For millennia, sailors have proven they can cross vast oceans on boats big and small. But what if you removed the crew, added two hulls, and fitted a hard wingsail and hydrofoils? Meet the Autonomous Unmanned Surface Vessel (AUSV), developed by Harbor Wing Technologies. It carries no crew, and it gathers the majority of its energy from thin solar films on its wingsail, solar panels on deck, and from props that spin in reverse as it sails.
In the mid-1990’s, veteran Hawaii sailor Mark Ott developed a performance trimaran that could handle Hawaii’s often-boisterous conditions. He collaborated with a team of experts, including David Hubbard and Duncan MacLane (who developed the hard sail used by Stars & Stripes in the 1988 America’s Cup campaign), veteran ocean-racing navigator Stan Honey, and others, to develop what he thought would be a one-off racer. Ultimately, the team realized they weren’t creating a raceboat but a workhorse vessel capable of dealing with rough weather without a crew. Ott presented his idea to the U.S. Navy, which is interested in autonomous surveillance devices for homeland defense. The result, the AUSV, uses a variety of computer-controlled mechanisms to pilot itself. What’s most interesting is how its hydrofoils, wingsail, and throttle all work together to sail at human-determined speeds.
That’s right, a throttle. To operate the boat a human first tells the AUSV (via remote control) how fast she wants the vessel to travel in order to arrive at a site of interest or to avoid a storm. Once the throttle is set, the computer trims the wingsail to deliver the desired speed.
The AUSV sails on self-adjusting, symmetrical, inverted T-shaped hydrofoils with control flaps located on their trailing edges. There is one beneath each ama and a rudder foil on the main hull. The hydrofoil flaps are controlled by sensor wands attached to levers, which connect to pushrods running down each blade to move each foil’s flap. The rudder foil is not adjustable, but controls pitch, like an airplane’s tail assembly.
The result is a system that responds automatically, second by second, to individual waves. The harder the water pushes against a wand, the more its hydrofoil flap pivots downwards, thus lifting the boat out of the water. The two foils, on the windward and leeward amas, are constantly fighting each other, explains Ott. The leeward foil works to lift the boat out of the water, while the windward foil sucks it back down. “The result,” says Ott, “is an infinitely stable platform.”
While the AUSV will be capable of rising up to 8 feet out of the water, Ott says the purpose of the hydrofoils is to increase stability. “Because we’re dealing with a wide beam and triangular-positioned foils, the AUSV has unlimited righting moment. The idea is to give the boat sea legs, not speed.” Because of this righting moment, the AUSV’s rig is always perpendicular to the water’s surface, which is important, given the craft’s unique rig and sailplan.
Mounted on deck is a carbon-fiber post extending up to the middle of the mast. The freestanding wing sail rotates freely on this axle; all the control and power cables run through a slip ring at the top of the post. The wing is a symmetrical airfoil with a pair of trailing-edge flaps. In the 30-foot AUSV prototype (see photo) the wing is a single airfoil with twin steering tails; in the 50-foot version (illustrated) the wing will have two independent vertical airfoils, each with twin tails. Extending forward from the bottom of the wing’s leading edge is a counterweighted boom housing the batteries that run the flap and the tail motors.
The two trailing-edge flaps can pivot up to 20 degrees. While sailing, the wing is kept “nose to the wind” by two pairs of steering tails. The tails control the wing’s angle of attack to the wind, determining the direction and amount of force generated by the wing. If necessary, the flaps can increase that force.
The tails are controlled by a powered winch. The computerized-control system analyzes boatspeed and heading, wing-and-tail positions, wind velocity and direction, the boat’s GPS position and destination waypoint, plus the next waypoint.
As David Hubbard explains, these tails act as a mainsheet does on a normal boat; when the tails are angled, the wing generates lift and the boat accelerates; when they are centered, the wing goes head-to-wind and loses drive force. A symmetrical airfoil produces lift proportional to its apparent wind angle up to about 12 degrees before stalling out. With flaps deployed, the angle can be increased and the wing’s power can be augmented by as much as 80 percent.
In high wind the upper flap is centered and the wing feathers into the wind. The driving force is instead concentrated on the wingsail’s lower section via its flap (much like reefing on a conventional boat). Even with the wing pointed directly into the wind and the upper flap in neutral, the lower flap can create enough lift for the boat to sail.
