Naval architect Sergio Cutolo of Hydro Tec takes stock of hydrofoils.
NauticExpo e-Magazine: How do hydrofoil work ?
Sergio Cutolo: How hydrofoils work is simple in theory:
- Basically, the hydrofoil consists of a wing-like structure mounted on struts below the hull.
- At low speeds the hull still glides through the water; as the boat’s speed increases, the hydrofoils create lift.
- At a certain speed, the lift produced by the hydrofoils equals the boat’s weight. Therefore the foils lift the hull out of the water and the boat rides, or planes on the foils,
- Instead of having an increase in drag with increasing speed—contrary to what happens in traditional boats due to pressure drag—the hydrofoils provide a more efficient way of cruising. Decreasing the drag increases speed and fuel efficiency.
There are two main hydrofoils configuration: surface-piercing and fully submerged hydrofoils. In the former configuration, part of the V-shaped hydrofoils rises above the water surface; operations become limited as wave height exceeds the hydrofoil’s strut length. Fully submerged hydrofoils are less subject to the effects of wave action and assure better seaworthiness. However, they need quite sophisticated stabilization systems.
NauticExpo e-Magazine: What are the main pros and cons?
- As the boat slides on hydrofoils, it becomes extremely fuel efficient and provides a smooth and comfortable ride.
- A wide adoption of hydrofoils is prevented by the complexity of building and maintaining them.
- It is necessary to install powerful engines that will be used just to have the boat in foilborne trim.
There are a few challenges that need to be addressed. Foils generally have sharp, potentially dangerous edges, which also entail maintenance issues.
NauticExpo e-Magazine: Any significant application in the yachting industry?
Sergio Cutolo: Apart from a few, one-of-a-kind examples or prototypes, the luxury yacht industry has never embraced the hydrofoil system that, on the other hand, benefited from the success of medium-sized passenger crafts and small leisure motor-boats and water toys with both traditional and electric propulsion.
Hydrofoils application on sailing boats deserves its own chapter. They are indeed widely used on highly sophisticated sailing boats, America’s Cup boats included.
NauticExpo e-Magazine: What about stern foils and the Hull Vane?
Sergio Cutolo: It is worth pointing out that a vast array of sophisticated stern hydrofoils are widely used on boats of different size and type to reduce pitching while improving fuel efficiency. This kind of hydrofoils gets the boat on plane more quickly and not only benefits water-skiers and powerboat racers, they also improve safety and efficiency by keeping the bow down.
The Hull Vane designed and patented by Dutch Van Oossanen and Associates Naval Architects, is a purposely shaped underwater foil at the stern of the yacht. Tailored to the individual yacht, the foil is fixed at the aft end of the hull creating lift and a resulting forward thrust while also reducing the stern wave. It also reduces the running trim—advantageous with a bulbous bow—and dampens pitching motion, which not only enhances efficiency but also increases comfort.