The stern post acts as a support for a beam. The beam is not structural, and merely supports the main sheet traveller, boarding ramp and the like. It would have been easy to have cut the notch before fitting it into the boat, but at the time I was minded to make an “I” shaped beam resting flat on top, and not the circular aluminium tube as shown on the plans. I’ve changed my mind and will now have a hollow wooden beam and so I needed to cut a 140mm diameter semicircle through 60mm of solid oak and ply. Not an easy thing to do neatly. After much head scratching I finally devised a jig cobbled from a router side fence and a steel rod. I cut it with several passes of a palm router. It went without a hitch and very nice it looks too!