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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. 
For a description of the politics surrounding the Virginia/Merrimack and Monitor, information about their construction, graphic accounts of their actions in battle, 'Iron Dawn' by Richard Snow is highly recommended . This book, along with the excellent Wikipedia entry, provide 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. Modesty was no part of his character.
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 remarkably tight timescale. Just 100 days were proposed for the design and construction of a warship bristling with novel features. There was endless scope for things to go wrong, and, as it turned out, there would be no time to make significant changes if fundamental problems had arisen.
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 low-profile trunk engine, made by the DeLamater Iron Works of New York, incorporated several novel features. 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 for rotation it was raised by a wedge under the central spindle's bottom bearing, drawn in by tightening a nut. 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. The hull sat very low in the water, offering adversaries little in the way of a target. There were obvious risks in having such a small freeboard. Ericsson had designed the turret's circular base to be sealed, when not required to rotate, by metal-to-metal contact. However, before the ship left Brooklyn Navy Yard a gasket of hemp rope had been placed between the sealing faces. 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. Inevitably, Ericsson had been unreceptive to the criticisms.
Incoming water was sprayed around by the fans, drenching everything including the fans' drive belts. The blowers stopped, and the boilers were starved of air. Steam pressure fell. The main and auxiliary engines lost power. Carbon monoxide from the furnaces started to accumulate. 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 short 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 mudbank, was at risk of destruction by Virginia, and Capt Worden decided to go to her aid. Minnesota's captain was by no means reassured by the sight of this curious semi-submerged vessel with just two guns.
Captain Worden, as he bravely headed into close-quarters battle with Virginia, would have been all too aware of of Virginia's success against well-armed wooden warships, and of Monitor's own shortcomings. He had already experienced the effect of having such a low freeboard in heavy seas. He was unhappy about the lot of the helmsman in the pilot hose, a square structure made from rectangular iron slabs, bolted together and interlocking in the manner of a log cabin, a shape not conducive to deflecting shot and shell. He was more concerned about the slow firing rate, and the vulnerability of his armament: just one pair of guns in a single turret, whose cylindrical shape made it vulnerable to direct impact. . As for the gun crew, they would soon come to know the horror of being confined in the turret, unable to see what was going on outside, disorientated by noise and the heat and the smoke of the guns and the shock of shells striking the armour.
Monitor and Virginia were engaged in a 4 hour duel on 9 March.
Neither ship sustained major damage, but Monitor's wheelhouse was hit, putting Captain Worden out of the action.
With the low, flat deck, and limited defensive capability (just the two turret-mounted heavy guns), Monitor would have been very vulnerable to boarding. The wheelhouse and turret might then have been easily put out of action, but the opportunity did not arise.
Virginia was seriously hampered by poor manoueverability. Monitor had a poor firing rate (7 - 8 minutes between shots) and the turret's controllability proved to be poor.
The capability of both ships' armamament had been constrained: Virginia's by the limited availabiity of solid shot, Monitor's by restrictions on the amount of charge allowed to be used.
Monitor had to temporarily disengage to transfer ammunition from the hull to the turret. At the time, Virginia was becoming very short of powder. Also, the lower unarmoured part of the hull was becoming exposed as the weight reduced as coal was consumed. Despite the reducing draught, Virginia became stuck on a sandbank for a time.
Following the engagement, it was determined that Monitor had been hit 22 times, including nine hits to the turret and two hits to the pilothouse. She had managed to fire 41 shots. Virginia's armour sustained 97 indentations from the fire of Monitor and other ships.
Monitor was involved in the Battle of Drewry's Bluff.
The ship was sent to the Washington Navy Yard for repairs, arriving on 3 October.
Modifications included an iron shield around the top of the turret. To improve seaworthiness , a 30-foot funnel and taller fresh air vents were added. A large blower with its own engine was installed which drew fresh air down through the pilothouse. By November the ship was ready to return to service.
The US Navy quickly placed contracts for more 'Monitor-type' vessels designed by Ericsson, starting with the Passaic-class followed by the Canonicus-class monitors. The Passaic class had nearly twice the displacement of Monitor, and had one 11-inch and one 15-inch gun. Seaworthiness was improved. Some of the ships saw very long service.
In late December 1862, Monitor was ordered to set off to proceed to Beaufort, North Carolina, to join two Passaic-class monitors. The prospect was not relished by the crew, who did all they could to make the vessel watertight, including caulking the turret with oakum. Severe weather was encountered, and the vessel and crew were overwhelmed by the amount of water ingress. A consequence of having a low freeboard and shallow draft was low tolerance to water ingress. The ship was abandoned and sank south east of Cape Hatteras with the loss of sixteen men.
The achievement of Ericsson and all concerned in designing and constructing this novel warship in little over 100 days is astonishing.
The timing was critical, allowing Monitor to arrive at Hampton Roads in time to arrest Virginia's onslaught against the Union's wooden ships.
The actions involving CSS Virginia and USS Monitor provided a graphic illustration to the world at large of the new era in naval warfare. Wars do bring about step changes in technology, but it should not be thought that the technology demonstrated at Hampton Roads had appeared totally out of the blue. It may be interesting to examine the broader context.
The duel between Monitor and Virginia in 1862 was the first battle between ironclads. It was not, though, the first use of ironclads in battle. This claim belongs to the three French armoured batteries Dévastation, Lave and Tonnante, which attacked the Russian fort at Kinburn in October 1855. The garrison soon surrendered. The effectiveness of the ships' armour was clearly demonstrated by the number of hits received against the number of casualties. However, the Russians only had 24 lb short fired at about 1000 yards range, against 4-inch wrought iron armour. France built five of the Dévastation-class ironclad floating batteries, while Britain embarked on building five of the same basic design. One of these, HMS Trusty (1855), was later used in gun turret trials (see below). More were ordered in December 1855, with iron hulls. HMS Erebus (1856) had a 30-degree 'tumble-home' to help deflect shot.
Clearly the French and British navies were well aware of the benefits of armour plating. On the other hand, they were also concerned about the weight penalty, and its effect of speed and manoueverability, especially with the prospect of having to progressively increase the amount of armour to counter the inevitable improvements in firepower. The action at Hampton Roads certainly focused minds. CSS Virginia had dramatically illustrated the vulnerability of her unarmoured rivals. Then, the 'drawn' result of the duel between Monitor and Virginia in 1862 highlighted the limitations of their firepower, even though Monitor had the largest naval guns obtainable. Monitor fired 168-lb shot. In contrast, the largest guns provided for HMS Warrior in 1861 were just 100-pounders.
It is often stated that as a result of the Battle of Hampton Roads in 1862, the navies of Britain and France halted further construction of wooden-hulled ships. This is misleading. They were already locked in an ironclad arms race following France's laying-down of the Gloire in 1858. By the end of 1861 France had laid down sixteen ironclad ships of the line, while Britain had laid down ten and ordered the armouring of another nine wooden ships then under construction. Most of the early ironclads had wooden hulls protected by armour, but the floating batteries Terror, Erebus and Thunderbolt (1856) had iron hulls, as did the larger warship HMS Warrior, ordered in 1859. Wooden warship construction continued in parallel, while non-armoured iron ships were also being built, in recognition of both the advantages and disadvantages of iron hulls compared with wood, and in the light of accumulating experience with iron hulls. Only by the mid-1870s had wooden warship building ceased in Royal Navy Dockyards.. The Royal Navy's figures for ironclad ships in mid-1862 show 14 iron hulled ironclads under construction or afloat, 5 wooden-hulled ironclads afloat and 7 under construction. In the latter category, the Royal Sovereign was to be fitted with a Coles turret (see below).
An aspect of the design of Monitor's rival, CSS Virginia (Merrimack) which had a marked influence on the world's navies, was the provision of a ram.
Theodore Ruggles Timby had patented a revolving gun turret in the USA in 1843. Much to Ericsson's annoyance, his business partners initially paid Timby a royalty for the use of gun turrets.
In September 1861, shipbuilder Thomas F. Rowland of the Continental Iron Works approached Welles with a scheme to build a vessel with a turret mounted on a railroad turntable. This was rejected, but Ericsson invited Rowland to visit him. According to Ericsson's friend and biographer, William C. Church, Rowland 'was shown the model sent to Napoleon in 1854, and satisfied that he could claim no priority for his idea of a turret.'
Capt. Cowper Phipps Coles in the UK had started to develop gun turrets for ships in the mid 1850s, and in early 1861 his design for an iron ship with turrets, sloping armoured upper works, and double-walled iron hull was described and illustrated in 'The Engineer'. However Ericsson claimed to have perfected the invention more than seven years before Captain Coles brought out his 'abortive scheme'. This claim presumably relates to the scheme he offered to Napoléon III in September 1854, but no record of any such submission has been found. Coles had started with an ad hoc arrangement of a cannon on a raft during the Crimean War in 1855. He soon followed this with another raft, which was provided with hemispherical iron shield over the gun. In late 1861 he had fitted a revolving gun turret to HMS Trusty for trials. With a view to deflecting shot, Trusty's turret was in the form of a truncated cone. The trials of Trusty's turret were very widely reported in the press in the autumn of 1861. At that time the Royal Navy still needed to use sail in addition to steam power (long ocean voyages, inefficient and unreliable steam machinery, limited bunker capacity), and a penalty of Coles' turret's success at deflecting projectiles was consequential damage to the ship's own masts and rigging! Coles soon determined to do away with masts.
Ericsson was presumably aware of Cowper Coles's proposed iron ship with turrets, sloping armoured upper works, and low freeboard, published in early 1861. It is not known whether Ericsson was influenced by any aspects of the British iron-hulled 'Mark II' armoured batteries built for Crimean War service. Of course, with their fairly conventional arrangement they bore little resemblance to Monitor, with its minimal freeboard, heavily armoured turret. They did, though, have a broad beam, shallow draft, flat bottom, had engine-driven fans for ventilation, armoured houses on the deck for lookouts, with communication by rubber speaking tubes. They were relatively cheap armoured batteries, capable of only 5-6 knots, not 'ships of the line', but they did not fear the oceans. They were rigged for ocean crossings, and from 1857 HMS Terror was based in Bermuda.
The arrival of Monitor on the scene certainly shocked the Admiralty into action. After years of half-hearted development, it was decided to convert the recently built 131-gun HMS Royal Sovereign to accommodate Coles's turrets, and work started with dismasting at Portsmouth on 7 April 1862
Ericsson wanted his engine to have low height, narrow width, and to have its weight balanced either side of the vessel's axis. He acheived is aims with his 'vibrating lever' or 'half trunk' engine. The individual features largely followed established practice, but Ericsson arranged them in an ingenious way to acheive his aims.
In order to obtain symmetry about the longitudinal axis, the two cylinders were coaxial (the photo above gives the impression of one cylinder, but they are separated in the middle. Ericsson contrived to minimise the length penalty of piston rods, crossheads, connecting rods and cranks in two ways. First, he adopted the trunk engine arrangement (introduced by John Penn in the 1840s). Then, to further minimise length and to avoid having two crankshafts, he adopted the 'vibrating lever' or 'bell crank' arrangement. Ericsson had used something similar on his curious 'vibrating engine' of 1841, designed for USS Princeton. Excellent animation here. It may have been inspired the vibrating engine patented by Elijah Galloway in 1829.
According to Ericsson's biographer, the 'Monitor type' of engine had been fully tested in the Judith, Daylight, and other vessels 
An award-winning accurate scale model of Monitor's engine was constructed by Rich Carlstedt, who undertook a considerable amount of research, including examination of the salvaged engine. See here for background information, here for photographs, and here for a forum discussion.
One of Monitor's officers, Lt. S. Dana Greene, complained in an official report that on the perilous voyage to Hampton Roads, 'she has not the steam-power to go against a head-wind or sea.' He also stated that 'I do not consider this steamer a sea-going vessel'. In contrast, Engineer Stimer had written to Ericsson referring to 'the stormy passage which proved us to be the finest sea-boat I was ever in.' Ericsson chose to 'trust to the judgement of a practical engineer as to the real damage done, than to the gallant commanders of these vessels, ...'
Some of the Monitor's artefacts have been salvaged and conserved, including the turret, guns, engine, condenser, Worthington pump, and many smaller items. See USS Monitor Center website here.
Photographs of (inverted) turret here. This photograph in particular indicates how the cylinder of the turret was assembled from numerous wrought iron plates having their long axis vertical. Each layer appears to comprise a ring assembled from about 24-26 individual plates. The edges of the plates butted together, and the butt joints were offset between layers. The layers of plates were held together by numerous bolts. The inboard ends of the bolts and their nuts were originally covered by thin plates, perhaps to prevent injury if the gun crew were thrown against the sides. Bolted iron rings at the top and bottom were not covered, presumably being out of harm's way. Alternatively the covering plates might have been fitted to reduce the consequences of nuts and bolts flying out due to impact. This would have been a serious hazard under severe pounding (as the Royal Navy had found out in trials in 1858), but the cover plates seem thin for this duty.
There appear to be seven layers of plate near the camera, plus a thinner ring which protrudes beyond the others. This seems to be a continuous ring, fairly short in height. It was designed to make contact with the copper alloy sealing fixed to the hull. It is not known how the constructors set about making the sealing faces flat, but it would be easier to deal with a narrow protruding ring rather than the full width of the face. The copper alloy ring was assembled from 12 segments (Fig. 171 in book of drawings), and that would certainly have had its critical faces machined before assembly in the hull.
The photograph shows the roof of the turret comprising a grid of iron bars (including railway rails). The grid received some support at mid-span from struts connected to a forged bar at the bottom of the turret. The photograph also shows the coffin-shaped pivoted blocks used to cover the gun ports.
This high resolution contemporary photograph shows that the outboard ends of the turret plates' bolts were countersunk and peened to form hemispherical heads. We can also see the effect of three hits which caused minor damage, perilously close to a gun port. We can also see that each gun port was formed by boring three overlapping holes. Eight layers of plate are visible in the nearest port, and beyond those is a collar bolted to the inside of the turret, against which the pivoted covers located. The collar was required because the gun barrels were not long enough to project through the gun ports. The collar made up for the gap and protected the turret crew from the muzzle blast..
The high resolution 1862 photograph also shows, in the background, the wheelhouse, modified after the battle of Hampton Roads by the addition of sloping plates aimed at deflecting shot. We can also see how the deck and upper part of the hull are protected by riveted plates. Protection of the deck and the top of the turret was very limited, but there was no need to guard against plunging shot.
Also see here for drawings of the turret.
It was necessary to exit the hull by going through the turret. This was only possible when an opening in the hull was aligned with an opening in the turret's floor, which only occurred when the turret was trained on the centre line.
Part of Monitor's vulnerability to rough seas stems from water ingress at the turret's seating ring. It is said that the problem arose due to the insertion of a gasket, contrary to Ericsson's plan of having metal-to-metal contact. This sounds fine in principle, but it would be interesting to know to what extent adequately flat surfaces had been obtained on the seating faces of these 20 ft diameter components. Was there a problem, perhaps with the mating of these surfaces, which led to the decision to insert 60+ feet of plaited rope? If the faces were uneven, there is a limit to the ability of a rope gasket to provide a seal at every point.