Page 19 - Theory and Design of Air Cushion Craft

4 Introduction to hovercraft

immersed in salt water an important design parameter for marine vehicles.
Additionally, it suggested a number of alternatives to the basic principle of pumping air
into a cavity under a hull, using a modified wing form instead, to achieve vehicles with
speeds closer to that of aircraft. Several vehicle concepts have developed from this work.

Amphibious hovercraft (or ACV)

The amphibious hovercraft (Fig. 1.3) is supported totally by its air cushion, with an
air curtain (high pressure jet) or a flexible skirt system around its periphery to seal the
cushion air. These craft possess a shallow draft (or a negative draft of the hull struc-
ture itself) and amphibious characteristics. They are either passive (being towed by
other equipment) or active, i.e. propelled by air propellers or fans. Some 'hybrid' craft
have used surface stroking, balloon wheels, outboard motors and water jets to achieve
different utility requirements.

Fig. 1.3 First Chinese medium-size amphibious hovercraft model 722-1.

Sidewall hovercraft (or SES)

This concept (Figs 1.4 and 1.5) reduces the flexible skirt to a seal at the bow and stern
of a marine (non-amphibious) craft, using walls or hulls like a catamaran at the sides.
The walls or hulls at both sides of the craft, and the bow/stern seal installation, are
designed to minimize the lift power.
Due to the lack of air leakage at the craft sides, lift power can be reduced significantly
compared with an ACV. Also, it is possible to install conventional water propellers or
waterjet propulsion, with rather smaller machinery space requirements compared to that
for air propellers or fans used on ACVs. This more compact machinery arrangement,
combined with the possibility for higher cushion pressure supporting higher specific pay-
load, has made a transition to larger size much easier for this concept than for the ACV.