve area. And build it on a substantial flat base which is subsequently cut in two. The biggest advantage of this arrangement is it reduces the actual opening distance and allows the seam to sit more smoothly when joining it.The 43% position of maximum draft produces a sail with slightly less acceleration but higher pointing ability due to a shallower entrance angle. Another option suggested which might be easier to construct is to a use graduated, tapered wedge on either side. Sliding the wedge in to the required lift height would certainly save on having all those pieces of ply of different thickness that seem to get lost. 2,Original board: Use the modified NACA curve to produce a curved sail building board. Unlike many of the suggestions for making such a board from solid timber,you can cut and shaped four ribs from 19 mm thick timber to match the NACA curve. You can make these about 470 mm long ie the modified NACA section is 470 mm chord. A piece of 4 mm ply approx 470 mm X 500 mm was then attached to the 4 ribs using small screws - one at each end and the other two in the middle separated by about 20 mm. The main grain of the ply was place across the ribs to allow it to bend more readily and conform to the ribs,then added strengthening timber battens across the ends of the it at the leading and trailing edge of the airfoil to keep then straight. This process produces a flat building board with no V-join in the middle. This is achieved by carefully marking the centre of the board midway between the two middle ribs and cutting the board in two. The two exposed ends are sanded smooth and rounded. The two sections are then taped together at the outer ends of the join on the top surface. This allows the two halves of the board to be tilted by placing temporary blocks under the two centre ribs to angle the two halves of the board to each other.
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