BEST OF 2014 – Foiling explained by those who know better

Foil technology, which dates back as far as the 1950s but has experienced wide media exposure since the exploits of the trimaran Hydroptère and the last America’s Cup, has been the subject of much development. We’ve talked a lot about it too: with the arrival of the Moth and more recently the GC32, Flying Phantom and SL33 catamarans on the market, anyone can experience the thrill of flying on water.

But are you sure you know how “foiling” navigation really works? We had it told to us in the August issue of GdV by an exceptional “professor”: Mario Caponnetto, a Genoese naval architect born in 1961 who won two America’s Cups with Oracle (in 2010 and 2013) as a key member of the fluid dynamics design (CFD) team, and who has now landed in Luna Rossa’s court.

THE KIWI’S PIGEON EGG
“Those who have tested boats such as GC32 and Flying Phantom,” Caponnetto began, “have been surprised by the ability to achieve stable and safe ‘flight’ immediately. The key to this lies in the type of foils employed, which are characterized by a high foil tip angle: this is the egg of Columbus discovered (seemingly by chance) by the New Zealanders during the design of their AC72. When a boat is in flight, the vertical hydrodynamic force produced by the foil (lift) is exactly equal to the weight of the boat (and opposite direction). If due to any disturbance (waves, gusts, acceleration or deceleration in maneuvering) the two forces become slightly different the boat will tend either to go out of the water (if the lift becomes greater than the weight) or to go down until the hull touches the water. One way to compensate for these disturbances,” Caponnetto continued, “is to act on the angle of the foil relative to the water flow. If the angle increases the lift increases and vice versa. If, for example, the foil is losing lift and the boat starts to go down, one can increase the angle by acting on the foil’s rake (i.e., longitudinal tilt) to reestablish the exact lift: but the reaction time of the ‘crew + rake control mechanism’ system is not fast enough and the system becomes unstable.”

THE “PASSIVE” STABILITY
“The foils of these ‘for everyone’ flying cats, however, exploit another effect that gives them a ‘passive’ flight stability that is independent of human control of the rake. If in fact the horizontal part of the foil (tip) is to produce a lift equal to the weight of the boat, the vertical part (strut) has the classic function of the drift of any sailboat, that is, to produce a lateral or side-force (in the upwind direction) equal and opposite to that produced by the sails (which push downwind). If, for example, the foil begins to push too hard and the boat begins to heave, the lateral surface area of the drift will be reduced, and the drift, forced to produce the same lateral force all the time, will only be able to do so at a greater angle to the boat’s drift.”

THE KEY LIES IN THE DRIFT
Let’s see now how the foil tip can stabilize the flight: “If the foil tip is horizontal,” Caponnetto explains, “this does not actually happen. But if it is angled upward instead, the lateral velocity causing the drift has a component perpendicular to the surface of the foil itself. Put simply, the water begins to push the foil back down until balance is restored (the top left and right figures show the unbalanced and balanced situations). In practice, the greater the angle of the foil tip relative to the horizontal (dihedral angle), the more stable the boat will be and manual corrections will not be required. Finally, it should be mentioned that foil drag increases with dihedral angle, and normally the best compromise between flight stability and boat speed should be sought.”