“The feeling for me of foil is like flying. When you start gaining speed on a small plane, and then instantly take off and leave the ground. It’s very similar on a boat”
-Jimmy Spithill, Skipper Oracle Team USA 2014 
No discussion of hydrofoils is complete without addressing their application to the 2013 America’s Cup yachts. Catamarans screamed across the ocean, flying on innovative hydrofoils. (Figure 1‑1) The crew constantly struggled to balance the foils against the wind and extract every last point of speed. It made excellent drama and energized the sport. But with all that excitement, we sometimes forget how the crew jeopardize their lives in every race.
Figure 1‑1: America’s Cup Hydrofoils 101 
DMS engineers seek to preserve life while pushing for new innovation and greater achievements. This article presents an engineering perspective on the America’s Cup hydrofoils of 2013, with options for improvement.
Hydrofoils work fine on sailing vessels; we possess the knowledge to design a stable sailing vessel with hydrofoils. (See article on hydrofoil control.) The roll stability under hydrofoils reacts similar to a normal displacement yacht. Under wind forces, the yacht heels over. This changes the angle and submersion of the hydrofoils. Various systems of control react and adjust the foil forces to balance the heeling moment. On a tack, the vessel switches direction of heel, and the foil on the opposite side takes control. The opposite side foil . . .
We encounter a stability problem with racing hydrofoils in the 2013 America’s Cup. These yachts target extreme performance. In pursuing those extremes, the yachts include the ability to retract the windward hydrofoil from the water. It improves the speed, . . . and it eliminates any passive stability in the foil system. By all rights, the yacht should fall off foil at that point.
The only thing that sustains foil flight in this unstable condition is the crew. They carefully balance the heeling moment of the wind against the hydrofoils. This is equivalent to balancing on a high wire, while someone keeps shaking the wire. While running at 40 – 60 knots. Used in this fashion, sailing hydrofoils are inherently unstable, always trying to fall off of foil.
The challenge of sailing hydrofoils adds to the fun, but as an engineer, I focus on the large increased risk to life. Sadly, those risks already progressed beyond theory. While training for the 2013 America’s Cup, sailor Andrew Simpson was killed.  The yacht capsized, and Mr. Simpson became trapped under the solid sections of the yacht. Emergency divers failed to locate him in time. The vessel owners prepared for the potential of a capsize with a myriad of safety features :
As an engineer, this tells me that the racers were not reckless; they cared about safety. But those preparations were not enough. I accept that extreme sportsmen demand new challenges, but I can’t accept that a sport warrants jeopardizing your life.
Max Sirena added: “The boat is basically too powerful. At the same time, this is our sport. This is a risk we take.” 
Beyond all other requirements, ships must protect the lives of their crew and passengers. The current designs for sailing hydrofoils disappoint in this area. But we don’t throw in the towel and forget about hydrofoils. Engineering gives us the tools to work the problem and develop new solutions. Maybe the vessels protected against the wrong challenge. We need to understand the fundamental risk of sailing hydrofoils.
The basic problem is the speed inherent to hydrofoil sailing. If a small dinghy capsizes at 5 knots, the consequences are minor. But capsizing at 40+ knots carries a much higher chance of injury. As a heuristic comparison, Figure 4‑1 indicates fatality rates, compared to ship speed. Although this was based on pedestrian collisions with motor vehicles, it shows much more severe danger above 30 knots.
The combination of instability and extreme speed mean too high of a risk. No one wants to remove the hydrofoils. Members of the racing industry desire that challenge of the hydrofoil instability. In that case, we need to accept and anticipate more crash stops and capsizes. If we want yachts designed like fighter jets, give the crew the protection of a fighter jet:
At DMS, we do not see engineering as an obstruction to new challenges. Engineering gives us the tools to try new ideas and evolve them into safe technology. With the proper planning and risk management, we can implement hydrofoil sailboats in a safe manner.
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