Passenger Hydrofoil with Fully-Submerged Foils: Kawasaki’s JETFOIL

The JETFOIL, a type of passenger hydrofoil with fully-submerged foils, is considered essential for traveling to and from remote islands. Powered by gas turbine engines, waterjet propulsors (pumps) draw in and then discharge seawater, generating lift by means of a set of fully-submerged forward and aft foils, thereby raising the hull above the water’s surface. Achieving a maximum speed of 45 knots (83 km/h), it cruises stably even in rough seas with wave heights as much as 3.5 m. It certainly deserves to be called a “flying ship.” A hydrofoil with fully-submerged foils was originally developed by The Boeing Company of the U.S. in an effort to apply aerospace technology to marine products. 

In 1974, a passenger-carrying hydrofoil was launched, which was named “JETFOIL,” as it resembled a jet with sharp, leaf-like wings. In 1987, Kawasaki acquired the manufacturing and sales licenses from Boeing, and 15 units were manufactured in Japan between 1989 and 1995.

A quarter-century later, a new JETFOIL was produced and delivered at the end of June to Tokai Kisen Co., Ltd., a Japanese maritime transport company. This 241-passenger ship, which was christened Seven Islands Yui (“yui” means “to knot, to connect”), has replaced the 36-year-old Seven Islands Niji (“rainbow”). The JETFOIL connects Tokyo and Oshima Island in an hour and 45 minutes, a trip which takes six hours on large passenger ships, marking a new chapter in its history.

JETFOIL’s propulsion system is comprised of two gas turbines, installed on the port and starboard sides, reduction gearboxes, and waterjet propulsors. More specifically, it is equipped with Rolls-Royce-produced gas turbines with an output of 2,794 kW each, Kawasaki Powerjet 20 waterjet propulsors that achieve 2,060 rpm, and reduction gearboxes that transfer the turbines’ rotational force to the propulsors. These elements work together to draw in seawater and then discharge it at high pressure from the rear of the vessel, creating the thrust needed for the ship to sail. In addition, according to Assistant Manager of the High-Speed Ship Designing Section, Shinsuke Kikuno, “The vessel incorporates a fail-safe design, enabling it to ‘land’ on the water and cruise safely even if one system fails.”

The water inlet located at the center of the aft foil draws in and then discharges 180 t of water every minute, which is equivalent to approximately half of a 25 m x 10 m x 1.5 m swimming pool. The highly pressurized nature of the water being drawn in and discharged enables the vessel to reach its top speed of 45 knots (83 km/h) in just three minutes.

The cockpit is equipped with a cutting-edge digital liquid crystal display (LCD) control panel.

The forward foil has flaps that move vertically and a forward strut that rotates horizontally. Working with the flaps on the aft foil, the forward foil achieves optimal course change. The ship’s captain only needs to operate the helm and the depth control lever.