The Titanic’s center propeller had four blades. It was the smallest of the ship’s three propellers and was driven by a centrally placed Parsons low-pressure steam turbine.
The two outer, or wing, propellers were larger and each had three blades. They were connected directly to Titanic’s two four-cylinder, triple-expansion reciprocating engines. The central turbine-and-propeller arrangement was a distinctive feature of the Olympic-class propulsion system.
The center propeller was not simply a smaller duplicate of the others. It could be stopped but could not be reversed, because the turbine was designed to rotate in one direction. During manoeuvres, the wing propellers provided most of the ship’s reversing and steering assistance.
This four-bladed centre screw helped turn steam energy into additional forward thrust without requiring a third reciprocating engine. Together, the three propellers supported Titanic’s normal rated output of 46,000 horsepower. The different blade counts are an easy source of confusion: the outer propellers had three blades each, while the central propeller had four.