Apart from the distinction in sizes mentioned above, two main categories of amphibious vehicle are immediately apparent: those that travel on an air-cushion (Hovercraft) and those that do not. Among the latter, many designs were prompted by the desire to expand the off-road capabilities of land-vehicles to an “all-terrain” ability, in some cases not only focused on creating a transport that will work on land and water, but also on intermediates like ice, snow, mud, marsh, swamp etc. This explains why many designs use tracks in addition to or instead of wheels, and in some cases even resort to articulated body configurations or other unconventional designs such as screw-propelled vehicles which use auger-like barrels which propel a vehicle through muddy terrain with a twisting motion.
Most land vehicles – even lightly armored ones – can be made amphibious simply by providing them with a waterproof hull and perhaps a propeller. This is possible as a vehicle’s displacement is usually greater than its weight, and thus it will float. Heavily armored vehicles however sometimes have a density greater than water (their weight in kilograms exceeds their volume in liters), and will need additional buoyancy measures. These can take the form of inflatable flotation devices, much like the sides of a rubber dinghy, or a waterproof fabric skirt raised from the top perimeter of the vehicle, to increase its displacement.
For propulsion in or on the water some vehicles simply make do by spinning their wheels or tracks, while others can power their way forward more effectively using (additional) screw propeller(s) or water jet(s). Most amphibians will work only as a displacement hull when in the water – only a small number of designs have the capability to raise out of the water when speed is gained, to achieve high velocity hydroplaning, skimming over the water surface like speedboats.