In practice, none of the similarity principles can be strictly applied. For example, if an object has a large aspect ratio, scaling will produce a model, one of whose dimensions will be excessive small (or large) for the purpose of fabrication. Consider, as an example the motion of a submarine in sea water. It is of interest to determine the drag force acting on it. It will be shown below that dynamic similarity leads to a requirement

where is the characteristic speed of the submarine and is its transverse length scale. It is assumed here that the laboratory experiment is also carried out in sea water. For a prototype, one can expect =20 m/s and = 10 m. Under experimental conditions, one would like to specify = 0.4 m leading to = 400 m/s. Producing such high speeds of water may be impossible under laboratory conditions; if it were to be possible free surface and compressibility effects would drastically corrupt the force data. Hence, the extension of laboratory data to the field scale is a daunting task and must be carried out with care.