Lots of testing in the ‘off season’. We completed some very productive sessions over the past few days and have more planned. As some of you may have observed, we went through some different concepts to the initial S foil setup. We are working on several areas for both numbers and feel, testing initially alone, then against other As.
The first area of investigation is properly understanding the behaviour of the foil system as the relationship varies between main foil and rudder foil force.
Rudder foil force can be manipulated by raking the rudder (tuning) and altering rudder winglet area (permanent change). The latter also influences aspect ratio which in turn modifies the overall lift-to-drag ratio at different angles of attack.
Aside from qualitative information such as visual observations and feedback from the skipper, we identified the need to work methodically through the range of possible combinations to gain quantitative ‘proof’ of how any concept performs.
The outcomes we are interested in are performance and stability (in that order).
We know that we can make the foils work much harder than with a conventional geometry (meaning they can be set up to carry a much higher percentage of boat weight without loss of control upon ‘takeoff’ – Takeoff simply being when that percentage reaches 100, keeping in mind that ride height is intended to be minimal).
What we don’t know yet is whether this mode is faster in terms of VMG to the bottom mark.
The trade-off between hull drag and foil drag is a fascinating, subtle and complex one given the A Class rules. It certainly seems to be less clear-cut than in some other classes.
So we are working through a range of settings for different values of rudder area to correlate our measurements with theoretical predictions.
We started with (gen 2) rudder winglets with Xcm ‘chopped off’ and tested the full range of useful rudder rake angles. Then repeated for the same rudders with a bit more length chopped off…
At the same time we monitor the ‘load share’ of the main foils and the behaviour of the platform as a whole.
Obviously we want to nail the minimum foil force required for stability (as that corresponds to the smallest drag penalty) then ascertain the best combination of area and angle to achieve the desired force.
A positive finding is that once set for crew weight little adjustment is required. Fine tuning is achieved by stepping forward and aft on the gunnel. Since rudder winglet angle is always positive in normal conditions (the exception being an incipient nosedive when they can go beyond neutral and start pulling the sterns back down), trimming the bow down neutralises their effect and thus reduces induced drag.
Similarly there are various options for foil immersion (connected to dihedral) and rake (connected to up/down lift) when sailing upwind.
The second area of investigation is the optimum transition point/s between different modes. Such as between sailing ‘conventionally’ with moderate heel to reduce immersed (windward) foil area and flat with the traveller down to get both foils working and carrying the entire weight of the platform together with the rudder winglets.
Finally we are looking carefully into the ‘human factors’ or interface issue.
Some additional complexity is acceptable if the reward is increased performance. However we are working hard to simplify the systems to make them easy to understand and to use effectively with minimal training.
As previously described, foil rake is (after some fine tuning of the setup) easily adjusted with a single line.
Foil immersion is a bit more tricky as the foil should be able to be raised and lowered remotely.
It remains to be seen whether vertical adjustment will be deemed worthwhile in terms of the cost/benefit trade-off between the workload of making the adjustment and the performance reward.
Vertical adjustment is predicted to only be required in sub trapezing conditions (less than 100% Righting Moment).
It is also worth noting that at least two other manufacturers have released information to the effect that they will also be incorporating dihedral change through vertical foil adjustment.
It will be interesting to see whether the predicted gains on offer can be realised in the real world during close racing.
In an ideal world the windward foil would always be fully up when sailing upwind/not foiling, but the constraints of a singlehanded boat make the practical ‘bancability’ of that option rather finely balanced.