form a watertight seal but in service this proved to leak heavily, despite caulking by the crew. The gap between the turret and the deck proved to be another kind of problem for several s, which used the same turret design, as debris and shell fragments entered the gap and jammed the turrets during the First Battle of Charleston Harbor in April 1863. Direct hits at the turret with heavy shot also had the potential to bend the spindle, which could also jam the turret. Monitor was originally intended to mount a pair of smoothbore Dahlgren guns, but they were not ready in time and guns were substituted, each gun weighing approximately . Monitors guns used the standard propellant charge of specified by the 1860 ordnance instructions for targets "distant", "near", and "ordinary", established by the gun's designer Dahlgren himself. They could fire a round shot or shell up to a range of at an elevation of +15°. Later designs (1872) represented the culmination of this pioneering work. An ironclad turret ship designed by Edward James Reed, she was equipped with revolving turrets that used pioneering hydraulic turret machinery to maneouvre the guns. She was also the world's first mastless battleship, built with a central superstructure layout, and became the prototype for all subsequent warships. With her sister of 1871 she was another pivotal design, and led directly to the modern battleship. The US Navy tried to save weight and deck space, and allow the much faster firing 8-inch to shoot during the long reload time necessary for 12-inch guns by superposing secondary gun turrets directly on top of the primary turrets (as in the and s), but the idea proved to be practically unworkable and was soon abandoned. With the advent of the s in 1908, the main battery turrets were designed so as to superfire, to improve fire arcs on centerline mounted weapons. This was necessitated by a need to move all main battery turrets to the vessel's centerline for improved structural support. The 1906 , while revolutionary in many other ways, had retained wing turrets due to concerns about muzzle blast affecting the sighting mechanisms of a turret below. A similar advancement was in the s and s, which dispensed with the "Q" turret amidships in favour of heavier guns in fewer mountings. Like pre-dreadnoughts, the first dreadnoughts had two guns in each turret; however, later ships