Whether it’s making the most of the spectacular conditions enjoyed by the 2025 ILCA Youth Worlds in Dublin Bay (left) or Alain Gautier delicately finessing his way upwind to win the 1989 Figaro Solitaire, back when the marathon singlehanded event took place in tweaky IOR Half Tonners, a perfectly balanced boat is a boat at or close to minimum drag for the platform and conditions. Gautier will soon after go on to finish 6th in his first Vendée Globe in 1989/90 aboard Open 60 Generali Concord before winning the 1992/93 Vendée with his Groupe Finot ketch-rigged 60 Bagages Superior. Gautier’s company Lanic Sport continues to manage and support offshore racing projects – counting Ellen MacArthur and more recently Isabelle Joschke among many satisfied customers
angle the greater will be the magnitude of that forward pointing component of force; and if there were no aerodynamic drag the greater would be what we will now call the driving force component. Still ignoring drag for the moment, the
greater this driving force component the faster the boat will sail and, as the magni- tude of that driving force component increases with the increasing angle between the track angle and the apparent wind angle, in other words the further off the wind we sail, as we all know, the faster we will go. However, velocity made good (VMG) is a combination of speed and pointing angle so that there will be a speed beyond which VMG will diminish and that will be the best speed and pointing angle for best VMG for that boat with that rig in that true windspeed. Now if we throw drag into the mix, that
will angle the total force vector (lift and drag, the one that matters) backwards, so that the drive force vector becomes shorter and thus the driving force becomes less. To maintain the required driving force the boat then has to sail at a wider angle. The greater the aerodynamic drag the lower the boat has to point to maintain the required driving force for best VMG – conversely the less the aero drag the higher the boat can point with net benefit. Thus it can be clearly seen that it is drag that determines just how high a boat can point, not lift. The same reasoning applies to the hydrodynamic forces as well. The lower
the hydrodynamic drag the less driving force is required to drive the boat forward at a given speed, and that lower required driving force can be obtained from the rig at a closer angle to the wind. Thus the boat will again be capable of pointing higher. Put in a nutshell, aerodynamic drag
reduces the driving force at any particular wind angle. If the drag component, the drag angle, is 12° and we halve the drag, the drag angle will reduce to around 6° and our rig will be able to produce the same driving force while sailing approxi- mately 6° closer to the wind. Likewise for the hydrodynamic forces.
If the hydrodynamic drag angle was 18° and we halved that drag the hydrodynamic drag angle would reduce to about 9° and since, for equilibrium, hydrodynamic drag must be equal and opposite to driving force, that could be achieved by sailing around 9° closer to the wind. Of course, equilibrium would also be achieved by holding the same course but the result would then be that you sailed faster at the same wind angle… but the ability to point higher, if required, would still be there. It is any drag that reduces the ability of
the boat to point. It is the viscous and wave drag that is produced by the boat’s progress through the water and also the induced drag, which is a product of the production of hydrodynamic lift or side force. It is the profile drag of the sails caused by the friction of the air over the sails plus the pressure drag caused by any
separation of the flow. It is the aerody- namic-induced drag caused by the produc- tion of aerodynamic lift and it is also the parasitic drag of the mast, the spreaders, the shrouds and the stays. It is the aerody- namic drag of the above-water part of the boat together with the parasitic drag of the crew and all the deck fittings. Indeed, it is anything on or attached to the boat that is swept by either air or water. These drags can be divided into viscous,
wave and lift-induced drags, and the way in which they vary and react with each other is also important in affecting how high a boat will point. Fig A, overleaf shows the way in which viscous and induced drag vary with speed. As can be seen, induced drag reduces with velocity while viscous drag rises with velocity. Both these drags change approximately as the square of velocity, induced drag inversely as the square of velocity and viscous drag directly as the square of velocity (actually a little bit less than that). Wave drag, of course, depending on Froude number, varies at a very much greater rate. It is a fact of life that just about any-
thing that reduces viscous drag increases lift-induced drag and vice versa. Wings on keels reduce induced drag but increase viscous drag. Deep, small chord fins reduce induced
drag but, because the small chord reduces Reynolds Number (Re) and because drag coefficient (Cd) rises as Re falls, viscous drag rises. Crescent-shaped wing tips
SEAHORSE 57
HENRI THIBAULT/DPPI
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