[quote=ACE11I disagree on the amount of energy lost though when the platform twists and returns. Unlike the watch spring that return energy isn't applied to the original job - that is forward motion. I think a substantial amount of it is lost by the platform "wriggling" about in the water rather than applying it to smooth forward motion.
I'm over this now - off to sail on my glued carbon beamed rocketship. Mmmmm - wonder how I can make it stiffer!
[/quote]

A flexing body in a fluid can create propulsion, think of the wings of a bird or a fish swimming in water. I don't say you will recover all the energy. But if your foils are moving around, this is much more efficient as you might think.

The more I think about the stiffness subject, the more I see it is a not explored and understood subject.
Image I am sailing on one hull and a puff hits the boat. What happens. More lift from the sail, which will heel up the boat and create more leeway. The increased leeway increases the forces on the boards, which will heel up more the boat. As a result I open the sheet/change the course to reduce the heeling.
Now, I have a properly designed flexible board:
The puff hits the boat, more forces in the sail, more leeway, but now the board flexes in a c-shape. It produces less sideforce, hence lessheeling moment, but some lift. The lift pushs the hull a bit out of the water, which generates less drag.
Thus with a flexible board I could have less additional heeling moment and less drag, which might be faster...

You can only improve a design if you open your mindset and if you don't get blinded by simplistic and too early conclusions like "stiffer is faster"

Anyway enjoy your sailing on whatever stiff boat.

Cheers

Klaus