Figure A
Top: Figure A – plotting Angle of Attack along the span of a deformed foil for different rake adjustments and foil/shaft rotations. Above: foil deformation calculated for a single specific sailing condition using Fluid Structure Interaction (FSI) combining CFD and FEA analysis to generate the ‘flying form’ of a foil in a not dissimilar way to a modern sail study. Sébastien Simon’s Juan K-designed Imoca Arkea Paprec (left) during the Défi Azimut. Simon won the 2018 Figaro before teaming up with team manager and co-skipper Vincent Riou. Prior to launching the new Arkea the pair campaigned Riou’s previous PRB and won this year’s Bermuda 1000 race in Arkea colours. Juan K’s latest Imoca pushes the Imoca rule on scow bows to the edge with the very blunt stem and flat forefoot
the in/out or up/down movement. For today’s updated rule a second degree of freedom was introduced on top of the previous one, which essentially allows for the foil to be also regulated in such a way that the distribution of forces varies depending on this new adjustment – which for most means the rake adjustment. The rule accepts this second degree of
freedom as long as it is a simple rotation, which means around a single axis. For any foil that is not straight, ie that has either a constant curvature throughout (such as Hugo Boss 7) or a variety of curvatures that can denote what is regularly called the shaft, elbow and tip, the relative position in space along the span of the foil impacts the loading along the foil span as a function of this rake adjustment. And this opens up the game enormously when it
comes to having a given foil perform from upwind in the light to downwind in heavy air. It opens the door to making the whole design more versatile and to not having to choose which specific condition one wants to target at the expense of others.
Figure A (above) In a typical deformed foil shape such as shown in Figure A one can differentiate clearly the shaft, elbow and tip. Then variation of geometrical Angle of Attack (AoA) is plotted along the span for differ- ent rake adjustments. Blue and orange represent curves for a
rotation axis and the yellow curves for another axis. We can see that the distribu- tion and relative change between tip and shaft varies dramatically with rake. In the upwind case you want to load the
tip for a more efficient side force and unload the shaft to reduce drag. Down- wind it is the opposite when side force becomes less important compared to vertical force. To illustrate this better we need to go
into the detail of three different but equally relevant sailing modes: Upwind In this mode the boat is heeled and sailing in a very Archimedean way, since the boatspeed is not high enough to take full advantage of the vertical force of the foil. At the same time righting moment is created mechanically by the water ballast and fully canted keel. This means that the boat is heavier and
the hull is quite submerged and due to the heel also very asymmetrical. This means that in order to improve hydro-aerody- namic efficiency the optimal solution is to
SEAHORSE 45
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