Battleships Class Caio Duilio

Caio Duilio and Andrea Doria

During the last world conflict, the Italian Navy utilized three distinct class of battleships, two of which were modernized vessels dating back to World War I (Conte di Cavour and Duilio Class) and one of new construction (Vittorio Veneto or Littorio Class). The Duilio Class dated back to 1909 when, by royal decree, Italy ordered the construction of two additional battleships, the Caio Duilio and Andrea Doria to the ones already under construction  (Conte di Cavour, Caio Giulio Cesare and Leonardo da Vinci).  The two battleships were delivered to the Navy in 1915 (the Doria was actually completed in 1916).  This article will focus on the modernization of the original design performed in the 30’s because , besides retaining the original hull, the ships should be considered mostly completely new.

Caio Duilio before the modernization

Victorious after the epic but bloody battles of World War I, Italy’s economy entered a period a deep recession and eventually depression similar to many western countries. The enormous financial expenditures of the conflict, and the heavy reliance on foreign imports, had left Italy nearly bankrupt.   During this period, Italy retained the , Doria and Duilio and the two battleships of the other class Cavour, Cesare in the naval reserve.  The Leonardo da Vinci had been lost due to Austrian sabotage.

With foreign powers building new battleships – Germany with the Deutschland (technically a pocket battleship) and France with the Dunkerque  (technically a battle cruiser) –  it was strategically and military impelling for the Italian Navy to respond with similar vessel to provide for a balance of power. Construction of new units was at this point financially prohibitive, thus a study was initiated to modernized the existing battleships. In 1937, Admiral (E) Francesco Rotundi [i] and the ‘Comitato Progetti Navi’, the bureau in charge of naval constructions, began a project to transform the Duiilio and Doria based on the experience acquired during the modernization of the Cesare and Cavour. Furthermore, at the same time Italy had begun constructing two new battleships (Vittorio Veneto or Littorio Class) and some the experience from this new design would be incorporated in the modernized units. Contracts were assigned to the Cantieri del Tirreno in Genoa (Duilio) and C.R.D.A.  in Trieste (Doria). Over a period of three years, more than 60% of each vessel was completely replaced.  The end result was more powerful armaments, higher speed, increased protection, and a radically new, and more elegant silhouette.  Work on these units required less time that the previous ones thanks to the experience already acquired.

The Hull

With the original hull completely emptied – only the external hull was left untouched – the inside was fully redesign to give room to decompression cylinders of the Pugliese type. These devices were particularly designed to absorb the impact of underwater explosions by providing a protected expansion chamber. Technically, the two options available were to either build anti torpedo protections external to the hull, similarly to the Barham, or internally as it was decided. The design  chosen provided for a better shaped hull, thus retaining speed which, at the time, was considered a critical advantage over foreign units.

The original Pugliese cylinders tested on the ships Brennero and Tarvisio had to be reduced in size, and thus effectiveness, due to the limited space available.  At the same time, a double hull bottom and lateral compartments were greatly improved, even though they were not able to save the Duilio from serious damages in the on December 11th, 1940 in Taranto due to the unusual location on the explosion directly under the hull.  Since the ship was close to shore, quick action saved her by having it lay on shallow waters near the shore. In that case, as it had happened to  the Cavour, the explosion wave, reflected by the bottom of the harbor, magnified the devastating effects causing serious damage. To improve the overall shape of the hull,  old style bow was removed and a new, modern shaped one fitted instead.   This new design extended the length of the ship by about 10 meters. Astern,  two of the four axels were removed,   but the overall shape was left untouched, including the rudders. 

The belt armor was left at 250 mm, but the base of the turrets (barbettes) received an additional 50 mm of armor.  The difference from the previous class was that this belt was attached to the old one rather than being placed at a certain distance. The new control and command tower was protected by 260 mm of steel, while the deck, originally protected only by two layers of 12 mm,  received an additional 80 mm of protection.


The decks were named:

Ponte di Coperta (Upper deck)
Primo Corridoio (First Desk)
Second Corridoio (Second Deck)
Copertino Superiore (Upper  Deck)
Copertino Inferiore Lower Deck)
Piano di Stiva (Hold)


The power plant was completely replaced removing the old 4-propellers, 3-turbines, 12-boilers systems producing 32,000 HP, with a modern 2-propeller,  2-turbines, 8-boilers system producing 75,000 HP. The boilers were of the Yarrow type and equally distributed between seaside and portside. The 22 kg/cm2 steam powered the Belluzzo system which incorporated a high pressure and two low pressure turbines.

The power plants were offset, one forward and one aft, and could receive steam from any of the boiler systems.   The funnel were placed closer to each other, thus making this class easily distinguishable from the Cavour and, at a distance, more similar to the new Littorio Class.  At the sea trials, the power plants were discovered to have much more power than the originally contracted values, still this class resulted at least 1 knot slower than the Cavour.


With the introduction of several new electrical instruments, the electrical system was redesign and the old steam dynamos replaced by more powerful power generating units operating both off the main boilers’ steam and also on diesel fuel. The diesel units guaranteed powered even in case on a complete failure of the boiler system.  Both vessels were equipped with two redundant gyrocompass of the latest generation with 12 repeaters each.  There were both protected and unprotected radio shacks and four 120 mm Galileo projectors.  During the conflict, these battleships were never equipped with radar equipment , even though there was a study conducted in 1943 to equip the Duilio and the Doria with a German or Italian apparatus. After the war, the ships received a British  L.W.S. radar of no practical use but training the crew.


Similarly to the Cavour Class, the most creative part of the modernization process took place around the main artillery.  The original Armstrong Vickers [ii] 305 mm guns were considered grossly inferior to what other navies were utilizing, but the cost for total replacement was prohibitive.  The original guns, which were made of an outer shell, coiled steel cables and an inner shell, or riffled tube, were disassembled. The coil was reduced in thickness by Ansaldo in La Spezia and the inner tube replaced with one of greater caliber bringing the guns up to 320 mm (12.6 “).  This new gun was designated as the Ansaldo 320 mm/44 1934. This technically challenging alteration resulted very successful as the lateral resistance of the gun barrel, while weakened, was not compromised. Furthermore, the elimination of the fifth turret, located amidships, gave extra material for the alteration.  At the end, the battleships were left with 10 guns each, three on the lower gun turrets and two on the upper ones, five aft and five forward.  

The 320 mm guns had a maximum elevation of 30˚ , three more than the previous class, and a maximum depression of -5 ˚and a range of 28,600 meters. The projective weighted 525 Kg. and had a speed of 830 m/sec (meters per second) at the muzzle. The rate of fire was 2 rounds per minute.  The length of the barrel was  48.8 calibers or 15.616 meters. Each gun weighted 64 metric tons.

The medium caliber 152 mm guns were completely eliminated as well as the 76/59. The original underwater 450 mm torpedo tube launchers were also removed.  The medium caliber guns on this class resulted radically different from the Cavour mostly due to the experience in the meantime acquired on the Littorio project.  The primary medium caliber armament consisted in 4 turrets each armed with three 134/45 O.T.O. 1937 guns for a total of 12. These turrets were located aft near turret  number 2, two on each side, and provided for an excellent rate of fire.  There were also 10 modern 90 mm Ansaldo 1939 guns similar to the ones used on the Littorio. These single guns were installed on stabilized platforms and provided for excellent antiaircraft protection. With an elevation of up to 75˚ and a rate of fire of 12 rounds per minute, these were excellent weapons.  Further antiaircraft protection was provided by 3 single 37/54 Breada 1932 and 12 double 37/54. During the conflict some of this lighter armament was altered resulting in a total of 16 dual 37/54 mm and 16 dual 20/65 mm.


All ammunitions were kept in four distinct magazines located under the armor deck and near the turrets.  The magazines could be easily flooded and were accessible to the outside via a modern system of rolling doors.  The standard ordnance included 800 shells for the 320 mm guns, 2,900 shells for the 120/50 mm, and 2,460 for the 100/47 mm.

The transfer of the large projectiles from the magazine to the guns was complex and very secure. Projectiles were picked up via electric winches and deposited on a loading dock. Subsequently, four begs containing the charges were added and the full charge elevated to the guns. The charges were introduced into the guns, after the projectile, two each time.  The whole system was automated, excluding the initial handling of the powder bags.

The 135 mm and 90 mm shells were loaded by elevators to the main deck and from there manually transported to the magazines usually located at the base of each turret. In total there were 440 armor piercing shells, 210 explosive shells for the 230/43.8 mm guns, – 572 armor piercing shells, 873 explosive naval shells and 328 explosive aerial shells for the 135/45 mm guns – 4,000 explosive shells and 190 start shells for the 90/50mm guns.  The smaller caliber guns had about 1,800 rounds for the 37/54 mm and 2,400 for the 20/65 mm each.

Firing Control

The firing control mechanisms and apparatus was completely replaced and substituted with  modern equipment which proved itself up to the task for the duration of the conflict.  The equipment itself and the space allocated to the operator was vastly improved over the Cavour Class. The main telemetry system was housed in a movable compartment located 23 meters above the waterline and positioned above the main control tower. There were two telemetry systems  each 7.20 meters wide.  The telemetry station was connected to the firing station which could control all guns automatically and fire them at once. In case of failure of the automated system, there was a  failover station situated on top of turret  number 2 (forward) and a 9 meters wide telemeter. Another similar station was also placed atop turret number 3 aft. The 135 mm turrets had their own centralized stations, but could also operate independently. Smaller guns and antiaircraft guns had their own independent aiming and fire control mechanisms.


After the experience acquired on the Cavour, this class was never equipped with aircrafts..


Like all other vessels in the navy since 1929, the Cavour Class battleships were painted light gray. During the conflict, after study conducted on methods to make the enemy’s telemetry more difficult to focused, and based experience d acquired on the Littorio and the cruiser Duca d’Aosta, the Duilio received a two-color mimetic paint schema.  The Doria instead received a three-color mimetic schema very similar to the one applied to the Cavour class and originally designed by the famous naval painter Claudius.


The Duilio class had a nominal displacement of 28,700 tons (29,000 metric) with a length of 186.9 meters, a width of 28 and a draft of 10.4 meters.  Armor represented 33.9% of normal displacement. The nominal power of 75,000 HP was calculated to be during trials as high as 87,000 HP. The maximum speed during these trial was 27 knots. Maximum speed at sea was about 27 knots, but the machinery could be stressed up to 26 knots.  The ships had a range of 4,250 (4,680 Dulio) miles at 13 knots, 3,390 (2,780 Duilio) at 20 knots and 1,700 at 24 knots. The bunkers could hold up to 2,552 (2,548 Duilio) tons of fuel.  The crew consisted of 36 officers and 1,400 between petty officers and sailors.


A debate over the option to modernizing these two ships versus building a new one – since these units were being  modernized at the same time the Vittorio Veneto Class  was being built– still rages.  The limitation of the Duilio Class, as with the Cavour before,  compared to the more powerful British battleships was quite evident during the conflict since  the Italian 320 mm could not compete against the British 381 mm. While the Duilio Class was an improvement over the Cavour Class, it still shared most of its limitations.   Eventually, only the Vittorio Veneto (Littorio) Class battleships represented a serious threat to the British Navy.  Thus, one has to conclude that building a single, more powerful battleship would have been preferable, but considering the technological  innovation, the ingenuity and the results achieved, much credit has to be given to the Italian naval engineers who collaborated on this project.   Let’s not forget that both units continued serving in the Italian Navy until 1956; this longevity  gives credit to its design and construction.

[i] Rotundi (Foggia, 10 July 1885 – Rome, 25 October 1945)  is universally known for having designed the Italian training ship Amerigo Vespucci.

[ii] Armstrong Vickers 12” 1909. There are some references to some of the guns being produced by Elswick and designated Pattern “T”. Both utilized Welin breech-blocks.