Here we go again – Part 1
At AC36 Team New Zealand won the battle of the foils for the second America’s Cup in succession… Dave Hollom is already tracking the odds of them making it three in a row
In my articles about the last edition of the America’s Cup we discussed the shape of both foils and their arrangement and also hulls, all of which were then very diverse. Come 2023 and the early renditions of foils this time around seem to be just as diverse as before whereas hulls do seem to be coming to some consensus regarding basic shape. Turning first to foils, Ineos, who had the
most tapered foils last time, now seem to have gone in the other direction towards foils with an elliptical or nearly elliptical spanwise area distribution, while those that previously had elliptical foils seem to have gone to straight and highly tapered planforms. Why should that be? Well, there have been some rule
changes. The maximum allowed span of the foils has increased and the weight has gone down, both of which make take-off a
48 SEAHORSE
lot easier which was the aim of the rule change. Lift-induced drag is the largest foil drag during take-off and, as achieving speed to take-off is largely determined by the amount of drag to be overcome, reduc- ing that induced drag is of vital importance to an early take-off. Induced drag varies as (Lift/Span)2
so it
can be seen that increasing span and reduc- ing lift (the weight of the boat) reduces induced drag and, because the product of the terms are squared, that reduction is large. Under the old rule, at a take-off speed of 16kt induced drag was around 4.5 times as great as the profile drag of the foils. Under the new rules, again at a take-off
speed of 16kt, that proportion has been reduced to about 3.2 times that of profile drag. Put another way, induced drag under the new weight and span restrictions will be the same as under the old restrictions at a lower speed of about 12.75kt, so that take-off should occur at something like 3kt less than previously (although that might not be totally true because design teams may well have eaten up some of that take-off improvement by improving other areas of performance). Induced drag also varies inversely as the square of velocity (1/V)2 so that as speed
builds after take-off or a manoeuvre, induced drag reduces very rapidly. For instance, under the new rule restrictions, at 32kt induced drag is only about a third of total hydrodynamic foil drag, leaving profile drag (obviously) more like two- thirds of total foil drag. This is because as speed rises not only is induced drag falling as the square of velocity but also profile drag is rising at about the same rate. Where on a graph these two curves cross,
drag is at a minimum (opposite). Below the speed at which minimum drag occurs (Fig 1) total drag falls with increases in speed and above this speed total drag rises with increases in speed. This gives rise to the classic horseshoe total drag curve of almost any flying machine… which in turn gives rise to some strange handling characteristics. In powered aircraft the area below the
point of minimum drag is sometimes known as the backside of the power curve. Under normal flying conditions, above this point of minimum drag altitude is changed using the elevator and speed by adjusting power. Below this point of minimum drag the reverse occurs. Altitude is controlled by adjustments in power – more power to climb, less to descend; and speed by using the elevator, pushing forward on the stick
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