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 fuel-inefficient 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 differentiation 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 sustained duration with little regard for fuel expended, a top consideration of oil-powered ships.
In US nuclear submarine propulsion, full speed is 50% reactor power. Flank speed is 100% 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% reactor power is reached. In addition, 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: (1) "Slow Ahead/Astern", the number of revolutions is standardized for the individual ship, and as such is unstated; (2) "Half Ahead/Astern", accompanied by an order for a specific power setting (e.g., "half ahead both engines, revolutions 1500"); (3) "Full Speed Ahead/Astern". This is reserved for emergencies, and as such the word "speed" is included to distinguish it from the other commands previously mentioned. No specific power setting is expressed, it being implicit that maximum power is required.