 The ship was powered by two back-acting steam engines driving one four-bladed propeller 21 feet (6.4 m) feet in diameter, using steam generated by six tubular boilers at a working pressure of 25 psi (172 kPa; 2 kgf / cm2). The engines, designed to produce 4,500 indicated horsepower (3,400 kW), and boilers were both subcontracted by Webb to the Etna Iron Works of New York City. The engines were originally intended to have a bore of 90 inches (2,286 mm) and a stroke of 45 inches (1,143 mm), but Webb increased the bore to 100 inches (2,540 mm) to insure that Dunderberg reached her contract speed. During the ship's first sea trial on 22 February 1867, she reached a speed of about 12.5 knots (23.2 km / h; 14.4 mph); during a demonstration for her French buyers on 12 June, Dunderberg reached 13.9 knots (25.7 km / h; 16.0 mph). She had two auxiliary boilers to provide steam for the steam engines that powered her pumps, ventilation fans and rotated the gun turrets. These engines, with their bore and stroke of 36 inches (914 mm), were larger than the main engines of the Passaic-class monitors. All of the boilers exhausted through a retractable funnel. The ship normally carried 540 long tons (550 t) of coal, but could hold a maximum of 1,000 long tons (1,000 t). She had a light brigantine rig that had a sail area of 11,170 square feet (1,038 m2).
