Windsurfing is where it all began! When I took up windsurfing in the early 1980s, the sails were triangles of Dacron, the masts were floppy, and the booms very long. Great fun in a light wind, but increasingly difficult to control if the wind freshened. This prompted me to take up sail design and making as a hobby, and before long the Transition Rig idea emerged. I took inspiration from the wings of birds and bats which could be adjusted according to need, and began the long process of trying to understand how to make variable geometry sailing rigs. Now, 30 years on, I have made progress, but there is still more to learn.

Here is a video of one of the windsurfing rigs filmed in 2001:

Shape-changing

In light winds, the board sailor stands more upright and exerts little downforce on the boom. The mast is tensioned in such a way that it and the attached sail take the fully extended form as their default configuration. In stronger winds, the sailor uses their bodyweight to counteract the forces being generated by the sail, and by hanging from the boom they exert more downforce on it. This downforce opposes the tensioning of the mast and causes it to begin to flex into a more zig-zag shape. The sail, which is made of a resilient material, adjusts its shape to become more streamlined and its centre of effort (the place on the sail from which all the sailing forces seem to arise) moves lower, making it easier to control.

Here is a short video explaining the shape-changing that is possible with the Transition Rig: