Page 89 - Theory and Design of Air Cushion Craft

The 'wave pumping' concept 73

(2.34a)
> c
This relates the required flow to the escape area and should result in a small free air
gap under the inflated segment tips of a loop and segment skirted craft over concrete.

2.6 The 'wave pumping' concept

The flow rate, calculated by equation (2.34), may only meet the requirements of skirt
clearance for a craft hovering on calm water. As a matter of fact, craft often operate
in rough seas, in which the craft pitches and heaves. Therefore designers have to cal-
culate the vertical motion of craft in waves so as to determine the average required
flow; this will be demonstrated in detail in Chapter 8.
Here we introduce a concept [16], namely wave pumping, which deals with the
extreme hovering attitude of craft in waves. We assume that the cushion inflow rate of
craft operating in waves will stay constant, namely the same as that in the static hov-
ering condition. Thus the cushion flow changes as the volume occupied by the wave
which is passing through the craft changes, as shown in Fig. 2.23.
Consequent to this, the cushion pressure will fluctuate because of the fluctuation of
cushion outflow while constant inflow rate and the incompressibility of cushion air
are assumed. Thus, the motion caused by fluctuating cushion pressure is called 'wave
pumping' motion. To simplify the calculation, we assume as follows:
• Cushion air is incompressible.
• Waves are simple sinusoidal waves.
• Skirt clearances at bow/stern seals are constant, while the craft operates in head
waves.
• The wave peak will never contact the wet deck of craft.
• The lowest edge of the cushion (i.e. the base line of sidewalls) coincides with the
horizontal line of trough, namely no air leakage under the sidewalls.
Two typical situations for wave pumping motion of craft operating in waves are
shown in Table 2.6. In fact we may assume that the SES can operate in one of three
following modes.

First operation mode - platforming

In this mode of operation, the ACV or SES cannot respond to the waves, normally
short steep chop, and so as wave peaks pass through cushion pressure is raised, and

Fig. 2.23 Platforming of SES in waves.