Page 58 - Theory and Design of Air Cushion Craft

42 Introduction to hovercraft

passenger and car ferries, fast military logistics vessels, utility vehicles and, at larger
size, short sea container feeder transport.
In order to accommodate weapon systems on marine craft such as aeroplanes and
helicopters, conventional displacement craft sometimes have to be enlarged to provide
the required deck area and hangar space, and also follow by increasing main engine
output, construction and maintenance cost. The SES solves this problem efficiently,
and creates a new concept of ship design philosophy. For instance, a large conven-
tional aircraft carrier of 30,000 tonne displacement can be replaced by a lighter SES
only weighing several thousand tonnes. Helicopters can land or take off on or from
the SES weighing only 200-300t, compared to a conventional ship which displaces at
least a thousand tons.
A 100 ton ACV/SES can accommodate up to 300 passengers. This can also be
achieved on a conventional monohull with the same displacement, unlike smaller
craft. In order to accommodate twenty berths on a conventional planing hull, design-
ers have to select a craft displacement of about 30-50t and power the craft by two sets
of marine diesel 12V150 to achieve a speed of about 35 km/h. In contrast, owing to
the spacious cabin, an SES weighing only 20t can satisfy the requirement of berth
arrangement and can reach a speed of up to 50 km/h with the same main engines.
It is probably safe to assume that a 300t anti-submarine SES with upper deck
of about 50 X 12 = 600m 2 could provide a suitable flying deck/platform for land-
ing anti-submarine helicopters. This would improve anti-submarine capability sig-
nificantly by comparison with conventional anti-submarine warfare displacement
ships with the same displacement.

Development to larger size
For a fast boat, the 'fast' is always limited by its displacement. This means that
fast craft always appear with small displacement. Using the air cushion to sup-
port most of the weight, and with the existence of rigid sidewalls, it is relatively
simple to develop the SES to a large size (up to thousands of tons displace-
ment) without difficulty, with a selection of water propulsors such as water pro-
pellers, waterjet propulsion, etc. The air cushion distributes loads evenly over the
primary structure, so that while an SES hull is large, lightweight structural
design can be employed effectively, minimizing capital cost.
Similar to other high performance vehicles such as planing boats and hydro-
foils, ACV/SES also belong to the hydrodynamic support group of marine craft
(c.f. static or buoyancy support). The difference between the ACV/SES and plan-
ing hull and hydrofoil craft are that the ACV/SES lift system operates at very
low interface pressure, so that significant overload only leads to reduced craft
speed, and does not seriously affect take off capability.
One can also combine SES with other high performance vehicle characteristics to
create a hybrid craft obtaining higher performance. There are two modes of operation
for air cushion catamarans: off cushion and on cushion modes. The SES with ordi-
nary thin sidewalls has a large difference of speed between the two modes: the off
cushion speed is low at about 10-20 knots. In the case of an air cushion catamaran
with thick sidewalls, it will operate as a high speed catamaran in the case of off