Figure 5: Lift = f(t)


structure. These springs are calibrated so as to resolve the nominal inboard vertical force. When that force increases the head block will rotate and pitch down the foil, thereby reducing the angle of incidence of the flow on the foil. Differential setting of spring tensions


Figure 6: Drag dynamic range


will then allow for changing the mean angle of incidence of the foil. Tuning the springs’ stiffness and pre-load will allow the skipper to adjust the dynamic behav- iour of the foil system to various boat speeds and sea states. It may, for example, be desirable to employ different spring stiffnesses and pre-loads side to side. Finally, a passive solution as outlined


above could also be employed on a medium to large IRC yacht equipped with lateral foils such as DSS, as this solution is fundamentally designed to reduce the additional drag induced by the cyclic pitching of the boat in a seaway. And also of course to provide a less punishing motion that will benefit the yacht’s crew as well as its structure.


Figure 7: Drag = f(t)


Addenda This article describes the additional drag due to dynamic fluctuation of flow inci- dence on foils under the current Imoca rule. The pair of experimental fully foiling Imocas recently observed trialling out-of- class rudder elevators present a very differ- ent case deserving of a future paper analysing in detail this three-point configu- ration in offshore conditions. Certainly flying on three points


(foil+keel+rudder) is a lot safer and a lot less punishing than flying on two points, as is the case today. However… even when flying on three


Figure 8: Archimedean mode


points in a seaway stability will be com- promised unless there is either active control of the foil lift (and that will not be cheap) or they use the ‘trick’ that I have proposed in this article, to limit the lift of the forward foil while keeping the rudder foil fully immersed. In both cases the absolute limit will be


the point at which the rudder foil venti- lates, when passing the crest of a wave, or cavitates, probably at around 40kt of boat speed. We have seen that situation during the early races of the last America’s Cup, although cavitation on the AC75 mono- hulls took place at somewhat higher speeds in New Zealand on account of sailing in flat water. Interesting times…


Figure 9: Foiling mode Figure 10


q


SEAHORSE 59


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