Having observed the new 15-inch Rodman cannon in fortifications and disappointed with the performance of the 11-inch Dahlgren versus , Assistant Secretary of the Navy Gustavus Fox required the new monitors to be equipped with at least one gun of 15-inch caliber, resulting in rush production of a new 15-inch Dahlgren. The turret was in diameter inside with the gun mounted flush because the muzzle diameter was too large for the turret opening. The large volume of propellant gases released inside the turret required the addition of a "smoke box" at the muzzle in the interior of the turret. As a result, the 15 in gunners could not see their targets and had to aim with the or guns. "Lehigh" had her 11-inch smoothbore replaced with an 8-inch Parrot. "Passaic" also had this modification by July 1863, and, eventually, all surviving members of this class had an additional 15-inch smoothbore added.