# Sea Drones Have a Voice Too: Detecting Sea Drones and Small Boats with Hydrophones

> Uncrewed attack boats have sunk warships and a merchant ship. They are low and small for radar, but their engines and propellers are loud underwater.

- Published: 6 November 2026
- Author: Ghulam Mujtaba, Ph.D. (https://drosonic.com/about.html)
- Publisher: DroSonic OSINT (https://osint.drosonic.com/), the research arm of DroSonic (https://drosonic.com/)
- Topics: Maritime, Acoustic Detection, Ukraine
- Confidence: moderate
- Web page: https://osint.drosonic.com/assessments/sea-drones-underwater-acoustic-detection.html
- Cite as: Mujtaba, G. (2026, November 6). Sea Drones Have a Voice Too: Detecting Sea Drones and Small Boats with Hydrophones. DroSonic OSINT. https://osint.drosonic.com/assessments/sea-drones-underwater-acoustic-detection.html
- Reuse: Text and original diagrams © DroSonic. You may quote this report and share its link with credit to DroSonic OSINT; to republish it in full, reuse our diagrams or use it commercially, contact us first. Photos and third-party figures keep the licences shown in their credits.

## Key questions

**How can you detect a sea drone?**
Radar struggles with small, low uncrewed boats, and they carry no AIS transponder. Their engines and propellers, though, produce steady underwater tones that hydrophones can pick up: a low-power Stevens Institute buoy detected a small boat without AIS at 1.1 km in the shallow Hudson River, and another boat's track began 8.7 km away in an earlier recording.

**Can hydrophones detect small boats automatically?**
Yes. A deep-learning method applied to about 171,000 two-minute recordings from a South Carolina estuary identified vessel noise with around 99% accuracy against human review, after filtering out snapping-shrimp noise.

**What sinks have sea drones caused?**
Ukrainian sources credit Magura V5 sea drones with eight Russian vessels, including the corvette Ivanovets, the landing ship Tsezar Kunikov and the patrol ship Sergey Kotov in early 2024. In June 2024 a Houthi uncrewed surface vessel struck the merchant ship Tutor in the Red Sea, which sank.

## Key numbers

- **8** Russian vessels credited to Magura V5 sea drones (Ukrainian claims, 2023–2024)
- **1.1 km** small boat without AIS detected by a 1 W buoy (shallow Hudson River, radar-confirmed)
- **8.7 km** start of one boat's track in an earlier recording (Stevens Institute)
- **~99%** vessel detection accuracy over 171,262 recordings (South Carolina estuary, vs human review)

## Bottom line

**Sea drones are hard to see and easy to hear.** Uncrewed attack boats ride low in the water, carry no transponder and present radar with a small target among the waves, which is why they have been able to reach and sink warships in the Black Sea and a merchant ship in the Red Sea. But they cannot run silent: an engine and a propeller pushing a boat at speed put a steady, recognisable signature into the water. Research on passive hydrophones shows how well that signature carries. A Stevens Institute buoy drawing 1 watt detected a small boat with no transponder at 1.1 km in a shallow, busy river; in an earlier recording a boat's track began 8.7 km out; and a deep-learning method picked vessel noise out of three years of estuary recordings with around 99% accuracy. None of these trials involved combat sea drones, and shallow or noisy water cuts range sharply. But the evidence points to cheap, networked hydrophones as a credible early-warning layer for ports, anchorages and coastal infrastructure.

## What each sensor sees of a sea drone

| Sensor — at sea | What it gives | Against a small uncrewed boat |
|---|---|---|
| AIS — the ship's own transponder | Identity and position of cooperating ships | Nothing: small boats need not carry it, and attackers will not |
| Coastal radar | Tracks on the surface | A low, small craft in wave clutter is hard to hold; masts and power needed |
| Cameras — visible and thermal | Identification once seen | Line of sight, weather and darkness limit it |
| Hydrophones — underwater sound | Detection and bearing from engine and propeller tones | Passive, cheap, works at night; range shrinks in shallow, noisy water |

## Small boats that sink big ships

Ukraine turned the uncrewed surface vessel into a weapon of attrition. Its defence intelligence agency says six Magura V5 sea drones attacked the Russian missile corvette Ivanovets on 1 February 2024 and sank it. ([Defence Intelligence of Ukraine](https://gur.gov.ua/en/content/morski-drony-hur-dominuiut-u-chornomu-mori)) Ukrainian claims credit the Magura V5, a 5.5-metre boat with a 300 kg payload, with eight Russian vessels between November 2023 and August 2024, among them the landing ship Tsezar Kunikov and the patrol ship Sergey Kotov; the larger Magura V7 was reported in May 2025 to have shot down two Su-30 fighters with missiles. ([Wikipedia](https://en.wikipedia.org/wiki/MAGURA_V5))

The weapon has spread. On 12 June 2024 a Houthi uncrewed surface vessel struck the stern of the bulk carrier Tutor in the southern Red Sea; the crew was airlifted off by the U.S. Navy, one sailor was reported missing, and the ship sank. ([gCaptain](https://gcaptain.com/m-v-tutor-sinks-after-houthi-attack-in-red-sea/))

## No transponder, little to reflect

Coastal surveillance relies mostly on two things: radar, which needs significant infrastructure to deploy and still finds small boats challenging, and AIS, the transponder system larger ships must carry. The small, low-profile boats used for smuggling and other illicit activity belong to a class not required to carry AIS. Researchers at Stevens Institute of Technology, who build passive acoustic systems for maritime surveillance, describe the kamikaze sea drone as a similar but more dangerous target: a craft with a low radar cross-section for which acoustics may be one of the best means of detection. ([IEEE HST 2022](https://doi.org/10.1109/HST56032.2022.10025447))

## Engines leave a signature

An engine and propeller put narrow, steady tones into the water. Stevens researchers report that small-boat engine sound is tied to the propeller's blade-passing frequency, between about 8 and 130 Hz, and to the shaft rate, between 8 and 45 Hz; water-jet craft, such as personal watercraft and some fast boats, emit higher frequencies still. Their processing looks for those harmonics, and for the rhythm they impose on broadband noise (a technique called DEMON), and tracks them over time so that intermittent sounds such as a cable twanging in the current are ignored. ([IEEE HST 2022](https://doi.org/10.1109/HST56032.2022.10025447)) It is also the principle DroSonic applies in air: its sensors listen for aerial drones' propellers, not boats. ([DroSonic](https://drosonic.com/#technology))

*Figure: What a hydrophone hears, and what shallow water takes away.* Diagram of underwater frequency bands for a small boat on a logarithmic axis: shaft rate 8 to 45 Hz, propeller blade-pass 8 to 130 Hz, water-jets higher still, with energy below about 150 Hz lost in 5 m deep river water; detection ranges of 8.7 km in an earlier recording and 1.1 km in the shallow Hudson River. The same tones that make a boat detectable are the ones shallow water filters out, which is why range varies so much from site to site.

This is the same principle as listening for an aerial drone: propulsion makes tones, and the tones identify the propulsion. Our report [Every Drone Has a Voice](https://osint.drosonic.com/assessments/every-drone-has-a-voice.html) sets out the aerial version.

## A buoy, a river array and three years of estuary sound

**A low-power buoy.** Stevens built a "smart acoustic buoy" from off-the-shelf parts: an in-house hydrophone, a microcontroller that processes sound on board, GPS, and a long-range radio link to shore. It draws about 1 watt and runs for roughly five days on a 121 Wh battery. Moored in about 5 metres of water in the Hudson River in August 2022, it detected a small boat with no AIS at 1.1 km, confirmed by radar. The shallow water stripped out most energy below about 150 Hz, so the harmonic method worked less well there, but the DEMON method stayed effective. In an earlier recording of a 25-foot outboard boat, the tracked tone began when the boat was 8.7 km away. ([IEEE HST 2022](https://doi.org/10.1109/HST56032.2022.10025447))

**A river array.** Stevens' SPADES-2 is a low-cost, bottom-mounted ring of eight hydrophones 2.2 metres across, with a long cable for power and data. Its software finds the direction of sound sources in real time and separates overlapping ones. It was tested for seven months tracking boats in the Hudson River. ([Stevens Institute of Technology](https://researchwith.stevens.edu/en/publications/stevens-passive-acoustic-detection-system-spades-2-and-its-prospe/))

**Three years of estuary sound.** Researchers in South Carolina applied a two-step deep-learning method to recordings from three hydrophone stations in the May River estuary: about 171,262 two-minute files from 2017, 2018 and 2021. Compared with human review of each file, it identified the presence of vessels with an average accuracy of around 99%. The recordings were full of competing sounds, from fish choruses to snapping shrimp, which had to be filtered out first. ([PLOS ONE](https://doi.org/10.1371/journal.pone.0302497))

*Figure: May River recorders, before and after a deployment.* Two photos: six cylindrical underwater sound recorders in black metal frames on a boat deck, and a scientist lifting one barnacle-covered frame out of murky estuary water. The estuary network's hydrophone recorders, which logged sound for years at fixed stations.

*Figure: Four boat passes, four different shapes.* Four spectrograms of two-minute underwater recordings, each with a dashed ellipse around a boat's sound: a narrow vertical burst, a wider burst, a long variable broadband patch, and a faint low-frequency band. Spectrograms of boat noise in the estuary. A detector has to recognise all of these, among shrimp and fish.

## Shallow water, busy water, no combat data

- **No sea-drone trials in the open record.** Everything above concerns ordinary small boats. Military sea drones may be quieter or louder; water-jet propulsion would move their sound higher, where the Stevens work found different processing necessary.
- **Range depends on the water.** The same buoy hardware heard one boat 8.7 km away and another only 1.1 km away in shallow river water. Harbours, the places most worth protecting, are often shallow and noisy.
- **Detection is not identification.** A hydrophone hears a boat; knowing it is hostile needs AIS records, radar, cameras or patrols. Even the Stevens buoy trial relied on radar, AIS and cameras on shore to confirm what it heard. ([IEEE HST 2022](https://doi.org/10.1109/HST56032.2022.10025447))
- **Lab-to-field gaps apply here too.** A model trained in one estuary may not transfer to another port; models learn the background sounds of the place they were trained.

### Confidence: How sure we are, and what would change it

**High confidence** that uncrewed surface vessels have sunk warships and a merchant ship, based on official statements and multiple reports, though the full Magura V5 tally rests on Ukrainian claims.

**Moderate confidence** that passive hydrophones can provide useful early warning against small attack boats. The physics and the small-boat results are solid; direct evidence against combat sea drones is not public.

**What would change it:** published trials of hydrophone networks against military uncrewed surface vessels, or evidence of sea drones designed for low underwater noise.

## The same principle, in air

DroSonic does not build hydrophones. Its sensors listen in air, for the propellers of aerial drones, using neural networks on low-cost edge hardware. ([DroSonic](https://drosonic.com/#technology)) We include this report because the physics is shared: propulsion makes tones, tones identify propulsion, and passive listening works at night and emits nothing.

A coastal site that faces drones on the water and in the air would need both layers: hydrophones below the surface and microphones above it. A DroSonic prototype was tested against live aerial drones at DIRT Days 26.04 in September 2026. ([DroSonic](https://drosonic.com/news/dirt-days-field-test.html))

## Listen below the surface

The sea drone repeats a pattern familiar from the air. A cheap, uncrewed craft defeats the sensors built for bigger, cooperative targets: it carries no transponder, offers radar little to reflect, and arrives low and fast. And like the aerial drone, it cannot hide what drives it. Propellers make tones; tones carry. For a port or an anchorage, a ring of low-power hydrophones is an inexpensive way to hear a fast boat coming from beyond visual range, especially at night.

At [DroSonic](https://drosonic.com/) we work on the airborne half of that problem: [passive acoustic detection](https://drosonic.com/#technology) of drones by their rotor sound. The principles carry across the waterline. A coastal site facing both threats needs both kinds of ears, with each feeding the same picture so that operators can see a fast boat and the drones flying above it in one place.

## Related reports

- [Every Drone Has a Voice: How Drones Sound: FPV, Shahed, Lancet and Jet](https://osint.drosonic.com/assessments/every-drone-has-a-voice.html): Radar sorts drones by size, radio detectors by their links. A microphone sorts them by what turns the propeller, into five families with five warning times.
- [What a Microphone Can Hear: Acoustic Drone Detection Explained](https://osint.drosonic.com/assessments/acoustic-drone-detection-explained.html): Acoustic sensors detect drones by the sound they can't switch off. How they work, how far they hear, and where they beat radar and RF.
- [Listening to the Sky: Acoustic Drone Detection: WW1 to Sky Fortress](https://osint.drosonic.com/assessments/listening-to-the-sky.html): Ukraine rebuilt a century-old idea, listening for aircraft, with thousands of cheap microphones. How it works, where it fails, and how the U.S. could use it.