Stability is unaffected but the boat is more responsive to changes in longitudinal trim.
In the following sequences Tom Stuchbery is deliberately ‘provoking’ the boat with aggressive steering inputs to get a feel for how it responds.
Existing foil assisted A Cats with C foils would continue in a ‘pitch-up’ feedback loop until the foils stall, making it very hard for the skipper to stay in control.
The pitch up is initiated by a slight downward component in the steering force generated by the rudders (this is present due to heel and is independent of any T, L or + foils on the rudders).
The pitch-up feedback loop is not just due to pitch instability. It is due to heave instability inherent in C and J foils: Even if the C or J foils are combined with rudder winglets to give pitch damping, heave instability remains because all the lift is generated at the bottom of such foils (this is where the horizontal area is concentrated) so the lifting area stays fully immersed. Therefore an increase in ride height does not cause a decrease in lift so it is not automatically corrected.
Our S foils differ by having the horizontal lifting part close to the hull so that this area immediately decreases as ride height goes up (the lifting segment of the foil immediately comes out of the water as ride height increases).
Paradox responds less ‘wildly’ to upsetting trimming forces.
One drwaback of having the lift just under the surface, however, is that the foils ventilate quite easily. A problem that could be solvable by optimising foil section and/or adding boundary layer devices to keep the flow attached…
To be clear, these are handling issues, not performance issues.
In most conditions C and J foils can be managed by setting them up so that their lift does not exceed the weight of the whole boat.
As long as the hull takes some weight (even if just the stern is ‘skimming’), heave and pitch stability are not an issue.
However if such speeds are reached that foil lift exceeds boat mass (and corrections are not made such as partially raising the foils to reduce dihedral angle), then a loss of control will be inevitable.
Our future testing will be aimed at weighing the drag penalties associated with maintaining stability, and determining whether they are worth accepting in normal racing situations.
Right now we are foiling but not claiming definitively that this is faster than foil assisted sailing in the A class. That remains to be seen.
To exploit the gains, one must understand the way the foils work. The boat must be kept flat so both foils can work together. The traveller seems to work best slightly lowered to direct the sail vector forward.
We are confident that we can regain our upwind superiority by incorporating an automatic toe-in adjustment in the foil bearings.
The aim remains to simplify the systems and evaluate whether Martin’s ingenious foil configuration is exploitable around the course.
As already mentioned, we are also exploring other configurations that have most of the advantages of the S foils but require less ‘retraining’ to exploit.
Our focus is sharply on getting around the course as quickly as possible. That is the basis for every design choice.
It is important to keep an open mind so testing will inform our understanding regardless of the attractiveness of each initial theory.
The process is about testing the theory with a view to refining it to gain an understanding of its validity.