You have to go back to that period of time and what was being done for marine engines. Basically there were 3 designs , Vertical engines , 2 cylinder "V" Oscillating engines, and Horizontal (mill style) engines. But before we review those,we must consider that the Monitor was a totally new ship design, as all the war ships those days were wooden frigates with iron sheets on their sides ( Thus the term "Ironclad" ) . The Monitor was really the first "Stealth" warship. The ship was only 10-1/2 feet tall ( w/o Turret) and drew 9 feet of water. The propeller was 9 feet in Diameter, and this meant that the engine crank Center Line was at 4-1/2 feet above the keel. The engine could be no more than 4-1/2 feet above that in the engine room , which immediately ruled out Vertical and oscillating styles and forced the designer to a horizontal mode. But that was a problem as horizontal engines had to be mounted 90 degrees to the propeller shaft ( Called athwartship) and that required a very wide ship ( Beam) and the Monitor was narrow and shallow by design .
Now Horizontal engines mounted athwartship have the cylinder one one side ( say Left) and the crank on the other (Right) and the crank should match the propeller C/L making them extremely wide , but they did have second cranks on some called back-acting engines where the power is transferred back to Propeller C/L .....even the Monitors engine does that. But what John Ericsson did was put the cylinders (2) together in the center of the ship --Go here to see his 1858 ( pre Monitor design ) engine in action .. note the "back acting"connecting rods turning the propeller shaft ( very short videos )
http://www.stationarysteam.com/linked/p ... y_2-10.mp4
Now that engine was still too wide, so he went to a Trunk Design which is seen here in the next link
Only half the engine is shown , but it is a cutaway to see the trunk design, and this is exactly scaled - note size of the 9' propeller
http://www.stationarysteam.com/linked/r ... rom_sw.mp4
The cylinders were 40 bore with a 22 inch stroke and ideal for the Monitor, which is not only the first all steel warship but very low in the water so it is hard for the enemy to hit .. There are over 40 inventions in that ship. and John Ericsson gave them all to the US Gov.
Rich