Length |
126 metres |
Wide |
16 meters |
Draught |
7.5 metres |
Tonnage |
4,995 |
Propulsion |
quadruple-expansion steam engine with two boilers |
Armour |
– |
Armament |
– |
Mercant Ship Olterra |
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The tanker Olterra begins her life as the Osage, constructed in 1913 by Palmer’s Ship Building and Iron Co. Ltd on Tyneside, United Kingdom, for a German shipping company. In 1914, she is sold to the Standard Oil Company in New York and renamed Baton Rouge. The ship is a medium-sized tanker with a gross registered tonnage of 4,995, a cargo capacity of 7,600 tonnes, and dimensions of 126 metres in length, 16 meters in beam, and a draught of 7.5 metres. Powered by a quadruple-expansion steam engine with two boilers, it has a maximum speed of 9 knots.
Eleven years later, in 1925, the vessel changes hands again, being sold to the European Shipping Co. Ltd of London and rechristened Olterra. After passing briefly through the ownership of the British Oil Shipping Co. Ltd, Olterra is purchased in 1930 by Andrea Zanchi of Genoa.
As Italy enters the Second World War by declaring war on France and the United Kingdom, Olterra finds herself anchored in the Bay of Gibraltar, near Algeciras, Spain. Under Captain Amoretti of Imperia, the Olterra is interned in the Spanish port of Algeciras, only six miles west of Gibraltar, after its crew scuttles it by opening the Kingston valves on June 10th, 1940, the day Italy enters the war.
Xa Flottiglia MAS |
Between September 24th, 1940 and September 15th, 1942, there are six submarine-borne assaults on Gibraltar by the Xa Flottiglia MAS. Three of these result in the destruction or sinking of several Allied freighters, with a combined tonnage of approximately 40,000 tonnes. Three of the assaults are executed by Siluro a Lenta Corsa’s launched from the submarine Scirè, while the other two are carried out by combat swimmers of Gruppo Gramma.
Following the attacks carried out by Gruppo Gamma and the submarine Scirè, the commander of the Xa Flottiglia MAS recognises the limitations of employing a submarine as a mother ship for targeting Gibraltar. He concludes that establishing a covert base in neutral Spain offers a more practical solution. This eliminates the need to transport the craft via submarines from Italy, significantly reducing risks and operational delays. To initiate this plan, Antonio Ramognino, a Xa Flottiglia MAS operative, rents a bungalow along the coastal road near Algeciras, strategically located opposite a bay frequently used by Allied convoys as an anchorage. This property, later named Villa Carmela, becomes a base for combat swimmer operations. Ramognino, aided by his Spanish wife, encounters little resistance in setting up residence.
Between July and September 1942, frogmen operating from Villa Carmela successfully sink or damage five merchant ships using limpet mines. During these missions, Olterra serves as a vital observation post, enabling intelligence gathering and operational support for these underwater attacks.
Around the same time, Lieutenant Licio Visintini, a seasoned veteran of prior submarine incursions against Gibraltar, identifies Olterra as a potential covert mother ship for Siluro a Lenta Corsa’s. Under the guise of raising the scuttled vessel for sale to a Spanish buyer, a Xa Flottiglia MAS team, disguised as civilian workers, assumes control of the tanker. The ship is later raised and towed to Algeciras. Here it is moored alongside the outer mole near another interned Italian vessel. Now, “repairs” can commence. Two civilian crew members, who had remained on board alongside a Spanish guard for over two years to safeguard the Italian company’s ownership rights, assist with the operation.
Inside the ship, Lieutenant Visintini’s team modifies several cargo holds and a boiler room into workshops for assembling and maintaining human torpedoes. A concealed observation post is constructed on the forecastle to monitor Allied shipping in Gibraltar Bay. To maintain the appearance of legitimate activity, the team stages scenes of workers engaged in standard maintenance tasks, deceiving both British and Spanish authorities.
Essential components for the Siluro a Lenta Corsa’s and other equipment are covertly transported into Spain by Xa Flottiglia MAS operatives, disguised as materials for the ship’s refurbishment. Using a pretext of hull cleaning, the stern is flooded to lift the bow above the waterline, while the vessel is heeled to starboard. This allows a hidden opening to be cut into the port side beneath a tarpaulin approximately 1.8 metres below the waterline. Once the ship is levelled, the compartment behind the opening becomes a launch area for the torpedoes, which can exit and re-enter undetected. This hidden exit allows the human torpedoes to be launched directly from a flooding bilge beneath the workshop. The unit tasked with overseeing these operations is designated Squadriglia Ursa Major, named after the constellation.
The original crew is gradually replaced with Xa Flottiglia MAS operatives disguised as sailors, leaving only Captain Amoretti and the chief engineer, De Nigris, as civilian members. Despite the ship’s position under the watchful eyes of British agents and Spanish authorities, rumors of repair work and a potential sale provide cover for the activities on board. A Carabineros picket is stationed on the ship, ostensibly to ensure neutrality, but the Xa Flottiglia MAS team’s ingenuity prevents any suspicions from materializing.
By the summer of 1942, the Olterra is fully operational as a base for attacks on Gibraltar. Lieutenant Visintini commands the base initially, succeeded by Lieutenant Commander Notari after Visintini’s death in action. Three operations are launched from the Olterra.
BG 5 on December 8th, 1942. |
On December 6th, 1942, following their involvement in Operation Torch, a British naval squadron arrives in Gibraltar. The fleet includes the battleship H.M.S. Nelson, the battlecruiser H.M.S. Renown, the aircraft carriers H.M.S. Furious and H.M.S. Formidable, along with several escort vessels. In response, Italian Lieutenant Licio Visintini plans an ambitious mission using three Siluro a Lenta Corsa, each crewed by two divers. Their targets are strategically selected: Visintini and Petty Officer Magro aim for H.M.S. Nelson, Midshipman Manisco and Petty Officer Varini target H.M.S. Formidable, while 2nd Lieutenant Cella and Sergeant Leone focus on H.M.S. Furious.
In the early hours of December 8th, 1942, the torpedoes deploy covertly from the disguised tanker Olterra. At 02:15, the first Siluro a Lenta Corsa reaches the boom defenses surrounding Gibraltar’s harbour. British security is heightened, as prior attacks by Italian combat swimmers have prompted the deployment of a Royal Navy Mine Clearance Divirs unit, led by Lieutenant Lionel Crabb. Motor launches patrol the area, dropping depth charges every three minutes to deter intruders.
Visintini and Magro’s Siluro a Lenta Corsa encounters the steel net protecting the harbour and appears to be struck by a depth charge. The explosion destroys their craft, and their bodies are later recovered by the British, who bury them at sea with full military honors.
The second Siluro a Lenta Corsa, piloted by Cella and Leone, faces a similar challenge. A searchlight exposes their presence, and anti-submarine vessels pursue them aggressively. Unable to evade capture, the crew scuttles their craft and swims to a nearby American freighter. They abandon their swimsuits to disguise their identity before surrendering to British authorities in Gibraltar.
Meanwhile, the third Siluro a Lenta Corsa, manned by Manisco and Varini, initially evades detection but is caught in the heightened alarm. During the chase, Leone disappears and is presumed lost at sea. Cella, separated from his copilot, abandons the craft in the belief that he is either near Gibraltar or the Spanish coast. Surfacing with plans to surrender to either British or Spanish forces, he is shocked to find himself mere meters from the Olterra. His torpedo is salvaged by the Italians the following day.
The two captured divers mislead British interrogators, claiming their mission was launched from a submarine. This ruse successfully deceives Allied intelligence about the true origins of the attack. Despite the ingenuity of the operation, the mission concludes in failure, with none of the designated targets successfully engaged.
BG 6 on May 8th, 1943 |
After the death of Lieutenant Licio Visintini, Lieutenant Ernesto Notari assumes command of the Ursa Major unit aboard the Olterra. Reinforcements in personnel and equipment are dispatched from Italy to support ongoing operations. With Gibraltar’s boom defences strengthened, future missions are directed at merchant and logistics vessels anchored near the naval base. The conclusion of the North African campaign and the Allied landings in Sicily make targeting supply ships a strategic priority.
Lieutenant Notari plans a new assault for the night of May 8th, 1943, timing it to coincide with poor weather and a favorable moon phase. Leading a wave of three Siluro a Lenta Corsa’s, Notari is accompanied by his second-in-command, Petty Officer Ario Lazzari. The second Siluro a Lenta Corsa is manned by Lieutenant Tadini and Petty Officer Mattera, while the third is piloted by Second Lieutenant Cella and Petty Officer Montalenti.
To avoid drawing British suspicion toward the Olterra, the team selects targets among merchant vessels anchored farthest from Algeciras. However, a strong gale complicates the operation, forcing the divers to navigate challenging currents as they approach their targets and attach limpet mines to the hulls.
By dawn, all three Siluro a Lenta Corsa’s return safely to their base. The mission proves highly successful: the mines damage three ships. The American Liberty ship Pat Harrison (7,000 tonnes) is heavily damaged and declared a total loss, with one American sailor killed in the explosion. The British freighter Mahsud (7,500 tonnes) settles on the seabed with part of its structure still visible above water, while the Camerata (4,875 tonnes) sinks completely.
To further mislead British intelligence, members of the Italian secret service scatter diving equipment along the shore, creating the illusion that the attack was carried out by combat swimmers rather than Siluro a Lenta Corsa’s.
BG 7 on August 3th, 1943. |
On the night of August 3th, 1943, the Ursa Major unit launches its final mission against Gibraltar. Three Siluro a Lenta Corsa’s, depart the Olterra, aiming for three transport ships anchored in the bay. Lieutenant Ernesto Notari leads the operation, keeping close to the Spanish coastline to avoid British searchlights scanning the open sea. Notari’s copilot for the mission, Petty Officer Andrea Gianoli, lacks extensive training on the piloted torpedoes, complicating the operation.
While attempting to attach an explosive charge to the keel of the American Liberty ship Harrison Grey Otis (7,700 tonnes), their torpedo malfunctions and spins out of control. As Notari opens the diving valves, the craft unexpectedly plunges to a depth of 34 metres before surfacing just meters away from the target. Disoriented and struggling to regain control, Notari discovers that the torpedo’s diving mechanism is no longer operational. Despite these challenges, he manages to retreat at full speed, aided by a school of porpoises masking his wake. Gianoli, however, is left behind during the chaos.
After clinging to the rudder of the Otis for two hours, Gianoli calls for help and is eventually rescued by the ship’s crew. Suspecting the vessel has been mined, a British motor launch with a member of Lieutenant Lionel Crabb’s Royal Navy Mine Clearance Diving unit, Petty Officer Bell, is dispatched to investigate. Just as Bell prepares to enter the water, the warhead detonates. The explosion severely damages the Otis, rendering it a constructive total loss, kills one sailor, and injures eight others.
Simultaneously, two other Allied vessels are targeted and destroyed. The Norwegian ship Thorshøvdi (9,900 tonnes) is split in two by a powerful blast, while the British freighter Stanridge (6,000 tonnes) sinks in shallow water. By 04:00 on August 4th, 1943, the mission has accounted for the destruction of 23,000 tonnes of Allied shipping.
These missions target Allied shipping in Gibraltar’s harbor, achieving notable successes. The ship’s secret is only uncovered on September 8th, 1943, after the Italian armistice, much to the surprise of the British. Following Italy’s armistice, Spanish authorities attempt to conceal evidence of the Olterra’s involvement. However, when the Royal Navy Mine Clearance Diving unit under command of Lieutenant Lionel Crabb stationed in Gibraltar boards the tanker, they uncover spare parts from three different torpedoes, enabling them to reassemble a complete manned torpedo, which they name Emily. Despite successful reconstruction, the craft is lost after six trials conducted in open seas.
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