A short video from a recent testing session.

Our understanding of the settings necessary for sustained stable foilborne sailing is steadily improving.

It is worth re-iterating the definition of stable flight with reference to the feedback loop that arises when external forces upset pitch angle and ride height.
A stable setup settles on an attitude and altitude without input from the crew.

We announced that Paradox could foil in a stable mode only when we were sure we had proven that it could.

Stability comes at some cost and we are open about the uncertainty regarding whether the benefits outweigh the costs. I believe we are close to finding an answer and I will describe our findings in detail in later posts.

Recent experiments by other manufacturers have shown that an unstable setup can be ‘tamed’: a well practiced skipper on a small boat can anticipate departures from the desired attitude and altitude given certain provisos, and make corrections, akin to balancing a ball on top of an inverted salad bowl.
In a racing context the conditions when this becomes unmanageable may not occur very often so overall an unstable setup can be competitive.
Think of it as riding a unicycle instead of a tricycle. Obviously humans are capable of learning to ride unicycles so the question becomes one of costs vs. benefits.
Exploiting an unstable platform is a muscle memory skill that can be learned ‘by feel’ with practice and is arguably more ‘natural’ once mastered than the mechanical and intellectual skill of adjusting foils for optimum trim.

In any case, our conclusions will reflect what we learned in testing. We will adopt the fastest configuration for getting around a course. Since we have been learning from our testing it will be different from the original setup.
A few notes on this video:

The discontinuities in the editing correspond to where Tom backed off to make adjustments using a control system that we would like to keep to ourselves for now.
The ‘flight’ was reliably uninterrupted for the whole run. The boat was safe and controllable throughout.

Looking at the wake carefully you can see the occasional disturbance due to ventilation of the surface piercing foil. This is an issue inherent in highly loaded surface piercing foils and we are experimenting with ways to mitigate it.
Fences are an obvious solution but have practical drawbacks as the foils must be retracted through the bottom bearing.
More promising options are leading edge discontinuities (cuffs) and boundary layer turbulators
Better still is an optimised foil section with ideal pressure distribution to prevent ventilation.
On the one hand this is a satisfying first:  Stable flight of a rule-legal A Class cat with VMG better than the lowriding benchmark.  On the other hand, it is still a tiny fraction of the learning curve we have before us!

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