Cagni class Submersible

Oceanic Submersibles: 1939-41: Ammiraglio Cagni, Ammiraglio Caracciolo, Ammiraglio Millo, Ammiraglio Saint-Bon.

The “admirals” was the ending point of almost two decades of Italian oceanic submarine designs, with sixteen planned but due to the war breaking out, only four were completed. At 2200 tonnes underwater and 88 m long they were also among the largest and possessed more fuel oil and torpedoes for longer patrols, but tailored against trade, reverting to smaller 450 mm tubes. Their two deck guns for surface action were a good complement for this. One requirement was to sail non-stop from Italy to Kismayu in Italian Somaliland (new submarine base) to operate in the Indian Ocean. But the war did not fit that plans. Instead, Cagni operated in the Atlantic and the three others acted as transport submarines to resupply the axis in North Africa. Two fall prey to U-class subs, one to a Hunt class escort destroyer and only Cagni survived the war, BU in 1948.

Design of the Cagni class

Development

The Cagni class were the last Italian oceanic submersibles designed prewar, and they were specifically tasked to lead long range commerce raiding operations, unlike previous designs that were more “universal” in nature. In particular, it was planned a class capable of rampaging the Indian Ocean’s British and French trade lines (from Indian and Indochina notably) that were either directed to South Africa or the Suez Canal. The commerce raiding only capacity led to the most interesting of their features, a return to 450 mm torpedoes, as this of World War I, either modern, or the stocks of late WWI torpedoes, still available. But this was combined with a much, much larger and more powerful submarine which internal pressure hull enables more torpedo tubes to be fitted in.

It was soon clear on paper that the adoption of smaller torpedoes on a very large hull enable the installation not only of extra tubes, but of a reserve of no less than 38, an absolute world record both for the time, still unbeaten today. For this, instead of six and four, they were given eight and six respectively in the bow and the stern. There was still an older WWI stick but to optimize performance, new 450 mm torpedoes, early developed for newly built torpedo boats of the Regia Marina, were developed. This new type was halfway between the typical 450 mm and the 533 mm, being of the lower calibre but long and much more capable. These regular 450 mm carried indeed a massive 110 kg warhead, instead of 533mm’s 270 kg warhead, but the new submarine models were designed instead to carry the same 200 kg payload, and they derived actually from air-dropped models, those used in particular by the famous trimotor SM.79 Sparviero with Gusto in the Mediterranean.

This choice of torpedoes was no accident: These submarines were designed to attack isolated merchant ships in distant seas. It was theorized they would have a cruiser lasting 136 days, so about four months, and even spare food and fuel supplies for another month. The large number of torpedoes enabled to make it for possible misses, and the large number of tubes ensured a high hit probability on important targets in the first place. It was only comparable, but superior, to the British T-class (ten 533 mm tubes, partly in the outer hull and not reloadable at sea). In that case, fourteen tubes was an absolute record, only matched by some paper projects. It was expected numerous torpedoes would be sacrificed for single targets. There was another unique feature, that was a rail guide allowing to send any spare torpedo from bow to stern and vice versa via the bulkhead hatches as well as even rotating them in the forward torpedo room, which was roomy indeed. To complete this arsenal, were the now standard two OTO 100mm/47 Modello 1938 deck guns on raised platforms. The AA defence also comprised four 13.2 mm anti-aircraft machine guns in two retractable twin Breda Modello 1931 mounts.

All this was on paper when defined in 1938. In practice, this didn’t work, partly due to the technical shortcomings common to all Italian submarines and partly due to changing operational requirements. In wartime, instead of their mission they ended supplying North Africa, less likely to be intercepted by the Royal Navy as when the torpedoes were out, they had sufficient space to carry a sizeable amount of supplies. All previous classes were simply too small. However, these ocean-going submarines were already considered too large, not ideal for the Mediterranean, notably not able to dive fast, not diving deep enough for mostly clear waters, and still somewhat slow and unwieldy. In fact, only Admiral Cagni managed to operate as planned in the Atlantic, but was only capable of claiming a meagre 5,500 tons of merchant shipping. The realization of transport tasked would lead to the construction of the dedicated and even larger Romolo class.

Construction and Names

These shortcomings and the war breaking up ensuring the initial, optimistic plan for 12 submarine was curtained to just the first batch. Internally these were soon known as the “ammiragli class” as all were to be named after admirals. They were ordered in 1939 and laid down later that year at CDRA in Montfalcone.
Here is an overview of the namesakes:

Hull and general layout

Cagni in 1942 with the new U-Boat type CT.

The Cagni class were the largest Italian submarine since a while, to be precise since the 1927 Balilla class and Ettore Fieramosca (1929) or the Calvi class (1935). Otherwise, they looked like stretched out Marconi class, and were similar in overall proportions, with a well refined transition between the deck and ballasts, two rows of water scoops, and a raised platform fore and aft for her main guns. The conning tower was designed to continue operating while surface in rough seas in the monsoon, as the poor visibility will prevent air reconnaissance and spotting while concealing her to possible escorts. For a displacement of 1,653 tons surfaced, larger than any Italian submarine to date, they rose to 2,136 tonnes submerged.
They measured 87.95 m (288.5 ft) long, more the Calvi class or any previous class, with a greater beam at 7.76 m (25.5 ft) and draught at 5.72 m (18.8 ft).

There was no plan for a radar, however. This large conning tower was still classic, with a nose in the breakwater, an enclose helmsman post with windows, and an open bridge above and aft of it. Then came the two periscopes, one for attack (smaller) and one for navigation (larger) at the core of the conning tower. Then came the aft platform for two pintle mounts holding the Breda HMGs that can be left strapped on deck, dismounted in a canvas wrapping, or installed when the sub surfaced. The deck guns received taps to remove before use, protecting their barrels, and sometimes canvas, but they were designed for underwater operations with all steel elements generously painted with waterproof coating.
See modifications below.

Powerplant

All four boats were given relatively powerful engines. On two shafts, they were powered by two diesel FIAT rated for 2,185 bhp each (1,630 Kw.) or 4,370 bhp total (3,260 kW). This was for surface running at 17 knots (31 km/h). Underwater, they relied on two 2 CRDA electric generators rated for 640 shp. each (477 Kw.) for a grand total of 1,280 shp. (1,300 Kw.) and 8.5 knots (15.7 km/h) top speed. As for range, based on 180 tonnes of fuel oil, the class was a record-setter, with 19,500 nautical miles (36,114 km or 22,440 miles) at 9 knots (17 kph or 10 mph) surfaced, and 107 nautical miles (198 km or 123 miles) at 3 knots (5.5 kph or 3.5 mph) submerged.

To put things into perspective, this would allow them to sail from Italy, cross Suez (in peacetime), crossed the Indian Ocean to the Pacific and even managed to reach Panama and arrive in the Caribbean and a point in Mid-Atlantic in one go. In reality, they mostly left their Italian bases to venture in the North or South Atlantic and down to the Cape, then cross into the Indian Ocean once Suez was closed. From Taranto to the Cape, this was 7,100 nautical miles already. But they needed margins for full speed runs while surfaced and escape attacks as well. Still, they had a considerable margin.

Armament of the Cagni class

Like many contemporary Italian Oceanic submarine classes, they were armed with two main guns and an AA defence with two twin Breda heavy machine guns. But the bread and butter of the design was their massive number of torpedo tubes. Eight forward, six aft was quite unique and only possible if using 450 mm tubes. This required an internal torpedo chamber space, vertically, of more than two meters. It was quite spacious in beam as well, with enough room to make a full gyration or 360 degree of a torpedo, which for this models meant an internal space of at least 5.80 meters (19 feet). This made them the roomiest Italian submarine ever built at this point. There was also a whole railing system that enable the transfer through bulkhead doors of any torpedoes from the bow to stern torpedo rooms.

Cannone da 100/47 Mod. 1938

100mm/47
Placed on the forward and aft raised platform or bandstand, well integrated into the hull, acting as wave breaker forward to avoid sea spray. These 100 mm/47 (3.9″) Modello 1938 benefited from developments and improvements made on the earlier Modello 1928, specifically intended for submarines but also used on a few auxiliary vessels, all manufactured by OTO. The Modello 1931, 1935 and 1938 for Submarines were present on the Glauco, Marcello, Cappellini, Liuzzi, Marconi, Cagni, Sirena, Perla, Adua, Argo, Acciaio and Flutto classes.

Specs

Gun Length oa: 194.5 in (4.94 m), 8-10 rpm.
Fixed Ammunition (HE) 30.4 lbs. (13.8 kg) 2,756 fps (840 mps)
Range: 35° 13,800 yards (12,600 m), OTO 1932
Weight 4.6 tons (4.7 mt) -5 /+32 or 35°.
To compare the previous Cannone da 100/43 modello 1927 had the following specs:
170 in (4.300 m) long, 8-10 rof. HE shells: 30.4 lbs. (13.8 kg), muzzle Velocity 2,625 fps (800 mps)
Elevation -5/+35°. Range: 30.4 lbs. (13.8 kg) HE Elevation/35° 12,000 yards (11,000 m).

AA: 2×2 Breda Mod. 31

The anti-aircraft defence counted on two twin Breda M1931 13.2 mm L/76 heavy machine guns, placed aft of the conning tower. Each mount weighed 695 kg, but ensured an elevation of -10° to 90°. They fired 125 g unitary rounds, with a muzzle velocity of 790 m/s, maximum range of 6,000 meters and effective range of 2,000 meters. Their rate of fire was 500 rounds/min, so 2000 rounds when both twin mounts fired in concert, however.

36x 450 mm (21 inches) torpedoes

The 45 cm (17.7″) W 200/450 x 5.75 Siluranti were unique to the Cagni class submersibles. No other submarine in the RM would carry those. They were essential to the success of the design overall. Derived from an airborne model, the Si 200/400, they were longer, as the former could either be 5.46m or 5.36m. The W200 were 5.75 meters long (18 ft. 10.5 in).
They weighted probably above 2,200 lbs. (1000 kg) and carried a 441 lbs. (200 kg) warhead, like the airborne models. To compare, the 450 mm used by Torpedo Boats and MAS boats had the same, but they were lighter and shorter.
Performance wise, the W200 used a classic Wet-heater to power them to 3,300 yards (3,000 m) at 44 knots or 8,750 yards (8,000 m) at 30 knots, better settings than other models. It was developed by Whitehead-Fiume. The model could be fired by other submarines as the company also provided cradle-like fittings which enable them to be fired from 533 mm tubes as well.
As for the numbers carried, it went from 38 as initially planned, down to 36 for realistic combat operations and compromises. Still, that was enormous, but the management is unknown. On average that means two reloads per tubes, so possibly 6 aft and 16 forward not counting those preloaded in the tubes. One can imagine that when these were absent, as well as all the associated gear, a considerable room was available to store any payloads.
These boats also came with hydrophones, albeit little info is available.

The Cagni class were famously camouflages, generally with the standard light grey/Dark Grey pattern of the time. Or here, a pattern with dark green stripes. These subs were mostly used for transport and could be often spotted close to shore. The deck was in a dark grey uniform pattern.

⚙ specifications
Displacement	1,653 tons surfaced, 2,136 submerged
Dimensions	87.95 x 7.76 x 5.72 m (288.5 x 25.5 x 18.8 ft)
Propulsion	2 shaft Fiat diesel engines 4,370 hp, 2× CRDA electric motors 1,800 hp
Speed	17 kn surfaced, 8.5 kn submerged
Range	13,500 nmi (25,000 km; 15,500 mi) at 9 knots (17 km/h; 10 mph)
Armament	14× 450 mm TTs (8 bow, 6 stern, 36), 2x 100 mm/47 OTO, 4× 13.2 mm Breda AA
Test depth	90 m (300 ft)+
Crew	85

Modifications and Evaluation

In 1940, they were the largest attack submarines of WWII for the Italian Navy. The design was single-hulled with external bulges and if not very agile, they had a good seaworthiness and habitability. However, there was an important change upon delivery in 1941: Their initial large conning tower was removed and replaced by a much smaller German style U-Boat conning tower, with only an open bridge and diminutive rear platform. This made them less conspicuous when surfaced, and helped to dive faster as well. In 1943, Ammiraglio Cagni was converted to a fully dedicated transport submarine for the routes to Japan. This included the removal of all torpedoes and associated pulleys, gantry and rails. The tubes were only kept for additional “cold storage”. Still, the payload was limited to the size of the loading hatches for the torpedoes. This fit well for fuel barrels for example.

Overall, the class never performed in its intended role (only Cagni did, and that was almost a footnote). So they ended with a poor tally for their cost and size. However “Supermarina” was pressed to find sustainable solution to supply axis troops, “stranded” in North Africa as long as the Royal Navy maintained its presence between Gibraltar, Malta and Alexandria to lead interception mission on surfaced Italian convoys. The cagni were thus stripped of their torpedoes and the space made free to carry a generous amount of supplies, up to 150-200 tonnes.

Cagni class Career

Ammiraglio Cagni (1940)

Cagni was the first ordered to CRDA Montfalcone in early 1939. She was laid down on 16 September 1939, launched on 20 July 1940 and completed on 1 April 1941. Her first operational patrol was in the Mediterranean, and in all she made five transport missions and 16 patrols.
Under the command of Lieutenant Commander Charles Liannazza on 15 October 1941, she transported 140 tons of fuel and ammunitions from Taranto to Bardia, and back at Taranto on 22 October, en route was she was spotted and depth charged but survived. Same on 18 November, then five offensive and sixteen more transport missions.
She was not transferred to Betasom (The Bordeaux Base) for Atlantic service, remained based in Taranto, but helped by her considerable range.
He first patrol from there was on 5-6 October 1942, she sailed from La Maddalena to the South-Atlantic against convoy “TS 23”. She crossed the Strait of Gibraltar on 12 October without any contact.
On 3 November, while submerged at daytime, she spotted and sank the 3,845 GRT Elder Dempster Lines motor ship Dagomba. On 29 November, while off Cape of Good Hope in Africa, she spotted and sank the 1,995 GRT Greek ship Argo. This was her longest mission with a record 136 days, but she had little to show for. In total, she made 21 missions in the Mediterranean, and was transferred via the South Atlantic for two missions, ending in the Indian Ocean in 1942–43 (to be updated 2026).
On 3 January 1943, she tried an at-sea torpedo and fuel transfer from Tazzoli, but it failed due to poor weather, however it succeeds on 13 January when she took 45 tonnes of fuel from a Milchsükhe (Type XIV U-Boat). On 15 February while in the always dangerous Bay of Biscay, she was spotted and strafed, bombed by an aircraft on patrol, leading to the death of Sergeant Gunner Michelangelo Cannistraro.

For her second Atlantic mission and 1st in the Indian Ocean also her second and last mission from 29 June 1943, she was on station off Durban on 20 September 1943, and stayed there for 84 days. She was at the time under command of Lieutenant Commander Joseph Roselli Lorenzini and was ordered to proceed to Singapore to attack merchant shipping there and return with a load of rubber and tin. On 17 July 1943 while off the Canary Islands, she spotted an unknown 5,500 GRT steamer, but missed. On 25 July, she torpedoed the 22,071 GRT UK armed merchant cruiser HMS Asturias. She survived, despite having her boiler room and machinery spaces flooded, loosing all power. Left for dead, she managed later to reach Freetown under tow. On 30 July Ammiraglio Cagni crossed the equator. On 28 August she entered into Indian Ocean.

By 8-9 September 1943, while on her second Atlantic sortie and 1,800 miles from Singapore, she was informed of the armistice, she was ordered to sail to Durban, South Africa on 20 September 1943. Spotted while surfaced by MS Jasmine Cagni was asked to surrender, but the commander refused, according to the terms of the Cassibile Armistice. She was returned in 1944 to Italy and became a training ship for ASW like most Italian subs in the co-belligerence. The Italian admiral Raffaele De Courten’s signed an agreement with Andrew Cunningham to avoid Italian submarines to have to surrender indeed, but this was not received throughout the RN, leading to some incidents.
On 8 November, she left for Taranto and arrived on 2 January 1944. Then she was moved to Palermo, she was used for anti-submarine training activities for Allied planes. On 10 February 1948, she was decommissioned and later broken up.

Ammiraglio Caracciolo (1940)

Ammiraglio Caracciolo, named after Francesco Caracciolo was laid down on 16 October 1939, launched on 16 October 1940 and commissioned on 1 June 1941. From June to December 1941, she was engaged in crew training. Then under orders, she was used for supplies transport, despite this being a risky and poorly performing service. On 8 December 1941, Carracciolo departed Taranto for her first mission under command of Lt. Cdr Alfredo Musotto with 138 tons of supplies for Libya, fuel and ammunition for the Afrika Korps in particular. Off the coast of Sollum she was strafed and reported by a British aircraft, making a casualty, Ensign Milos Baucer, but her AA managed to was shot it down. Ships arrived, she made a crash dive to 80 meters. On 10 December, she reached Bardia, unloaded her cargo and departed later that day with on board Italian and German soldiers, mainly officers of the Bersaglieri and Carabinieri to Suda and from there, load further supplies. On 11 December, about30 miles from Bardia, she spotted a British convoy bound for Alexandria. At 2:40 a.m. she went on the attack, launched two torpedoes but missed a merchantman, then two more, dodged by the destroyer HMS Farndale which came back and depth charged her. She had to dive to 160 metres but was severely ands forced to surface, immediately hit by gunfire from Farndale, down to its AA machine gunners. The captain order scuttling initiated shortly afterwards and at 3:00 a.m. survivors abandoned her. Caracciolo sank afterwards with 48 which disappeared at sea, 15 officers including Commander Musotto and 33 passengers, but 53 survivors were rescued and captured by Farndale.
In addition to a single transport mission, she made five transfer missions (1,445 miles surfaced).

Ammiraglio Millo (1940)

Ammiraglio Millo (named after Enrico Millo) was laid down on 16 October 1939, launched on 31 August 1940 and commissioned on 1 May 1941. From May to October 1941, Millo performed crew training and was noted operational on September 15. Transport missions started immediately. On November 21, 1941, she left Taranto with 138 tons of petrol cans, 6.8 tons of anti-tank ammunition to Derna, arrived on November 23, unloaded, and departed on the morning of the 24th, back to Taranto 2 days later. Other missions took place on November 30 towards Bardia and Benghazi, then Tripoli, and January 26, 1942 Tripoli. On her return off the coast of Libya, she was under air attacj but escaped unscathed.
In total, she transported 365.2 tons of fuel, 167 tons of ammunition, 91 tons of supplies. On March 6, 1942, she was deployed southeast of Malta (Operation V.5) to protect an Italian convoy for Libya, preventing sorties from Malta. Six days later, she left the area. At 1.23 pm on 14 March, Millo under the command of Lt.Cdr. Vincenzo D’Amato was zigzagging surfaced off Capo dell’Armi and towards Taranto to return to port when spotted by the British submarine HMS Ultimatum, which launched four torpedoes, two hit home amidships and aft. Millo sank almost with all hands off Punta Stilo (Commander D’Amato, 2 officers, 52 petty officers and sailors). Lieutenant Marcello Bertini, three other officers, two petty officers, eight sailors were on deck or in the CT and jumped overboard to be rescued and captured by Ultimatum. Sergeant Lingua, an electrician, was rescued by a boat later at 2:08 PM. Millo performed 6 missions, 4 transport, 4 transfer over 8,045 miles. Her wreck was rediscovered in September 2005 lying on its port side, under 65-72 m.

Ammiraglio Saint-Bon (1940)

Ammiraglio Saint-Bon, named after Simone Antonio Saint-Bon was laid down on 16 September 1939, launched on 6 June 1940 and commissioned on 12 June 1941. Delivery planned on March 1, 1941, was postponed to rpelace her CT by a German U-Boat type one, and she was commissioned in June. Until October, she was training. She was tasked to carry petrol for her deployment in the Atlantic, under LtCdr. Livio Piomarta. On October 10, 1941 under Gustavo Miniero she left Taranto for a first mission with 153 tons of fuel to Bardia. Two days later she was attacked 100 miles from Bardia by aviation but managed to hit and repel the aircraft. After unloading on the night of the 13th) she headed for Taranto but on October she was attacked with bombs by an aircraft about 75 miles northwest of Crete, but fierce AA allowed her to escape unscathed and reach port. On November 16th, she headed for Bardia, returned on the 22nd. A third mission to Derna started on the 27th until December 2nd. A fourth mission to Benghazi, closed on December 21st.
On January 4th, 1942 she left Taranto for Tripoli with 155 tons of fuel and ammunition. At 5:42 on the 5th while surfaced off Punta Milazzo, she was spotted by HMS Upholder (an ace submarine of the U class), launching all torpedoes: one hit home, detonating ammunition apparently as Saint Bon blew up. Commander Miniero, 7 officers, 49 petty officers and sailors went down, three men survived to be rescued, Lieutenant Luigi Como, Sergeant Valentino Ceccon, and Chief Torpedo Officer Ernesto Fiore, by Upholder. For 5 transport missions carrying 695.5 tons of fuel and lubricants, 16.7 tons of ammunition and 5 transfer missions she crossed 6927 miles.