Titanic, as with any ship, was her engines. Cunard had drawn heavily on Admiralty experience with turbines in building the Lusitania and Mauretania, but Harland and Wolff wasn’t able to utilize Admiralty design expertise, so the Titanic had to rely on “old- fashioned” reciprocating engines to drive her port and starboard wing screws, while her center screw was driven by a revolutionary low-pressure turbine. While not necessarily as up-to-date as the turbines of the big Cunard liners, the design proved to be a near-perfect compromise, generating very little vibration and being quite economical. The three engines together produced about 55,000-shaft horsepower, sufficient to push the 52,310-ton Titanic to a top speed (designed) of 24-25 knots. This was two knots slower than the Cunard ships, but it was fast enough to make her competitive on the North Atlantic, and there would be compensations for the slower speed.
The two reciprocating engines were the largest such engines ever built. As one of the leading technical journals of the day, Shipbuilder, explained, they were of the four-cylinder, triple-expansion, direct-acting inverted type, balanced on the Yarrow, Schlick and Tweedy system. (This latter bit was a system by which the throws on the engine’s crankshaft were set up in an asymmetrical but carefully calculated arrangement designed to maximize the thrust from each cylinder while minimizing vibration.) Each engine stood forty feet tall, with four cylinders–one high-pressure, one intermediate-pressure, and two low-pressure. Steam was fed into the high-pressure cylinder–which measured 54 inches in diameter–at 215 pounds per square inch (psi). As it expanded, the steam’s energy would drive the piston in that cylinder (this was the first “expansion”), then it would be vented into the 54-inch intermediate cylinder at 78 psi, driving that piston (this was the second “expansion”). The steam, now at 24 psi, would be ducted into the two low-pressure cylinders, each of which measured 97 inches across (this was the third “expansion”). The stroke of these engines–that is, the vertical distance each piston moved–was 75 inches; they could turn at a top speed of eighty revolutions per minute, driving massive three-bladed propellers that were twenty-three feet in diameter.
Once it left the low pressure cylinders, the steam wasn’t done working yet. Taking advantage of the exhaust steam venting from the two reciprocating engines, the steam was bled from the fourth cylinder of each engine and ducted to the low-pressure turbine at a pressure of 9 psia (pounds per square inch absolute–that’s right, it’s pressure was actually almost six pounds lower than atmospheric pressure, but because the steam loop in the Titanic‘s engines was a closed system, that is, it never vented to the atmosphere but continually recirculated, live steam could still do useful work even at such low pressures). This turbine drove the center shaft, on which was mounted a three-blade propellor sixteen feet in diameter. Spinning at a much higher speed than the reciprocating engines–as much as 175 rpm–the turbine created almost no vibration itself, while the other two engines turned in opposite directions, effectively damping each other out and creating one of the smoothest powerplants in operation, which translated only a very gentle movement to the structure of the ship. The center turbine did have one drawback: it could not be reversed, meaning that for the ship to go astern or if the other engines were reversed for an emergency stop, the turbine would be useless. However, this was considered to be no more than an inconvenience.