By 1941, the steam turbine was the preferred marine steam engine because of its greater efficiency compared to earlier reciprocating compound steam engines. Steam turbine engines required very precise manufacturing techniques and balancing and a complicated reduction gear, however, and the companies capable of manufacturing them already were committed to the large construction program for warships. Therefore, a 140-ton vertical triple expansion compound steam engine of obsolete design was selected to power Liberty ships because it was cheaper and easier to build in the numbers required for the Liberty ship program and because more companies could manufacture it. Eighteen different companies eventually built the engine. It had the additional advantage of ruggedness and simplicity. Parts manufactured by one company were interchangeable with those made by another, and the openness of its design made most of its moving parts easy to see, access, and oil. The engine - 21 feet (6.4 m) long and 19 feet (5.8 m) tall - was designed to operate at 76 rpm and propel a Liberty ship at about 11 knots (20 km/h; 13 mph).