Firstly the effect of foil rake on the angle of attack of the horizontal part of the foil, the part that provides lift in the up/down direction:
In the first picture the foils are in the fully aft position in the top bearing.
This is the ‘max lift’ position, resulting in the lowest takeoff speed for transitioning to full foiling.
With the new foil bearings this will also correspond to max toe-in, further augmenting lift.
Full scale testing will confirm the optimum angle (both rake and toe-in) for lowest overall drag.
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| Yellow lines show foil leading edge angle to vertical and the Angle of Attack (AoA) of the lifting portion when set as pictured. Orange lines show position for windward foil when sailing upwind. In this position the foil will pull down, adding ‘weight’ to the windward hull |
The trade-off is that more lift increases the drag of the foils. This additional foil drag buys a reduction in hull drag since less of the hull is in the water thanks to the lift from the foils.
If the reduction in hull drag is greater than the additional foil drag, then there will be a net gain.
Since hull drag and foil drag rise at different rates with speed, the exact ‘crossover’ (think of it as the optimum takeoff speed) remains to be seen. On an A, with current foil technology, it is somewhere in the teens when measured in knots. At lower speeds the efficiency of the slender leeward hull is formidable. Since sail area is limited, the option of increasing power is precluded so drag reduction becomes imperative.
Tuning the foil angles is key to assessing the true potential of this configuration. It is easy to do by simply swapping out the centre element of the top foil bearings. The difference in observed performance with different angles is significant. Getting these angles wrong means the true performance available is not realised.
Dihedral angle (and hence vertical lift fraction) is set by foil depth. It is zero with the foils fully down and maximum at the ‘reaching position’ as seen in the image below. This is the position always used except in consistently light (‘sub-trapezing’) conditions.
When underway, the effective dihedral angle is self regulating through the variation in curvature of the S foils. As ride height increases, the part of each foil that remains in the water becomes progressively more vertical, reducing dihedral.
The role of the rudder ‘L’s is to control pitch angle. Our new rudders will maintain stability in pitch with a considerable reduction in drag.
Remember that stability in pitch only comes into play when fully foiling. If full foiling proves to only pay in extreme conditions, then the Ls can be made even smaller (it would not make sense to carry the associated drag penalty, however small, at speeds where stability is not critical if such speeds are relevant most of the time). Since the effective split of vertical lift between foils and rudders changes with sail force, it is relatively straightforward to hone in on a winglet area that begins providing negative feedback only when the hulls leave the water completely.
Positioning the foils forward (ahead of the sidestays) has the principal advantage of reducing induced drag by letting the rudders share sideforce with the main foils.
A secondary advantage is that the Ls on the rudders have more leverage to control pitch attitude.
One question to be tested is whether the forward position of the foils adversely affects manoeuvrability.
Our policy is to share findings as we learn new things and develop new products.
The reason is simply that all the work we do is motivated by passion for the sport. Sharing this passion is profoundly rewarding.
Our customers realise why we do what we do. That is why they decide to become our customers.
In light of this, no sweeping claims have been or will be made. Martin Fischer’s stable foiling configuration makes sense and we are working to unlock its potential. We will only adopt it for production if it proves superior.
At the same time we are prototyping a less extreme arrangement that will be simpler to use and have a less critical performance profile. This is being designed by Dario Valenza and will be mated to a revised hull shape made to match the new concept.
This alternative arrangement will be unveiled very soon. Buyers will be eventually offered the configuration that emerges as superior after testing, modifications and more testing.
Much of the thinking behind the S foil arrangement will still be applicable and the eventual, hopefully simpler, production version/alternative will benefit from our extensive R&D process, carried out using the very best tools and methods.
As the simpler alternative is going through the final analysis stage, it appears that it will have a slight edge in light and moderate conditions. This makes sense as it will have less wetted area, simply because the emphasis is on drag reduction at moderate speeds before dynamic stability becomes dominant.
In the top end of the wind range, performance should be comparable but the sailing style will be more conventional. Safety margins will still be higher than existing foil assisted designs and full foiling will still be possible, albeit with higher takeoff speeds.
This may yet prove the fastest way around a course most of the time in a class where the displacement to length ratio is exceptionally low and sail area is limited.
The reason to buy a Paradox remains the same: carefully researched design, build quality that goes beyond mere performance into aesthetic excellence, personal customer support and, most importantly the driving passion we want to share with our customers.
