Attaching the stringers to the frames. Note the temporary support for frame 2 .

Having set all the frames to line and vertical (except frame 4 of course) I next added the gunwales on both sides of the boat.
Next the 20mm by 20mm stringers were added to both the outside edge and the botttom of each hull. Gluing these into position sets the spacing finally and checks and cross checks were made to ensure both the correct spacing , squareness of the frames and their verticality.
 
To assist in obtaining the correct curvature for the bottom stringers I cut frame 2 from 18mm plywood and assembled temporarily onto a piece of pine at the correct height. The stringers were not glued to this frame at this time.
  
   
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Having frame 2 on the temporary support proved helpful when assembling the keel onto the frames and stem. The stringers provided enough downward resistance that no further weights were necessary to maintain the correct height.
There certainly is some pressure required to bend the keel across frame 1 and along the stem. I added extra weights ( concrete blocks) to the front of the strongback to ensure nothing 'moved'.

 

Starting to look like a boat