Bolzano was a unique heavy cruiser, sometimes considered to be a member of the , built for the Italian Regia Marina (Royal Navy) in the early 1930s, the last vessel of the type to be built by Italy. A modified version of the earlier Trento class, she had a heavier displacement, slightly shorter length, a newer model of gun, and a more powerful propulsion system, among other differences influenced by the that had followed the Trentos. Bolzano was built by the Gio. Ansaldo & C. between her keel laying in June 1930 and her commissioning in August 1933.
Bolzano had a fairly uneventful peacetime career, which primarily consisted of naval reviews for Italian and foreign dignitaries. She saw extensive action in the first three years of Italy's participation in World War II. She took part in the Battles of Calabria, Taranto, Cape Spartivento, and Cape Matapan. The ship was lightly damaged at Calabria, but she emerged from the other engagements unscathed. She also frequently escorted convoys to North Africa in 1941 and 1942 and patrolled for British naval forces in the central Mediterranean Sea.
The ship was torpedoed twice by British submarines; the first, in July 1941, necessitated three months of repairs. The second, in August 1942, caused extensive damage and ended the ship's career. She was eventually towed back to La Spezia, where repairs were to be completed. Resources were unavailable, however, and Bolzano remained there, out of action. Plans to convert her into a hybrid cruiser-aircraft carrier came to nothing for the same reason. After Italy surrendered to the Allies in September 1943, La Spezia was occupied by German forces; to prevent them from using her as a blockship, Italian and British frogmen sank Bolzano using Chariot manned torpedoes in June 1944. The Italian Navy ultimately raised the ship in September 1949 and broke her up for scrap.
Design
Bolzano was ordered under the 1929–1930 construction program; the Regia Marina had initially planned on building six heavy cruisers in the 1920s and 1930s, which comprised the and es. These vessels were to have operated as two three-ship cruiser divisions, but by 1929, the Regia Marina had decided to use as a temporary fleet flagship while the s were being rebuilt. As a result, the Regia Marina, which was under intense pressure from Gio. Ansaldo & C. for a construction contract, ordered a seventh heavy cruiser. A new design was prepared that was based heavily on the Trento class, but also incorporated improvements from the Zara design. These included the new, longer gun, more powerful boilers, and a stepped up forecastle deck to improve seakeeping. The high command of the Regia Marina persisted in their belief that speed was more important than armor, and so the new vessel adopted a level of protection similar to the earlier Trentos rather than the more sturdily built Zaras. The sailors of the fleet referred to Bolzano as "un errore splendidamente riuscito—an error beautifully executed" owing to her weak armor protection.
General characteristics and machinery
Bolzano was long between perpendiculars and long overall. She had a beam of and a draft of . She displaced normally and up to at full load, though her displacement was nominally within the restriction set in place by the Washington Naval Treaty. She had a crew of 725 officers and enlisted men as originally configured, though her wartime complement increased to 28 officers and 788 enlisted men. The ship was fitted with two tripod masts, one atop the forward superstructure and the other just forward of the rear funnel. A large, rotating aircraft catapult was located amidships, between the two funnels. She could carry two or three IMAM Ro.43 seaplanes for aerial reconnaissance.
Her power plant consisted of four Parsons geared steam turbines, which drove four screw propellers, and were powered by ten oil-fired water-tube boilers, which were trunked into two widely spaced funnels. Her engines were rated at and produced a top speed of . While on speed trials with a light displacement of , she reached from . Electricity was provided by generators with a total output of .