Wikipedia
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.
Usage examples of "flank speed".
Last we had him, he was at flank speed on a steady northeasterly bearing.
It just happened to look like Raj's own personal army was heading back to the East Residence at flank speed.
He too went to flank speed and chased after her for half an hour, coming down to five knots three minutes before the torpedoes ran out of fuel.
Instruct them to change vector to pursue the pirate at flank speed!
The normal procedure for countering a torpedo was turn and run, flank speed, in the opposite direction.
All ships move at flank speed to positions from which you can Fuse.
If the Kilrathi came on at flank speed, they'd get to Sirius a day and a half before the other two carriers could join up.
The flank speed time from the system where we were to Kessel was eighteen hours, and that was if the astrogator wanted to pull a Solo and skirt the fringes of the Maw.
If the Americans increased certain if their leading ships to flank speed, they would fire surface-to-surface missiles by 1000.
I bounced the Beetle out onto the street, turned left, and charged away at flank speed.
There was damn-all Brainard could even attempt now that the CO had called for flank speed.