While unmanned aircraft like the RAF’s Reapers get plenty of media attention, seagoing drones have recovered far less coverage.
Unmanned boats, known as Unmanned Surface Vehicles (USVs), have been quietly entering service around the globe – and some of them have already been used in action.
In May, the Royal Navy took delivery of its first autonomous minesweeper.
Ever since navies started clearing mines, minesweeping has been a cat-and-mouse game, with new weapons specially designed to destroy the minesweepers attempting to clear them.
Being able to get people out of the minesweeping boat is the maritime equivalent of using tracked robots for remote bomb disposal.
The new minesweeper is 11 metres long and weighs six tons. The hull is made of glass-reinforced plastic; non-metallic hulls are preferred for this role as they do not set off magnetic mines.
It can tow a variety of mine-clearing systems with acoustic, magnetic and electric sensors, as well as devices which emit signals to trick mines into exploding harmlessly.
One thing the new vessel lacks is a catchy name like Predator or Reaper. It’s known as the Mine Countermeasures and Hydrographic Capability Combined Influence Minesweeping System.
The Israeli Seagull, unveiled in 2016, is similar to the British unmanned boat, but may take on a wider variety of roles, some of them offensive as well as defensive.
It can operate effectively in Sea State 4 – ‘moderate’ seas with waves of up to 2.5 metres – and can survive Sea State 7 with waves of up to nine metres.
Seagull is modular and can be fitted with a wide variety of gear depending on the mission. The mine-hunting version includes a tethered robot submarine for closer investigation of underwater objects, and expendable explosive "mine disposal vehicles” to destroy them.
An alternative fit for anti-submarine warfare equips Seagull with an advanced sonar suite. Optional armaments include a remote-controlled 12.7mm machine gun for operations such as counter-piracy, and even torpedoes.
A Mission Control System, which may be mounted on a ship, allows operators to control two Seagulls simultaneously from sixty miles away on missions lasting for four days.
Even with no communication link to the operator, Seagull can avoid collisions and obey international navigation rules.
The biggest and most ambitious military unmanned vessel is currently the American Sea Hunter, developed by the Defense Advanced Research Projects Agency (DARPA).
This is a high-speed trimaran 40 metres long, with a slim hull and two outriggers for stability.
Sea Hunter is intended to carry out the whole process of locating and trailing submarines without human assistance.
It can operate autonomously on the high seas, mingling with other sea traffic on voyages of up to 70 days, without a human ever setting foot on deck.
The idea is that, with its small size and lack of crew, Sea Hunter will be a fraction of the cost to acquire and operate off a manned anti-submarine frigate. Large numbers of such drone vessels could be acquired to assist more conventional forces.
Sea Hunter possesses sophisticated artificial intelligence to help it outwit human submarine commanders.
Some of the algorithms controlling it were ‘crowdsourced’ by DARPA – the agency set up an online submarine hunting game for players to develop innovative ideas. It is, however, unarmed.
Sea Hunter is undergoing sea trials until the end of this year, and the developers are already considering additional roles for the robot boat.
At the other end of the range of sophistication are the crude remote-control speedboats deployed by Houthi rebels in Yemen.
One such boat was captured by the Yemeni Navy last week. These are based on the 10-metre Al Fattan patrol boat, with two 200 horsepower outboard motors and a maximum speed of 45 knots (52 mph).
The remote-control system comprises automated GPS navigation to get the boat into the target area, and a video camera for lining up the attack run.
Each robot boat carries a giant 500-kilo explosive warhead taken from a Soviet-built Styx anti-ship missile. This is an armour-piercing shaped charge capable of inflicting tremendous damage.
In January 2017, one such boat hit the Saudi frigate Al-Madinah killing two sailors, and there have been several other attacks on warships. The unmanned boats have also attacked oil terminals.
Unmanned vessels are not about to make sailors redundant, but they are becoming increasingly useful as auxiliaries.
However, they may also become an increasingly dangerous threat.