Underwater noise created by ships poses a danger to marine life. Military vessels, the oil and gas industry and shipping traffic all contribute to sound that can interfere with the ability of marine animals to hear, navigate, communicate and catch prey. The United Nations recognized this problem in 2018 at its Convention of Migratory Species and called for more research on the impact of underwater radiated noise (URN) and for efforts to mitigate ocean noise.

University of Strathclyde (Glasgow) researchers have adapted existing equipment to develop a waterproof aerial drone capable of measuring this noise. The Hydrone is equipped with a SoundTrap hydrophone recorder to measure the underwater radiated noise generated by marine craft propulsion systems.

The device can deploy from a boat, land on the sea, power off and float while the SoundTrap hydrophone suspends below it. After capturing the necessary data, it is flown back to the boat. Its unique configuration minimizes extraneous background noise from tidal current cross-flow, a problem that affects traditional weighted-line systems with a hydrophone attached to a buoy or support vessel.

The system can be launched quickly and inexpensively to sites 5 m to 1.5 km from the target vessel and Hydrone measures underwater radiated noise generated by marine craft propulsion systems. easily recovered, due to the aerial platform. The vessel does not have to stop as it would to recover a heavier in-water drone or buoyed system.

"Within this context, technology like the HyDrone can simplify the practical measurement of the URN from ship propellers and other sources. The unit can operate wherever the target ship is operational and so can measure noise levels in shallow waters," said Professor Patrick Fitzsimmons of the Department of Naval Architecture, Ocean and Marine Engineering.

The Hydrone can measure near-field noise next to the propeller and behind the vessel and can also record temperature and salinity with its on-board low-light cameras and observe a ship propeller’s tip vortex cavitation (the creation of vapor-filled cavities in the water). This type of cavitation induces cold-boiling, which results in blade erosion, vibration and noise when the cavities implode. The data gathered will validate advanced computational fluid dynamics simulations of cavitation-induced noise.

The Hydrone was tested at 10 mm immersion against a standard vertical array of tethered hydrophones in a trial that confirmed its capability to monitor background noise.