STIRLING MARBLEHEAD YACHT RACING CLUB Inc.

Woorabinda Lake - Stirling South Australia

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SAIL MAKING - Building Board Construction  
by Ben Morris (last edited 21/07/2013)

Shape in Sails Building Board Making Seams Set the Seam Curvature Making a Sail Sail Material Diagonal Seams etc Back to Intro page Setting the Sails The Claudio Tool Measuring Procedures
  1. The sail building board is constructed on a 500mm X 500mm piece of stable high density compressed board that is solid and stable. 

  2. Four identical ribs corresponding to the airfoil shape chosen (I chose the 43% parabolic section outlined in the spreadsheet) are cut out, smoothed and screwed/glued to the board leaving a 60mm or so gap between the centre two.  The outside two are on the edge of the board.  I used 16mm board for this

  3. The inner two ribs are pre-drilled for the hinges at a position about half-way down the parabolic section (see end elevation diagram).

  4. Construct two hinges made of 8-10mm threaded rod by making a 12mm cut out in the ends of 100mm sections and drilling and pinning the two interlocking pieces with a 2mm steel pin. See end of page for alternative hinge

  5. Fit the two hinges and secure using four nuts for each hinge.  Ensure that the pin is precisely in the centre of the space between the two  inner ribs and that the pivot pin is horizontal.  Make sure there is no force on the hinge pulling or pushing the ribs.

  6. Cut out the cross supports carefully and glue them in place with an epoxy glue ensuring their upper surface is parallel to the rib surface at their intersection.

  7. When you are satisfied with all the previous steps mark the centre of the board on the bottom surface and while inverted cut the base board into two by making two cuts 5mm or so either side of the centre line.

  8. Work from now on must be carried out on a flat firm table or bench.  The board should now pivot up in the middle in a firm but controlled way

  9. Cut two thin 3-5mm thick ply or similar sheets to cover the two sides of the board.  Each will be just larger than 250 mm X 500mm.  To reduce the tension on the board and to make the ply sit easily over the parabolic curve, I pre-bent the ply by gently spraying its surface with water and adding weights to the centre whilst supporting the ends with blocks of timber.  I repeated the moisture a couple of times and left the ply in the bent position for a week or so to make sure it was completely dry.

  10. Before securing the ply to the ribs/cross pieces the ends of the ply where they meet in the middle will need to be shaped slightly as the pivot position is higher than the ply at the front and rear ends of the parabola.  You can gauge how much to remove by test fitting the pieces on the board and raising the middle of the base board by 20mm or so. (There should be no need to ever raise it above that amount - probably 15mm would be OK)

  11. Secure the ply to the ribs and cross pieces using screws starting in the middle and working out.  The pre-bending will make this much easier.  Make absolutely sure that the two ply covers are touching in the middle and that they are in line with the hinge points.  I chose not to glue the ply down in case I had to later make alterations to the hinges.  Inspection holes could be made through the bottom base as an alternative and the ply glued in place.

  12. I draw guide lines across the surface with a thin permanent marker or biro at the 43% maximum curvature point and in pairs either side of this at equivalent distances so the lines represent where a seam would reach.  I number these pairs and have about 6 of them.  This makes it easy to lay a seam between the lines of the same number and ensures the maximum camber is at 43%.  Distances for these lines can be deduced from the Sail Curve Analysis spreadsheet

  13. The surface needs to be sealed and smoothed.  I used two coats of epoxy resin with a small amount of filler added and spread the thin mixture with a plastic straight edge.  Sand the surface to a clear smooth finish.  The guide lines show quite well through this finish.

  14. I made small blocks to go at the four corners of each base piece to act as pivot points for the lifting action required at the centre.

I  took a great deal of care with this construction and have been rewarded with a very firm board and one which produces repeatable results.  All of the seams produced have been very even and regular in the curve with no tendency to pucker.  The positioning of the hinges so high up the board means a minimum of gap is produced when the board is lifted.

Plan view of board before top ply surface added

 

End view of board before top surface added

 

Side view of board showing top ply surface and general parabolic shape and hinge location

Alternative hinge method

Rather than using the hinge described above at least two potential sail-makers have suggested using a piano hinge or simply a pair of ordinary hinges to allow the movement.  This involves adding a wooden batten on each end of the centre ribs.  The height of this should match the position of the original pivot line.  The hinge is then screwed on the top of these two battens.  Clearly the top surface must be added after these hinges are secured.  In order to allow adjustment in future the battens could be secured in position with screws or metal threads to the end rib.  The two faces of the battens should be separated by sufficient distance or planed back at an angle to allow the two sides to lift (about 15 mm is sufficient) as the bottom of the two battens will tend to move in towards each other.  Make sure the hinge is secured in a way as to allow this movement as well.  Examine the photo below which shows the board made by Michael Harris from SMMC.  Thanks to Michael and Alan Gold from our club SMYRC for these suggestions.