Above: Dennis Conner acknowledges the plaudits as he steers his Valentijn-designed Defender Liberty back towards Newport after winning race one of the 1983 Cup against Australia II. The body language suggests that, even after winning the first race, Conner knows how big a fight he has on his hands to pull off a second back-to-back defence. By now all of the Defender’s ultimately failed efforts to have Australia II’s keel ruled illegal only served to feed the pre-match intimidation of the US crew. Australia II was slippy in the light and easier to accelerate than Liberty, but the superbly sailed US boat was fast upwind and in the end it probably all came down to the smaller, more efficient spinnakers (right) that had been created for the Challenger by the mercurial Tom Schnackenberg
proved the case in Newport in 1983. Interestingly, I had the pleasure of
working with the late Peter Norlin during the Swedish challenge of 1992. For those who do not know, Peter was the designer of the IOR Half Ton champion Scampi and many very successful Six Metre boats and, besides being a bloody good designer who really did understand what makes a boat fast, he was a hell of a nice guy. One day, after a game of squash, Peter
and I were sitting in the sauna and talking about, what else but boats, and Peter bought up the fact that he had tried the inverse taper keel on his Sixes but it did not appear to be a fast solution and he wondered why. This, once again, rein- forced what we had found in the tank and what the Australian sailors had found when sailing the conventional Challenge 12 and the unconventional Australia II. So what is the reason for the apparent
design paradox? Well, the inverse taper keel has, as its name implies, a greater chord at the tip and a smaller chord at the root where it joins the hull. This is the reverse of a conventional keel, hence the nickname for the Ben Lexcen ‘upside down keel’. The problem with this arrangement is that it gives a very different spanwise lift distribu- tion from the traditionally tapered keel. Reverting for the moment to compari-
son with aircraft, an elliptical spanwise lift distribution gives minimum lift-induced drag for any particular span. In the absence of any wing twist an elliptical spanwise area distribution will give you this elliptical spanwise lift distribution, with the lift coefficient (Cl) constant along the span, as is the aerodynamic angle of attack, which is why it gives minimum induced drag. This is useful when span is limited but becomes relatively unimpor- tant when span is unlimited when wing bending moment becomes more important and other wing planforms are better. Now boats are different from aircraft
and thus the ideal spanwise lift distribution will be different and probably not elliptical – in fact, it may well alter at different boat speeds and thus differing Froude numbers (Fr). However, the same principle applies that reducing the chord on any planform increases the local Cl and increasing the chord reduces the local Cl in relation to the rest of the lifting surface. Thus, looking at the inverse taper keel,
one would expect that the Cl at the top, where it meets the canoe body and where its chord is small, will be high and looking at the whole boat as a lifting body rather than as a separate keel, rudder and canoe body, all producing lift, the Cl on the canoe body, in this area, will be low. This difference in Cl at the junction will result in much vortex shedding in this area and a consequent rise in induced drag. Also, junction or interference drag will be
high. Junction drag varies as the square of junction length and the cube of thickness/ chord ratio (T/C). By shortening the root chord you are swapping a reduction of the interference drag, by the square, for an increase in the interference drag, by the cube, so the interference drag must be greater. In actual fact it is even worse than that
because, for strength reasons, if you shorten the chord you must increase the thickness so that the T/C ratio rises even more quickly, making things even worse. This situation is when no lift is being
produced. As soon as the boat is producing side force things get worse still because junction drag also varies with angle of attack. A high Cl implies a high angle of attack and, as there is a high Cl at the junction, the consequently higher angle of attack increases junction drag yet again. Remember the Mickey Mouse ear keels that appeared at one stage in the develop- ment of IOR boats? They were meant to reduce junction drag but they didn’t last long. Do I need to wonder why? Some of this rise in lift-induced drag can
be offset by using winglets on the tip of the keel but they don’t come for free. There is the extra wetted surface area and also the extra junction drag of two or maybe even four extra junctions, depending upon how they are attached. There is a strange relationship between
lift induced and all the other drags (viscous and junction drag in an aircraft and, addi- tionally wave drag in a boat). Induced drag, for a given lift force and span, varies inversely as the square of velocity while all the other drags vary directly with velocity, by the square for viscous and very much more for wave drag. Thus, as one drag reduces with velocity, the other drags rise so that the resulting total drag curve is of a horseshoe nature. Minimum drag in the aircraft case
occurs where both viscous and induced drags are the same. As weight is a con- stant, in an aircraft (ignoring fuel) this is the point of best lift-drag ratio (L/D) and is therefore the speed that will give the best glide angle, which in a glider will give the greatest distance covered from a given altitude in still air. In a boat this point of minimum drag
represents the speed at which you will achieve the highest beneficial sailing angle to the wind. This will not necessarily be the point at which you will achieve the best velocity made good (VMG) – as that con- sists of both how high you point and also the speed through the water at whatever that pointing angle is. However, it will be the highest you can point with benefit, if you have to clear a mark or obstruction. If you pinch any higher you will not only sail more slowly, you will also sail at a lower angle and perhaps not clear the mark. Returning now to an aircraft, when
viscous drag is reduced as compared to reducing induced drag our glider would glide at the same angle in both cases, but it would fly faster, and thus get to wherever it was going sooner if viscous drag was
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