The Saudi-led coalition said it intercepted and destroyed an unmanned explosives-laden boat launched from Yemen by the Iran-aligned Houthi group on September 19, 2019, an incident that could further increase regional tensions after the attack on Saudi oil installations. Since 2017, in fact, there have been several reports of attacks or discovery of these unmanned explosive vessels in the country.
"The coalition's naval forces detected an attempt by the terrorist Houthi militia backed by Iran to carry out an imminent act of aggression and terrorism south of the Red Sea using an unmanned, rigged boat ... launched from Hodeidah province," coalition spokesman Colonel Turki al-Malki said in a statement. Malki said that the foiled attack represented a threat to regional and international security and the safety of maritime routes and international trade. He did not specify the intended target.
There was no confirmation by the Houthi movement, which had claimed responsibility for the September 14, 2019, attacks on Saudi oil facilities. Riyadh has rejected the claim and said those strikes did not come from Yemen.
The Saudi response
The Saudi-led coalition on September 19, 2019, launched a military operation north of Yemen’s port city of Hodeidah against Houthi military targets. The coalition airstrikes came hours after it intercepted and destroyed a bomb-laden boat on the Red Sea. The coalition said it had destroyed four sites used in assembling remote-controlled boats and sea mines to help protect the freedom of maritime navigation. “These sites are used to carry out attacks and terrorist operations that threaten shipping lines and international trade in the Bab al-Mandab Strait and the southern Red Sea,” coalition spokesman Colonel Turki al-Malki said in a statement.
The Houthi movement in Yemen said through its Masirah TV that the coalition had breached an UN-brokered cease-fire deal that reached in Stockholm last year to halt fight in Hodeidah, saying that they are ready to confront "any possible military escalation.
The unmanned craft of Iran and the Houthis
The IRGCN has concentrated on acquiring and developing small fast boats: patrol boats, commando boats, missile boats, torpedo boats and fast attack crafts. Those developments have included unmanned surface crafts. The unmanned fast boats have been in the Iranian inventory since 2010. Booby-trapped boats are among the most significant modifications made by Iranian-backed Houthi militia.
According to a U.S. military assessment on threats from unmanned craft, “utilizing suicide unmanned surface craft is an asymmetric strategy which both allows Iran and it's proxies to compete on an uneven playing field and poses a risk by allowing operators to pick and choose targets of opportunity. The asymmetric advantage the Iranians have provided the Houthi’s have been instrumental in extending the conflict.
The "Ya Mahdi" unmanned boat
The Iranian Revolutionary Guard Corps Navy's "Ya Mahdi" unmanned surface vessel was first publicly described in 2010 as a remotely controlled fast attack craft. During the Guard Corps exercise "The Great Prophet 5" held that April 2010 in the Straits of Hormuz, IRGCN commander Ali Reza Tangsiri said that "since the boat has high speed, it is less detectable by radar." 
The" Ya Mahdi "vessel is based on the civilian Bladerunner 51 speed boat, of which manned variants serve with the IRGCN. At least one Bladerunner was illegally sold to Iran in a sophisticated smuggling operation. The successful smuggling operation was well-detected by Western intelligence, but the continued supply of engines and other equipment for Ya Mahdi construction has never been satisfactorily explained.
The Bladerunner 51 is a fiberglass, Kevlar and carbon-fiber boat, 9.8-meters long that runs at 35 knots (40 mph) and is powered by two large outboard engines. The Navy of the Islamic Revolutionary Guard Corps produces unmanned surface vessel s in Iran. The technology is simple and this relatively low-tech vessel is stuffed with explosives, usually C-4 in the form of a shaped charge in order to penetrate the hulls of larger commercial and military ships.
The conflict in Yemen has seen the development of unmanned boat bombs by Houthi militants, manufactured with Iranian assistance.  There are at least two different types of Houthi remote control boats.
The Shark – 33
The Shark 33 is an unmanned surface vehicle developed by Iran and exported to Yemen's Houthi rebels. The vessel is propelled by outboard motor and filled with explosives for remote attacks on surface ships. A Shark 33 was allegedly used for the first time in an attack in the Red Sea on the Saudi Arabian frigate HMS al Madinah in January 2017 that killed two sailors and wounded three.
On December 14, 2017, Laura Seal, a US Defense Department spokesperson, displayed Shark 33 guidance components at Joint Base Anacostia-Bolling that had been seized by the United Arab Emirates. “There are more than half a dozen pieces of evidence demonstrating that these components are directly traceable to Iran,” she said.
In January 2018, Saudi Coalition forces captured a Shark 33 unmanned surface vehicle after a failed attack. The boat was a former UAE Coast Guard boat that had been donated to the Yemeni Navy and later captured by the Houthis. Many of the components used to transform the vessel into an unmanned platform were made in Iran.
The boat powered by two 200 horsepower Yamaha outboard motors and fitted with a camera, hydraulic steering, GPS antennas and a navigation computer housed in a waterproof container. Furthermore, researchers found that the vessel was equipped with the warhead of a P-15 Soviet-manufactured anti-ship missile, supplied by the Soviets to Yemen in 1989.
The second type of unmanned boat has been discovered on September 8, 2018, off of Fasht Island on Yemen’s western coast. The remote-controlled explosive boat belonging to Houthi rebels appears to be a much smaller and more heavily modified vessel, designed to have a lower profile on the water and exploit its stealthy characteristics to approach its target.
The boat is powered by a single 200 horsepower Yamaha outboard motor and reportedly had several different explosive charges placed in wooden boxes throughout the hull. According to a press conference held by Arab Coalition forces, the boat was fitted with a GPS system and had numerous electronic components. 
Houthi unmanned crafts attacks
Over the past three years, Saudi Arabia and the UAE have faced a number of attacks by the Houthis, using those boats targeting military and commercial targets including oil tankers:
On July 9, 2019, naval forces from the Saudi-led coalition in Yemen foiled an attempted attack on an unidentified commercial ship in the southern Red Sea by the Iran-aligned Houthis. Spokesman Colonel Turki al-Malki said the coalition had destroyed an unmanned boat laden with explosives which the militants had used for the attack. A Houthi military spokesman denied targeting commercial shipping in the area, the group's Al Masirah TV reported, calling the claims "pure slander and completely baseless".
On September 9, 2018, Yemeni naval forces foiled a bombing operation in which Houthi militias were planning to attack international shipping in the Red Sea with a booby-trapped boat. An improvised explosive device was planted on the boat and was speeding on international waters in the Red Sea. Yemeni naval forces were able to stop the boat and force it toward one of the uninhabited islands close by.
On August 23, 2018, the Arab coalition foiled an attack by Houthi militants using a booby-trapped speed boat full of explosives to target commercial vessels. The boat was launched from the shore near the port of Hodeidah.
On July 8, 2018, the Houthis attacked two Saudi tankers in the Red Sea, one of which sustained minimal damage. The Houthis used unmanned booby-trapped Blue Fish boat. Riyadh responded by temporarily suspending oil shipments through the strait, which the Houthis have in the past threatened to block.
On August 16, 2017, Yemeni forces destroyed an explosives-laden boat targeting a United Arab Emirates military vessel in Al-Mocha port. The Yemeni forces eliminated the threat before it reached the docked ship.
On July 29, 2017, Arab coalition forces in Yemen announced that the Houthi militias targeted Al-Mocha port with a remote-controlled booby-trapped boat. The boat hit a pier near a group of ships where it exploded but caused no losses.
On April 25, 2017, Saudi Arabian border guards in Jazan foiled a terror plot attempting to blow up a Saudi Aramco oil depot and distribution station using a booby-trapped boat. The boat was spotted at the time of its launch from one of the small islands in Yemeni waters. The boat’s speed increased to 34 knots when it entered Saudi waters and it was headed to the petroleum products distribution station. When the border patrols intercepted the boat, it became clear that there were no people on board and that it was remote controlled. The guards decided to fire at its engines and disable them before the boat could reach its target.
On January 30, 2017, a Saudi frigate Al Madinah came under attack while it was on patrol west of Yemen’s Hodeidah port. It was the first attack by an unmanned, remote-controlled craft in the Red Sea. A Houthi boat collided with the rear of the Saudi warship, resulting in the explosion of the boat and a fire at the rear of the ship. The crew was able to extinguish the fire but two members of the ship’s crew were killed and three others were injured. The Saudi frigate presumed military mission, enforcing a blockade on the nearby Houthi-controlled port of Hodeidah.
Since 2015, strategic sea trade routes near Yemen have come under increasing threats and vessels near Yemen’s coast have been attacked by Houthi militants. The Houthi rebels used different tactics and weapon systems including: anti-ship missiles, armed speed boats, remote-controlled booby-trapped boats and naval mines, all supplied to the Houthis by Iran.
Iran has provided the Houthis with advanced capabilities and Yemen has been a testing ground and a battle laboratory for Iran- where it can test out these new capabilities.
One of the weapons is the unmanned explosives-laden boat. A cheap and lethal system in the asymmetric naval warfare. Because the boats are primarily fiberglass and running close to the surface, picking them up on radar is difficult. The best way to detect them is with modern electro-optical sensors, but even these sensors have limited record against them. 
The Houthis, who have threatened to widen attacks on Saudi Arabia, have in the past targeted vessels off Yemen, which lies on one side of the Bab al-Mandeb strait at the southern mouth of the Red Sea, one of the world's most vital oil tanker routes.
The last incident came as the United States and Saudi Arabia consider responses to the assault on Saudi oil facilities, which Washington and Riyadh blame on common foe Iran. Tehran denies any involvement.
The escalating tension could hamper United Nations efforts to implement a stalled troop withdrawal deal in the main port of Hodeidah, a lifeline for millions of Yemenis, which was agreed at peace talks in Sweden last year.
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