Those splendid brainboxes at DARPA - the Pentagon's in-house bazaar of the bizarre - have outdone themselves this time. They now plan an entirely uncrewed, automated ghost frigate able to cruise the oceans of the world for months or years on end without human input.
The new project is called Anti-submarine warfare Continuous Trail Unmanned Vessel (ACTUV), and is intended to produce "an X-ship founded on the assumption that no person steps aboard at any point in its operating cycle". The uncrewed frigate would have enough range and endurance for "global, months long deployments with no underway human maintenance", being able to cross oceans largely without any human input - communications back to base would be "intermittent", according to DARPA.
In particular, the automated warship would need to avoid crashing into other vessels as it prowled the seas on the business of the US government, a function normally performed by bridge watchkeeping officers. DARPA specifies that the ACTUV must be able to conduct "safe navigation at sea within the framework of maritime law" - that is the International Rules for Prevention of Collisions at Sea, aka "Rule of the Road", which Royal Navy officers have to memorise almost word-perfect.
Then, while weaving in and out of other ships, the crewless frigate must be able to stay on the trail of a well-nigh silent diesel-electric submarine running beneath the waves. Such subs are operated - albeit in small numbers - by various minor powers around the world, and are considered by some in the major navies to be a very serious threat.
DARPA's idea would be that every time such a sub put to sea or was otherwise at a known location, an ACTUV would be put onto its tail - freeing up hugely expensive manned ships and subs from routine shadowing work. The thinking is that following such a submarine is fairly easily done compared to finding it in the first place.
That might be true in this case, as DARPA specify that the ACTUV should be able to carry out "continuous overt trail of threat submarines", as opposed to following them secretly as manned US forces might. The robo-frigate would be able to simply get a lock on its prey using powerful active sonar, sending loud "pings" of sound into the sea and detecting the echoes from the sub.
It could then hang close on the sub's tail where active sonar tracking is easy, as it would have "propulsive overmatch" - ie it would be much faster. Nuclear submarines can be speedy enough to lose a surface ship in some circumstances, but this isn't feasible for a diesel-electric boat. Better still, there would be no need for expensive silencing on the ACTUV (of the sort seen on British Type 23 frigates, for instance) as it would expect to be using active sonar anyway.