Page 63 - Theory and Design of Air Cushion Craft

SES and ACV design 47

directly by a small over-pressure of the air contained in the cushion. Our first aim is
to understand what defines the requirements for cushion air flow and pressure.
By reducing the contact area, drag is reduced. The air cushion interacts with the
water surface in a similar way to the classic potential theory developed by Lamb [208] in
the 1930s. In addition, flexible skirts, and the momentum of the cushion air, introduce
drag components. Once the means is available to estimate drag as well as the static
forces on an air cushion, stability, trim and manoeuvrability can be investigated.
Skirts are generally considered as a separate element for analysis, though they can
influence craft performance, particularly instabilities such as tuck under which may
lead to 'plough-in', and skirt bounce.
SES and ACV motions are complicated by the influence of the air cushion pressure
variations on the water surface, and resulting variations of dynamic trim. We present
non-linear analysis methods for solving the equations of motion in this text, as this
approach gives the most accurate means for estimation.
In the same way as for ships, it is often easier to carry out model tests than theo-
retical analyses, to determine some of the design parameters needed. SES and ACV
model testing needs careful interpretation, since responses which are susceptible to
different scaling laws (Froude and Reynolds) both have a significant effect on the total
response. Recommendations are discussed in Chapter 9.
The Design Methodology section of this book begins with a review of the basic
design requirements, including rules and regulations which have to be met. This is fol-
lowed by a discussion on the estimating methods to determine principal dimensions.
Once these have been estimated, the craft can be developed first by designing the lift
system, the skirt, the hull structure and propulsion system. Main engines can then be
formally selected, and after optimization of the main dimensions, consideration can
be given to the craft systems and controls, and internal outfit.