Left: when it was good it was very good… however, when Ineos Britannia did begin to bounce around it looked as if the binary ‘on-off’ nature of having a more seamless skeg/water seal probably worked against the British boat which often bounced higher than its New Zealand rival. Below: the ultimately unsuccessful X-29 forward- swept-wing experimental aircraft flies over Edwards Air Force Base in 1987 with a much younger Tom Cruise on the stick…
the greater the energy within the vortex the less energy there is to propel the boat for- ward. Thus, by keeping the small vortex that develops around this skeg attached all the way to the stern would appear to be better. They say in my neck of the woods ‘You
don’t get owt for nowt’: this vortex, devel- oped by the skeg and its vortex generator, does cost in energy consumed. But com- pared to the rather large vortex that would develop if the skeg and vortex generator were not there, it is small by comparison and the energy consumed is less. In the case of the 2024 generation,
though the skegs on Britannia and Patriot run pretty well the length of the boats and, on Britannia, at a considerable depth, they both have a large gap between the back of the skeg and the rudder/elevator. Just why is a bit of a mystery, unless there is some kind of symbiotic relationship between the skeg vortex and the rudder/elevator. Provided one can find a workable solu-
wing, the downwash starts later so that we have the effect of moving the wing further forward within this body of air, and thus to a point where the downwash slope is less steep so that induced drag is reduced. I must stress that this is my personal
theory. But whether it is correct or not, it is clear from Van Dam’s experiments that moving the tip back does reduce induced drag. Also, to reinforce Van Dam’s conclu- sions, it has long been known that sweep- back on a wing, which also moves the tip vortex aft, reduces induced drag. In visualising a physical phenomenon it
sometimes helps to look at extremes – or the opposite of what we are looking at. For instance, if we look at an aircraft with the opposite of sweep-back, forward sweep, we can imagine the tip vortex now forming in front of the main body of the wing such that the wing is now exposed to the full down- wash from those tip vortices. Because the downwash is steeper the drag component of local lift will be greater. Perhaps that’s one reason why forward-sweep experimental aircraft such as the Grumman X29 (above) remained just that: experimental aircraft. So now you are probably wondering,
quite reasonably, why New Zealand went the other way with their 2024 AC lift foils, with a straight leading edge and a curved trailing edge which, of course, moves the tip forward. Well, one of the problems with sweep, which you inevitably produce in the tip area if you move the tip back, is a loss of laminar flow in that region. This is caused by the stagnation point on the lead- ing edge of the wing in the swept area, migrating outboard towards the tip.
52 SEAHORSE As it moves along the leading edge of
the wing this crossflow tends to trip the laminar flow in this area with the result that the flow downstream of this distur- bance at the wing tip becomes turbulent in nature and turbulent flow has higher drag than laminar flow. Thus, although the swept tip will reduce lift-induced drag the viscous drag will be greater. For reasons I will not go into too deeply
here, in boats operating in a medium where the density of the fluid is more or less constant, it usually pays to reduce viscous drag. In aircraft, however, as they climb, the density of the air reduces so that, although they fly faster, which reduces Cl and thus lift-induced drag, the lower density has the opposite effect… The net result is that an aircraft is nearly
always flying at high Cls where induced drag is of prime importance, whereas foil- ing boats are only operating at high Cls when moving slowly. At the comparatively faster speeds, where they more normally operate, viscous drag thus becomes more important to the performance of the boat. So how does this affect the skeg? Well,
first-generation AC75s (and some later boats) had a skeg that was predominantly in the fore part of the boat, as in American Magic’s first Patriot, or tended to taper away towards the stern. This to me would be the equivalent of moving the tip of a wing forward which would appear wrong: you are allowing the tip vortex to develop in size as the distance between skeg and water in this area becomes greater as it moves aft, and the larger the vortex the greater the energy contained within it and
tion to the attachment of the rudder to the skeg that allows no gap, I would have thought it an advantage to have the two with no gap. In discussing the Moth we said that the daggerboard joining the hull to the water was an advantage as it extended the effective span of the whole lifting system. Having the rudder/elevator joining the hull of an AC75 would appear to be no different, except that the two join at the stern rather than in the middle of the boat. Personally, I don’t think this matters. Consider a wing with swept-back tips
placed in a wind tunnel with the tips attached to either wall. Does the flow anticipate that it can’t form a tip vortex as the tip downstream touches the wall, and thus treats the swept tips as if they were still part of the leading edge of the wing? Or does it form a vortex, which bursts when it reaches the extreme tip that is touching the tunnel wall? If it’s the former then it truly does double, or at least increase, the effective span – though there would probably be other drags associated with the tip’s proximity to the wall and, in our case, the water. However, this is analogous to the AC75
case with the rudder/elevator attached to the skeg. And if this analogy is correct per- haps no skeg vortex needs to be generated to take the lift right down to the water surface, which would save some drag. Looking at Britannia its defining feature
is an enormous skeg, deeper than any of the others and continuing to almost the stern. Perhaps the idea is to actually seal the bottom to the sea surface by having the skeg permanently, or nearly so, extending
DANIEL FORSTER/ALAMY
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