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Top left: the modelled surface pressure across an Imoca foil seen from the outside of the surface (extrados) at different foil rakes and (right) the same pressure display but now seen from the inside of the same foil (intrados). As you would expect (and certainly hope), there is an almost complete reversal of pressure moving from the outer to inner surface of the foil. The bow-on view (left) skimming just before take-off. The green dotted line is the application line of the force coming from the foil, blue is that from the keel alone and red is the total hydrodynamic force alignment. A side view (right) of the same situation. Note the aftwards (and draggy) orientation of the keel force (blue). It is not very efficient to rely on the keel for vertical force, and this should only be the case in specific downwind conditions when the consequences of burying the bow are worse than the inefficiency of the keel fin as a producer of vertical force


sail in negative leeway. This means that the hydro drag is reduced at the same time as the aero efficiency is improved. But to achieve negative leeway one needs to pro- duce the required side force in the ‘rotated’ leeway position and to achieve this the foil is raked forward as much as possible so that the tip increases its angle of attack, producing most of the side force. In prac- tice this is usually around -2° of leeway. Reaching In this mode the boat achieves speeds that can take full advantage of the foil and as such taking off is easiest. But here righting moment is king so the keel is dropped to a lower angle to maximise right- ing moment… I know this is counterintu- itive but this is the case because the hydro- dynamic heeling moment created by the keel is very quickly greater than the mechanical righting moment due to the mass of the bulb, so it comes down to a reduction of heeling moment rather than an increase of mechanical righting moment. At high speeds the keel is literally in the


middle. With the keel in the centred position it is worth using it to create the required side force rather than using the foil (adding to foil drag); this effect is further accentuated if the hull is skimming or fully flying, when the impact of hull drag due to leeway is a lot less important than sailing upwind or in full Archimedean mode. This means that now the foil is


46 SEAHORSE


primarily required to produce a vertical force and not a side force, so the rake is brought back to unload the tip. However, while wanting all the vertical


force from the foil we also want to main- tain its contribution to dynamic righting moment, so when reducing rake to reduce the load on the tip we don’t want to reduce the load on the elbow where the additional righting moment originates. This is where the rotation in space is so critical, allowing for the loading of the elbow to be almost constant across rake adjustment but at the same time loading and unloading the tip and shaft in inverse proportion. Downwind This is a more difficult mode to sail in since we are now heavily impacted by wave shape and general sea state. In relatively flat water the same notions of reaching could be applied. But with a heavy sea state (and there is always a sea state offshore!) the boat ends up having to sail faster than the waves (which usually travel at 21kt and up to 25kt in the Southern Ocean). To achieve an efficient mode in these


conditions, and to be able to sail past the waves, we can only do it with an exagger- ated bow-up trim, high enough that the bow does not bury. In this mode, flying is very difficult because of the continuous longitudinal instabilities created by the sea state. So our best mode ends up being very


bow-up and safe, leaning back on the stern and at most with the transom lifting out every now and again. At this wide TWA, righting moment is not a primary element of performance and we often struggle more to heel the boat over rather than the opposite, so side force from the foil is almost irrelevant. So how can we increase the vertical force with little or no side force? The answer is with the keel. So the keel is now canted again to its


maximum (38°) so that by virtue of the sloping keel pivot it can produce vertical force to lift the boat/bow and heel the boat at the same time. But for this to happen positive leeway is required and therefore the foil tip is completely unloaded so that the required side force is created by the keel alone. This unloading of the tip can be done in


different ways, depending on the foil in/out position (the foil extension). If the foil is fully out, rake is minimised so as to reduce the AoA of the tip and shift the loading in towards the shaft, pivoting around the elbow which also helps to reduce the dynamic righting moment. Another option is with the foil partially


retracted, when thanks to the shaft curva- ture the tip now becomes more horizontal and as such produces more vertical force than side force, at which point rake can be increased again accordingly. The problem





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