The Olympic-class liners were fitted with two triple expansion engines, which powered the port and starboard propellers, and a low-pressure turbine, which powered the center propeller. This propulsion system was first tested on the White Star sister ships Laurentic I(http://www.greatships.net/laurentic1.html) and Megantic (http://www.greatships.net/megantic.html), which entered service in 1909. Megantic was a conventional twin-screw steamer with two quadruple expansion engines, while Laurentic was a triple-screw steamer with two triple expansion engines and one turbine engine. Because Laurentic proved to be both more economical to operate and slightly faster, her engine configuration was selected for the Olympic-class liners, and was to remain a Harland & Wolff standard for years.