Fig A
Fig B
Dr FW Lanchester, another of the author’s important but largely forgotten figures in the history of aero and hydrodynamics. An engineering polymath, Lanchester filed many patents in different fields and in 1899 founded the Lanchester Motor Company which traded as late as 1955. The Lanchester Company was a major supplier of military vehicles during WWI
sometimes known as lift-induced vortex drag, while induced drag at the water surface is evidenced by the generation of waves and is thus sometimes known as lift-induced wave drag. At the water surface on the high-
reduce induced drag but, because of the migrating stagnation point on the leading edge of swept wings which destroy lami- nar flow, it becomes harder to maintain laminar flow over the swept tip area so viscous drag rises; it is much the same with other choices between induced and viscous drags. There is thus a trade to be made between one or other of these two drags. It is also of interest to see the effect of
reducing each of them while holding the other constant (Fig B, above). As can be seen minimum drag occurs at a higher speed if viscous drag is reduced and, although min- imum drag is the same, it occurs at a lower speed if induced drag is reduced. Reducing viscous drag would therefore seem to be preferable to reducing induced drag in that minimum drag is the same, so that both boats could point as high as each other but the boat with reduced viscous drag would sail faster through the water and would thus have a better VMG. The most damaging drag for windward
performance is, however, wave drag and, because it rises so rapidly with speed, at critical points in the performance enve- lope, it is the drag, the reduction of which will most improve the windward perfor- mance of most boats.
58 SEAHORSE Most boats typically sail to windward
at speeds just below those where there is a rapid rise in wave drag. By reducing wave drag it raises that speed, but also, because induced drag reduces as the square of velocity, it also reduces induced drag… Thus the reduction of one drag also
reduces another, so that our boat’s perfor- mance envelope is massively improved. It can not only point higher, it can sail faster at that pointing angle. Perhaps the most dramatic example of
the effect of reducing wave drag is the upwind performance of the International Fourteen dinghy, which, in the right condi- tions, will plane upwind at speeds of 12- 13kt. Not bad for a 14ft boat. There is another reason why reducing
wave drag is so important to the wind- ward performance of sailing boats. Induced drag is caused by the equalisation of pressure on the high and low-pressure sides of the lifting surface. Typically this occurs at the tip but, as
the famous aerodynamicist Theodore von Kármán noted in 1936, sailing boats have two tips: one at the tip or bottom of the keel and the other at the water surface. Induced drag at the tip is evidenced by the formation of vortices and is therefore
pressure side of the boat the water surface is elevated and on the low-pressure side it is depressed. However, there is already a wave system around the boat caused by the boat’s progress through the water so that this secondary wave system becomes super- imposed on the existing wave system. Now the energy in a wave system varies
approximately as the square of the wave system’s amplitude so that superimposing one wave system on another raises the combined wave drag by a huge amount. However. the lower the initial wave system the lower the combined system and thus the lower the combined wave drag. That is why reducing the hull’s basic wave drag is so important to improving a sailing boat’s performance. So the next time you are squeezed out of
your lane after the start it might be worth considering just why the other boats have less drag than you? Is it better sails or is it a better sail trim? Are their foils better fin- ished than yours and when was the last time you polished the bottom or, if it’s an open class, has their designer produced a better lower-drag hull than your own? Assuming that you are competing at the
top level in your class and that, therefore, all the foregoing have been attended to, if your boat still doesn’t point as high as the others perhaps you need a new design altogether?
q
ALAMY
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