Video: It's all about efficiency.
Why do catamarans “fly”?
Hydrofoils work exactly the same way as wings on an aeroplane – the only difference being that the necessary lift is not generated by the air, but from the water. At just two metres in length and 70 centimetres wide, the foils, like a wing, have a convex upper surface. This makes the distance the water must travel on the upper side longer than on the lower side. As such, the water must flow faster. This, in turn, generates negative pressure. This negative pressure sucks the foil upwards, creating the desired lift.
If the catamaran has achieved a high enough speed, it will lift completely out of the water. Like every other sailing boat, catamarans also heel in the wind. This results in only one foil and a rudder being in contact with the water, thus reducing water resistance to a minimum. This means that the displacement caused by the hull no longer hinders the speed – this is no less than a sailing revolution and has made aerodynamics and the reduction of air resistance a key factor in achieving maximum performance. No wonder, then, that the BMW Group’s aerodynamic test centre in Munich and the BMW specialists’ expertise play a major role in the design of ORACLE TEAM USA racing yachts.
Drag vs. lift.
The crucial efficiency criterion for yachts is the relationship between lift and drag. Above the water, the wing sail on AC catamarans generates a huge lift. Under the water, meanwhile, the foils generate the lift that minimises water resistance.
This combination has dramatically increased efficiency, along with other progressive factors, such as the reduction of weight through the use of the very latest lightweight technology – another area, in which BMW provides a valuable technology transfer for ORACLE TEAM USA. The pioneering automotive achievement of putting a carbon chassis into production plays a key role in the realisation of the fully-electric BMW i3 and the BMW i8 plug-in hybrid sports car. The weight saved by using carbon fibre allows the pioneering efficiency and dynamics of the BMW i models.
Balance is everything.
To allow the catamaran to fly continuously, helmsman Jimmy Spithill must adjust the angle of both foils and the rudder blades every second to suit the course, direction of the wind, speed and opposition. He needs absolute sensitivity for this task – and an intelligent steering mechanism. This is being developed by BMW Motorsport engineers, who are transferring technology from automobiles to competitive sailing. The constant readjustment is immensely important, as sailing on foils is extremely unstable – and if the hull hits the water, the boat loses an awful lot of speed.
Revolution through multihulls.
The drag of a classic monohull is proportional to the water displaced by the hull – this dramatically limits the speed potential. By using multihull boats and the boost from foiling in the America’s Cup, it has been possible to triple the top speed.
A glance at the history of the America’s Cup shows just how profound the change of paradigm has been: between the very first America’s Cup and 2007, the maximum speed of the boats increased by just four knots. Since then, the speed has increased by an incredible 30 knots.