This allowed a wider field of view and easier communications between captain, pilot and crew.

In combination, the significant hydrodynamic refinements and improved draught to the boilers resulted in a one-knot speed increase over the prototype monitor, despite the Passaic having a lower specific power than Monitor (Passaic being almost 400 tons heavier and yet with the same 320ihp-rated machinery).

Having observed the new 15-inch Rodman cannon in fortifications and disappointed with the performance of the 11-inch Dahlgren versus CSS Virginia, 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.

Passaic also had this modification by July 1863, and, eventually, all surviving members of this class had an additional 15-inch smoothbore added.

Later improvements included an additional 50 tons of deck plating over the magazines and machinery spaces as well as rings fitted around the turret and pilot houses to prevent their pivoting machinery from being jammed by shot.