It is easy to find things in common between a car and a motor boat. It is less simple when the boat is a sailing yacht: the hull (or the hulls, in the case of a catamaran or a trimaran) corresponds to the chassis and the set of sails - including the mast - matches the propeller. However, there’s a huge difference compared to an engine, whether combustion or electric: the ‘fuel’ is not available from a station and above all, even when it is available, it needs to be managed. Wind, even in areas considered as being reliable, changes in both intensity and direction. And this according to a logic that yachtsmen can sometimes predict, based on personal experience and increasingly sophisticated instrumentation. But sometimes, even the most skilled of them get it wrong, even spectacularly. And there is nothing more frustrating for a regatta racer waiting to make the best of the wind ‘power’ than to remain without any.
The two forces
Accordingly, in order to understand the importance of wind on the boat’s motion, it is important to clarify the relationship between aerodynamic force and hydrodynamic force. In a situation of zero wind that gradually increases, a stationary sailing boat – with the mainsail hoisted – starts to move and shifts laterally due to the effect of the aerodynamic force. The displacement of the hull immersed in the water in turn generates a hydrodynamic force. The result of the two forces combined is a new force - known as a headwind force - directed at the bow of the boat. The boat accelerates until the two forces become equal and opposite, at which point the speed becomes constant. As the wind drops, deceleration occurs, just like acceleration can occur if the wind picks up, keeping in mind that too much power is likely to cause trouble. The task of the ballast under the keel, which is a characteristic of traditional boats, is to prevent capsizing when close hauled – the point of sail into the wind – the wind ‘pushes’ too laterally on the sails and without a weight that contrasts its force, the hull tilts dangerously.
The battle of sails
The sails therefore have the task of ‘collecting’ the wind to the correct extent to seek the best performance possible, a feat which however also requires two other factors: the hydrodynamic quality of the hull and the capabilities of the crew. So now we can link this to the concept of motorsports: only a mix between chassis, engine and driver can lead to success: just two of these are hardly enough, and very rarely is one alone. Consequently, it is quite understandable how the design of the ‘rigging and sails’ has been vital since the very onset of yachting. As is the fact that the America’s Cup constitutes the main lab to test all technical progress: for a hundred years, it was the American title holders who launched just the right new features at just the right time, something that their rivals were often not permitted to do. But since the triumph of the Australia 3 – in Newport, in 1983 – many challengers have also been able to astound when it comes to sails. The positive aspect of the experiment was the subsequent adaptation of the new features on normal boats, just like what happens in car races, starting with F1. Materials such as kevlar, mylar, carbon fibre – now commonplace – were introduced for the America’s Cup.
Now there’s the soft wing
The AC 75 – the America’s Cup class – has one more feature which on the one hand enhances, and on the other makes things a little trickier. These are the foils: the foil cant arms that rest on the water to provide the necessary stability. On the sides of the course, the leeward foil provides the lifting thrust for the boat, while the windward foil provides the straightening momentum (since there is a bulb missing under the keel to act as a ballast) and prevents capsizing – except in the event of errors or forcing. Furthermore, the boat has a soft wing, which consists of a twin-skin mainsail. The double-sail skins combine with the spar to form a wing. The soft wing generates the same efficiency as a rigid wing – resisting to wind power more – but with an ease of use similar to that of a traditional sail. It is a highly sophisticated contraption: 20,000 km of carbon wires are used to make it, while another 5,000 are needed to weave each jib and 12,000 for each Code Zero, the sail for running downwind. The power of Aeolus, the god of wind, is not easy to master, but in the America’s Cup, they know how to handle it.