This paper has been written with the SRN-2 and VA-3 type of craft in mind; they are basically over water craft but with a capability of operating over land for the purpose of embarking or disembarking passengers and freight or of crossing mud flats or sandbanks. Immersed sidewall craft have not been considered since their problems are similar to those of high-speed launches. Overland air-cushion vehicles will, as Mr. Lamb has pointed out, most likely operate over prepared routes and therefore navigation should not present any problems.

The main advantage offered by air-cushion craft is speed. The second advantage is its amphibious capability. Because of this an air-cushion craft can operate routes that are out of the question for any other kind of vehicle except perhaps a helicopter. An example of such a route is the proposed Rhyl–Hoylake ferry, which at low water is almost an overland route.