Now you can look all you want. Delegates at a 1984 conference of naval architects at the Dutch Maritime Research Institute pore over the towing tank model of the final design for Ben Lexcen’s Australia II with her revolutionary keel… winner of the 1983 America’s Cup
reduced rather than induced drag. The same principle applies to a boat but, because wave drag rises so rapidly, this effect is even more pronounced and also minimum total drag is not the same for both cases. Thus for maximum VMG we need to be
reducing every other drag than induced drag, and even at the expense of induced drag, which it inevitably will be. Just about everything that you can do to reduce induced drag, other than altering the wing planform while retaining the same area, raises viscous drag and vice versa. So put wings on a keel – or make them
longer – and this will reduce induced drag but viscous drag will rise. Take the wings off or make them shorter and induced drag will rise but viscous drag will reduce. As we say in Yorkshire, ‘You don’t get owt for nowt’. You can perhaps now see why Australia
II had higher drag, as her skipper John Bertrand found out when both the conven- tional Challenge 12 and Australia II were on load cells being towed from the same tender. And also, as we discovered during our own tank tests and as Peter Norlin noted when he used an inverse taper keel on one of his Six Metres. To make the Australia II concept work
the keel needed wings and that put up both viscous and junction drag. It did, however, reduce induced drag which improved her acceleration out of a tack but at some expense to her straightline upwind speed and her downwind speed, which was even slower again, which seems to fit her performance profile. Australia II solved these problems, at
least the downwind problem, by working on better spinnakers, nevertheless they of course only increase the driving force; the
56 SEAHORSE
extra drag is still there. If Dennis Conner had had better prior knowledge of Aus- tralia II’s performance profile perhaps he too could have improved his own spin- nakers and retained a downwind speed advantage; but of course he didn’t know and even before the start of the Match he believed that he had the slower boat. Thus, on occasion, he took chances – and the rest is history. The final example is more about the
pundits getting the story wrong rather than about which was the faster boat, which was not in doubt. We are talking about AC35, the event sailed in Bermuda in 2017 in 50ft foiling cats. This was the first foiling Cup where elevators were allowed which altered the design side of the whole event, but only one team seemed to grasp the full extent of this change. For the previous Cup in AC72s elevators
were not allowed and ride height control had to be accomplished by pre-setting the horizontal stabiliser and by an automatic self-levelling foil system. The introduction of elevators for the 2017 event transformed the possible foil systems and allowed much more efficient lift systems to be used. Almost without exception teams used a
vertical foil to provide side force merged with a horizontal foil to provide lift. How- ever, Team New Zealand hit upon the very simple idea of using a single appropriately angled foil to produce both lift and side force, which allowed the required forces to be produced on a smaller area foil working at no greater a Cl than the two distinct foils. As the area was less the drag force was also less, and if you reduce drag for the same required lift forces you just have to sail faster, and New Zealand did just that. (Team NZ did face structural challenges with their thinner, narrower foils, but with
long nights put in by the build team their faster but more challenging foils just made it to the end of the Match.) The simple secret of this winning devel-
opment was the fact that the hypotenuse of a triangle is always shorter than the sum of the other two sides! If you set your foil (the hypotenuse) at the appropriate ‘average’ angle it will produce a force normal or at right angles to its rigging angle which, when resolved into vertical and horizontal com- ponents, will be of the same magnitude and direction as the lift forces produced on the more distinct vertical and horizontal foils – as were used by all the other Cup teams that year. However, it will be shorter by about
18% on the same chord and at the same Cl and Cd so that the area, and thus the viscous drag, will go down by about 18%. There are, however, other hydro -
dynamic drags such as induced and rudder drag, so the total drop in hydrodynamic drag will be more like 10% but that alone equates to an increase in speed of just over 3%. And in a racing boat, that is a killing advantage, and so it transpired with Team New walking the series. The New Zealanders also used cyclors
rather than grinders to produce the onboard power which the press preferred to use as the reason for their success; while this also might have been an advantage it was the angled lift foil that gave them the ultimate increase in speed. Such a simple idea, it is a wonder that
no one else thought about it. Actually, one or two teams did, but as usual it was the New Zealanders ‘throwing it out there’ and committing early and fully to a wholly different concept that got the job done. Remember that it is the winners who q
write history.
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