The forebeam is hollow and made up of 2 strip planked halves glued together. To provide extra support in the middle there is a plywood web. The web is cut from a sheet of 12mm ply and has to have the curve of the beam cut into it so that it can be glued on its edge, it cant be bent to fit on its edge. I am cutting it out of the edge of a 1200mm x 2400mm sheet. In order to cut the correct shape I decided to make stencils out of 3 sheets of 3mm mdf that came as packaging material with the kit.
You might think that cutting the mdf stencils is an extra set of cutting, why not just mark the ply and cut that. I decided that the mdf was free but the ply is expensive so it was worth the extra cutting to get it right. Also because I want to cut the 3 pieces of ply from the long edge of 1 1200mm x 2400mm sheet but that I have 3 sheets of mdf it was far easier to join the 3 sheets of mdf and mark the full length of the beam. Then I could use the 3 stencil parts to mark just 1 edge of the ply sheet and cut all 3 pieces to the correct curve without having to mess around with marks each time I have cut a piece off. In fact I think doing it this way was actually faster, the mdf took about 10 minutes to cut and then it took about 2 minutes to use the stencils to mark the ply. To mark the stencil, I transposed the measurements of the beam onto the sheets and using a piece of cedar strip I marked the curved shape. I then used a jigsaw to cut the stencil. Once I had the stencils and made sure the curve was correct by dry fitting it to the beam, I transferred the shape to the ply and cut them out with a jigsaw. I then dry fit them also and trimmed here and there where my jigsaw cutting drifted a little from the line.
With the ply web dry fitted to the beam, I am ready to glue it in. I will do that on the weekend. I have decided that instead of a scarf join to make the 3 pieces into 1 long web I will butt join them with ply plates overlapping the join on either side. This should be stronger than a scarfes and also provide extra surface to glue the top of the beam to. I also need to run a stringer for added gluing area along the length of the web and for this I will use either more ply cut to the same curved shape or cedar strips bent to shape. To glue the web in I will glue the 3 pieces together first (probably tomorrow so they are set for the weekend) then glue the whole web in. Once it is set I will run a cove along both sides and glass it in. Enough glue and I should be able to fill all of the voids made by my crap jig-sawing!
Then I start on the top and cover up the evidence!