The Pennsylvania-class battleships, of the United States Navy, were an enlargement of the Nevada class; having two additional 14-inch (356 mm) 45-caliber main battery guns, greater length and displacement, four propellers and slightly higher speed. They also had a relatively large secondary battery of 5-inch (127 mm) 51-caliber guns, which was soon reduced when many of the guns' locations proved vulnerable to high seas.
The General Board, having just finished the design of the Nevada class battleship, moved on towards the seventh class of US dreadnought design issuing the 1913 design parameters, 12 14”/45 main guns, 22 5”/51 secondary’s, 21 knots (39 km/h; 24 mph), and Nevada-class armor scheme. Minor rearranging of the secondary guns into fire control groups was designated. The strength of the General Board was again shown by demanding better ships from C&R Bureau and overcoming stagnation as it had done with the Nevadas. The resulting ship now carried twelve 14”/45 by using triple turrets in all 4 positions that would be carried on in each succeeding class up to the Colorado class when twin 16 inch (410 mm) turrets were introduced.
The designers had noted the increasing size, range and explosive power of torpedoes as torpedo designs improved. In addition the design of the Davis torpedo was a concern. The Davis torpedo delivered an 8 inches (200 mm) artillery shell in a gun barrel housed within a torpedo. As a result a series of caissons were built (actual sections of the hull as they were proposed for construction) and tested with both external armor and internal armor. The Davis gun could cause damage but the answer to the Davis gun was external armor, the external armor however greatly increased the amount of damage caused by a conventional torpedo. As a result of the caisson tests the Pennsylvania class battleships were designed with a 4 layer system of thin plating, air space, thin plate, oil space, thin plate, air space, followed with an armored layer 9.5 feet (2.9 m) inboard. This allowed the explosion to dissipate in the air space and deform the liquid loaded back plate. This section withstood the explosion of 300 pounds (140 kg) of dynamite place against the hull of the caisson. This was considerably in advance of any navy at the time.
This class returned to steam driven geared turbine propultion, though Oklahoma of the prior Nevada class had experimented with reciprocating engines for fuel economy. Fore River was able to demonstrate better economic performance with geared steam turbines. The Pennsylvania class introduced the 4-engine 4-propeller scheme used in all further U.S. battleship designs.
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