Relativistic speed
Relativistic speed refers to speed at which relativistic effects become significant to the desired accuracy of measurement of the phenomenon being observed. Relativistic effects are those discrepancies between values calculated by models considering and not considering relativity. Related words are velocity, rapidity, and celerity which is proper velocity. Speed is a scalar, being the magnitude of the velocity vector which in relativity is the four-velocity and in three-dimension Euclidean space a three-velocity. Speed is empirically measured as average speed, although current devices in common use can estimate speed over very small intervals and closely approximate instantaneous speed. Non-relativistic discrepancies include cosine error which occurs in speed detection devices when only one scalar component of the three-velocity is measured and the Doppler effect which may affect observations of wavelength and frequency.Relativistic effects are highly non-linear and for everyday purposes are insignificant because the Newtonian model closely approximates the relativity model. In special relativity the Lorentz factor is a measure of time dilation, length contraction and relativistic mass increase of a moving object.
Flank speed
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.