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| Two boat testing session |
As a general observation, we are in unexplored territory for this class and arguably for this type of boat at this scale simply because the relative effect of foil setup on overall performance is much greater when the foils are working hard enough to support most of the mass of boat and skipper most of the time (and all of it some of the time).
Put simply, when the foils are doing most of the work, getting the settings right is much more influential than if they were only helping out a little.
In hindsight this should be no surprise. Ask any ‘Mothista’ about the effect of a small fraction of a degree of foil angle and they will say it is like night and day. When the foils are the only part of the boat actively interacting with the water (in the case of Paradox the hull may still be ‘skimming’ but our measurements tell us it is supported by the foils, not by the water) their effect is dominant.
The saving grace is that the correct setup is mostly related to crew weight and remains constant for different conditions. Once the correlation is understood, it should be easily duplicated.
Now that we understand the (far reaching) effects of main foil shape, toe-in and rake, the key is getting the right amount of ‘lift share’ so that the sterns are supported by the rudder winglets, but a step back can still raise the bows up sufficiently to ‘pop’ the boat up onto the foils.
This is a function of some combination of winglet Angle of Attack (AoA) and winglet area.
Lets say we want X amount of lift from the rudders such that they will support enough weight to keep the sterns ‘flying’ but not so much that the stern cannot be made to sink somewhat when the skipper takes a step aft.
We could obtain the desired lift with small winglets at a big AoA or with big winglets at a smaller AoA.
Assuming aspect ratio can be optimised in both cases, the lowest drag solution will come down to the chosen foil section – and the lift coefficient (Cl) it is happiest at.
However the choice will also have an effect on stability: If the AoA is larger, then the boat will trim down further before the winglet goes through a neutral AoA and begins pulling down to restore the desired pitch attitude.
In reality having the winglets actually pull down will only happen in rare ‘extreme’ situations. However it is a helpful way to visualise the dynamics at play.
Simply reducing the AoA with bow-down trim is enough to introduce a stern-down restoring moment.
The vital part is the rate at which this moment increases since its rate of change is key to stability in pitch.
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| Optimum ride height with skipper not all the way aft and correct lift sharing by the rudder winglets. As the bow pitches down, rudder winglet AoA decreases. In fact we found that the most stable setups tend to takeoff ‘stern first’ and stay level or slightly bow-down in flight. The boat happily sits in this attitude when set up correctly. Notice the absence of wake other than spray. |
We are cristallising a useful map of how this foil system works and how it can be exploited.
It appears that performance is good when it is set up correctly.
The most impressive aspect has been the utter predictability and controllability of some foils (more than others) when pushing hard downwind. That is definitely an aspect of the brief that was met successfully.
What remains to be proven is whether the gains are exploitable around the course.
One finding for example has been that with some foil types there is quite a significant benefit in raising the windward foil when sailing upwind.
In a close racing situation this can only be exploited if the system to raise and lower the foils is extremely easy and fast to use with minimal distraction.
We have therefore experimented with a series of mechanical solutions and it seems the last iteration meets the criteria.
In short it uses elastics to raise the boards automatically and a single line with significant mechanical advantage to lower them. More detail on the evolution of these mechanical systems will be released later.
As already mentioned, this is a problem that some of our competitors will also have to solve as they adopt ‘S’ foils with outward inflection at the top that alters dihedral angle as a function of foil vertical position.
Having explored this development path we will only adopt in production a system that is reliable and easy to use without distracting from ‘keeping eyes out of the boat’.
If we are not satisfied that such a system can be engineered (meaning that, after friction is overcome and single line operation achieved, the burden on the skipper is still judged to be excessive) then we will change the foil system so that it does not need to be touched during racing.
This may involve changing foil shape and/or finding a compromise toe-in setting that is optimised for always having both foils down.
Referring to the design brief for Paradox, the final balance to be struck must be in favour of best achievable speed around the course.
If a certain setup cannot be sailed at a high percentage of its potential for a large percentage of the time around a course (by a ‘mere mortal’), then a slightly compromised variation that is more exploitable will be more competitive.
With this in mind, simplifying the boat is vitally important and what we are learning now will inform the choices for the next prototypes and the production setup.