Page 178 - Theory and Design of Air Cushion Craft

SES transverse dynamic stability 161

It can be seen that the transverse stability of the models reduces significantly dur-
ing take-off, particularly in the case of small heeling angle 9 = 2°. The transverse
stability even reduces to half of that at zero speed, though it increases rapidly above
hump speed.
Bogdanov showed that the craft bow was situated at the wave peak and the stern at
the trough when travelling at hump speed. The immersed sidewalls therefore cause
added wave-making at this speed. When a craft is heeling this will cause a deeper
trough at the stern for the immersed sidewall and in contrast, the trough would be
reduced at the stern of the emerged sidewall. The restoring moment is therefore
reduced due to such asymmetric drafts at both sidewalls and seals.
In the case where the craft speed is over the hump speed, the wave trough caused by
the sidewalls and air cushion system will be far behind the craft stern and the
immersed sidewall and seals will provide a large hydrodynamic force and righting
moment. The transverse righting moment therefore increases rapidly at speeds above
hump.

Transverse stability in waves
The transverse stability of hovercraft in waves needs to be considered together with
craft motions, particularly with respect to the roll characteristics of SES in waves. This
will be described further in Chapter 8.

Criteria and standards for the stability of SES
Criteria and standards for stability are a very important input to the design and con-
struction of SES. The standards derived from various national bodies are described in
Chapter 10. These vary somewhat. An approach to setting criteria is described below,
based on Andrew Blyth's work for the UK CAA reported in [42].
Designs should always be evaluated at several loading conditions within the
designed operating range, since this can often affect the results significantly. In order
to address the differing needs of different stages of the design process, as well as the
different levels of sophistication of analysis appropriate to craft ranging in size
between tens and thousands of tonnes in displacement, compliance with each crite-
rion may be demonstrated by a range of methods, ranging from simplistic formulae,
through more complex mathematical methods, to model tests or full-scale trials (if
appropriate).
Naturally, the more simplistic the method, the more important it is that the results
can be expected to be conservative. So the use of more sophisticated and hence expen-
sive techniques will often enable higher VCGs to be used with confidence. Failure to
pass the simple methods does not necessarily imply total unacceptability.

Static stability
The initial, lateral roll stiffness averaged over the range 0-5° of heel should not be less
than a transverse metacentric height (GMt) of 10% of the craft maximum beam,
when measured or calculated for a static longitudinal trim angle within about half
a degree of level keel. This is equivalent to a percentage CG shift per degree of
0.175. Calculation, model test or full-scale experiment are considered appropriate for
evaluation.