An Atlantic crossing speed does not represent every limit of a liner's machinery. A quoted non-emergency trial speed of 38.32 knots for SS United States leads to a distinction between ordinary cruising, sustained high-speed crossings and possible emergency performance. Commercial operation still required fuel economy, even when a ship was designed to maintain unusually high speed. Wartime accounts of Queen Mary and Queen Elizabeth exceeding 35 knots are presented as reports of emergency sprints, separate from their stated ability to sustain about 33 knots. Such speeds came with machinery costs, and liners could replace engines on schedules that navies would consider uneconomical.
Notable Point · 2:20:47-2:24:18
Sustained liner speed, trials and emergency power
Liners had different sustained, trial and emergency speeds, with fuel economy and machinery life determining which could be used routinely.