This is the mast beam on the port hull. It was purposely built over sized so that it can be trimmed to fit into the side wall of the hull. The angle of the upper section of the bulkheads were taken from the plans and tranfered to the mast beam.
Precise measurements were needed otherwise there may be a huge gap between the beam and the side wall.
The excess length was then sawn off.
Finally the side panel of the upper section of the port hull is glassed onto the hull panel.