Page 65 - Theory and Design of Air Cushion Craft

Introduction 49

flexible extensions to the air jet nozzles. These skirts significantly enhanced the obsta-
cle clearance, giving real capability in a seaway as well as over rough ground and truly
demonstrated amphibious capabilities.
The necessary condition for a hovercraft with peripheral jets to take off from static
hovering to planing is that the air clearance of such a craft is at least larger than
0.5-0.8 times the depression caused by the cushion pressure on the water surface. If
the hover gap is less than this, the hard structure will interact with the induced waves
as the craft accelerates, greatly increasing the drag. However, for a hovercraft with
flexible skirts the air gap underneath the skirt itself is not a key condition for the take-
off of the craft, since the skirts will deform under the action of the induced wave-
form.
The flexible skirted ACV is easier to accelerate through hump speed in rough water
as well as over calm water for the same reasons - the skirt deforms under the action
of the water surface. Nevertheless the power needed to traverse the drag peak (or
'hump') between the displacement and the planing condition is relatively high for a
jetted skirt.
Engineers looked at different concepts of cushion geometry and skirts (see Chapter
7) to reduce the drag. Jet extensions gave way to bag skirts with smaller jet extensions
and eventually the jet extensions were replaced with convolutions called fingers by the
end of the 1960s (see Fig. 2.3). Thus, although the actual air clearance decreased
because of the lower air cushion efficiency of such skirt types, the effective cushion
depth greatly increased and both drag and required power were reduced.
The bag and finger type skirt has had great vitality, its design evolving continuously
from the mid 1960s to the present, with gradually improving resistance characteristics
and responsiveness, improving ride quality over rough surfaces. To date specific lift
power has decreased to 14.7-19.4 kW/t (20-25 shp/t) for current craft compared with
73 kW/t (100 shp/t) for early ACVs (SR.N1), i.e. a factor of five, due to decreasing air
clearance requirements beneath the flexible skirt.
At the present stage of development of ACVs and SES, the problems with respect
to take-off from displacement mode to planing mode are no longer a key technical
issue. Rather, the flexible skirt should be designed to meet the designer's seaworthi-
ness requirements. In this respect, the concept of wave pumping has been identified
as an important issue (the physical concept will be described later in this chapter)
and designers consequently pay more attention to pitch and heave damping
characteristics.
It may safely be taken for granted from this that it is better for designers to deter-
mine the lift power according to non-dimensional flow rate rather than relative air
gap. It is also more convenient and reasonable to calculate lift power according to
non-dimensional flow rate. The air gap is not a unique criterion for deciding the air
cushion performance of craft; however, by experience, one still can use the air gap as
one measure for assessing the air cushion performance.
Static air cushion theories derived in the 1960s are still suitable for describing the
powering performance of craft with modern flexible skirt designs and so these are
summarized below. The text in this chapter will proceed with air cushion theory from
early research; then the flow rate coefficient method; the wave-pumping concept and
its requirements; the determination of the heaving damping coefficient and finally the
heave stability derivatives of ACVs.