WASHINGTON — The cat-and-mouse game of anti-submarine warfare (ASW) is always characterized by endurance. How long can a crew hold out? How long can a pursuer stay on top? If it's a non-nuclear sub, how long can the undersea boat's engines keep running?

Enter the unmanned vessel – still restricted by fuel, but unconstrained by the need to support human operators. While small unmanned surface and underwater vehicles have been around for decades, the US Navy is taking a significant leap forward with the development of the Anti-Submarine Warfare Continuous Trail Unmanned Vessel (ACTUV), a 132-foot long vessel intended to search for submarines at sea as long as three months at a time.

ACTUV — pronounced "active" — "is focused on providing a high degree of autonomy," program manager Scott Littlefield told reporters Wednesday. "It's not just a remote controlled boat."

Computers will drive and control the ship, Littlefield said, but a human will always be observing, able to take charge if necessary. The concept, called Sparse Supervisory Control, means "the human being is in control, but not joy sticking the vessel around," Littlefield said.

Prime contractor for the Defense Advanced Research Projects Agency (DARPA) ACTUV program is Leidos, and construction took place at the Vigor Shipyard in Portland, Oregon. Until a merger in 2014, the yard was known as the Oregon Iron Works, specializing in the design and construction of exotic special-mission craft.

Launched in January, the ACTUV vessel, named Sea Hunter, has been running trials in the Portland area. The vessel was commissioned today in a ceremony at Portland, and in a couple weeks will be sent to San Diego, where DARPA and the Office of Naval Research (ONR) will begin a two-year-long trial period to test the concept and various sensors that can be installed on the 145-ton full load displacement vessel. DARPA will conduct the initial trials and turn the vessel over to ONR later this year. The test phase will run through September 2018.

The Sea Hunter was launched by crane at Vigor Shipyards, Portland, Oregon in January 2016.

Photo Credit: DARPA

Sea Hunter will carry about 40 tons of fuel. Top speed on trials was around 27 knots, Littlefield said, although the actual top speed is dependent on the sea state and how much fuel is on board. The vessel is intended to  be operational through Sea State 5 – moderate waves up to about 6 and a half feet high and winds up to 21 knots — and be survivable through Sea State 7, considered to be rough weather with seas up to 20 feet high.

The configuration of the composite-construction design resembles a Polynesian war canoe, with a long, slim hull supported by outboard pontoons — called Amas — connected by outriggers. The composite hull features a foam core with a fiberglass skin, Littlefield said, while the outriggers are fabricated with higher stiffness.

Sea Hunter will not carry weapons, but will trail sensors designed to detect and track submarines. The vessel, Littlefield said, could operate with Littoral Combat Ships, in essence becoming an extension of the LCS ASW module.

A removable operator control station is installed on the craft, and a person will be on board throughout the test period, "for safety and backup," Littlefield said. "We'll be there for a while until we work through the reliability question."

A small, removable pilot house is fitted on the Sea Hunter so a human can be on board throughout the sea trials, ready to take over if necessary.

Photo Credit: DARPA

Like most DARPA projects, the Sea Hunter is not a prototype for an operational Navy platform, although it could be. The overall goal, Littlefield said, is "to build something very affordable which could probably be acquired in large numbers."

As the first hull, Sea Hunter is a bit more pricey than a potential follow-on vessel.

"It looks we're going to deliver the first one for a construction cost of between $22 million and $23 million dollars," Littlefield said. "We're trying to get to a series cost of about $20 million a copy — not cheap, but not as expensive as a manned warship." The daily operating cost is likely to be between $15,000 and $20,000, he added.

The cost figures do not include program costs, including development, design and software.