main belt before curving down to meet the bottom of the hull. This formed a void which housed an empty drum wide with thick walls; the rest of the void was filled with liquid. The drum ran the length of the torpedo defense system, and was designed to collapse to contain the explosive pressure of a torpedo hit. The torpedo bulkhead would prevent any splinters or explosive effects from entering the ships' vitals. The system was designed to protect the ship from torpedo warheads up to . The system did not perform as effectively as expected, however. This was due to two major defects in the design. The riveted joint that connected the interior torpedo bulkhead to the bottom of the hull was not strong enough to sustain the tremendous shear loadings associated with direct contact explosions. The joints failed even in cases of non-contact explosions, which prevented the hollow drum from collapsing as designed and resulted in massive flooding. The fineness of the hull shape prevented the 3800 mm thickness from being maintained for the entire central citadel; the width of the drum was reduced significantly abreast of the main battery, down to . The ability of the drum to absorb explosive shock correspondingly fell in relation to its size. Construction The keels for Vittorio Veneto and Littorio were laid on the same day, 28 October 1934, at the Cantieri Riuniti dell'Adriatico shipyard in Trieste and the Ansaldo shipyard in Genoa, respectively. Vittorio Veneto was launched on 22 July 1937, with Littorio following exactly one month later on 22 August. While incomplete, Vittorio Veneto went to sea on 23 October 1939 to conduct machinery trials. She was delivered to the Italian Navy in Trieste, still incomplete, some six months later on 28 April 1940. She departed Trieste on 1 May for final fitting out at the dockyard in La Spezia. After completion on 15 May 1940, she went to Taranto to join the fleet. Littorio underwent the same pattern of machinery trials prior to completion; she was delivered to the fleet on 6 May 1940. Two additional ships were laid down four years later. Roma was built by the CRDA shipyard, starting on 18 September 1938. She was launched on 9 June 1940 and was completed on 14 June 1942, after which she joined the fleet in La Spezia and replaced Littorio as the fleet flagship. Impero was laid