In Memoriam: RMS Titanic
Olympic Class Propulsion
Once the decision had been made to sacrifice speed for luxury, Pirrie and Ismay worked out the details of propulsion. Since turbines were still relatively new, neither was comfortable using them as the sole means of driving their biggest, most expensive ships. The builder and the owner reached agreement to employ a unique combination of the mature reciprocating technology and the newer, more efficient turbine technology. In this way, they could improve the speed of the ship without sacrificing economy of operation.
Each ship was fitted with two 4-cylinder, triple expansion, direct acting, inverted reciprocating engines, balanced with the Yarrow, Schlick, and Tweedy system. They were arranged to take steam at 215 pounds per square inch and exhaust it at a pressure of around 9 pounds per square inch. The cylinders were 54 inches (HP), 84 inches (IP), 97 inches (LP), and 97 (LP) inches in diameter, with a stroke of 75 inches. Each engine was expected to produce 15,000 horsepower at 75 revolutions per minute. These engines drove the wing propellers and were used for moving astern or maneuvering in port.
The center propeller was driven by a low pressure Parsons turbine. The turbine took the 9 pounds per square inch exhaust from the reciprocating engines and had an exhaust pressure to the condensers of one pound per square inch. The Parson's turbine was capable of generating 16,000 shaft horsepower at 165 rpm.
The turbine was not reversible. When maneuvering in port or backing up, the ship used only the two wing propellers. As will be seen, the time to effect "full astern" from "full ahead" and the impact of the center propeller on the rudder would have dramatic consequences.
