Houthi USVs: A Rising Threat to Global Trade

The Houthis’ campaign against merchant shipping is driven by a combination of ideological motivations and internal political pressures. Their anti-Israel stance plays a significant role, as reflected in their official slogan, which includes “Death to Israel, A Curse Upon the Jews.”

Additionally, internal challenges, such as declining domestic support due to the administration’s failure to provide basic services and enforce civil liberties, have likely pushed the Houthis to adopt aggressive external actions. By uniting the population against an external enemy, the Houthis aim to bolster domestic support.

The Bab al-Mandeb Strait is a crucial chokepoint in global maritime trade, connecting the Red Sea with the Gulf of Aden. Approximately 12% of global shipping passes through this narrow strait, making it a prime target for Houthi attacks. The alternative route around the Horn of Africa is not viable for most shipping due to the significant increase in transit time and fuel costs.

The impact of Houthi attacks has been substantial. Freight rates on routes between Asia and Europe have skyrocketed by as much as 284%, contributing to global inflation and increasing costs for consumers worldwide. The threat to maritime trade has prompted a US-led coalition to launch Operation Prosperity Guardian in December 2023 to counter the Houthi threat, although the frequency and scale of attacks remain high.

Uncrewed surface vehicles (USVs) offer several advantages in maritime warfare, particularly for groups like the Houthis. Unlike aerial drones or missiles, USVs are harder to detect on radar due to their small size and low profile. They can carry large explosive payloads and strike at a vessel’s waterline, causing significant damage. Additionally, USVs are cheaper to produce and maintain, allowing the Houthis to deploy them in large numbers.

The Houthis have used various types of USVs, including converted patrol boats, weaponised skiffs, and purpose-built designs like the Toofan series. These USVs are primarily assembled in Yemen, using components likely supplied by Iran. This local production capability makes it difficult to disrupt their supply chain, allowing the Houthis to continue their maritime operations with minimal external support.

Since November 2023, the Houthis in Yemen have engaged in a campaign against merchant vessels transiting the Red Sea, Bab al-Mandeb and the Gulf of Aden. This campaign has resulted in global ramifications due to its impact on maritime trade.

Through most of the campaign thus far, the Houthis had relied primarily upon missiles and uncrewed aerial vehicles (UAVs, or drones) to conduct their attacks on merchant shipping. This changed on 12 June, when a one-way attack uncrewed surface vessel (OWA-USV, informally known as a kamikaze drone boat) successfully attacked the MV TUTOR (IMO: 9942627), disabling the vessel. The TUTOR was subsequently sunk, bringing international attention to this new dangerous threat to maritime shipping – the USV.

FROM YEMENI CONFLICT TO RED SEA ATTACK

The Houthis, officially named Ansar Allah, are a Shia militant group controlling one-third of Yemen’s territory and 70-80 per cent of its population. Funded and armed by Iran, the Houthis possess an extensive arsenal of advanced weapon systems.

The Yemeni Civil War between the Houthis and the Republic of Yemen, backed by a Saudi Arabian-led coalition, began in 2014 when the Houthis seized control of Yemen’s capital, Sana’a. In 2022, a UN-brokered ceasefire curtailed the fighting, a truce that has informally held since, with negotiations ongoing. During this time, the Houthis have engaged in tentative state-building in their occupied territory.

Following the 7 October 2023 Hamas-led assault in Israel, Houthi leader Abdul Malik al-Houthi expressed intentions to join the conflict against Israel. Subsequently, on 19 October, the Houthis launched missiles and one-way attack uncrewed aerial vehicles (OWA-UAVs) towards Israel, marking the beginning of their involvement in the war.

In November, the Houthis announced that they would target Israeli-linked merchant shipping transiting the Red Sea and other adjacent waters. Since then, their campaign against merchant shipping has used advanced military hardware, including UAVs, anti-ship ballistic missiles (ASBMs), anti-ship cruise missiles (ASCMs), and USVs, posing significant risks to merchant vessels that are inadequately equipped to counter such threats.

While the Houthi anti-shipping campaign is purportedly targeted at vessels associated with Israel, many merchant vessels have been targeted that have no such associations or attribution. These even include vessels carrying Russian and Chinese cargo, and in one instance, a vessel destined for Iran. The indiscriminate nature of Houthi attacks has substantially raised the risk for vessels associated with any nation transiting in range of Houthi weapon systems.

While the Houthis’ attacks on merchant shipping are in part ideologically driven by their vehement anti-Israel stance, with “Death to Israel, A Curse Upon the Jews” being part of the Houthi official slogan that is displayed on their flag, the campaign has likely also been conditioned by internal considerations.

A key cause for their aggressive actions is likely to be declining domestic support –  the Houthi administration’s failure to provide basic services and refusal to pay public sector salaries, while simultaneously enforcing increasingly severe restrictions on civil liberties, has resulted in declining domestic support.

However, the Houthis have bolstered their domestic support by uniting the population against an external threat. This is likely to be a key reason for the Houthis’ aggressive anti-merchant shipping campaign.

The Bab al-Mandeb Strait connects the Red Sea and the Gulf of Aden, situated between Yemen,  Djibouti and  Eritrea.  Merchant vessels travelling between the Indian Ocean and the Mediterranean Sea, via the Suez Canal, must transit through the Bab al-Mandeb.

This makes the strait a critical maritime chokepoint for global shipping, with 12 per cent of all global shipping passing through it. At only 20 miles wide, the Bab al-Mandeb offers the Houthis a highly advantageous position to conduct attacks on maritime traffic.  The alternative shipping route involves travelling around the Horn of Africa, which would entail drastically increased transit time and fuel costs.

On the most affected maritime shipping routes, between Asia and Europe, container freight rates have increased by as much as 284 per cent. Such costs have a knock-on effect on the consumer, contributing to already problematic global inflation. Beyond the direct threat to seafarers, the Houthi campaign has global implications for maritime trade.

In  December  2023,  a  US-led coalition launched  Operation  Prosperity  Guardian to counter the Houthi threat to maritime commerce. Coalition warships have been successful in thwarting many Houthi attacks. However, the scale and frequency of Houthi attacks have kept the threat level high.

The Houthis have launched hundreds of missiles (primarily ASBMs) and UAVs at merchant vessels. Most of these attacks either miss or are intercepted by coalition vessels.  However,  approximately  40 merchant vessels have either been directly hit or damaged by Houthi attacks, with four of these vessels being abandoned and/or sinking.

On 4 January, US Vice Admiral Brad Cooper reported a  USV  being observed within a “couple miles” of US naval forces and commercial shipping, before detonating. This marked the first identified use of a USV by the Houthis in their current campaign. In the following months, the United States Central Command  (USCENTCOM)  sporadically reported the destruction of USVs as part of Operation Prosperity Guardian.

On 12 June, the MV TUTOR (IMO: 9942627) was hit by a USV, which immobilised the vessel and killed one crewmember. The disabled vessel was subsequently struck with ASBMs and sunk.

The  incident  was  a  highly  successful operation for the Houthis, the first time the Houthis impacted a vessel with a USV and only the third vessel sunk since the Houthis began  targeting  merchant  shipping  in November 2023.

Since then, the Houthis have conducted a far greater number of USV operations. The systemic integration of  USVs into Houthi maritime operations has increased both the lethality and accuracy of Houthi attacks, substantially increasing the threat to merchant shipping.

The concept of using an uncrewed vessel as a weapon is by no means a modern phenomenon. In ancient warfare, wooden vessels were set alight and sent into the enemy fleet, called fireships. By the 17th Century, fireships would be loaded with combustible materials such as pitch, tar and gunpowder.

With the advent of iron and then steel hulls, fireships became obsolete, though uncrewed vessels packed with explosives were used in WWII. In the 21st Century, the use of explosives delivered by small craft has become a potent threat to vessels. In October 2000, al-Qaeda suicide bombers attacked the USS COLE anchored at Aden, Yemen, with a small fibreglass boat laden with C4 explosives, killing 17 sailors and causing significant damage.

Such weapon systems are typically referred to as water-borne improvised explosive devices (WBIEDs) – the small boat acts as an improvised delivery system for a powerful explosive payload.  A  USV,  however, specifically refers to a vessel operated without the need for a pilot, in at least the

terminal phase of attack. USVs are instead operated either autonomously or via remote control. While some USVs are sometimes called WBIEDs if they are improvised designs, such as converted skiffs, it is more accurate to refer to such weapons as USVs.

The Houthis are pioneers in using USVs. In January 2017, the Houthis attacked the Saudi frigate AL MADINAH in the Red Sea using crude  USVs,  the first operational use of one-way attack uncrewed surface vessels (OWA-USVs) in combat recorded in modern warfare. Since then, in the Russia-Ukraine war, Ukraine has used OWA-USVs to great effect against  Russia’s  Black Sea Fleet, forcing its retrograde from its traditional home port in the Crimean town of Sevastopol.

USVs have several advantages over aerial weapons in the maritime battlespace.

HARDER TO DETECT 

USVs are more difficult to detect than airborne projectiles. USVs travel along the surface and are slower, generally resulting in a smaller radar cross-section, making them harder to detect on radar systems. USVs also have a reduced infrared signature compared to missiles, making them less detectable by thermal imaging systems.

STEALTH

The profile of a USV is difficult to distinguish from civilian craft, and therefore challenging to counter due to rules of engagement or the law of armed conflict. The Houthis have exploited this to great effect, even using dummies to suggest that the USVs are crewed. Many USVs have a civilian profile, making them challenging  to distinguish from the skiffs and fishing boats that are ubiquitous in the region.

A critical vulnerability of missiles and many UAV systems is that they require a launcher which often has a military profile and can be targeted. USVs conversely can sit in port until they are required.

COST-EFFECTIVE PRODUCTION

USVs  are  generally  cheaper  to  both manufacture  and  maintain  than  missile systems, which means they can be cheaply produced in larger quantities and used more extensively.

WATERLINE DETONATION

USVs carry large explosive payloads that detonate at the waterline, which can result in substantial water ingress,  especially when using some form of shaped charge. A direct hit on a vessel’s weak point, such as the engine compartment, can immobilise a merchant vessel and make it extremely vulnerable to secondary strikes by missiles and UAVs.

The Houthis have used multiple different kinds of USVs, including both purpose-built and more improvised variants. While typically equipped with what are highly likely Iran-made components, particularly the more advanced hardware such as computerised guidance systems,  they are primarily assembled in Yemen. This would make the supply of USVs much harder to counter, as the Houthis can maintain and expand their USV fleet using local workshops.

All of the USV variants used by the Houthis observed so far are remote-controlled, as opposed to being fully autonomous. As opposed to the Ukrainian OWA-USVs used in the Black Sea, they all require human piloting in their initial phase. After manually guiding the OWA-USV towards the target vessel, the pilot disembarks, and the OWA-USV is then navigated via remote control for the terminal phase of the attack.

All Houthi USVs therefore have cockpits and depend on motherships to retrieve the pilots. These motherships can also act as command and control centres for USV operations, forward-deployed reconnaissance assets, and be used to tow USVs for long-range operations by minimising the USV’s initial fuel expenditure. In some Houthi USV operations, these secondary skiffs/motherships have been observed, though not always. While there has been speculation that the Houthis may have the capability to remotely control USVs from land, this has not been verified.

The Houthis have been observed using the following three types of USV:

CONVERTED PATROL BOATS

The USVs the Houthis used in 2017 against the  Saudi  AL  MADINAH  frigate were converted patrol boats. This first iteration of Houthi USV used conversion kits, likely supplied by Iran, to convert conventional patrol boats into OWA-USVs.

WEAPONISED SKIFFS

First observed in 2020, the Houthis have USVs that have the appearance of the skiffs and fishing vessels that are commonplace in the region.  These skiffs have been weaponised and equipped with systems that enable them to be remotely controlled. While these typically do not have the same range, speed or warhead size as larger purpose-built designs, they do have a civilian profile, complicating the targeting process of coalition forces and often confusing merchant vessel security teams. The MV TUTOR was immobilised by a weaponised skiff OWA-USV that had dummies onboard, and the ship’s guards were reportedly “confused as to what they were observing”.

PURPOSE-BUILT  DESIGNS  –  TOOFAN/ BLOWFISH

Serially produced, the Houthis have three variants of purpose-built USVs they call Toofan (Deluge), also known as Blowfish. Compared to weaponised skiffs, Toofans generally carry larger warheads,  have greater speeds, and are typically fitted with sensor masts.

The  Houthis claim that they used the newest variant,  the  Toofan-3,  against MV  TRANSWORLD  NAVIGATOR  (IMO: 9469924) on 23 June. The Houthis claim that the Toofan-3 has a 1000-1500 kilogram warhead, a warhead that, at the high end, would be 30 times as large as that carried by a typical Houthi Waid UAV, or three times as large as that found on a Houthi Asef anti-ship ballistic missile (ASBM).

The  Houthis employ a variety of both sophisticated and rudimentary intelligence, surveillance and reconnaissance (ISR) methods to conduct target acquisition in the Red Sea, Bab al-Mandeb and Gulf of Aden.

OPEN SOURCE INTELLIGENCE (OSINT)

Merchant vessels are legally mandated by the IMO under the Safety of Life at Sea (SOLAS) Convention, Chapter V, Regulation 19, to broadcast their location through Automatic Identification System (AIS) transponders. AIS broadcasts the name, position, navigational status, course, speed and other safety-related information of a vessel every 2 to 10 seconds. AIS data is publicly available on maritime traffic monitoring platforms, and the Houthis, particularly in the earlier period of their campaign, are suspected to have exploited this to inform their targeting cycle.

Firstly, however, AIS tracking’s accuracy is limited. It is likely that the poor hit rate of Houthi missile strikes, especially in the early stages, is reflective of this. Secondly, merchant vessels have adapted and now often switch off their AIS transponders when transiting near  Houthi-controlled  Yemen. IMO provisions allow for AIS to be switched off if its continual operation compromises the security of the vessel.

COASTAL RADAR

The Houthis have historically used coastal radar systems to coordinate strikes. Radar systems provide more accurate data on a vessel of interest’s position, speed and course.  However,  static targeting infrastructure is highly susceptible to coalition strikes.  Operation  Prosperity Guardian has significantly degraded Houthi coastal radar capability, and the Houthis are now thought to have limited to no maritime radar coverage.

IRANIAN INTELLIGENCE

It is highly likely that the Houthis have been provided intelligence by Iranian surveillance vessels, namely the Islamic Revolutionary Guard Corps  Navy  (IRGCN)-linked  MV BEHSHAD (IMO: 9167289). A converted cargo ship, the BEHSHAD is believed to be retrofitted with a range of ISR capabilities including signals intelligence (SIGINT) equipment,  radars,  onboard observers, small boats and reconnaissance UAVs. The BEHSHAD has been linked to multiple Houthi strikes against merchant shipping. However, in April 2024, the BEHSHAD was withdrawn to Bandar Abbas, Iran. It is likely that this was done due to the threat posed by Israeli retaliation strikes against Iran that could target the high-profile vessel. Iran’s maritime ISR capabilities in the region, therefore, have lessened considerably.

UAV ISR 

While one-way attack uncrewed aerial vehicles  (OWA-UAVs,  colloquially called suicide/kamikaze drones)  have been used by the  Houthis to conduct aerial strikes, the Houthis also deploy a variety of reconnaissance UAVs to carry out ISR operations.  There have been multiple instances of merchant vessels observing UAVs prior to coming under missile and/or USV attack.

Using open-source data, primarily sourced from USCENTCOM which has been conducting operations against the Houthis as part of Operation Prosperity Guardian, the following is a non-exhaustive list of identified USV usage by the Houthis in 2024 in their anti-shipping campaign:

Changing TTPs

The evolution of Houthi TTPs in the Red Sea, Bab al-Mandeb and the Gulf of Aden towards greater deployment of  USVs considerably increases the threat posed to merchant shipping. This is due to the increased accuracy and lethality of Houthi USV operations compared to Houthi aerial weapon systems, and the complications involved in detecting and targeting a USV.

Even with the advanced weapon systems the Houthis have at their disposal, a modern merchant vessel is a difficult target to sink. This is firstly due to their survivability, and secondly due to challenges in target acquisition.

Survivability of Merchant Vessels

Due to their enormous size and compartmentalisation,  modern merchant vessels have a surprisingly high degree of survivability,  even against military-grade weapon systems. Because of this, even a successful strike by a Houthi ASBM or UAV is unlikely to immobilise or sink a merchant vessel.

USVs are More Lethal

USVs challenge the survivability of merchant vessels as they strike at the waterline and can carry larger payloads than aerial weapons. Out of 34 reported incidents of damage to merchant vessels being caused by aerial attacks in the Red Sea, Bab al-Mandeb and Gulf of Aden, three of these resulted in either the sinking or abandonment of the vessel.

Out of four reported incidents of damage caused by USVs, one resulted in a vessel sinking,  the  TUTOR.  Although the sample size is limited, these numbers indicate a 9 per cent critical success rate for accurate aerial attack and a 20 per cent critical success rate for accurate USV attack. This suggests a far greater lethality for USV attacks.

Inaccuracy of Houthi Attacks

The accuracy of Houthi attacks has also been conspicuously poor. The level of ISR required for consistently accurate long-range missile strikes against moving targets is generally beyond the capabilities of the Houthis.

The Houthi use of open-source AIS tracking, coastal radar,  and  Iranian intelligence to conduct target acquisition has been diminished, further impeding Houthi ISR and therefore the accuracy of Houthi attacks. It is highly likely that the Houthis relied heavily on AIS tracking to conduct target acquisition in the earlier period of their anti- merchant shipping campaign. However, the poor tracking accuracy and cessation of AIS broadcasts, the degradation of coastal radar, and the withdrawal of the BESHAD have diminished the Houthis’ maritime ISR capabilities, which in turn has provided major challenges for the accuracy of their strikes.

Additionally,  Houthi  ASBMs and  ASCMs rely on complex electro-optical guidance systems to lock on to a target in the terminal phase. While it is likely that the Houthis have gained experience and therefore proficiency in exploiting the capability of their missile guidance systems, the Iran-provided homing seeker technologies they use are not nearly as sophisticated as comparable Western guidance systems. The guidance systems of  Houthi missiles likely have several disadvantages that reduce accuracy, including weather dependency, more limited nighttime capability, as well as issues with clutter and discrimination.

Why USVs are More Accurate

USVs offer a more accurate alternative weapon system for the Houthis. A USV does not need to rely as heavily on long-range targeting data. Remotely controlled by an operator, a USV can make real-time adjustments to its course and speed in the approach and, most importantly, in the terminal phase.

The  Houthis do not require nearly as comprehensive maritime intelligence data to successfully conduct an   OWA-USV attack.  A  forward-deployed grouping of Houthi vessels consisting of a mothership, support skiffs and USVs can have only an approximate location for a merchant vessel and then find, fix and maintain custody of the target vessel.

In addition,  manual remote-controlled guidance has proven to be demonstrably more reliable than the electro-optical guidance of Houthi missiles, which ensures far greater accuracy in USV attacks than missile attacks during the terminal phase once the target has been acquired.

Lack of Vessel Countermeasures

Merchant vessels are ill-equipped to consistently counter USVs. Even warships equipped with Close-In Weapon Systems (CIWS), point-defence autocannons that use high-calibre ammunition with exceptionally fast fire rates, can struggle against USVs, as demonstrated by the successful Ukrainian operation against the Russian Tarantul-class corvette IVANOVETS despite its CIWS.

A notable exception to this may be the case of the Liberian-flagged container ship MV PUMBA (IMO: 9302566). On 20 July, the  PUMBA’s security team ostensibly destroyed a Houthi USV with small-arms fire. The security team opened fire on the USV, which then detonated approximately 30 metres away from the vessel, enough distance for the resulting blast to cause no notable damage. It is possible, however, that the USV was remotely detonated.

This is in stark contrast to the case of the TUTOR, where the security team, baffled by what they were observing, took no defensive measures as the USV approached. It is likely, particularly with the well-publicised PUMBA incident,  that merchant vessel security teams will adapt to the increasing USV threat by using small-arms fire much more readily as a countermeasure.

However, it is likely that if the PUMBA’s security team did indeed successfully destroy the USV, the countermeasure will be difficult to reliably replicate. The USV’s approach in that instance gave the security team an ideal firing position, it is challenging to accurately engage such a target from a  moving  platform,  it  likely  requires  a high degree of luck for small-arms fire to cause detonation, and the civilian profile of many Houthi USVs are problematic for security teams adhering to complex rules of engagement.

It is highly likely that the Houthis will continue to use USVs in significant numbers against merchant vessels in the immediate future.

The main current limitations of Houthi USVs are their requirement for a human pilot in their initial navigation and their reliance on support vessels. In the future, the Houthis may develop USVs to be fully remotely controlled at long range, increasing their operational flexibility and reducing their reliance on support vessels.

It is likely that the Houthis have ramped up their production of USVs in local workshops, as suggested by their increasing levels of deployment. Evolving Houthi maritime TTPs towards greater USV use is highly likely to have led to a redistribution of resources to prioritise USV production.

An increased supply of USVs could enable the Houthis to use USV swarm tactics, the coordinated and simultaneous deployment of multiple USVs against a single target, as Ukraine has used to great effect against Russian vessels in the Black Sea. A swarm attack is far more likely to sink a target vessel, as multiple breaches of the hull can sink a vessel despite compartmentalisation.

The Houthis’ maritime ISR capabilities have been greatly reduced due to merchant vessels switching off AIS broadcasts, the destruction of coastal radar and the withdrawal of the Iranian spy vessel BESHAD. To make up for this, another future development could be the adaptation and use of Houthi USVs for ISR purposes.

The Houthis increasingly rely on UAVs to conduct maritime ISR, and UAVs have been frequently sighted before USV attacks. The purpose-built Houthi Toofans are already typically equipped with sensor masts, and the Houthis may develop alternative models that are provisioned with more sensors with greater capabilities. USVs would offer several advantages over UAVs for Houthi maritime ISR,  including increased endurance and stealth. The integration of reconnaissance USVs would enhance the coverage and fidelity of Houthi ISR, which could optimise target acquisition for Houthi missile, OWA-UAV and OWA-USV attacks.

Another development in Houthi maritime TTPs could be the increased use of uncrewed underwater vehicles (UUVs). Unlike USVs, UUVs are submersible, providing stealth advantages and the ability to detonate below the waterline. Iranian-supplied UUV components  destined  for  the  Houthis were intercepted by coalition forces on 15 February, and on 17 February USCENTCOM reported destroying a Houthi UUV.

However, UUVs are limited by their greatly diminished access to the electromagnetic spectrum while underwater, complicating their control.  UUVs must carry smaller payloads and almost always have significantly reduced range and speed. Initial observations also indicate that Houthi UUVs are likely wire-guided, which provides further limitations on their operational flexibility. It is therefore likely that the Houthis will continue to prioritise the use of USVs over UUVs, although they may attempt to integrate UUVs into more layered operations.

The evolution of Houthis’ TTPs, through the increased use and operational success of USVs, substantially increases the threat to merchant shipping in the Red Sea, Bab al-Mandeb and Gulf of Aden. This use of USVs is firstly more lethal, able to better counter the survivability of merchant vessels. Given the limitations of Houthi maritime ISR, the Houthi use of USVs has secondly proven to be more accurate than missile strikes.

It is highly likely that the Houthis will continue to employ their current modus operandi of using USVs to immobilise a merchant vessel before secondary strikes with missiles and UAVs.

Merchant    vessels    have    limited countermeasures  against  this  emerging threat, with an anti-piracy industry aimed almost  entirely  at  countering  boardings. While it is likely that vessel security teams may seek to emulate the success of the MV PUMBA’s security team in ostensibly destroying a USV with small-arms fire, it is unlikely that this will be a consistently replicable countermeasure.

The Houthis are highly likely to continue developing their USV technology, with the assistance of Iran, and in future may develop more capable and operationally flexible USV designs. Increased production of USVs, which is likely to be a priority for the Houthis, may also enable the future use of USV swarm tactics. The Houthis may additionally seek to develop reconnaissance USVs to supplement their increasingly reduced maritime  ISR  capabilities.  While the limitations of UUVs, compared to USVs, pose challenges for effective use, it is a realistic possibility that the Houthis may seek to integrate UUVs into their maritime operations.

The effectiveness of the  Houthis’  anti-merchant shipping campaign in causing delays through rerouting and higher freight costs, disrupting global supply chains and impacting consumer prices and inflation, is likely to be worsened by the enhanced accuracy and lethality of USV attacks.

Despite being a relatively small non-state actor, the Houthis have leveraged their geographic position and capabilities to have a global impact. It is likely that they will seek to emulate this success during any future crises in Israel-Palestine and the wider region. The Houthis’ threat to global trade,  intensified  by  the  deployment  of advanced weapon systems like USVs, will likely become a staple component of the asymmetric strategies and tactics employed by Iran’s Axis of Resistance to counter Israel and the West.