Simple theory for calculating the pressure beneath a hovercraft is well known, see for example ref. 1. Of the two results, and , the former is usually preferred on the grounds of simplicity since over a large range of jet thickness/height ratios the difference between the two results is negligible. For large values of t\h the latter equation is preferred since it tends to a possible limiting value, i.e., whereas in the former equation. This latter is a consequence of one of the basic assumptions, i.e. that and since in the limiting case h=0 there can be no jet, then However, neither equation can be regarded as valid for x > 1 since for this condition a constant jet thickness equal to the slot width t cannot be maintained. In practice what happens in this condition (x > 1) is that the rate of mass flow in the jet is reduced.