The chief constructor, Sir Edward Reed, was ill, so the design of this ship was entrusted to his assistant and brother-in-law, Nathaniel Barnaby, himself a future chief constructor.
For reasons that have not survived, the Admiralty required that Penelope to be a ship of unusually shallow draught, possibly in light of the operations in the shallow Baltic Sea during the Crimean War of 1854–55.

The ship was 260 feet (79.2 m) long between perpendiculars and had a beam of 50 feet (15.2 m).
She had a draught of 15 feet 9 inches (4.8 m) forward and 17 feet 4 inches (5.3 m) aft.
Penelope displaced 6,034 long tons (6,131 t) and had a tonnage of 3,096 tons burthen.
She had a complement of 350 officers and ratings.
She was the first British capital ship to be fitted with a washroom.

Penelope had a pair of Maudslay three-cylinder, horizontal-return, connecting-rod steam engines, each driving a single 14-foot (4.3 m) propeller.
The engines used steam provided by four boilers with a working pressure of 30.5 psi (210 kPa; 2 kgf/cm²).
The ship reached a speed of 12.76 knots (23.63 km/h; 14.68 mph) from 4,703 indicated horsepower (3,507 kW) during her sea trials on 1 July 1868.
She carried a maximum of 500 tonnes (510 t) of coal, enough to steam 1,360 nautical miles (2,520 km; 1,570 mi) at 10 knots (19 km/h; 12 mph).

The shallow-draught requirement forced Barnaby to build her with twin screws, as a single screw of larger diameter would have been mounted insufficiently deep to be effective.
The Admiralty also wanted hoistable propellers as the reports from Pallas and Favorite, with their fixed propellers, were distinctly uncomplimentary about their sailing qualities.
She was the only twin-screw ship ever to have hoisting screws.
Provision for the hoisting frames and twin rudders forced a very unusual shape to the stern, which unintentionally greatly increased drag.
The other issue was that the shallowness of her draught made her very unhandy under sail, and she was described as "drifting to leeward in a wind like a tea tray".
Penelope was ship-rigged with three masts and a sail area of 18,250 square feet (1,695 m²).
Her speed under sail alone was only 8.5 knots (15.7 km/h; 9.8 mph).
Her shallow draught gave her a metacentric height of 2.7 feet (0.8 m) at deep load, which made her a very steady gun platform.

Penelope's main armament of eight rifled muzzle-loading (RML) 8-inch (203 mm) guns was concentrated amidships in a box battery.
The guns at the corners of the battery were given additional gun ports, embrasured into the sides of the hull, to give her a limited amount of end-on fire.
The shell of the 8-inch gun weighed 175 pounds (79.4 kg) and was rated with the ability to penetrate 9.6 inches (244 mm) of wrought-iron armour.
The ship mounted three rifled breech-loading (RBL) 5-inch (127 mm) Armstrong guns as chase guns, one in the stern and two under the forecastle in the bow, although these were judged to be very ineffective weapons.
She also carried a pair of RBL 20-pounder 3.75-inch (95 mm) Armstrong saluting guns.

The waterline wrought iron armour belt of Penelope covered her entire length.
It was 6 inches (152 mm) thick amidships, backed by 10–11 inches (254–279 mm) of wood, and thinned to 5 inches towards the ends of the ship.
It had a total height of 5 feet 6 inches (1.7 m), of which 4 feet (1.2 m) was below water and 1 foot 6 inches (0.5 m) above.
The sides of the 68-foot-long (20.7 m) box battery were also 6 inches thick, and its ends were protected by 4.5-inch (114 mm) bulkheads.
Between the battery and the belt was a 96-foot-long (29 m) strake of 6-inch armour, also closed off by 4.5-inch bulkheads.
The chief constructor, Sir Edward Reed, was ill, so the design of this ship was entrusted to his assistant and brother-in-law, Nathaniel Barnaby, himself a future chief constructor. For reasons that have not survived, the Admiralty required that Penelope to be a ship of unusually shallow draught, possibly in light of the operations in the shallow Baltic Sea during the Crimean War of 1854–55.
The ship was 260 feet (79.2 m) long between perpendiculars and had a beam of 50 feet (15.2 m). She had a draught of 15 feet 9 inches (4.8 m) forward and 17 feet 4 inches (5.3 m) aft. Penelope displaced 6,034 long tons (6,131 t) and had a tonnage of 3,096 tons burthen. She had a complement of 350 officers and ratings. She was the first British capital ship to be fitted with a washroom.
Penelope had a pair of Maudslay three-cylinder, horizontal-return, connecting-rod steam engines, each driving a single 14-foot (4.3 m) propeller. The engines used steam provided by four boilers with a working pressure of 30.5 psi (210 kPa; 2 kgf/cm²). The ship reached a speed of 12.76 knots (23.63 km/h; 14.68 mph) from 4,703 indicated horsepower (3,507 kW) during her sea trials on 1 July 1868. She carried a maximum of 500 tonnes (510 t) of coal, enough to steam 1,360 nautical miles (2,520 km; 1,570 mi) at 10 knots (19 km/h; 12 mph).
The shallow-draught requirement forced Barnaby to build her with twin screws, as a single screw of larger diameter would have been mounted insufficiently deep to be effective. The Admiralty also wanted hoistable propellers as the reports from Pallas and Favorite, with their fixed propellers, were distinctly uncomplimentary about their sailing qualities. She was the only twin-screw ship ever to have hoisting screws. Provision for the hoisting frames and twin rudders forced a very unusual shape to the stern, which unintentionally greatly increased drag. The other issue was that the shallowness of her draught made her very unhandy under sail, and she was described as "drifting to leeward in a wind like a tea tray". Penelope was ship-rigged with three masts and a sail area of 18,250 square feet (1,695 m²). Her speed under sail alone was only 8.5 knots (15.7 km/h; 9.8 mph). Her shallow draught gave her a metacentric height of 2.7 feet (0.8 m) at deep load, which made her a very steady gun platform.
Penelope's main armament of eight rifled muzzle-loading (RML) 8-inch (203 mm) guns was concentrated amidships in a box battery. The guns at the corners of the battery were given additional gun ports, embrasured into the sides of the hull, to give her a limited amount of end-on fire. The shell of the 8-inch gun weighed 175 pounds (79.4 kg) and was rated with the ability to penetrate 9.6 inches (244 mm) of wrought-iron armour. The ship mounted three rifled breech-loading (RBL) 5-inch (127 mm) Armstrong guns as chase guns, one in the stern and two under the forecastle in the bow, although these were judged to be very ineffective weapons. She also carried a pair of RBL 20-pounder 3.75-inch (95 mm) Armstrong saluting guns.
The waterline wrought iron armour belt of Penelope covered her entire length. It was 6 inches (152 mm) thick amidships, backed by 10–11 inches (254–279 mm) of wood, and thinned to 5 inches towards the ends of the ship. It had a total height of 5 feet 6 inches (1.7 m), of which 4 feet (1.2 m) was below water and 1 foot 6 inches (0.5 m) above. The sides of the 68-foot-long (20.7 m) box battery were also 6 inches thick, and its ends were protected by 4.5-inch (114 mm) bulkheads. Between the battery and the belt was a 96-foot-long (29 m) strake of 6-inch armour, also closed off by 4.5-inch bulkheads.
HMS Penelope (1867)