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The previous Hugo Boss broke new ground, lighter and narrower than her rivals, placing more emphasis on her long, flat DSS-style foils; ironically the 2016 Imoca that depended more than any other on her foils for performance lost a large chunk of one of them in the South Atlantic. This time (above) Alex Thomson’s team are again pressing into new territory with these giant curved foils with one constant radius throughout. A knock-on benefit is that, unlike the ‘clashing’ foils on Charal, Thomson can retract both foils in light air


with this at the top end of the speed range is that the foil might not be deeply enough immersed and will regularly ventilate due to the proximity of the free surface of the water when the hull elevates above it. But at least we have something to play with as we try to adapt to the conditions! Skimming versus flying, and when to start flying? If we analyse the range of modes in which an Imoca (or similar type) hull can sail, we can start at one end with a very Archimedean mode when there is no con- tribution from the appendages and the displaced volume of the submerged hull is equivalent to the mass of the boat. At the other end of the range the hull is com- pletely flying. In between of course we have a continu-


ous range of assisted modes, ending with ‘skimming’, which is the mode immedi- ately prior to take-off and flight. The position along this range at which a boat will sail at any given time is completely dependent on hydrodynamic efficiency. On one side of the equation there is the


hull drag reduction due to the increasing support of the hull and on the other side the drag cost that is being paid for creating that support that the hull is receiving. The net value of this equation should be an overall drag reduction for it to be worth


48 SEAHORSE


doing, the solution value being propor- tional to the square of flow speed and the mass to be supported. This means that at low speeds it is not


worth supporting the hull but if one keeps accelerating, there is a moment, a boat- speed, when such support nets a total drag reduction. For an Imoca the speed when support starts paying off is around 10kt. Then at 18kt full flight drops drag further. But it gets a bit more complicated than


this. At first sight one could easily be tempted to design a foil and hull to earlier take-off criteria, since 18kt is very easily achieved by these boats nowadays; the downside is that this foil will be big and thick and although it would satisfy the goal of a low-speed take-off, it will not be the right answer for all those cases when early take-off is not the main parameter. In other words, at speeds above and below 18kt. Furthermore, within the context of


singlehanded sailing, it is difficult to believe that a boat can be constantly pushed to its maximum performance – in fact, we know for certain this is almost never the case in long solo races. Hence if we are sailing a lot of the time below maximum potential, then getting stuck with something draggy is not ideal. Today this whole balance of performance elements and criteria is the


subject of very lively debate and I’m not sure we know the answer yet. Where to position take-off is a key


question, but I’d venture to say that any- where between 18kt and 22kt of boatspeed is the right range. For a crewed Imoca for the Ocean Race that early take-off will be more obviously desirable since at higher speeds the full crew can drive their boat much harder than a solo skipper. Foil deformation Long foils will (indeed should) deform a lot when loaded, some current Imoca foils deflect by more than 800mm at the tip. This means that the foil geometry is substantially modified under sailing loads and as such the relationship between the vertical and side components of its hydrodynamic force changes a lot. This change occurs in the direction of


increasing side force at the expense of reducing vertical lift, and the magnitude of the change is generally in the range of 10-20 per cent. This deformation therefore needs to be taken into account – it is currently addressed via a simulation called fluid structure interaction (FSI) that essen- tially combines CFD and FEA, within a single yet iterative solution. Accounting for foil deformation within design is one of the key areas from which the next iteration of substantial gains will come.


q


MARK LLOYD/ATR


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