The U.S. Navy is investing heavily in robotic ASW (Anti-Submarine Warfare) vehicles. In addition to very small ones (the size of a torpedo and even smaller) but has also been developing small robotic surface ships. The latest one is the largest USVs (unmanned surface vessels) yet called ACTUV (Anti-Submarine Warfare Continuous Trail Unmanned Vessel). This is a 41 meter (132 foot) 145 ton ship designed to operate autonomously for up to 90 days, patrolling the high seas while used a towed sonar array and other sensors to hunt for submarines. Top speed is 48 kilometers but is cruises at about half that while working. There is no crew on board so up to 40 tons of fuel can be carried. The ship is slender but has two out-riggers to stability. ACTUV is designed to operate in up to Sea State 5 (six meter waves and 38 kilometer and hour winds and survive Sea State 7 (nine meter waves and 59 kilometers an hour winds). Since ACTUV will always have humans monitoring it and able to take control the vessel can be move out of the way of major storms (that go up to Sea State 12). The first ACTUV (called Sea Hunter) is now starting two years of sea trials. Initially there will be a small compartment on deck to hold a human operator who can intervene immediately is there is some unforeseen emergency. There will also be a manned ship accompanying ACTUV for the initial tests before ACTUV is turned lose on its own. It appears that ACTUV is built to operate in the Pacific (where the Chinese fleet is) or the North Atlantic (where the rebuilt Russian fleet is). If successful, mass produced ACTUVs will cost less than $25 million each and cost less than $20,000 a day to operate. That is less than a tenth of what a manned ASW vessel costs to build and operate.
Because ACTUV type vessels operate on the surface without crews there is the risk of someone getting on board and damaging the vessel or stealing technology (or the entire vessel). This is a known problem with USVs, which have been used to patrol harbors and coastal waters for years. Security measures have been developed which apparently work and because of that no one wants to provide any details.
ACTUV is a step from an earlier anti-submarine USV built to operate from a LCS (Littoral Combat Ships). Officially called the Fleet class ASWUSV (Anti-Submarine Warfare Unmanned Surface Vehicle), the 12.6 meter (39 foot) long boats weigh 8.5 tons and can carry 2.5 tons of sensors and other equipment. This USV can move up to 63 kilometers an hour and stay at sea for up to 24 hours. Most of the time, it would be moving slowly, using its sonar to search for subs. The ASWUSV is equipped with GPS and a computerized navigation system that allows it to automatically run search patterns. Thus the sailors controlling the boat remotely, can move it to an area that helicopter or aircraft dropped sonobuoys have picked up a contact, and pursue it more intensively with the more powerful sensors it has on board. Such pattern searching, worked out with algorithms derived from experience (with what subs can do), have been a successful tactic since World War II. So far ASWUSV has spent over 2,000 hours at sea operating autonomously.