Denmark uses overwater drones to monitor the North Sea and Baltic Sea

The Danish armed forces are using four autonomous unmanned surface drones with fixed sails of the “Voyager” type from US boat builder Saildrone for a military surveillance mission. The deployment of the boats, which are powered by wind and electric motors, is initially limited to three months. The robot boats are to operate in the Danish waters of the North and Baltic Seas.

Since Russia's war of aggression against Ukraine, NATO has been on heightened alert in the North and Baltic Seas. Acts of sabotage, such as those on the Nord Stream pipelines and various underwater cables, are forcing the two seas to be monitored more closely. Denmark is doing this with autonomous surface drones. Two Voyagers left Køge Marina, a seaport 40 km south of the Danish capital Copenhagen, on Monday. Two of these boats had already set sail on July 6.

The Voyager boats are around 10 m long and have a fixed sail that can be adjusted using an electric motor to make optimum use of the wind for propulsion. This allows the drones to remain in use for longer because they do not have to rely solely on motorized propulsion and are energy-efficient. An additional drive is nevertheless installed. An electric motor, which is powered by a backup battery, is used when there is not enough wind for propulsion and the boat is in danger of becoming unmaneuverable. The boat obtains the energy for the battery from solar cells embedded in the surface of the hull.

On board are all kinds of autonomous systems that make the Voyager suitable for carrying out military surveillance missions on its own. These include autonomous control and navigation systems as well as various sensors such as radar, infrared and optical cameras, sonar and acoustic sensors. This enables the boats to collect data that can be evaluated using machine learning and artificial intelligence (AI) and provide a comprehensive picture of the surface and underwater situation. Specifically, the aim is to detect potentially threatening ships and operations at an early stage and forward these findings to a military base where a decision is made on a possible response.

Surveillance mission

Initially, the efforts will focus on monitoring underwater cables in Danish waters. Damage to maritime infrastructure has increased since 2023. Most recently, the fiber optic connection between Latvia and the Swedish island of Gotland was interrupted in January 2025. In total, at least eleven submarine cables have been damaged in the past two years. The two Nord Stream pipelines were previously destroyed in 2022.

All incidents are presumably due to acts of sabotage by hostile actors. Some of them are believed to have been caused by ships from the Russian shadow fleet, which consists of obsolete oil tankers whose origin is obscured by unclear ownership structures. This includes, for example, the tanker Eagle S, which flies the flag of the Cook Islands. The ship was detained and confiscated by Finnish security forces due to damage to an undersea cable in the Baltic Sea.

The four autonomous Voyager boats work with other surveillance operations, such as by ship and drones, to detect suspicious ship movements in the North and Baltic Seas. The autonomous boats are more durable and cheaper to operate than manned vessels.

(olb)