Passaic class monitor

USS Lehigh on the James River.
Class overview
Builders:	Continental Iron Works Harlan & Hollingsworth Harrison Loring Reaney, Son & Archbold Donohue, Ryan & Secor Atlantic Iron Works
Preceded by:	USS Monitor
Succeeded by:	Canonicus class
In commission:	25 November 1862 – 1899
Completed:	10
Lost:	2 USS Weehawken, foundered 6 December 1863 USS Patapsco, mined 15 January 1865
General characteristics
Displacement:	1,335  tons
Tons burthen:	844 (bm)
Length:	200 ft (61 m) overall
Beam:	46 ft (14 m)
Draught:	10 ft 6 in (3.20 m)
Propulsion:	2 Martin boilers, 1-shaft Ericsson vibrating lever engine, 320  ihp (235  kW)
Speed:	7  knots
Complement:	75
Armament:	1 × 15  in (381  mm) smoothbore 1 × 11  in (279  mm) smoothbore Lehigh, Patapsco: 1 × 15  in (381  mm) smoothbore 1 × 8  in (203  mm) Parrott rifle Camanche: 2 × 15  in (381  mm) smoothbore
Armor:	Iron Side: 5 – 3  in (12.7 – 7.6  cm) Turret: 11  in (27.9  cm) Deck: 1  in (2.5  cm)

The Passaic-class ironclad monitors of the U.S. Navy saw service in the U.S. Civil War and the Spanish–American War. This highly successful class was an improved version of the Monitor equipped with a XV-inch Dahlgren gun in place of one of the XI-inchers.

Naval architect and engineer John Ericsson designed the Passaic-class warships, drawing upon lessons learned from the first USS Monitor, which he also designed. The Passaic monitors were larger than the original Monitor and had their pilothouses atop the turret, rather than near the bow. This allowed a wider field of view and easier communications between captain, pilot and crew. The shape of the hull was an improvement with a less pronounced overhang than the Monitor. The Passaic class featured an 18 ft (5.5 m) funnel and improved ventilation.

Having observed the new 15" Rodman cannon in fortifications and disappointed with the performance of the XI-inch Dahlgren versus the CSS Virginia, Assistant Secretary of the Navy Gustavus Fox required the new monitors to be equipped with at least one gun of 15" caliber, resulting in rush production of a new XV-inch Dahlgren. The turret was 21 ft (6.4 m) in diameter inside with the 15 in (380 mm) 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 8 in (200 mm) or 11 in (280 mm) guns. The Lehigh had her 11 inch smoothbore replaced with an 8 inch Parrot. The 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.