Italian Ironclad Caio Duilio - Design

Design

In her day the Caio Duilio was as revolutionary in her design as the French La Gloire had been a decade before. The ship exemplified the concept of the 'monster gun', a vessel carrying the heaviest calibre weapons possible, capable of incapacitating an enemy warship with a single hit.

Initially designed to carry four 38-ton rifled muzzle loaders, similar to those carried on the British HMS Devastation the design was sufficiently flexible to permit the gun calibre to be increased during construction. When the British armaments manufacturer Armstrongs announced their 50-ton 355 mm RML (rifled muzzle loader), Duilio was modified to receive it instead, which required considerable modifications to the ship's armour to save the necessary weight. Eventually the gun calibre was increased to accept an even newer and larger Armstrong gun, the 100-ton 450 mm RML.

To accommodate such massive weapons, the armour belt was restricted to the central third of the vessel, forming a citadel on which the turrets were mounted en-echelon (forward turret to starboard). This arrangement enabled all guns to bear on either beam, but end-on fire was compromised by the severity of the muzzle blast causing damage to the ship's unarmoured upperworks.

Either end of the ship, outside the citadel, was subdivided into watertight compartments, so that flooding due to damage to the ends would be contained, and would be insufficient to compromise stability.

Weight of armour and armament precluded auxiliary sail propulsion, which for the limited range required to operate in the Mediterranean, was in any case unnecessary. The ship was a mastless design. The design speed of 15 knots was high for a battleship of this era, but there was insufficient space within the citadel to accommodate all the machinery. The boilers and engine spaces were protected by an armoured deck 50 mm thick extending the full length of the waterline. This protection scheme was described as a 'raft-body' design.

The compromises in protection, and general structural weakness, raise doubts as to whether the large guns could keep firing across the deck. The unarmoured ends were vulnerable to rapid firing light weapons, and could conceivably flood sufficiently to cause her to turn turtle without penetrating the thick armour at all.

These perceived limitations were probably overstated for ships designed to operate in the sheltered waters of the Mediterranean.