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Paradox Version 2.0 Foil Concept

The new foil concept is simple, easy to use and low drag.
After lots of R&D to validate theoretical predictions, it seams that for most sailors the benefits of ease-of-use outweigh those of ‘trick’ configurations that need to be learned, tacked, and trimmed to suit changing conditions.

The brief for Paradox calls for a boat within the A Class rules that can win races.
This means getting around the course quickly, without overly taxing the skipper who has the only pair of hands on board. Straight line speed must be combined with good tacking and down-speed performance as well as predictable behaviour.

We will continue to test ‘radical’ foil geometries as part of long term development, but right now the best way to fulfill the brief is with our new production foils that you can see in the pictures below.

Their shape resembles an apostrophe (‘) or comma, rather than any letter of the alphabet.
They are polyhedral foils made up of three straight segments connected by two tight radii.

The working span of the foil is planar simply to minimise wetted area: A straight line gives the least frontal and wetted area for a given dihedral angle.
The foil goes from the hull exit point to the inward beam limit in a straight line which is shorter than any curve, whether C, S, or J shaped.

The bend just below the hull is to minimise junction interference drag: It makes the included angle between hull and foil closer to 90 degrees.

The inward bend near the top of the foil is a solution unique to the requirements of the A Class: Popular thinking in modern multihulls is to reduce dihedral angle as the foils are lowered.
The idea being that in light winds the leeward foil is pushed all the way down, increasing span and area while reducing the vertical lift component at the same time. This gives a high-aspect upright foil for light air sailing.
However this approach requires that the windward foil be retracted since the leeward one alone is deep enough to provide all the necessary sideforce.

Our foils take the opposite approach: they become more upright as they are retracted.
In light airs you sail with them both partially up, giving a pair of smaller vertical foils that are both contributing sideforce.
This means no need to ‘tack’ the foils every time you change direction.

As soon as there is enough wind to fully power up, you lower both foils all the way and leave them there for the whole race.
By putting the top ‘handle’ rope through different holes in the head of the foil, you can pre-set the ‘max’ dihedral angle to taste. A small change in foil immersion gives a relatively large change in dihedral.

Partially raised foil (shown red) is more upright than fully lowered.

Eliminating foil curvature makes the lifting surface efficient and gives a positive feel for the ‘bite’ that the foil has on the water.

Foil rake and toe-in are pre-set by our custom hull and deck bearings.
Since angle of attack in the horizontal plane is coupled with leeway angle, foil rake adjustment on the water is no longer necessary. Stepping aft to trim the stern down automatically increases both sideforce and vertical lift.

Combined with other revisions that will be described in later posts, Version 2.0 represents a return to a guiding philosophy of simplicity and ease of use.

This is a great example of the truism that profound simplicity is inherently much more challenging to design well than complexity.
To simplify a product, the designer must understand which elements are essential and how they can be combined, excluding the superfluous, in a way that enhances the user experience while maximising performance.

It is extremely satisfying to come full circle and be able to present a product that is simple not through elimination but through integration.

Lessons Learned

Our development journey has taken us in just under a year from the initial S foil concept to other ideas (from different camps) including various iterations of J and L foils adapted to be Class legal.
Throughout we kept an open mind, learning without prejudice from experimentation.

With L foils we achieved reliable stable flight and impressive top speeds. However the demands of these configurations, connected with the radical fore-and-aft positioning they called for, their need for active adjustments and, especially, their inherent asymmetry (requiring that they be tacked, jibed, and re-configured for different points of sail), made them demanding around the course. In a race situation, the straight-line gains did not justify the impositions they placed on the skipper.

The new foils will give stable flight at higher speeds because their area decreases lineally with increasing ride height. Take-off speed can be lowered by increasing toe-in angle, however this will not give a net drag reduction.
The inherent efficiency of the long, slender, light A Class hull, combined with limited power and sail area, call for a middle road of stable foil assisted sailing with a ‘late’ transition to full foiling in an automated fashion.

We will continue to experiment, explore new ideas and share our findings while the production version of Paradox is out there getting runs on the board.

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