Page 90 - Theory and Design of Air Cushion Craft

74 Air cushion theory

Table 2.6 Craft operational modes with respect to the wave pumping motion
Operation mode due Running attitude Cushion over wave Cushion over wave
to wave pumping crest trough

Mode 1 Platforming Air blown off from Air feed to cushion to
cushion fill cavity
Mode 2 Cushion volume constant Craft lifted up Craft drops down

as a trough passes, the air gap under the skirts increases and volume flow increases.
The result is a rapid oscillation in the fan characteristic and vibration felt by opera-
tors. If lift power is not increased, skirt drag increases and speed reduces, often with
a bow-down trim induced and in very short chop possibly a plough-in tendency. In
very small sea states, small vibrations can be induced, which feel rather like driving a
car over cobbles, hence the effect is called 'cobblestoning'. Normally this only occurs
in craft which have a cushion with high volume flow rate.

Second operation mode - constant cushion volume
If the flow rate and cushion volume are held constant, keeping the lift power output
at a minimum, then a definite vertical acceleration will exert on the craft because of
wave pumping motion. Thus the maximum vertical acceleration can be derived under
the action of pumping as follows:
2
= 7rv ]/[10x/ c ] (2.35)
[
(d^) max
This calculation is approximate, because a lot of assumptions have been made. In par-
ticular, the heave and pitch motion of the craft in waves and air leakage around the
sides of cushion have not been considered, therefore the calculation is very simple and
does not demonstrate the seaworthiness quality of the craft.
It does, however, indicate the acceleration which will occur if the craft follows the
wave surface profile, where no reserve lift power or inflow rate is available. To reduce
this, it is necessary to allow the skirt to respond to the waves, which will then allow air
to be pumped out of the cushion. An example calculation for this is given below. The
aim of this calculation is to help designers to consider the reserve of lift power which
is needed to be available to counteract the extreme motion of craft operating in rough
seas.

Third operation mode - combination of first and second modes

The cushion pressure, cushion volume and the height of wet deck relative to the water
surface are changed together, namely trading-off both the foregoing motions. In prac-
tice this is the mode which practical ACVs operate in.

Platforming analysis

The first mode is platforming, i.e. the cushion pressure and the vertical position of the
wet deck remain constant, then the vertical acceleration will also be constant. This is
the ideal operating attitude of craft and what the designer requires. However, one has