Page 85 - Theory and Design of Air Cushion Craft

Static air cushion characteristics on a water surface 69

vertical distance between the lower tip of the skirt and an undisturbed water surface.
Owing to the application of bag and finger skirts and the improvement in perfor-
mance through development, skirt clearance on a water surface has decreased year by
year.
One can observe that the skirt clearance on a water surface is very small on mod-
ern ACVs and sometimes the value may be negative for larger craft with responsive
skirts. Therefore, it is suggested that cushion air flow can be calculated by plenum
chamber theory or the foregoing methods applied to SESs. The peripheral jet requires
much higher air flow to seal the air cushion in the case where the craft hovers with a
significant gap to the calm water surface.
It is noted that the hovering process for an ACV with flexible skirt is more compli-
cated, and is shown in Fig. 2.20, in which the numbers are explained as follows:
1. This represents that the craft floats off cushion statically on a water surface, the
draft of craft is T 0.
2. Lift fan starts to operate, but owing to the low revolutions, fan pressure is low,
therefore T < T 0. Though the craft is partially supported by air pressure, the draft
of the buoyancy tank is still larger than zero to provide partial support of the craft.
3. Fans speed is continuously increasing. In the case of T = 0, namely zero draft of
the buoyancy tank, then the weight of the craft will be completely supported by
cushion lift.
4. The fan speed is increasing further, pressure remains almost constant while flow
rate is increased, thus the skirts begin to inflate. A positive hull clearance h' begins
to be gained, but smaller than design hull clearance.
5. The hull clearance is equal to design value h' s, a large amount of cushion air is now
leaking under the peripheral skirt, the volume being dependent on the fan charac-
teristic and lift power.

Fig. 2.20 Various static hovering positions of an ACV.