This video is nearly real-time. I cut a few bits where I walked off camera and when I was setting up the camera. It shows how quickly and accurately the internal strongback can be assembled.
Due to the rocker of the Petrel Play, I decided the strongback would fit best if I laid it flat. Normally, the 4 inch dimension is vertical. By laying it horizontal I was able to have a longer strongback. It is still only 10.5 feet (320 cm).
The strongback holes in the forms are cut oversize with tabs sticking in to match the aluminum tube dimensions. The goal was to reduce the friction while putting together the forms, while still having solid register points centering the forms.
I cut the end forms to slide into the center of the aluminum tube that I use for a the strongback. Because of the horizontal orientation of the strongback tube I had to cut thicker spacers to hold the forms centered. Once the end forms were screwed in, I slipped the spacers in between the forms. The spacing on these forms is 10 inches (25.4) because that is what I had pre-made spacers for. The numbers on the forms indicates the number of inches from the bow of the finished boat.
The wedged spacer in the middle clamps all the forms in place. After the forms were secured, I double checked the alignment. There is some room for the forms to rotate around the axis of the strongback. A couple taps of a hammer twists them to the correct orientation.