Page 59 - Theory and Design of Air Cushion Craft

Applications for ACV/SES 43

cushion mode, and an SES in on cushion mode. The US BH-110 craft had thick side-
walls, and a maximum speed of 38 knots. In off cushion mode, its cruising speed was
18 knots. Such craft possess an advantage for military applications, where loitering is
part of the mission.
There seems to be a misunderstanding on the seakeeping quality of SES:
some commentators considered in the developmental stage of ACV/SES that sea-
keeping was poor, and this view seems to persist. Following a series of measures to
improve seakeeping quality, SES are better than conventional displacement vessels
with the same displacement. For this reason the missions which in general are under-
taken by conventional vesssels could be undertaken by SES with lighter craft weight.
The air cushion catamaran 719-11, being operated between Shanghai municipality and
Chong-Ming Island, can be operated reliably in the same limiting sea state as for the
conventional catamaran weighing a thousand tons on the same route, though the all
up weight of the SES is only 220t. The seakeeping quality of SES can be improved
still further by measures such as improving skirt design, adopting high cushion length
beam ratio, improving sidewall configuration or adding anti-pitch hydrofoils, opti-
mising sidewall lines and installation of cushion damping systems.
The ride quality of fine hulled catamarans has improved greatly during the last
decade, partly due to the competition between the concepts of catamaran, SES and
hydrofoil. The catamaran concept is currently very attractive for speeds of up to 50
knots, for vessels in the up to 120m size range. It is likely that the SES will prove
attractive for applications in this size range at speeds above 50 knots, and for rather
larger craft in the future.

Speed

The air cushion is a device to reduce surface friction or over water drag. ACV and SES
have lower installed total power than other transport concepts for service speeds in
excess of 40 knots. This creates the prospect of lower operating costs for high speed
designs. These characteristics suggest that ACV and SES craft may be most effectively
applied where there are special requirements which cannot be fulfilled by any other
vehicle, or where there is a clear margin of efficiency which can justify a more complex
craft from the operational and maintenance point of view. An overview is given below.

Military applications

The ACV can be used effectively as an amphibious assault craft, across the shore land-
ing craft, guided missile craft, mine sweeper, mine layer or amphibious coastal patrol
craft. As one example, the US Navy continues to develop its amphibious landing fleet
with the LCAC, each of which can accommodate heavy or medium sized tanks and
landing troops. Landing ships constructed in the future must possess the capability to
accommodate the LCAC. The effectiveness of the US Navy craft, and Russia's equiv-
alent, has resulted in Japan forming its own squadron for coastal defence duties.
During the 1990s the design of the 55 tonne capacity LCAC and it's equivalent have
matured as service experience has suggested ways to cut build cost, and maintenance
analysis has shown approaches to minimize the operational cost. In the meantime