In between each of those goes two evenly spaced smaller stingers, two 20mm x 25mm so only 40mm wide. I scarf cut and joined wood to make the first four pieces that are full length. Then I had to mark out and cut the notches in each frame for them.
To do this I measured the distance between each of the main stringers and marked the frames. Then I got the straightest of the four and lay it along the frames so that it sat on each of the marks. A few minor adjustments and I could mark out the bottom of the notches. I used a metal rule to take the angle of the stringer looking from the side and marked a line on each frame, and carried it around onto the the fore and aft face of each frame. Then using the stringer as a guide measure the distance you need to let the stringer into the frame to maintain the curve and mark it. One side will be 25mm where the edge of the frame touches the stringer, the other will be more to allow the stringer to curve in.
Remove the stringer and use two pieces of the wood to mark the top lines, carry the lines around the side and mark the 'let in' measurement up and you have your notch marked. I used a hand saw cutting from the outside towards the centreline and followed the lines. It seemed to work okay, make sure you are checking each side of the frame as you cut as they are not the same depth front and back.
Now do this so you end up with 78 notches marked (don't count them until you have finished) all hand sawn to the correct depth and angle. I decided to mark and cut them all first before fitting any stringers, it just felt more efficient as I was into the swing of marking out.
Feel free to curse, particularly when you lean in to check a measurement and bang your head on the middle main stringer or crack your shins on the bottom one.
Using a chisel carefully take out the waste material working from each side. The cuts will prevent the frame wood splitting, and some will almost come out with a few hits from each side. You will be surprised how angled rear to front the final result is towards frames 1, 2 and 3.
Below is part way done, note the extra depth to be taken out on the forward side of the frames (up to 30-35mm in on the very front ones to allow the curve. If you look closely on frame two, three and four right hand side you will see that the notches are waiting for cutting, chisel work and finishing.
The below shot is a close up of the side to show hopefully the different depths:
And a final shot from the front:
Next update should show the inbetween stringers in place, things should move along a bit quicker but after the Christmas break.
