Top: a study of relative performance of Olympic kayaks from the mid-1970s onwards in various depths and widths of different bodies of water has provided a good insight into the effects of these parameters on drag and achievable performance. The deterioration in performance in ‘restricted water’ further demonstrates the critical nature of leading-edge design and finish for hulls as well as foils. Above: Watin/Bethwaite empirical measure of yaw drag – side-by-side towing two identical 49er hulls on each side of a powerboat with the towing array forward so the hulls are not in the powerboat’s wake; one hull is towed straight, the other skewed at 2°. At hull speed a 15 per cent increase in hull drag is recorded with 2° of yaw. That equals 0.3kt difference in speed (the short green line). A 49er, due to its shallow skimming form, enjoys low yaw drag. For a deep forefooted multihull or displacement yacht the yaw drag would be much higher. The smaller 29er (right) has often been employed as a second validation platform during Bethwaite’s studies
us to generate similarly precise LE sections. Then began a series of comparisons. The scenario was that a 49er, like an
18ft skiff, comes out of a tack at approxi- mately 6kt, and then accelerates upwind at 8-9-10 even 11kt. Compare that to a Laser that tacks at 3-3.5kt and hits hull speed at 4.5kt. If you apply the square law (pres- sure increases as the square of velocity) the Laser ends up at 162% potential excess side-loading capacity from its relatively fat foils, whereas the 49er ends up at possibly as high as 270% increase in side-loading capacity. Plus there are various other ‘form drag’ considerations that mean that any skiff with anything like a Laser section centreboard will get crucified. (The area of a 49er centreboard below the keel is 0.296m2. At, say, 6kt it is enough to generate approximately 90kg of side
66 SEAHORSE load, so if 62 xC = 90kg (C is a constant
accounting for Centre of Lift [CoL], Angle of Attack [AoA] and Area) then at 10kt if you maintain CoL, AoA and Area that same foil will be able to generate 102
/62 =
100/36 = 2.77-times or 250kg of lift. Obvi- ously Yaw (AoA) reduces and that reduces CoL, plus effective area reduces because the tip feathers. Same sum for the Laser). There are of course other considerations
that come into this. In anything over about 5.6kt hull speed (HS) a 49er is starting to plane, and for every, say, 0.5kt increase there is a notable increase in lift up out of the water and that extra speed reduces yaw (therefore yaw drag). To cut an equally long story short, if we
concentrated on section uniformity to a high level, and equally if we tapered down to the tip, not just in sectional width
(chord) but also in percentage camber (and we did this in a very controlled and theo- retical manner exploiting exponential tapers), then we were able to make a quan- tum jump in drag reduction, via section, LE radius and reduction in tip vortex drag. That in turn meant that the boat moved
faster and lifted higher, and that in turn resulted in higher side-loading capacity which in turn meant less leeway, and lower sectional drag (we were well and truly inside the [drag] bucket). Enter circular/ domino conundrum! As I touched on last month, we commis-
sioned three sets of 49er tooling and they are massive – I am guessing the centre- board mould weighs close to 70kg (the actual centreboard weighs 3kg, interesting comparison). Those moulds have been in constant use since and there are no others.
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