It was a lovely calm day out on Lake Maggiore in Northern Italy. Conditions were just perfect for aeronautical engineer Enrico Forlanini’s test run for the latest model of his hydrofoil, a long motor hull on the underside of which he had affixed a ladder system of foils he hoped would lift the boat out of the water as she moved.
The test was a success. The year was 1911. “The foil era began that very day,” explains 57-year-old naval and mechanical engineer Mario Caponnetto, a veteran of the 1992 America’s Cup with Il Moro di Venezia. He followed this up a stint with Luna Rossa between 2004 and 2007 before going on to head Oracle’s Computational Fluid Dynamics Department, delivering victory in the Cup in both 2010 and 2013.
In 2004, Caponnetto began collaborating with Francis Hueber also, a partnership that led to the foundation in Valencia of Caponnetto-Hueber SL, a specialist naval engineering company that was also commissioned to design Luna Rossa’s foils for the 36th America’s Cup.
When did people starting talking about foils in sailing?
People started talking about foiling for sailing craft in the 1960s but there are earlier examples still, such as Monitor from 1955. However, these were experimental craft.
When did you start working on foils?
With Oracle for the 2010 America’s Cup when we were up against the catamaran Alinghi. We investigated the possibility of getting our 34-metre trimaran with its 70-metre wingsail mast, foiling. We had also tested a shorter high-wind foil-assisted trimaran: the forward half of the hull was out of the water and the stern was immersed. But in the end we liked the big displacement foil-free trimaran more which was optimised for the calms that we were expecting at Valencia in February. Then in the Cup at San Francisco, we started out pretty conservatively in the Oracle Design Team but we responded well to the Kiwis’ foiling with a boat that was superior in terms of its aerodynamics.
What are foils and how do they work?
They are essentially wings that work in the water rather than the air. Centreboards and rudders are wings too. There is no difference between them in physics terms. But rather than being vertical wings, foils are horizontal to produce greater vertical rather than lateral thrust and so be able to lift the boat out of the water. They also work like any other wings. The shape and angle of the wing to the air or water flow create an asymmetry in the pressure field of the fluid itself and thus a pressure difference between the upper and lower surfaces of the wing. This difference in pressure is what creates lift which raises the aircraft or boat up.
We’ve seen many different types of foil. Why?
First and foremost, the big difference is between submerged and semi-submerged foils. The semi-submerged foils can self-stabilise. If there is excessive lift and the boat rises too high, a part of the wing comes out of the water and thus no further lift is created. Consequently, the boat lowers itself until the foil establishes equilibrium again. In fully submerged foils, which are always entirely under water, this doesn’t happen and, as with aircraft, you need a system that will allow you to adjust the angle of the entire foil or a part of it at least (flaps, ed.’s note) to suit the lift that you need to generate. There are also hybrid foils which is why so many different shapes are out there.
Aside from its shape, what makes a foil more or less efficient?
Foils aren’t just useful in terms of upping speed. One of our projects, Seabubble, is a taxi-boat for the Seine where the speed limit is 15 knots. The advantage of foils in this instance is that they create practically no waves and can cut through waves caused by other boats without impacting passengers.
But there are also the rudder surfaces…In theory, there could be such a thing as a single foil boat but really you need at least two points to keep the boat balanced. On a foiling sail boat you have to try to put maximum load on the wing that is more downwind to boost righting moment. The rudder, or tail wing, thus works to stabilise pitching. Aboard the AC50s in the 2017 Cup, because there were always two rudder surfaces in the water, you could play around also with the angle between them to add righting moment and thus power on the sails.
The next Cup will be competed using the AC75, a foiling monohull. What will make the difference between the teams?
The AC75 Class Rule is completely unprecedented and everyone is starting from scratch. Although there are elements that are the same for everyone (including the foil movement system, ed.’s note) and a relatively strict rule, I’m sure all the teams will come up with some original ideas and figure out any way possible of getting around the rules that exists. It’s a lateral thinking game. Whoever does that best will win. As is always the case.
Is it possible that foils will make the transition to pleasure sailing craft?
You don’t always have to go fast. You can go slow and be relaxed too. But wherever performance is a priority, foiling will used more and more. The area I see most potential in is motorboats. There is no comparison between cutting through the waves in silence on a foiler and slapping against them in a conventional hull.
Are foils an end point or is there more to come?
I believe and I also hope that there will always been something new to invent. In the short term, I see potential in automatically-controlled fully-submerged foils which provide both improved performance and comfort at sea.
Then, above certain speeds, aerodynamic lift in the air rather than water becomes efficient. I’m thinking of revisitations of the Ekranoplan which is a cross between a hydrofoil and a hovercraft, and used ground effect. Working in racing has taught me to keep a very open mind to the unusual. The naval sector does suffer from a certain inertia when it comes to innovation. I think progress needs people prepared to gamble on change.