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The USS Monitor was a revolutionary iron-hulled steam warship, designed by John Ericsson.
The Monitor, the first iron warship commissioned by the United States Navy, was built during the American Civil War as an urgent response to the Confederacy's construction of the ironclad CSS Virginia (based on the former wooden steam frigate USS Merrimack). Monitor became famous for her central role in the Battle of Hampton Roads on 9 March 1862, where, under the command of John Lorimer Worden, she fought the Virginia to a standoff. 
'Iron Dawn' by Richard Snow provides an excellent account of the background to the politics and construction of Virginia/Merrimack and Monitor, and gives graphic accounts of their actions at sea. The book also provides much of the following information.
The impetus to build an ironclad in 1861 came from the Secretary of the Navy, Gideon Welles and Cornelius Bushnell (a 'venture capitalist' in today's terms). They established an Ironclad Board. Bushnell was introduced to Cornelius Delamater, who in turn brought his friend John Ericsson into the frame. Ericsson was endowed with quite remarkable ingenuity, engineering knowledge and dynamism. He was by no means bound by convention. He was also dogmatic and hot-tempered.
Ericsson produced a preliminary proposal for a novel all-iron vessel whose hull was nearly submerged, and which carried a single cannon in a revolving turret. At the end of August 1861 he wrote direct to Abraham Lincoln with his proposals. Lincoln apparently did not see the letter, but Benjamin Isherwood did, and rejected the idea. It should be noted that Ericsson's standing with the government had been tarnished by technical failures which were, rightly or wrongly, laid at his door. However, following Bushnell's intervention, Ericsson was ordered to proceed, and construct a vessel within a very tight timescale.
A partnership was formed, with Bushnell and Ericsson sharing a half interest, and John Winslow and John Griswold the other half (Winslow and Griswold were partners in the Rensselaer Iron Works and the Albany Iron Works). Ericsson was to be in charge of the design. His chief draughtsman was Charles MacCord.
The armour plates were produced by H. Abbott and Sons of Baltimore. The gun turret was made by the Novelty Iron Works of New York. The novel low-profile engine was made by the DeLamater Iron Works of New York. The hull was constructed by the Continental Iron Works. The two guns were designed by John A. Dahlgren. Ericsson wanted 15-inch guns, but he had to settle for 12-inch, and finally 11-inch. Even then, they were to be limited to a charge of 15 lbs of powder, rather than the 30 lbs for which they were designed.
The turret was 9 ft high and its internal diameter was 20 ft. The cylindrical part was assembled from 1-inch plates bolted together to the thickness of 8 inches, increased to 11 inches at the gun port area. At rest, the turret rested on a smooth ring of 'composition metal', but when being rotated it was raised by a wedge under the central spindle's bottom bearing. A large spur gear mounted on the spindle engaged with a train of gears driven by a small steam engine.
The hull was launched on 30 January 1862, without the gun turret. The 120-ton turret was installed soon after, and was first rotated using steam on 17 February. The vessel first moved under steam on 19 February.
On 8 March, the CSS Virginia engaged with warships of the US Navy, with devastating effect, sinking the Cumberland and destroying the Congress. Virginia was seen to be practically invulnerable, with nothing to prevent it proceeding to Washington and laying the city to waste. Panic ensued.
Prepared for action, Monotor headed south on 6 March, proving very stable in calm water. On the following date, rough sea conditions were enountered. Ericsson had intended the turret's circular base to be sealed by metal-to-metal contact, but mechanics at Brooklyn Navy Yard had inserted a gasket of hemp rope. In the rough sea, the penalty of the low freeboard made itself felt, and water readily found its way through the turret seal, soaking any crew in the vicinity. In the separate armoured wheelhouse, the helmsman was battered by water forcing its way in through small openings.
As the seas became heavier, water found its way into the short funnels and into the air intakes for the centrifugal fans which provided air for the boilers and for ventialtion. Worden had previously raised concern about the height of the funnels, and Engineer Stivens had criticised the height of the air intakes, at just 4 ft above the deck. Inevitabley, Ericsson was unreceptive to the criticisms.
Incoming water was sprayed around by the fans, drenching their drive belts. The blowers stopped, and the boilers were starved of air. Steam pressure fell. The main and auxiliary engines lost power. Carbon monoxide started to accumaulate. Crew members began to lose consciousness or passed out. others bravely went down to rescue them. Alban Stimers made repeated returns to the engine room after breaks to breathe fresh air.
Monitor was taken in tow by one of the accompanying vessels - the Seth Low - and towed to calmer waters near land, and normal conditions were restored. On returning to rough conditions, Monitor was in trouble again, both with the blowers and with the rudder's controlling ropes jumping off their guides. Again, the Seth Low towed Monitor to calmer conditions. Later, conditions improved, and Monitor continued to head towards Chesapeake Bay, while the crew set to work dealing with the effects of corrosion which resulted from the ingress of seawater, causing seizure of various parts of the machinery and guns. Monitor picked up a pilot, who provided news of the Virginia/Merrimack's exploits, with the warning that Virginia's destructive work would continue with the daylight and favourable tide. The USS Minnesota, 44 guns, stuck on a mudabnk, was at risk and Capt Worden decided to go to her aid. Minnesota's captain was not reassured by the sight of this risible semi-submerged vessel with just two guns. .............................
Note: Theodore Ruggles Timby had patented a revolving gun turret in the USA in 1843. In the UK Capt. Cowper Phipps Coles had started to develop gun turrets for ships in the 1850s, but Ericsson claimed that he had perfected the invention more than seven years before Captain Coles brought out his 'abortive scheme'.
Some of the Monitor's artefacts have been salvaged and conserved. See USS Monitor Center website here.