Flank speed

Flank speed is a nautical term referring to a ship's true maximum speed, beyond the speed that can be reached by traveling at full speed. 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 engine overheating issues.

For example, the most economic speed of the Littoral combat ship is 20 knots (37 km/h; 23 mph) providing a range of 4,300 nautical miles (8,000 km; 4,900 mi), and endurance of 215 hours. This ship has a flank speed of 50 knots (93 km/h; 58 mph) but can travel only 1,500 nautical miles (2,800 km; 1,700 mi) at flank speed, exhausting its fuel in 30 hours. Thus, its "flank speed" consumes fuel over seven times faster than most economic.

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. Emergency may not be any faster than flank, but indicates the ship should be brought up to maximum speed in the shortest possible time.

In surface ship nuclear marine propulsion, the differentiation between full speed and flank speed is of lesser significance, because these machines can be run at or very near their true maximum speed for virtually unlimited periods of time. In US Nuclear Submarine propulsion, full speed is 50% reactor power. Flank speed is 100% power, although depending upon the specifics of the individual propulsion plant, limits for the propulsion turbine first stage pressure, or limits for reactor thermal power (in MW) may be reached before 100% reactor power is reached. In addition, for flank speed, the reactor's main coolant pumps must also be shifted into fast speed.