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Flank speed is a nautical term referring to a ship's true maximum speed but it is not equivalent to the term full speed ahead. Usually, flank speed is reserved for situations in which a ship finds itself in imminent danger, such as coming under attack by aircraft. Flank speed is very demanding of fuel and often unsustainable because of propulsion system limitations. The related term emergency may not be any faster than flank but it indicates that the ship should be brought up to maximum speed in the shortest possible time.
Other speeds include one-third, two-thirds, standard and full. One-third and two-thirds are the respective fractions of standard speed. Full is greater than standard but not as great as flank. In surface ship nuclear marine propulsion, the difference between full speed and flank speed is of lesser significance, because vessels can be run at or very near their true maximum speed for a long time with little regard for fuel expended, an important consideration for oil fuelled ships.
In US nuclear submarine propulsion, full speed is 50 percent reactor power. Flank speed is 100 percent power, although limits may be reached for the propulsion turbine first stage pressure or for reactor thermal power (in MW) (depending upon the specifics of the individual propulsion plant) before 100 percent reactor power is reached. For flank speed, the reactor's main coolant pumps must also be shifted into fast speed.
"Flank speed" is exclusively an American phrase and as such is unknown in Commonwealth ("White Ensign") navies. The Commonwealth navies use the following telegraph commands:
- Slow ahead/astern, the number of revolutions is standardized for the individual ship and is unstated
- Half ahead/astern, accompanied by an order for a power setting (e.g., "half ahead both engines, revolutions 1,500")
- Full speed ahead/astern. This is reserved for emergencies and as such the word "speed" is included to distinguish it from the other commands mentioned. No power setting is expressed, it being implicit that maximum power is required
- Stavridis, James; Girrier, Robert (2007) [First published 1911]. Watch Officer's Guide: A Handbook for All Deck Watch Officers (Fifteenth ed.). Annapolis, Maryland: Naval Institute Press. p. 146. Retrieved 13 May 2015.
- Slade, Stuart (29 April 1999). "Speed Thrills III - Max speed of nuclear-powered aircraft carriers". NavWeaps. Retrieved 13 May 2015.